Patent Application
20120028149
The present invention relates to a method for shutting down an indirect internal reforming solid oxide fuel cell having a reformer in the vicinity of the fuel cell.
A solid oxide fuel cell (hereinafter sometimes referred to as SOFC) system usually includes a reformer for reforming a hydrocarbon-based fuel, such as kerosene and city gas, to generate a reformed gas as a hydrogen-containing gas, and an SOFC for electrochemically reacting the reformed gas and air for electric power generation.
The SOFC is usually operated at a high temperature of 550 to 1000° C.
Various reactions, such as steam reforming (SR), partial oxidation reforming (POX), and autothermal reforming (ATR), are used for reforming, and heating to a temperature at which catalytic activity is exhibited is necessary for using a reforming catalyst.
Steam reforming is a very large endothermic reaction. Also, the reaction temperature of the steam reforming is 550 to 750° C., which is relatively high, and the steam reforming requires a high temperature heat source. Therefore, an indirect internal reforming SOFC is known in which a reformer (internal reformer) is installed near an SOFC, and the reformer is heated using radiant heat from the SOFC and the combustion heat of the anode off-gas (gas discharged from the anode) of the SOFC as heat sources (Patent Literature 1).
Also, Patent Literature 2 discloses a method for shutting down the operation of a fuel cell, in which the stack temperature is decreased, while the fuel electrode layer side is maintained in a reducing condition, by supplying water, and hydrogen or a hydrocarbon-based fuel to the fuel cell, while decreasing their flow rates, in stopping electric power generation.
It is considered that when the method described in Patent Literature 2 is used, the anode can be maintained in a reducing atmosphere during the shutdown of the fuel cell, and the oxidative degradation of the anode can be prevented.
But, in the method described in Patent Literature 2, reliable reforming is not ensured when the SOFC anode is maintained in a reducing condition, using a hydrogen-containing gas obtained by reforming a hydrocarbon-based fuel. In other words, an unreformed hydrocarbon-based fuel may be discharged from the reformer and flow into the anode.
Particularly, in a case where a heavy hydrocarbon, such as kerosene, is used, when the heavy hydrocarbon leaks from the reformer and flows into the SOFC, the performance of the SOFC may be degraded due to carbon deposition.
It is an object of the present invention to provide a method for shutting down an indirect internal reforming SOFC, in which it is possible to prevent the oxidative degradation of the anode by a reformed gas, while reliably reforming a hydrocarbon-based fuel.
A first embodiment of the present invention provides a shutdown method for shutting down an indirect internal reforming solid oxide fuel cell including
a reformer having a reforming catalyst layer, for reforming a hydrocarbon-based fuel to produce a reformed gas,
a solid oxide fuel cell for generating electric power using the reformed gas, a combustion region for combusting an anode off-gas discharged from the solid oxide fuel cell, and
an enclosure for housing the reformer, the solid oxide fuel cell, and the combustion region,
wherein
a flow rate of the hydrocarbon-based fuel supplied to the reformer in a state in which the following conditions i to iv are all satisfied is represented as FkE, and a temperature condition of the reforming catalyst layer in the state is represented as TrE,
i) an anode temperature of the solid oxide fuel cell is steady,
ii) the anode temperature is less than an oxidative degradation temperature,
iii) in the reformer, the hydrocarbon-based fuel is reformed, and a reformed gas having a composition suitable to be supplied to an anode is produced, and
iv) an amount of the reformed gas produced is equal to or more than a requisite minimum flow rate FrMin for preventing oxidative degradation of the anode when the anode temperature of the solid oxide fuel cell is a temperature that is equal to or more than the oxidative degradation temperature,
stepwise flow rates Fk(j) of the hydrocarbon-based fuel are predetermined (wherein j is an integer of 1 or more and M or less, where M is an integer of 2 or more), where Fk(j) increases with an increase in j, Fk(M) which is the largest among Fk(j) is equal to FkE, and Fk(j) is equal to or more than a minimum value of hydrocarbon-based fuel flow rates at which the reformed gas at a flow rate that is equal to or more than FrMin can be obtained by a reforming method in a reaction temperature range of this reforming method, a type of this reforming method being a type of a reforming method which is performed after start of the shutdown method,
a flow rate of the hydrocarbon-based fuel supplied to the reformer at a point of time of the start of the shutdown method is represented as Fk0,
one or more temperature conditions Tr(j) of the reforming catalyst layer are found beforehand (wherein j is an integer of 1 or more and M−1 or less), in the temperature condition Tr(j) a flow rate of the reformed gas obtained when the hydrocarbon-based fuel at the flow rate Fk(j) is reformed in the reforming catalyst layer by a reforming method being FrMin, a type of this reforming method being a type of a reforming method which is performed after the start of the shutdown method,
when the anode temperature falls below the oxidative degradation temperature, supply of the hydrocarbon-based fuel to the reformer is stopped to complete the shutdown method, and
while the anode temperature does not fall below the oxidative degradation temperature, the shutdown method includes the following steps:
A1) measuring a reforming catalyst layer temperature T and comparing this measured temperature T with TrE;
B1) when T<TrE in step A1, performing the following steps B11 to B14 in order:
B11) increasing a temperature of the reforming catalyst layer,
B12) measuring the reforming catalyst layer temperature and comparing this measured temperature T with TrE,
B13) when T<TrE in step B12, returning to step B11, and
B14) when T≧TrE in step B12, adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer from Fk0 to FkE and moving on to step D1;
C1) when T≧TrE in step A1,
if there does not exist, among the predetermined hydrocarbon-based fuel flow rates Fk(j), a flow rate Fk(j) at which a corresponding temperature condition Tr(j) is equal to or less than the reforming catalyst layer temperature T measured in step A1 and which is equal to or more than Fk(1) and is smaller than FkE, then adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer from Fk0 to FkE and moving on to step D1, and
if there exists, among the predetermined hydrocarbon-based fuel flow rates Fk(j), one or more flow rates Fk(j) at which corresponding temperature conditions Tr(j) are equal to or less than the reforming catalyst layer temperature T measured in step A1 and which are equal to or more than Fk(1) and are smaller than FkE, then performing the following steps C11 to C17 in order:
C11) adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer from Fk0 to Fk(J),
where J represents j that gives the smallest Fk(j) among one or more flow rates Fk(j) at which corresponding temperature conditions Tr(j) are equal to or less than the reforming catalyst layer temperature T measured in step A1 and which are equal to or more than Fk(1) and are smaller than FkE,
C12) measuring the reforming catalyst layer temperature and comparing this measured temperature T with TrE,
C13) when T≦TrE in step C12, adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer to FkE and moving on to step D1,
C14) when T>TrE in step C12, comparing this T with Tr(J),
C15) when T>Tr(J) in step C14, returning to step C12,
C16) when T≦Tr(J) in step C14, increasing the flow rate of the hydrocarbon-based fuel supplied to the reformer from Fk(J) to Fk(J+1) and increasing J by 1, and
C17) after step C16, comparing J with M, and if J≠M, then returning to step C12, and if J=M, then moving on to step D1; and
D1) waiting for the anode temperature to fall below the oxidative degradation temperature.
The hydrocarbon-based fuel may include a hydrocarbon-based fuel having a carbon number of two or more.
It is preferred that the concentration of a compound having a carbon number of two or more in the reformed gas be 50 ppb or less on a mass basis.
A second embodiment of the present invention provides a shutdown method for shutting down an indirect internal reforming solid oxide fuel cell including
a reformer having a reforming catalyst layer, for reforming a hydrocarbon-based fuel to produce a reformed gas,
a solid oxide fuel cell for generating electric power using the reformed gas,
a combustion region for combusting an anode off-gas discharged from the solid oxide fuel cell, and
an enclosure for housing the reformer, the solid oxide fuel cell, and the combustion region,
wherein
a flow rate of the hydrocarbon-based fuel supplied to the reformer in a state in which the following conditions i to iv are all satisfied is represented as FkE,
i) an anode temperature of the solid oxide fuel cell is steady,
ii) the anode temperature is less than an oxidative degradation temperature,
iii) in the reformer, the hydrocarbon-based fuel is reformed, and a reformed gas having a composition suitable to be supplied to an anode is produced, and
iv) an amount of the reformed gas produced is equal to or more than a requisite minimum flow rate FrMin for preventing oxidative degradation of the anode when the anode temperature of the solid oxide fuel cell is a temperature that is equal to or more than the oxidative degradation temperature,
a flow rate of the hydrocarbon-based fuel supplied to the reformer at a point of time of start of the shutdown method is represented as Fk0,
a calculated value of a flow rate of the hydrocarbon-based fuel capable of being reformed at a measured temperature of the reforming catalyst layer by a reforming method is represented as FkCALC, a type of this reforming method being a type of a reforming method which is performed after the start of the shutdown method,
stepwise flow rates Fk(j) of the hydrocarbon-based fuel are predetermined (wherein j is an integer of 1 or more and M or less, where M is an integer of 2 or more), where Fk(j) increases with an increase in j, Fk(M) which is the largest among Fk(j) is equal to FkE, and Fk(j) is equal to or more than a minimum value of hydrocarbon-based fuel flow rates at which the reformed gas at a flow rate that is equal to or more than FrMin can be obtained by a reforming method in a reaction temperature range of this reforming method, a type of this reforming method being a type of a reforming method which is performed after the start of the shutdown method,
one or more temperature conditions Tr(j) of the reforming catalyst layer are found beforehand (wherein j is an integer of 1 or more and M−1 or less), in the temperature condition Tr(j) a flow rate of the reformed gas obtained when the hydrocarbon-based fuel at the flow rate Fk(j) is reformed in the reforming catalyst layer by a reforming method being FrMin, a type of this reforming method being a type of a reforming method which is performed after the start of the shutdown method,
when the anode temperature falls below the oxidative degradation temperature, supply of the hydrocarbon-based fuel to the reformer is stopped to complete the shutdown method, and
while the anode temperature does not fall below the oxidative degradation temperature, the shutdown method includes the following steps:
A2) measuring a reforming catalyst layer temperature T, calculating FkCALC using this measured temperature T, and comparing values of this FkCALC and FkE;
B2) when FkCALC<FkE in step A2, performing the following steps B21 to B24 in order:
B21) increasing a temperature of the reforming catalyst layer,
B22) measuring the reforming catalyst layer temperature T, calculating FkCALC using this measured temperature T, and comparing values of this FkCALC and FkE,
B23) when FkCALC<FkE in step B22, returning to step B21, and
B24) when FkCALC FkE in step B22, adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer from Fk0 to FkE and moving on to step D2;
C2) when FkCALC≧FkE in step A2,
if there does not exist, among the predetermined hydrocarbon-based fuel flow rates Fk(j), a flow rate Fk(j) at which a corresponding temperature condition Tr(j) is equal to or less than the reforming catalyst layer temperature T measured in step A2 and which is smaller than FkE, then adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer from Fk0 to FkE and moving on to step D2, and
if there exists, among the predetermined hydrocarbon-based fuel flow rates Fk(j), one or more flow rates Fk(j) at which corresponding temperature conditions Tr(j) are equal to or less than the reforming catalyst layer temperature T measured in step A2 and which are smaller than FkE, then performing the following steps C21 to C27 in order:
C21) adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer from Fk0 to Fk(J),
where J represents j that gives the smallest Fk(j) among one or more flow rates Fk(j) at which corresponding temperature conditions Tr(j) are equal to or less than the reforming catalyst layer temperature T measured in step A2 and which are smaller than FkE,
C22) measuring the reforming catalyst layer temperature, calculating FkCALC using this measured temperature T, and comparing values of this FkCALC and FkE,
C23) when FkCALC≦FkE in step C22, adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer to FkE and moving on to step D2,
C24) when FkCALC>FkE in step C22, comparing the reforming catalyst layer temperature T measured in step C22 with Tr(J),
C25) when T>Tr(J) in step C24, returning to step C22,
C26) when T≦Tr(J) in step C24, increasing the flow rate of the hydrocarbon-based fuel supplied to the reformer from Fk(J) to Fk(J+1) and increasing J by 1, and
C27) after step C26, comparing J with M, and if J≠M, then returning to step C22, and if J=M, then moving on to step D2; and
D2) waiting for the anode temperature to fall below the oxidative degradation temperature.
The hydrocarbon-based fuel may include a hydrocarbon-based fuel having a carbon number of two or more.
In this case, it is preferred that the concentration of a compound having a carbon number of two or more in the reformed gas be 50 ppb or less on a mass basis.
A third embodiment of the present invention provides a shutdown method for shutting down an indirect internal reforming solid oxide fuel cell including
a reformer having a reforming catalyst layer, for reforming a hydrocarbon-based fuel to produce a reformed gas,
a solid oxide fuel cell for generating electric power using the reformed gas,
a combustion region for combusting an anode off-gas discharged from the solid oxide fuel cell, and
an enclosure for housing the reformer, the solid oxide fuel cell, and the combustion region,
wherein
a flow rate of the hydrocarbon-based fuel supplied to the reformer in a state in which the following conditions i to iv are all satisfied is represented as FkE, and a temperature condition of the reforming catalyst layer in the state is represented as TrE,
i) an anode temperature of the solid oxide fuel cell is steady,
ii) the anode temperature is less than an oxidative degradation temperature,
iii) in the reformer, the hydrocarbon-based fuel is reformed, and a reformed gas having a composition suitable to be supplied to an anode is produced, and
iv) an amount of the reformed gas produced is equal to or more than a requisite minimum flow rate FrMin for preventing oxidative degradation of the anode when the anode temperature of the solid oxide fuel cell is a temperature that is equal to or more than the oxidative degradation temperature,
a flow rate of the hydrocarbon-based fuel supplied to the reformer at a point of time of start of the shutdown method is represented as Fk0,
when the anode temperature falls below the oxidative degradation temperature, supply of the hydrocarbon-based fuel to the reformer is stopped to complete the shutdown method, and
while the anode temperature does not fall below the oxidative degradation temperature, the shutdown method includes the following steps:
A3) measuring a reforming catalyst layer temperature T and comparing this measured temperature T with TrE;
B3) when T<TrE in step A3, performing the following steps B31 to B34 in order:
B31) increasing a temperature of the reforming catalyst layer,
B32) measuring the reforming catalyst layer temperature and comparing this measured temperature T with TrE,
B33) when T<TrE in step B32, returning to step B31, and
B34) when T≧TrE in step B32, adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer from Fk0 to FkE and moving on to step D3;
C3) when T≧TrE in step A3,
performing the following steps C31 to C35 in order:
C31) measuring the reforming catalyst layer temperature and comparing this measured temperature T with TrE,
C32) when T≦TrE in step C31, adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer to FkE and moving on to step D3,
C33) when T>TrE in step C31, calculating a flow rate FkMinCALC of the hydrocarbon-based fuel at which the reformed gas at the flow rate FrMin can be produced in the reformer at the reforming catalyst layer temperature T measured in step C32 by a reforming method, a type of this reforming method being a type of a reforming method which is performed after the start of the shutdown method, and comparing values of this FkMinCALC and FkE,
C34) when FkMinCALC<FkE in step C33, adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer to FkMinCALC and returning to step C31, and
C35) when FkMinCALC≧FkE in step C33, adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer to FkE and moving on to step D3; and
D3) waiting for the anode temperature to fall below the oxidative degradation temperature.
The hydrocarbon-based fuel may include a hydrocarbon-based fuel having a carbon number of two or more.
In this case, it is preferred that the concentration of a compound having a carbon number of two or more in the reformed gas be 50 ppb or less on a mass basis.
A fourth embodiment of the present invention provides
a shutdown method for shutting down an indirect internal reforming solid oxide fuel cell including
a reformer having a reforming catalyst layer, for reforming a hydrocarbon-based fuel to produce a reformed gas,
a solid oxide fuel cell for generating electric power using the reformed gas,
a combustion region for combusting an anode off-gas discharged from the solid oxide fuel cell, and
an enclosure for housing the reformer, the solid oxide fuel cell, and the combustion region,
wherein
a flow rate of the hydrocarbon-based fuel supplied to the reformer in a state in which the following conditions i to iv are all satisfied is represented as FkE,
i) an anode temperature of the solid oxide fuel cell is steady,
ii) the anode temperature is less than an oxidative degradation temperature,
iii) in the reformer, the hydrocarbon-based fuel is reformed, and a reformed gas having a composition suitable to be supplied to an anode is produced, and
iv) an amount of the reformed gas produced is equal to or more than a requisite minimum flow rate FrMin for preventing oxidative degradation of the anode when the anode temperature of the solid oxide fuel cell is a temperature that is equal to or more than the oxidative degradation temperature,
a flow rate of the hydrocarbon-based fuel supplied to the reformer at a point of time of start of the shutdown method is represented as Fk0,
a calculated value of a flow rate of the hydrocarbon-based fuel capable of being reformed at a measured temperature of the reforming catalyst layer by a reforming method is represented as FkCALC, a type of this reforming method being a type of a reforming method which is performed after the start of the shutdown method,
when the anode temperature falls below the oxidative degradation temperature, supply of the hydrocarbon-based fuel to the reformer is stopped to complete the shutdown method, and
while the anode temperature does not fall below the oxidative degradation temperature, the shutdown method includes the following steps:
A4) measuring a reforming catalyst layer temperature T, calculating FkCALC using this measured temperature T, and comparing values of this FkCALC and FkE;
B4) when FkCALC<FkE in step A4, performing the following steps B41 to B44 in order:
B41) increasing a temperature of the reforming catalyst layer,
B42) measuring the reforming catalyst layer temperature T, calculating FkCALC using this measured temperature T, and comparing values of this FkCALC and FkE,
B43) when FkCALC<FkE in step B42, returning to step B41, and
B44) when FkCALC≧FkE in step B42, adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer from Fk0 to FkE and moving on to step D4;
C4) when FkCALC≧FkE in step A4, adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer from Fk0 to FkE and moving on to step D4; and
D4) waiting for the anode temperature to fall below the oxidative degradation temperature.
The hydrocarbon-based fuel may include a hydrocarbon-based fuel having a carbon number of two or more.
In this case, it is preferred that the concentration of a compound having a carbon number of two or more in the reformed gas be 50 ppb or less on a mass basis.
A fifth embodiment of the present invention provides
a shutdown method for shutting down an indirect internal reforming solid oxide fuel cell including
a reformer having a reforming catalyst layer, for reforming a hydrocarbon-based fuel to produce a reformed gas,
a solid oxide fuel cell for generating electric power using the reformed gas,
a combustion region for combusting an anode off-gas discharged from the solid oxide fuel cell, and
an enclosure for housing the reformer, the solid oxide fuel cell, and the combustion region,
wherein
a flow rate of the hydrocarbon-based fuel supplied to the reformer in a state in which the following conditions i to iv are all satisfied is represented as FkE, and a temperature condition of the reforming catalyst layer in the state is represented as TrE,
i) an anode temperature of the solid oxide fuel cell is steady,
ii) the anode temperature is less than an oxidative degradation temperature,
iii) in the reformer, the hydrocarbon-based fuel is reformed, and a reformed gas having a composition suitable to be supplied to an anode is produced, and
iv) an amount of the reformed gas produced is equal to or more than a requisite minimum flow rate FrMin for preventing oxidative degradation of the anode when the anode temperature of the solid oxide fuel cell is a temperature that is equal to or more than the oxidative degradation temperature,
a flow rate of the hydrocarbon-based fuel supplied to the reformer at a point of time of start of the shutdown method is represented as Fk0,
when the anode temperature falls below the oxidative degradation temperature, supply of the hydrocarbon-based fuel to the reformer is stopped to complete the shutdown method, and
while the anode temperature does not fall below the oxidative degradation temperature, the shutdown method includes the following steps:
A5) measuring a reforming catalyst layer temperature T and comparing this measured temperature T with TrE;
B5) when T<TrE in step A5, performing the following steps B51 to B54 in order:
B51) increasing a temperature of the reforming catalyst layer,
B52) measuring the reforming catalyst layer temperature T and comparing this measured temperature T with TrE,
B53) when T<TrE in step B52, returning to step B51, and
B54) when T≧TrE in step B52, adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer from Fk0 to FkE and moving on to step D5;
C5) when T≧TrE in step A5, adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer from Fk0 to FkE and moving on to step D5; and
D5) waiting for the anode temperature to fall below the oxidative degradation temperature.
The hydrocarbon-based fuel may include a hydrocarbon-based fuel having a carbon number of two or more.
In this case, it is preferred that the concentration of a compound having a carbon number of two or more in the reformed gas be 50 ppb or less on a mass basis.
The present invention provides a method for shutting down an indirect internal reforming SOFC, in which it is possible to prevent the oxidative degradation of the anode by a reformed gas, while reliably reforming a hydrocarbon-based fuel.
Embodiments of the present invention will be described below, using drawings, but the present invention is not limited thereto.
In this specification, a “steam/carbon ratio” refers to a ratio of the number of moles of water molecules to the number of moles of carbon atoms in a gas supplied to a reforming catalyst layer, and an “oxygen/carbon ratio” refers to a ratio of the number of moles of oxygen molecules to the number of moles of carbon atoms in a gas supplied to the reforming catalyst layer.
One embodiment of an indirect internal reforming SOFC in which the present invention can be carried out is schematically shown in
The indirect internal reforming SOFC includes a reformer 3 for reforming a hydrocarbon-based fuel to produce a reformed gas (hydrogen-containing gas). The reformer includes a reforming catalyst layer 4.
The indirect internal reforming SOFC includes an SOFC 6 for generating electric power using the above reformed gas, and also includes a combustion region 5 for combusting an anode off-gas discharged from the SOFC (particularly the anode of the SOFC).
The indirect internal reforming SOFC includes an enclosure 8 for housing the reformer, the solid oxide fuel cell, and the combustion region.
The indirect internal reforming SOFC refers to the enclosure (module container) 8 and equipment included in the interior of the enclosure.
In the indirect internal reforming SOFC in the embodiment shown in
Each supply gas is supplied to the reformer or the SOFC, after being appropriately preheated as required.
A water vaporizer 1 equipped with an electrical heater 2 is connected to the indirect internal reforming SOFC, and piping for supplying the hydrocarbon-based fuel to the reformer is connected to the midstream of connection piping for the water vaporizer 1. The water vaporizer 1 generates steam by heating with the electrical heater 2. The steam may be supplied to the reforming catalyst layer after being appropriately superheated in the water vaporizer or downstream thereof.
Also, air is supplied to the reforming catalyst layer, and here, air can be supplied to the reforming catalyst layer after being preheated in the water vaporizer. Steam or a mixed gas of air and steam can be obtained from the water vaporizer.
The steam or the mixed gas of air and steam is mixed with the hydrocarbon-based fuel and supplied to the reformer 3, particularly to the reforming catalyst layer 4 of the reformer 3. When a liquid fuel, such as kerosene, is used as the hydrocarbon-based fuel, the hydrocarbon-based fuel may be supplied to the reforming catalyst layer after being appropriately vaporized.
The reformed gas obtained from the reformer is supplied to the SOFC 6, particularly to the anode of the SOFC 6. Although not shown, air is appropriately preheated and supplied to the cathode of the SOFC.
Combustible components in the anode off-gas (gas discharged from the anode) are combusted by oxygen contained in a cathode off-gas (gas discharged from the cathode) at the SOFC outlet. In order to do this, ignition using the igniter 7 is possible. The outlets of both the anode and the cathode are open in the module container 8. The combustion gas is appropriately discharged from the module container.
The reformer and the SOFC are housed in one module container and modularized. The reformer is disposed at a position where it can receive heat from the SOFC. For example, when the reformer is located at a position where it receives thermal radiation from the SOFC, the reformer is heated by thermal radiation from the SOFC during electric power generation.
In the indirect internal reforming SOFC, the reformer is preferably disposed at a position where radiation heat can be directly transferred from the SOFC to the outer surface of the reformer. Therefore, it is preferred that there be substantially no obstacle between the reformer and the SOFC, that is, it is preferred to make the region between the reformer and the SOFC be an empty space. Also, the distance between the reformer and the SOFC is preferably as short as possible.
The reformer 3 is heated by the combustion heat of the anode off-gas generated in the combustion region 5. Also, when the temperature of the SOFC is higher than that of the reformer, the reformer is also heated by radiation heat from the SOFC.
Further, the reformer may be heated by heat generation by reforming. When the reforming is partial oxidation reforming, or when the reforming is autothermal reforming and heat generation by a partial oxidation reforming reaction is larger than endothermic heat by a steam reforming reaction, heat is generated with the reforming.
In this specification, a state in which all of the following conditions i to iv are satisfied is referred to as a reforming-stoppable state.
i) The anode temperature of the SOFC is steady.
ii) The above-described anode temperature is less than an oxidative degradation temperature.
iii) In the reformer, the hydrocarbon-based fuel is reformed, and a reformed gas having a composition suitable to be supplied to the anode is produced.
iv) The amount of this reformed gas produced is equal to or more than the requisite minimum flow rate FrMin for preventing the oxidative degradation of the anode when the anode temperature of the SOFC is a temperature that is equal to or more than the oxidative degradation temperature.
<Conditions i and ii>
The anode temperature means the temperature of the anode electrode, but may be the temperature of a stack-constituting member, such as a separator, near the anode when it is difficult to physically directly measure the temperature of the anode electrode. With respect to the location for the measurement of the anode temperature, it is preferred to use a position where the temperature becomes relatively high, more preferably a position where the temperature becomes the highest, in terms of safe control. A location where the temperature becomes high may be found by preliminary experiment or simulation.
The oxidative degradation temperature is a temperature at which the anode is oxidatively degraded. For example, the electrical conductivity of the anode material is measured by a DC four-terminal method, with the temperature varied, in a reducing or oxidizing gas atmosphere, and the oxidative degradation temperature may be determined as the lowest temperature at which the electrical conductivity in the oxidizing gas atmosphere becomes lower than that in the reducing gas atmosphere.
<Condition iii>
The condition iii means a state in which in the reformer, the hydrocarbon-based fuel is reformed, and a reformed gas having a composition suitable to be supplied to the anode is obtained. For example, when the hydrocarbon-based fuel includes a hydrocarbon-based fuel(s) having a carbon number of two or more, the condition iii means a state in which the reformed gas is reducing, and a concentration of a C2+ component(s) (one or more compounds having a carbon number of two or more) in the reformed gas is not more than a concentration which does not cause any problem in view of anode degradation and flow blockage due to carbon deposition. The concentration of the C2+ component(s) in this case is preferably 50 ppb or less as a mass fraction in the reformed gas.
<Condition iv>
The requisite minimum reformed gas flow rate FrMin for preventing the oxidative degradation of the anode is the smallest flow rate among the flow rates at which the anode electrode is not oxidatively degraded by the diffusion of the cathode off-gas into the interior of the anode from the anode outlet. This reformed gas flow rate may be found beforehand by performing an experiment or a simulation, while varying a reformed gas flow rate, in a state in which the anode temperature is maintained at the oxidative degradation temperature or higher.
The oxidative degradation of the anode may be judged, for example, by measuring the electrical conductivity of the anode electrode by experiment and comparing it with that of an anode electrode not oxidatively degraded. Alternatively, the oxidative degradation of the anode may be judged by calculating the compositional partial pressure of the anode gas by simulation using an equation including an advection-diffusion term and comparing it with equilibrium partial pressure in the oxidation reaction of the anode electrode. For example, when the anode electrode material is Ni, the equilibrium partial pressure of oxygen in an anode electrode oxidation reaction represented by the following formula is 1.2×10−14 atm (1.2×10−9 Pa) at 800° C., and if the calculated value of the oxygen partial pressure of the anode is smaller than this value, then it can be judged that the anode electrode is not oxidatively degraded. Also when the anode temperature is a temperature other than 800° C., the maximum value of oxygen partial pressures at which the anode electrode is not oxidatively degraded may be found by equilibrium calculation, and if a calculated value of the oxygen partial pressure of the anode is smaller than this value, then it can be judged that the anode electrode is not oxidatively degraded.
Ni+0.5O2NiO
The flow rate of the reformed gas supplied to the SOFC (the amount of the reformed gas produced in the reformer) in order to prevent the oxidative degradation of the anode is preferably a flow rate such that the reformed gas is combustible at the stage of being discharged from the anode after passing through the SOFC. When the smallest flow rate among the flow rates of thus combustible reformed gas is larger than the above-described requisite minimum reformed gas flow rate, the smallest flow rate among the flow rates of the combustible reformed gas may be considered to be a reformed gas flow rate “equal to or more than the requisite minimum flow rate” referred to in the condition iv. It is possible to judge whether a gas is combustible or not, for example, by sampling a gas in the combustion gas discharge line and performing composition analysis in experiment, or by calculating in simulation.
The first embodiment of the shutdown method of the present invention will be described.
The flow rate of the hydrocarbon-based fuel supplied to the reformer (particularly, the reforming catalyst layer) in the reforming-stoppable state is represented as FkE.
FkE may be obtained beforehand by experiment or simulation. FkE may be found by performing an experiment or a simulation, while varying flow rates of fluids supplied to the indirect internal reforming SOFC, such as the flow rate of water (including steam) for steam reforming or autothermal reforming and the flow rate of air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, a cathode air flow rate, the flow rates of a fuel and air supplied to a burner, and flow rates of fluids, such as water and air, supplied to a heat exchanger; and electrical input and output to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, water and liquid fuel evaporators, the SOFC, fluid supply piping, and the like, and electrical input taken out from a thermoelectric conversion module and the like, that is, varying the operation conditions of the indirect internal reforming SOFC, and searching for FkE that steadily satisfies the conditions i to iv. FkE may be any value as long as the conditions i to iv are satisfied, but in terms of thermal efficiency, the smallest FkE is preferably used. The operation conditions of the indirect internal reforming SOFC, including the FkE, are determined beforehand as operation conditions in the reforming-stoppable state.
The temperature condition of the reforming catalyst layer in the reforming-stoppable state is represented as TrE. TrE may be found together with FkE in the process of search for FkE, and TrE is a temperature condition of the reforming catalyst layer that corresponds to the single FkE used.
M different stepwise hydrocarbon-based fuel flow rates Fk(j) are determined beforehand. Here, j is an integer of 1 or more and M or less, where M is an integer of 2 or more.
Here, Fk(j) increases with the increase in j. In other words, Fk(j)<Fk(j+1). Also, Fk(M) which is the largest among Fk(j) has a value that is equal to FkE.
In this case, a steam/carbon ratio and/or an oxygen/carbon ratio that correspond to each Fk(j) may be set for each Fk(j). Also, when a gas not contributing to a reaction is supplied to the reforming catalyst layer, its flow rate may be set for each Fk(j). The steam/carbon ratios, the oxygen/carbon ratios, or the flow rates of the gas not contributing to a reaction, which are set correspondingly to Fk(j), may have the same value for all Fk(j) or may differ from one Fk(j) to another.
Further, Fk(j) is equal to or more than the minimum value (Fkmin(j)) of hydrocarbon-based fuel flow rates at which the reformed gas at a flow rate that is equal to or more than FrMin can be obtained by a reforming method. A type of this reforming method is that of a reforming method which is performed after the start of the shutdown method.
This minimum value Fkmin(j) may be obtained by preliminary experiment or simulation. For example, a Fkmin(j) may be found by measuring or calculating reformed gas flow rates by using the reforming method of the type performed after the start of the shutdown method as a reforming method, varying the flow rate of the hydrocarbon-based fuel and the temperature of the reforming catalyst layer, setting the flow rates of steam and/or oxygen to flow rates obtained from a steam/carbon ratio and/or an oxygen carbon ratio that correspond to the flow rate of the hydrocarbon-based fuel and Fk(j), and setting, when supplying a gas not contributing to a reaction to the reforming catalyst layer, a flow rate of this gas to the flow rate that corresponds to the Fk(j); and obtaining the minimum value of flow rates of the hydrocarbon-based fuel at which the reformed gas flow rate is FrMin. The reforming method is, for example, steam reforming, autothermal reforming, or partial oxidation reforming. Alternatively, for example, when a rate of a reaction other than a reaction accompanied by the decrease of the hydrocarbon-based fuel (raw fuel) supplied to the reforming catalyst layer is much faster than that of the reaction accompanied by the decrease of the raw fuel, and it can be considered that components other than the raw fuel instantaneously reach an equilibrium composition, a Fkmin(j) may be obtained by equilibrium calculation using a Gibbs energy minimization method or the like. For example, using a Fk(j), a steam/carbon ratio and/or an oxygen carbon ratio that correspond to the Fk(j), and, when supplying a gas not contributing to a reaction, a gas flow rate of this gas, a composition of a gas supplied to the reforming catalyst layer is obtained, and a gas composition excluding the maximum value of the raw fuel composition suitable to be supplied to the anode (an allowable raw fuel composition) is obtained. By using the obtained composition, and total pressure or the partial pressure of each component, and varying the flow rate of the raw fuel and temperature, equilibrium calculation is performed to calculate gas flow rates for an equilibrium composition. By adding the gas flow rate of the allowable raw fuel composition to the calculated gas flow rates for the equilibrium composition, the flow rates of a reformed gas having a composition in which the raw fuel flow rate is the largest, among reformed gas compositions suitable to be supplied to the anode, are calculated, and the minimum value of the raw fuel flow rates at which the reformed gas flow rate is equal to or more than FrMin may be set as the Fkmin(j).
It is assumed that there exist one or more Fk(j) that are smaller than FkE and are equal to or more than Fkmin(j). When there do not exist one or more such Fk(j), it is possible to perform step A1, and when T<TrE in step A1, perform step B1, and when T≧TrE in step A1, adjust the flow rate of the hydrocarbon-based fuel supplied to the reformer from Fk0 to FkE and move on to step D1.
Also, it is assumed that when the reforming catalyst layer temperature is TrE, Fk(j) that is smaller than FkE is reformable. When there do not exist one or more such Fk(j), it is possible to perform step A1, and when T<TrE in step A1, perform step B1, and when T≧TrE in step A1, adjust the flow rate of the hydrocarbon-based fuel supplied to the reformer from Fk0 to FkE and move on to step D1. Whether Fk(j) that is smaller than FkE is reformable when the reforming catalyst layer temperature is TrE may be found, for example, by finding beforehand temperature conditions TR(j) in which reforming is possible when the hydrocarbon-based fuel at the flow rates Fk(j) is reformed in the reforming catalyst layer, and comparing TR(j) with TrE. In this case, if TrE≧TR(j), then it may be considered that when the reforming catalyst layer temperature is TrE, Fk(j) that is smaller than FkE is reformable. The TR(j) may be obtained by preliminary experiment or simulation. For example, a TR(j) may be found by analyzing or calculating the concentrations of the reformed gas components by using the reforming method of the type performed after the start of the shutdown method as a reforming method, setting the flow rate of the hydrocarbon-based fuel to a Fk(j), setting steam and/or oxygen flow rates to flow rates obtained from steam/carbon ratios and/or oxygen carbon ratios that correspond to the flow rate of the hydrocarbon-based fuel and Fk(j), supplying, when supplying a gas not contributing to a reaction to the reforming catalyst layer, this gas at a flow rate that corresponds to the Fk(j), and varying the temperature of the reforming catalyst layer; and obtaining the minimum temperature of the reforming catalyst layer at which a reformed gas with a composition suitable to be supplied to the anode is obtained.
Fk(j) may be set, for example, at equal intervals.
It is preferred to make M as large as possible and make the interval between Fk(j) small, in terms of thermal efficiency. For example, it is possible to make M as large as possible and make the interval between Fk(j) small, within the allowable range of the memory consumption of a flow rate controlling means, and within a range in which the interval exceeds the precision of a pressure increasing means and flow rate controlling and measuring means.
The flow rate of the hydrocarbon-based fuel supplied to the reformer at the point of time of the start of the shutdown method is represented as Fk0.
[Tr(j) that Corresponds to Fk(j)]
The temperature conditions Tr(j) of the reforming catalyst layer are found beforehand (wherein j is an integer of 1 or more and M−1 or less). In a temperature condition Tr(j), the flow rate of the reformed gas obtained when the hydrocarbon-based fuel at a flow rate Fk(j) is reformed in the reforming catalyst layer is FrMin. The temperature conditions Tr(j) may be obtained by preliminary experiment or simulation. For example, a temperature condition Tr(j) may be found by measuring or calculating reformed gas flow rates by using the reforming method of the type performed after the start of the shutdown method as a reforming method, setting the flow rate of the hydrocarbon-based fuel to a Fk(j), setting the steam and/or oxygen flow rates to flow rates obtained from a steam/carbon ratio and/or an oxygen carbon ratio that correspond to the flow rate of the hydrocarbon-based fuel and Fk(j), setting, when supplying a gas not contributing to a reaction to the reforming catalyst layer, a flow rate of this gas to a flow rate that correspond to the Fk(j), and varying the temperature of the reforming catalyst layer; and obtaining a temperature of the reforming catalyst layer at which the reformed gas flow rate is FrMin. Alternatively, for example, when a rate of a reaction other than a reaction accompanied by the decrease of the hydrocarbon-based fuel (raw fuel) supplied to the reforming catalyst layer is much faster than that of the reaction accompanied by the decrease of the raw fuel, and it can be considered that components other than the raw fuel instantaneously reach an equilibrium composition, a Tr(j) may be obtained by equilibrium calculation using a Gibbs energy minimization method or the like. For example, using a Fk(j), a steam/carbon ratio and/or an oxygen carbon ratio that correspond to the Fk(j), and, when supplying a gas not contributing to a reaction, a gas flow rate of this gas, a composition of a gas supplied to the reforming catalyst layer is obtained, and a gas composition excluding the maximum value of the raw fuel composition suitable to be supplied to the anode (an allowable raw fuel composition) is obtained. By using the obtained composition, and total pressure or the partial pressure of each component, and the Fk(j), and varying the temperature, equilibrium calculation is performed to calculate gas flow rates for an equilibrium composition. By adding the gas flow rate of the allowable raw fuel composition to the calculated gas flow rates for the equilibrium composition, the flow rates of a reformed gas having a composition in which the raw fuel flow rate is the largest, among reformed gas compositions suitable to be supplied to the anode, are calculated, and a temperature at which the reformed gas flow rate is FrMin is obtained, and this temperature may be set as the Tr(j).
Specifically, when a certain type of reforming is performed before the start of the shutdown method, the same type of reforming as this (for example, steam reforming) may be performed after the start of the shutdown method. In this case, the temperature conditions of the reforming catalyst layer in which the flow rate of the reformed gas obtained by reforming the hydrocarbon-based fuel at the flow rates Fk(j) in the reforming catalyst layer is FrMin, when this type of reforming is performed in the reformer, are Tr(j). For example, when steam reforming is performed before the start of the shutdown method, steam reforming may also be continuously performed after the start of the shutdown method, and the temperature conditions of the reforming catalyst layer in which the reformed gas at the flow rate FrMin is obtained from the hydrocarbon-based fuel at the flow rates Fk(j) when steam reforming is performed in the reformer are Tr(j).
Alternatively, when a certain type of reforming (a first type of reforming) is performed before the start of the shutdown method, a different type of reforming from this (a second type of reforming) may be performed after the start of the shutdown method. In this case, the temperature conditions of the reforming catalyst layer in which the flow rate of the reformed gas obtained by reforming the hydrocarbon-based fuel at the flow rates Fk(j) is FrMin, when the second type of reforming is performed in the reformer, are Tr(j). For example, when autothermal reforming is performed before the start of the shutdown method, the reforming may be switched to steam reforming after the start of the shutdown method. In this case, the temperature conditions of the reforming catalyst layer in which the reformed gas at the flow rate FrMin is obtained from the hydrocarbon-based fuel at the flow rates Fk(j) when steam reforming is performed are Tr(j).
In order to determine a flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer, the measured value of the reforming catalyst layer temperature is compared with the above TrE or the above temperature conditions Tr(j). In order to do this, the reforming catalyst layer temperature is measured. For example, the reforming catalyst layer temperature may be monitored (continuously measured).
When the monitoring of the temperature of the reforming catalyst layer has been performed since before the start of the shutdown method, the temperature monitoring may be continuously performed as it has been.
When the anode temperature falls below the oxidative degradation temperature, the reducing gas becomes unnecessary, and therefore, the supply of the hydrocarbon-based fuel to the reformer can be stopped to complete the shutdown method. Therefore, the monitoring of the temperature of the reforming catalyst layer may be continuously performed until the anode temperature falls below the oxidative degradation temperature.
An appropriate temperature sensor, such as a thermocouple, may be used for the measurement of the reforming catalyst layer temperature.
[Case where Reforming Method is Changed Before and after Start of Shutdown Method]
When the reforming method is changed before and after the start of the shutdown method, the above-described FkE and TrE are determined for a stoppable state when reforming after the change of the reforming method is performed. Also, the above-described FrMin is determined for the stoppable state when reforming after the change of the reforming method is performed.
While the anode temperature does not fall below the oxidative degradation temperature, the following steps A1 to D1 are performed. When the anode temperature falls below the oxidative degradation temperature, the supply of the hydrocarbon-based fuel to the reformer can be stopped, regardless of the status of the implementation of steps A1 to D1, to complete the shutdown method. It is possible to stop the supply of fluids supplied to the indirect internal reforming SOFC, such as water (including steam) for steam reforming or autothermal reforming and air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, cathode air, the fuel and air supplied to the burner, and fluids, such as water and air, supplied to the heat exchanger; and the input and output of electricity to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, the water and liquid fuel evaporators, the cell stack, the fluid supply piping, and the like, and electrical input taken out from the thermoelectric conversion module and the like, according to the stop of the supply of the hydrocarbon-based fuel to the reformer.
The shutdown method includes steps A1 to D1, but it is not necessary to actually perform all of steps A1 to D1, and only part of steps A1 to D1 may be performed according to the circumstances.
First, a reforming catalyst layer temperature T is measured. Then, the magnitude relationship between this temperature T and the above-described TrE is checked.
When T<TrE in step A1, the following steps B11 to B14 are performed in order. “T<TrE” means that the hydrocarbon-based fuel at the flow rate FkE cannot be reformed.
Step B11
First, step B11 is performed. In other words, the step of increasing the temperature of the reforming catalyst layer is performed.
For example, the temperature of the reforming catalyst layer is increased using an appropriate heat source, such as a heater or a burner annexed to the reformer.
Step B12
Then, step B12 is performed. In other words, the step of measuring a reforming catalyst layer temperature T and comparing the values of this T and TrE is performed.
Step B13
When T<TrE in step B12, the method returns to step B11. In other words, while T<TrE, steps B11 to B13 are repeatedly performed. During this time, the temperature of the reforming catalyst layer increases.
In performing steps B12 and B13, the temperature increase in step B11 may be stopped once, but while steps B12 and B13 are performed, step B11 may be continued.
Step B14
When T≧TrE in step B12, the step of adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer (represented as Fk) from Fk0 to FkE and going to step D1 is performed. “T≧TrE” means that the hydrocarbon-based fuel at a flow rate that is equal to or less than FkE can be reformed.
At this time, in a case where the reforming method should be changed before and after the start of the shutdown method, the fuel flow rate is adjusted from Fk0 to FkE, and the reforming method is changed. By this method, it is possible to prevent the oxidative degradation of the anode with the reformed gas, while reliably reforming the hydrocarbon-based fuel.
When T≧TrE in step A1, step C1 is performed. “T≧TrE” means that the hydrocarbon-based fuel at a flow rate that is equal to or less than FkE can be reformed. In this case, if there does not exist, among the hydrocarbon-based fuel flow rates Fk(j) determined beforehand, a flow rate Fk(j) at which the corresponding temperature condition Tr(j) is equal to or less than the reforming catalyst layer temperature T measured in step A1 and which is equal to or more than Fk(1) and is smaller than FkE (this flow rate Fk(j) is hereinafter referred to as a “selective flow rate”), then the flow rate of the hydrocarbon-based fuel supplied to the reformer is adjusted from Fk0 to FkE, and the method goes to step D1. At this time, in a case where the reforming method should be changed before and after the start of the shutdown method, it is possible to adjust the fuel flow rate from Fk0 to FkE and change the reforming method.
For example, if j=2 is considered, and if Tr(2) is equal to or less than the reforming catalyst layer temperature T measured in step A1, and Fk(1)≦Fk(2)<FkE, then there exists a selective flow rate Fk(2). If there does not exist any such a selective flow rate, then the above-described operation is performed.
On the other hand, when T≧TrE in step A1, and if there exists, among the predetermined hydrocarbon-based fuel flow rates Fk(j), the above-described selective flow rate Fk(j), then the following steps C11 to C17 are performed in order.
Step C11
j that gives the smallest Fk(j) among the selective flow rates is represented as J, and the flow rate of the hydrocarbon-based fuel supplied to the reformer is adjusted from Fk0 to Fk(J). At this time, in a case where the reforming method should be changed before and after the start of the shutdown method, the flow rate Fk of the hydrocarbon-based fuel is adjusted from Fk0 to Fk(J), and the reforming method is changed.
Step C12
The reforming catalyst layer temperature is measured, and this measured temperature T is compared with TrE.
Step C13
When T≦TrE in step C12, the flow rate of the hydrocarbon-based fuel supplied to the reformer is set to FkE, and the method goes to step D1.
Step C14
When T>TrE in step C12, this T is compared with Tr(J).
Step C15
When T>Tr(J) in step C14, the method returns to step C12. In other words, when T>TrE in step C12 and T>Tr(J) in step C14, steps C12, C13, and C15 are repeatedly performed. During this time, the reforming catalyst layer temperature decreases. Therefore, eventually, T≦Tr(J) is satisfied.
Step C16
When T≦Tr(J) in step C14, the flow rate of the hydrocarbon-based fuel supplied to the reformer is increased from Fk(J) to Fk(J+1), and J is increased by 1.
Step C17
After step C16, J is compared with M, and if J≠M, then the method returns to step C12, and if J=M, then the method goes to step D1.
In step C1 and the subsequent steps, from a state in which T≧TrE, it is possible to set operation conditions to the operation conditions in the reforming-stoppable state, and bring the internal reforming solid oxide fuel cell to the reforming-stoppable state without allowing the unreformed hydrocarbon-based fuel to flow into the anode. But, generally, within a temperature range preferred for reforming, as the reforming catalyst layer temperature T becomes higher, the reformed gas flow rate becomes larger. Therefore, while T≧TrE, the reformed gas flow rate is equal to or more than FrMin, and this means that an excessive hydrocarbon-based fuel is supplied.
On the other hand, in step C1 and the subsequent steps, by supplying the hydrocarbon-based fuel at a flow rate as small as possible, among the flow rates of the hydrocarbon-based fuel at which the reformed gas at a flow rate that is equal to or more than FrMin can be produced and which are smaller than FkE, to the reformer, it is possible to make the hydrocarbon-based fuel supplied to the reformer as small as possible, while producing a reformed gas at a flow rate that is equal to or more than FrMin in the reformer. But, if the supply of the hydrocarbon-based fuel at a flow rate as small as possible, among the flow rates of the hydrocarbon-based fuel at which the reformed gas at a flow rate that is equal to or more than FrMin can be produced, is continued, then T<TrE may be satisfied due to the decrease of the reforming catalyst layer temperature, and there may be a case where the hydrocarbon-based fuel at the flow rate as small as possible, among the flow rates of the hydrocarbon-based fuel at which the reformed gas at a flow rate that is equal to or more than FrMin can be produced, cannot be reformed. Further, there may be a case where it is impossible to set operation conditions to the operation conditions in the reforming-stoppable state and bring the internal reforming solid oxide fuel cell to the reforming-stoppable state without allowing the unreformed hydrocarbon-based fuel to flow into the anode.
Therefore, when T≦TrE is satisfied, by setting operation conditions to the operation conditions in the reforming-stoppable state, it is possible to bring the internal reforming solid oxide fuel cell to the reforming-stoppable state without allowing the unreformed hydrocarbon-based fuel to flow into the anode.
Also, when T>TrE, by supplying the hydrocarbon-based fuel at a flow rate as small as possible, among the flow rates of the hydrocarbon-based fuel at which the reformed gas at FrMin or more can be produced, to the reformer, it is possible to make the hydrocarbon-based fuel supplied to the reformer as small as possible, while producing a reformed gas at a flow rate that is equal to or more than FrMin in the reformer.
In step D1, the method waits for the anode temperature to fall below the oxidative degradation temperature. During this time, the flow rate of the hydrocarbon-based fuel is maintained at FkE, and the flow rates of fluids supplied to the indirect internal reforming SOFC, such as the flow rate of water (including steam) for steam reforming or autothermal reforming and the flow rate of air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, the cathode air flow rate, the flow rates of the fuel and air supplied to the burner, and the flow rates of fluids, such as water and air, supplied to the heat exchanger; and the input and output of electricity to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, the water and liquid fuel evaporators, the cell stack, the fluid supply piping, and the like, and electrical input taken out from the thermoelectric conversion module and the like, are maintained in the operation conditions in the reforming-stoppable state determined beforehand. In other words, the operation conditions of the indirect internal reforming SOFC are maintained in the operation conditions of the indirect internal reforming SOFC in the reforming-stoppable state determined beforehand. The anode temperature decreases with time, and therefore, eventually, the anode temperature falls below the oxidative degradation temperature. The anode temperature may be appropriately monitored (continuously measured) using a temperature sensor, such as a thermocouple.
The monitoring of the anode temperature is preferably started immediately after the shutdown method is started. If the temperature monitoring has been performed since before the start of the shutdown method, then the temperature monitoring may be continued as it has been, also when the shutdown method is performed.
When the anode temperature falls below the oxidative degradation temperature, the supply of the hydrocarbon-based fuel to the reformer can be stopped to complete the shutdown method.
One example of the first embodiment will be described using
Stepwise flow rates Fk(1) and Fk(2)=FkE are determined beforehand. In this case, M=2.
The monitoring of the reforming catalyst layer temperature and the monitoring of the anode temperature have been continuously performed since before the point of time of the start of the shutdown method (the same applies to the subsequent cases).
Immediately after the shutdown method is started, step A1 is performed. In other words, the reforming catalyst layer temperature T is measured, and this T is compared with TrE.
In this case, T≧TrE (
Tr(1) that corresponds to Fk(1) is equal to or less than the reforming catalyst layer temperature T measured in step A1 (
In step C11, the smallest Fk(j) among the selective flow rates is Fk(1), and j that gives Fk(1) is 1, and therefore, J=1. And the flow rate of the hydrocarbon fuel is adjusted from Fk0 to Fk(1). When the reforming method should be changed before and after the start of the shutdown method, the flow rate of the hydrocarbon-based fuel is adjusted from Fk0 to Fk(1), and the reforming method is changed, in step C11.
In step C12, the reforming catalyst layer temperature T is measured, and this T is compared with TrE.
In this case, T>TrE (
For a while, steps C12, C14, and C15 are repeated, and during this time, the reforming catalyst layer temperature decreases with time. While T≧TrE, the hydrocarbon-based fuel at the flow rate Fk(1) that is a flow rate that is equal to or less than FkE can be reformed. Also, while T≧Tr(1), by supplying the hydrocarbon-based fuel at the flow rate Fk(1) to the reformer, the reformed gas at a flow rate that is equal to or more than FrMin continues to be supplied to the anode.
In the case shown in
In step D1, the method waits for the anode temperature to fall below the oxidative degradation temperature.
If the anode temperature becomes less than the oxidative degradation temperature, then the supply of the hydrocarbon-based fuel to the reformer can be stopped to complete the shutdown method.
If the anode temperature falls below the oxidative degradation temperature before T becomes equal to or less than TrE in step C12, then step C13 need not be performed.
By operating in this manner, it is possible to supply the reformed gas at the requisite minimum flow rate or more to the anode, while reliably performing reforming.
In the above case, TrE>Tr(1), and therefore, in step C13, at a point of time when T becomes equal to or less than TrE, Fk is set to FkE. In this case, Tr(1)>TrE, and at a point of time when T becomes equal to or less than Tr(1) in step C14, Fk is set to Fk(2)=FkE (step C16). This case will be described using
Until a point of time when T≦Tr(1) is satisfied after the shutdown method is started, the operation is similar to that of case 1-1. In other words, after the method goes from step A1 to step C1, and step C11 is performed, steps C12, C14, and C15 are repeated until a point of time when T≦Tr(1) is satisfied. While T≧TrE, the hydrocarbon-based fuel at the flow rate Fk(1) that is a flow rate equal to or less than FkE can be reformed. Also, while T≧Tr(1), by supplying the hydrocarbon-based fuel at the flow rate Fk(1) to the reformer, the reformed gas at a flow rate that is equal to or more than FrMin continues to be supplied to the anode.
At a point of time when T becomes equal to or less than Tr(1) in step C14, immediately, Fk is increased from Fk(1) to Fk(2)=FkE and J is increased by 1 to become 2 (step C16). In step C17, J is compared with M, and J=M=2, and therefore, the method goes to step D1.
Step D1 and the subsequent steps are similar to those of case 1-1.
If the anode temperature falls below the oxidative degradation temperature before T becomes equal to or less than Tr(1), then step C16 need not be performed.
By operating in this manner, it is possible to supply the reformed gas at the requisite minimum flow rate or more to the anode, while reliably performing reforming.
The case of M=3 will be described using
Until a point of time when T≦Tr(1) is satisfied after the shutdown method is started, the operation is similar to that of case 1-1. In other words, after the method goes from step A1 to step C1, and step C11 is performed, steps C12, C14, and C15 are repeated until a point of time when T≦Tr(1) is satisfied. While T≧TrE, the hydrocarbon-based fuel at the flow rates Fk(1) and Fk(2) that are flow rates equal to or less than FkE can be reformed. Also, while T≧Tr(1), by supplying the hydrocarbon-based fuel at the flow rate Fk(1) to the reformer, the reformed gas at a flow rate that is equal to or more than FrMin continues to be supplied to the anode.
At a point of time when T becomes equal to or less than Tr(1) in step C14, immediately, Fk is increased from Fk(1) to Fk(2) and J is increased by 1 to become 2 (step C16). In step C17, J is compared with M, and J≠M=3, and therefore, the method returns to step C12, and for a while, steps C12, C14, and C15 are repeated. While T≧Tr(2), by supplying the hydrocarbon-based fuel at the flow rate Fk(2) to the reformer, the reformed gas at a flow rate that is equal to or more than FrMin continues to be supplied to the anode.
At a point of time when the reforming catalyst layer temperature decreases and T becomes equal to or less than Tr(2) in step C14, immediately, Fk is increased from Fk(2) to Fk(3)=FkE and J is increased by 1 to become 3 (step C16). In step C17, J is compared with M, and J=M=3, and therefore, the method goes to step D1.
Step D1 and the subsequent steps are similar to those of case 1-1.
Of course, also in this case, if the anode temperature falls below the oxidative degradation temperature, then the supply of the hydrocarbon-based fuel to the reformer may be stopped at this point of time to complete the shutdown method.
In case 1-3, it is possible to reduce the amount of the hydrocarbon-based fuel supplied until the stop of reforming and shorten shutdown time (time from the start of the shutdown method until the anode temperature falls below the oxidative degradation temperature) compared with case 1-2.
A case where T<TrE in step A1, that is, a case where step B1 is performed, will be described using
After the start of the shutdown method, step A1 is immediately performed, and the measurement of the reforming catalyst layer temperature T, and the comparison of this T with TrE are performed. T<TrE (
In this case, the temperature of the reforming catalyst layer is increased by an appropriate heat source, such as a burner or a heater annexed to the reformer, until the reforming catalyst layer temperature becomes equal to or more than TrE, so that the hydrocarbon-based fuel at the flow rate FkE can be reformed, as shown in
When T≧TrE is satisfied in step B12, the flow rate Fk of the hydrocarbon-based fuel supplied to the reformer is adjusted from Fk0 to FkE (step B14). When the reforming method should be changed before and after the start of the shutdown method, the fuel flow rate is adjusted from Fk0 to FkE, and the reforming method is changed. Then, the method goes to step D1 (step B14).
Step D1 and the subsequent steps are similar to those of case 1-1.
“The hydrocarbon-based fuel at a certain flow rate can be reformed (or is capable of being reformed) in the reforming catalyst layer” refers to that when the hydrocarbon-based fuel at this flow rate is supplied to the reforming catalyst layer, the composition of the gas discharged from the reforming catalyst layer becomes a composition suitable to be supplied to the anode of the SOFC.
For example, “can be reformed in the reforming catalyst layer” may be that the supplied hydrocarbon-based fuel can be decomposed to a C1 compound(s) (a compound(s) having a carbon number of 1). In other words, “can be reformed in the reforming catalyst layer” means a case where reforming can proceed in the reforming catalyst layer until a composition is obtained in which a C2+ component(s) (a component(s) having a carbon number of 2 or more) in the gas at the outlet of the reforming catalyst layer has a concentration, which does not cause the problems of anode degradation and flow blockage due to carbon deposition, or less. The concentration of the C2+ component(s) in this case is preferably 50 ppb or less as a mass fraction in the reformed gas. And in this case, it is enough that the gas at the outlet of the reforming catalyst layer is reducing gas. Methane is permitted to be contained in the gas at the outlet of the reforming catalyst layer. In the reforming of the hydrocarbon-based fuel, usually, methane remains in the equilibrium theory. Even if carbon is contained in the gas at the outlet of the reforming catalyst layer in the form of methane, CO, or CO2, carbon deposition can be prevented by adding steam as required. When methane is used as the hydrocarbon-based fuel, it is enough that reforming proceeds so that the gas at the outlet of the reforming catalyst layer becomes reducing.
With respect to the reducing property of the gas at the outlet of the reforming catalyst layer, it is enough that the property is to the extent that if this gas is supplied to the anode, the oxidative degradation of the anode is suppressed. In order to do this, for example, the partial pressures of oxidizing O2, H2O, CO2, and the like contained in the gas at the outlet of the reforming catalyst layer may be lower than their equilibrium partial pressures of oxidation reactions of the anode electrode. For example, when the anode electrode material is Ni, and the anode temperature is 800° C., the partial pressure of O2 contained in the gas at the outlet of the reforming catalyst layer may be less than 1.2×10−14 atm (1.2×10−9 Pa), the partial pressure ratio of H2O to H2 may be less than 1.7×102, and the partial pressure ratio of CO2 to CO may be less than 1.8×102.
A position for the measurement of the reforming catalyst layer temperature will be described in detail below. This measurement position may be used when TrE, Tr(j), and TR(j) are found beforehand, and when the temperature of the reforming catalyst layer is measured in steps A1 to C1.
<Case where there is One Temperature Measurement Point>
Temperature Measurement Position
When there is a single temperature measurement point in the reforming catalyst layer, it is preferred to use preferably a position where the temperature becomes relatively low in the reforming catalyst layer, more preferably a position where the temperature becomes the lowest in the reforming catalyst layer, as the position for the measurement of temperature, in terms of safe side control. When the reaction heat in the reforming catalyst layer is endothermic, the vicinity of the center of the catalyst layer may be selected as the temperature measurement position. When the reaction heat in the reforming catalyst layer is exothermic, and the temperatures of the end positions are lower than that of the center portion due to heat release, an end of the catalyst layer may be selected as the temperature measurement position. A location where the temperature becomes low may be found by preliminary experiment or simulation.
<Case where there are Plurality of Temperature Measurement Points>
The point for the measurement of temperature need not be one. Two or more temperature measurement points are preferred in terms of more accurate control. For example, it is possible to measure the inlet temperature and outlet temperature of the reforming catalyst layer and use their average temperature as the above-described reforming catalyst layer temperature T. However, in a case where the rate of a reaction other than a reaction accompanied by the decrease of the hydrocarbon-based fuel (raw fuel) supplied to the reforming catalyst layer is much faster than that of the reaction accompanied by the decrease of the raw fuel, and it can be considered that components other than the raw fuel instantaneously reach an equilibrium composition, even if there are a plurality of temperature measurement points in the reforming catalyst layer, it is preferred to use the temperature of a point nearest to the outlet of the reforming catalyst layer, among the temperatures measured at the plurality of points, as the temperature to be compared with Tr(j) in step C1. When there are a plurality of temperatures of points nearest to the outlet of the reforming catalyst layer, a calculated value, such as the lowest value of them or their average value, may be appropriately used as a representative value.
Alternatively, for example, it is possible to consider regions Zi obtained by dividing the reforming catalyst layer into N (N is an integer of 2 or more, and i is an integer of 1 or more and N or less), find the temperature Ti of each divided region Zi, and find TrE(j) (={TrE1, TrE2, . . . , TrEN}) and Tr(j) (={Tr(j)1, Tr(j)2, . . . , Tr(j)N}) for each divided region beforehand. In this case, when any of Ti becomes equal to or less than TrEi, the flow rate of the hydrocarbon-based fuel may be set to FkE. Also, in this case, when any of Ti becomes equal to or less than Tr(j)i, Fk(j) may be increased to Fk(j+1). Alternatively, in a case where the rate of a reaction other than a reaction accompanied by the decrease of the hydrocarbon-based fuel (raw fuel) supplied to the reforming catalyst layer is much faster than that of the reaction accompanied by the decrease of the raw fuel, and it can be considered that components other than the raw fuel instantaneously reach an equilibrium composition, when the temperature of a point nearest to the outlet of the reforming catalyst layer, among Ti, becomes equal to or less than Tr(j), Fk(j) may be increased to Fk(j+1).
When N divided regions Zi are considered, temperatures of all divided regions may be set as the temperature condition, or temperature(s) of one or some (not all) regions among the N divided regions may be set as the temperature condition. The catalyst layer regions for the temperature condition may be appropriately changed according to the feed rate of the hydrocarbon-based fuel.
As the temperature of the divided region Zi, actually measured temperature may be used as it is, but a calculated value, such as the average value of the inlet temperature and outlet temperature of the divided region, may be appropriately used as a representative value.
Also, it is not necessary to measure temperatures for all divided regions Zi. Also, the number of divisions of the catalyst layer, N, and the number of temperature measurement point(s) may be independently set.
It is also possible to measure temperature(s) of one or some (not all) of the N divided regions and find temperature(s) of the remaining divided region(s) by appropriate interpolation from the measured temperature(s).
For example, as a temperature of a divided region where no temperature sensor is installed, a temperature of a divided region nearest to this divided region may be used. When there are two nearest divided regions, a temperature of either of the two divided regions may be used, or the average value of temperatures of the two divided regions may be used.
It is also possible to measure temperatures at a plurality of points in the reforming catalyst layer (at different positions along the gas flow direction), independently of the divided regions, and find a temperature of each divided region from the measured temperatures at the plurality of points. For example, it is possible to measure temperatures of the inlet and outlet of the reforming catalyst layer (a temperature of any position in the middle portion may be further measured), interpolate the temperature of the reforming catalyst layer from these measured temperatures by an approximation method, such as a least squares method, and find temperatures of the divided regions from the interpolation curve.
In order to find temperatures of all divided regions, temperatures of the following positions may be measured.
The inlet and outlet of each divided region.
The interior (one point or a plurality of points) of each divided region (inner side of the inlet and the outlet).
The inlet, outlet, and interior (one point or a plurality of points for one divided region) of each divided region.
In order to find a temperature of one or some (not all) of the divided regions, temperatures of the following positions may be measured.
The inlet and outlet of one or some (not all) of the divided regions.
The interior (one point or a plurality of points) of one or some (not all) of the divided regions (inner side of the inlet and the outlet).
The inlet, outlet, and interior (one point or a plurality of points for one divided region) of one or some (not all) of the divided regions.
When the flow rate Fk of the hydrocarbon-based fuel is set to FkE, the flow rates of fluids supplied to the indirect internal reforming SOFC, such as the flow rate of water (including steam) for steam reforming or autothermal reforming and the flow rate of air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, the cathode air flow rate, the flow rates of the fuel and air supplied to the burner, and the flow rates of fluids, such as water and air, supplied to the heat exchanger; and the input and output of electricity to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, the water and liquid fuel evaporators, the cell stack, the fluid supply piping, and the like, and electrical input taken out from the thermoelectric conversion module and the like, are accordingly set, as required, to the operation conditions in the reforming-stoppable state determined beforehand. In other words, the operation conditions of the indirect internal reforming SOFC are set to the operation conditions of the indirect internal reforming SOFC in the reforming-stoppable state determined beforehand.
When the flow rate of the hydrocarbon-based fuel supplied to the reformer is changed in step C11 and step C16, the flow rates of fluids supplied to the indirect internal reforming SOFC, and the input and output of electricity to and from the indirect internal reforming SOFC may be accordingly set to operation conditions determined beforehand, as required, as in the above. For example, with respect to the flow rate of water supplied to the reformer, in order to suppress carbon deposition, the water flow rate may be decreased with the decrease of the fuel flow rate, so that a predetermined value of the steam/carbon ratio is maintained. With respect to the flow rate of air supplied to the reformer, the air flow rate may be decreased with the decrease of the fuel flow rate, so that a predetermined value of the oxygen/carbon ratio is maintained. The flow rates of fluids supplied to the indirect internal reforming SOFC, other than the water and air supplied to the reformer, and the input and output of electricity to and from the indirect internal reforming SOFC may be set to the operation conditions in the reforming-stoppable state determined beforehand, or may be set to operation conditions determined beforehand as functions of the fuel flow rate.
When the reforming method is changed, the flow rates of fluids supplied to the indirect internal reforming SOFC, and the input and output of electricity to and from the indirect internal reforming SOFC may be accordingly set to operation conditions determined beforehand, as required, as in the above. For example, in order to suppress carbon deposition, the flow rate of water supplied to the reformer may be changed to a flow rate at which a steam/carbon ratio determined beforehand is obtained. The flow rate of air supplied to the reformer may be changed to a flow rate at which an oxygen/carbon ratio determined beforehand is obtained. The flow rates of fluids supplied to the indirect internal reforming SOFC, other than the water and air supplied to the reformer, and the input and output of electricity to and from the indirect internal reforming SOFC may be set to the operation conditions in the reforming-stoppable state determined beforehand, or may be set to operation conditions determined beforehand as functions of the fuel flow rate.
When a steam reforming reaction is performed, that is, steam reforming or autothermal reforming is performed, steam is supplied to the reforming catalyst layer. When a partial oxidation reforming reaction is performed, that is, partial oxidation reforming or autothermal reforming is performed, an oxygen-containing gas is supplied to the reforming catalyst layer. As the oxygen-containing gas, a gas containing oxygen may be appropriately used, but in terms of the ease of availability, air is preferred.
The present invention is particularly effective when the hydrocarbon-based fuel has a carbon number of 2 or more, because in the case of such a fuel, particularly, reliable reforming is required.
In order to perform the method of the present invention, appropriate instrumentation and controlling equipment, including a computing means, such as a computer, may be used.
Next, the second embodiment of the shutdown method of the present invention will be described.
The flow rate of the hydrocarbon-based fuel supplied to the reformer (particularly, the reforming catalyst layer) in the reforming-stoppable state is represented as FkE.
FkE may be obtained beforehand by experiment or simulation. FkE may be found by performing an experiment or a simulation, while varying flow rates of fluids supplied to the indirect internal reforming SOFC, such as the flow rate of water (including steam) for steam reforming or autothermal reforming and the flow rate of air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, a cathode air flow rate, the flow rates of a fuel and air supplied to a burner, and flow rates of fluids, such as water and air, supplied to a heat exchanger; and electrical input and output to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, water and liquid fuel evaporators, the SOFC, fluid supply piping, and the like, and electrical input taken out from a thermoelectric conversion module and the like, that is, varying the operation conditions of the indirect internal reforming SOFC, and searching for FkE that steadily satisfies the conditions i to iv. FkE may be any value as long as the conditions i to iv are satisfied, but in terms of thermal efficiency, the smallest FkE is preferably used. The operation conditions of the indirect internal reforming SOFC, including the FkE, are determined beforehand as operation conditions in the reforming-stoppable state.
The temperature of the reforming catalyst layer included in the above operation conditions in the reforming-stoppable state determined beforehand, is represented as TrE. TrE may be found together with FkE in the process of search for FkE, and TrE is a temperature condition of the reforming catalyst layer that corresponds to the single FkE used.
The flow rate of the hydrocarbon-based fuel supplied to the reformer at the point of time of the start of the shutdown method is represented as Fk0.
The calculated value of the flow rate of the hydrocarbon-based fuel capable of being reformed at a measured reforming catalyst layer temperature by a reforming method of a type performed after the start of the shutdown method (this flow rate is hereinafter sometimes referred to as a “reformable flow rate”) is represented as FkCALC. In other words, FkCALC may be obtained by measuring the temperature of the reforming catalyst layer, and calculating the flow rate of the hydrocarbon-based fuel capable of being reformed in the reforming catalyst layer when the reforming catalyst layer has this temperature. At this time, it is assumed that the reforming method of the type performed after the start of the shutdown method is performed in the reforming catalyst layer (the type of the reforming method is hereinafter sometimes referred to as a reforming type). The reforming type is, for example, steam reforming, autothermal reforming, or partial oxidation reforming.
Specifically, when a certain type of reforming is performed before the start of the shutdown method, the same type of reforming as this may be performed after the start of the shutdown method. In this case, the flow rate (calculated value) of the hydrocarbon-based fuel capable of being reformed, when this type of reforming is performed in the reformer, is FkCALC. For example, when steam reforming is performed before the start of the shutdown method, steam reforming may also be continuously performed after the start of the shutdown method, and the flow rate of the hydrocarbon-based fuel capable of being reformed at the measured temperature of the reforming catalyst layer when steam reforming is performed in the reformer is FkCALC.
Alternatively, when a certain type of reforming (a first type of reforming) is performed before the start of the shutdown method, a different type of reforming from this (a second type of reforming) may be performed after the start of the shutdown method. In this case, the flow rate of the hydrocarbon-based fuel capable of being reformed, when the second type of reforming is performed in the reformer, is FkCALC. For example, when autothermal reforming is performed before the start of the shutdown method, the reforming may be switched to steam reforming after the start of the shutdown method. In this case, the flow rate (calculated value) of the hydrocarbon-based fuel capable of being reformed at the measured temperature of the reforming catalyst layer when steam reforming is performed is FkCALC.
M different stepwise hydrocarbon-based fuel flow rates Fk(j) are determined beforehand. Here, j is an integer of 1 or more and M or less, where M is an integer of 2 or more.
Here, Fk(j) increases with the increase in j. In other words, Fk(j)<Fk(j+1). Also, Fk(M) which is the largest among Fk(j) has a value that is equal to FkE.
In this case, a steam/carbon ratio and/or an oxygen/carbon ratio that correspond to each Fk(j) may be set for each Fk(j). Also, when a gas not contributing to a reaction is supplied to the reforming catalyst layer, its flow rate may be set for each Fk(j). The steam/carbon ratios, the oxygen/carbon ratios, or the flow rates of the gas not contributing to a reaction, which are set correspondingly to Fk(j), may have the same value for all Fk(j) or may differ from one Fk(j) to another.
Further, Fk(j) is equal to or more than the minimum value (Fkmin(j)) of hydrocarbon-based fuel flow rates at which the reformed gas at a flow rate that is equal to or more than FrMin can be obtained by the reforming method. A type of this reforming method is that of a reforming method which is performed after the start of the shutdown method.
This minimum value Fkmin(j) may be obtained by preliminary experiment or simulation. For example, a Fkmin(j) may be found by measuring or calculating reformed gas flow rates by using the reforming method of the type performed after the start of the shutdown method as a reforming method, using the flow rate of the hydrocarbon-based fuel as a variable, using the temperature of the reforming catalyst layer as a variable within a range considered, setting the flow rates of steam and/or oxygen to flow rates obtained from a steam/carbon ratio and/or an oxygen carbon ratio that correspond to the flow rate of the hydrocarbon-based fuel and Fk(j), and setting, when supplying a gas not contributing to a reaction to the reforming catalyst layer, a flow rate of this gas to flow rate that corresponds to the Fk(j); and obtaining the minimum value of flow rates of the hydrocarbon-based fuel at which the reformed gas flow rate is FrMin. The reforming method is, for example, steam reforming, autothermal reforming, or partial oxidation reforming. Alternatively, for example, when a rate of a reaction other than a reaction accompanied by the decrease of the hydrocarbon-based fuel (raw fuel) supplied to the reforming catalyst layer is much faster than that of the reaction accompanied by the decrease of the raw fuel, and it can be considered that components other than the raw fuel instantaneously reach an equilibrium composition, a Fkmin(j) may be obtained by equilibrium calculation using a Gibbs energy minimization method or the like. For example, using a Fk(j), a steam/carbon ratio and/or an oxygen carbon ratio that correspond to the Fk(j), and, when supplying a gas not contributing to a reaction, a gas flow rate of this gas, a composition of a gas supplied to the reforming catalyst layer is obtained, and a gas compositions excluding the maximum value of the raw fuel composition suitable to be supplied to the anode (an allowable raw fuel composition) are obtained. By using the obtained composition, and total pressure or the partial pressure of each component, using the flow rate of the raw fuel as a variable, and using the temperature of the reforming catalyst layer as a variable within a range considered, equilibrium calculation is performed to calculate gas flow rates for an equilibrium composition. By adding a gas flow rate of the allowable raw fuel composition to the calculated gas flow rates for the equilibrium composition, the flow rates of a reformed gas having a composition in which the raw fuel flow rate is the largest, among reformed gas compositions suitable to be supplied to the anode, are calculated, and the minimum value of the raw fuel flow rates at which the reformed gas flow rate is equal to or more than FrMin may be set as the Fkmin(j).
It is assumed that there exist one or more Fk(j) that are smaller than FkE and are equal to or more than Fkmin(j). When there do not exist one or more such Fk(j), it is possible to perform step A2, and when FkCALC<FkE in step A2, perform step B2, and when FkCALC≧FkE in step A2, adjust the flow rate of the hydrocarbon-based fuel supplied to the reformer from Fk0 to FkE and move on to step D2.
Also, when the values of at least one or more of the steam/carbon ratio, the oxygen/carbon ratio, and the flow rate of the gas not contributing to a reaction, which are set correspondingly to each Fk(j), are different from values in the operation conditions in the reforming-stoppable state determined beforehand, it is assumed that when the reforming catalyst layer temperature is the temperature in the operation conditions in the reforming-stoppable state determined beforehand, TrE, Fk(j) that is smaller than FkE is reformable. When there do not exist one or more such Fk(j), it is possible to perform step A2, and when FkCALC<FkE in step A2, perform step B2, and when FkCALC≧FkE in step A2, adjust the flow rate of the hydrocarbon-based fuel supplied to the reformer from Fk0 to FkE and go to step D2. Whether Fk(j) that is smaller than FkE is reformable when the reforming catalyst layer temperature is TrE may be found, for example, by finding beforehand temperature conditions TR(j) in which the hydrocarbon-based fuel at the flow rates Fk(j) can be reformed in the reforming catalyst layer, and comparing TR(j) with TrE. In this case, if TrE≧TR(j), then it may be considered that when the reforming catalyst layer temperature is TrE, Fk(j) that is smaller than FkE is reformable. The TR(j) may be obtained by preliminary experiment or simulation. For example, a TR(j) may be found by analyzing or calculating the concentrations of the reformed gas components by using the reforming method of the type performed after the start of the shutdown method as a reforming method, setting the flow rate of the hydrocarbon-based fuel to a Fk(j), setting steam and/or oxygen flow rates to flow rates obtained from a steam/carbon ratio and/or an oxygen/carbon ratio that correspond to the flow rate of the hydrocarbon-based fuel and Fk(j), supplying, when supplying a gas not contributing to a reaction to the reforming catalyst layer, this gas at a flow rate that corresponds to the Fk(j), and using the temperature of the reforming catalyst layer as a variable within a range considered; and obtaining the minimum temperature of the reforming catalyst layer at which a reformed gas with a composition suitable to be supplied to the anode is obtained.
Fk(j) may be set, for example, at equal intervals.
It is preferred to make M as large as possible and make the interval between Fk(j) small, in terms of thermal efficiency. For example, it is possible to make M as large as possible and make the interval between Fk(j) small, within the allowable range of the memory consumption of a flow rate controlling means, and within a range in which the interval exceeds the precision of a pressure increasing means and flow rate controlling and measuring means.
[Tr(j) that Corresponds to Fk(j)]
The temperature conditions Tr(j) of the reforming catalyst layer are found beforehand (wherein j is an integer of 1 or more and M−1 or less). In a temperature condition Tr(j), the flow rate of the reformed gas obtained when the hydrocarbon-based fuel at a flow rate Fk(j) is reformed in the reforming catalyst layer is FrMin. The temperature conditions Tr(j) may be obtained by preliminary experiment or simulation. For example, a temperature condition Tr(j) may be found by measuring or calculating reformed gas flow rates by using the reforming method of the type performed after the start of the shutdown method as a reforming method, setting the flow rate of the hydrocarbon-based fuel to a Fk(j), setting the steam and/or oxygen flow rates to flow rates obtained from a steam/carbon ratio and/or an oxygen carbon ratio that correspond to the flow rate of the hydrocarbon-based fuel and Fk(j), setting, when supplying a gas not contributing to a reaction to the reforming catalyst layer, a flow rate of this gas to a flow rate that correspond to the Fk(j), and using the temperature of the reforming catalyst layer as a variable within a range considered; and obtaining a temperature of the reforming catalyst layer at which the reformed gas flow rate is FrMin. Alternatively, for example, when a rate of a reaction other than a reaction accompanied by the decrease of the hydrocarbon-based fuel (raw fuel) supplied to the reforming catalyst layer is much faster than that of the reaction accompanied by the decrease of the raw fuel, and it can be considered that components other than the raw fuel instantaneously reach an equilibrium composition, a Tr(j) may be obtained by equilibrium calculation using a Gibbs energy minimization method or the like. For example, using a Fk(j), a steam/carbon ratio and/or an oxygen/carbon ratio that correspond to the Fk(j), and, when supplying a gas not contributing to a reaction, a gas flow rate of this gas, a composition of a gas supplied to the reforming catalyst layer is obtained, and a gas composition excluding the maximum value of the raw fuel composition suitable to be supplied to the anode (an allowable raw fuel composition) is obtained. By using the obtained composition, and total pressure or the partial pressure of each component, and the Fk(j), and using the temperature as a variable within a range considered, equilibrium calculation is performed to calculate gas flow rates for an equilibrium composition. By adding the gas flow rate of the allowable raw fuel composition to the calculated gas flow rates for the equilibrium composition, the flow rates of a reformed gas having a composition in which the raw fuel flow rate is the largest, among reformed gas compositions suitable to be supplied to the anode, are calculated, and a temperatures at which the reformed gas flow rate is FrMin is obtained, and this temperature may be set as the Tr(j).
Specifically, when a certain type of reforming is performed before the start of the shutdown method, the same type of reforming as this (for example, steam reforming) may be performed after the start of the shutdown method. In this case, the temperature conditions of the reforming catalyst layer in which the flow rate of the reformed gas obtained by reforming the hydrocarbon-based fuel at the flow rates Fk(j) in the reforming catalyst layer is FrMin, when this type of reforming is performed in the reformer, are Tr(j). For example, when steam reforming is performed before the start of the shutdown method, steam reforming may also be continuously performed after the start of the shutdown method, and the temperature conditions of the reforming catalyst layer in which the reformed gas at the flow rate FrMin is obtained from the hydrocarbon-based fuel at the flow rates Fk(j) when steam reforming is performed in the reformer are Tr(j).
Alternatively, when a certain type of reforming (a first type of reforming) is performed before the start of the shutdown method, a different type of reforming from this (a second type of reforming) may be performed after the start of the shutdown method. In this case, the temperature conditions of the reforming catalyst layer in which the flow rate of the reformed gas obtained by reforming the hydrocarbon-based fuel at the flow rates Fk(j) is FrMin, when the second type of reforming is performed in the reformer, are Tr(j). For example, when autothermal reforming is performed before the start of the shutdown method, the reforming may be switched to steam reforming after the start of the shutdown method. In this case, the temperature conditions of the reforming catalyst layer in which the reformed gas at the flow rate FrMin is obtained from the hydrocarbon-based fuel at the flow rates Fk(j) when steam reforming is performed are Tr(j).
In order to determine a flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer, the measured value of the reforming catalyst layer temperature is compared with the above temperature conditions Tr(j). Also, the measured value of the reforming catalyst layer temperature is used for the calculation of FkCALC. In order to do this, the reforming catalyst layer temperature is measured. For example, the reforming catalyst layer temperature may be monitored (continuously measured).
When the monitoring of the temperature of the reforming catalyst layer has been performed since before the start of the shutdown method, the temperature monitoring may be continuously performed as it has been.
If the anode temperature falls below the oxidative degradation temperature, then the reducing gas becomes unnecessary, and therefore, the supply of the hydrocarbon-based fuel to the reformer can be stopped to complete the shutdown method. Therefore, the monitoring of the temperature of the reforming catalyst layer may be continuously performed until the anode temperature falls below the oxidative degradation temperature.
An appropriate temperature sensor, such as a thermocouple, may be used for the measurement of the reforming catalyst layer temperature.
[Case where Reforming Method is Changed Before and after Start of Shutdown Method]
When the reforming method is changed before and after the start of the shutdown method, the above-described FkE, FrMin, and TrE used when the values of at least one or more of the steam/carbon ratio, the oxygen/carbon ratio, and the flow rate of the gas not contributing to a reaction, which are set correspondingly to each Fk(j), are different from values in the operation conditions in the reforming-stoppable state determined beforehand, are determined for a reforming-stoppable state when reforming after the change of the reforming method is performed. Also, the above-described TR(j) used when the values of at least one or more of the steam/carbon ratio, the oxygen/carbon ratio, and the flow rate of the gas not contributing to a reaction, which are set correspondingly to each Fk(j), are different from values in the operation conditions in the reforming-stoppable state determined beforehand, is determined for a case where reforming after the change of the reforming method is performed.
While the anode temperature does not fall below the oxidative degradation temperature, the following steps A2 to D2 are performed. When the anode temperature falls below the oxidative degradation temperature, the supply of the hydrocarbon-based fuel to the reformer can be stopped, regardless of the status of the implementation of steps A2 to D2, to complete the shutdown method. It is possible to stop the supply of fluids supplied to the indirect internal reforming SOFC, such as water (including steam) for steam reforming or autothermal reforming and air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, cathode air, the fuel and air supplied to the burner, and fluids, such as water and air, supplied to the heat exchanger; and the input and output of electricity to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, the water and liquid fuel evaporators, the cell stack, the fluid supply piping, and the like, and electrical input taken out from the thermoelectric conversion module and the like, according to the stop of the supply of the hydrocarbon-based fuel to the reformer.
The shutdown method includes steps A2 to D2, but it is not necessary to actually perform all of steps A2 to D2, and only part of steps A2 to D2 may be performed according to the circumstances.
First, a reforming catalyst layer temperature T is measured. Then, a reformable flow rate FkCALC is calculated based on this temperature T. Further, the magnitude relationship between the flow rate FkE of the hydrocarbon-based fuel supplied to the reformer in the above-described reforming-stoppable state and this FkCALC is checked.
When FkCALC<FkE in step A2, the following steps B21 to B24 are performed in order. “FkCALC<FkE” is considered to mean that the hydrocarbon-based fuel at the flow rate FkE cannot be reformed in the reformer (by a reforming type after change, if the reforming type is changed).
Step B21
First, step B21 is performed. In other words, the step of increasing the temperature of the reforming catalyst layer is performed.
For example, the temperature of the reforming catalyst layer is increased using an appropriate heat source, such as a heater or a burner annexed to the reformer.
Step B22
Then, step B22 is performed. In other words, the step of measuring the reforming catalyst layer temperature T, calculating FkCALC using this T, and comparing the values of this FkCALC and FkE is performed.
Step B23
When FkCALC<FkE in step B22, the step of returning to step B21 is performed. In other words, while FkCALC<FkE, steps B21 to B23 are repeatedly performed. During this time, the temperature of the reforming catalyst layer increases.
In performing steps B22 and B23, the temperature increase in step B21 may be stopped once, but while steps B22 and B23 are performed, step B21 may be continued.
Step B24
When FkCALC≧FkE in step B22, the step of adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer (represented as Fk) from Fk0 to FkE and going to step D2 is performed. “FkCALC≧FkE” is considered to mean that the hydrocarbon-based fuel at the flow rate FkE can be reformed in the reforming catalyst layer (by a reforming type after change, if the reforming type is changed).
At this time, in a case where the reforming type should be changed before and after the start of the shutdown method, the fuel flow rate is adjusted from Fk0 to FkE, and the reforming type is changed. By this method, it is possible to prevent the oxidative degradation of the anode with the reformed gas, while reliably reforming the hydrocarbon-based fuel.
When FkCALC≧FkE in step A2, step C2 is performed. “FkCALC≧FkE” is considered to mean that the hydrocarbon-based fuel at the flow rate FkE can be reformed in the reformer (by a reforming type after change, if the reforming type is changed before and after the start of the shutdown method).
In this case, if there does not exist, among the hydrocarbon-based fuel flow rates Fk(j) determined beforehand, a flow rate Fk(j) at which the corresponding temperature condition Tr(j) is equal to or less than the reforming catalyst layer temperature T measured in step A2 and which is smaller than FkE (this flow rate Fk(j) is hereinafter referred to as a “selective flow rate”), then the flow rate of the hydrocarbon-based fuel supplied to the reformer is adjusted from Fk0 to FkE, and the method goes to step D2. At this time, in a case where the reforming method is changed before and after the start of the shutdown method, it is possible to adjust the fuel flow rate from Fk0 to FkE and change the reforming method.
For example, if j=2 is considered, and if Tr(2) is equal to or less than the reforming catalyst layer temperature T measured in step A2, and Fk(2)<FkE, then there exists a selective flow rate Fk(2). If there does not exist any such a selective flow rate, then the above-described operation is performed.
On the other hand, when FkCALC≧FkE in step A2, and if there exists, among the hydrocarbon-based fuel flow rates Fk(j) determined beforehand, the above-described selective flow rate Fk(j), then the following steps C21 to C27 are performed in order.
Step C21
j that gives the smallest Fk(j) among the selective flow rates is represented as J, and the flow rate of the hydrocarbon-based fuel supplied to the reformer (represented as Fk) is adjusted from Fk0 to Fk(J). At this time, in a case where the reforming method should be changed before and after the start of the shutdown method, the flow rate Fk of the hydrocarbon-based fuel is adjusted from Fk0 to Fk(J), and the reforming method is changed.
Step C22
The reforming catalyst layer temperature is measured, FkCALC is calculated based on this T, and the values of FkCALC and FkE are compared.
Step C23
When FkCALC≦FkE in step C22, the flow rate of the hydrocarbon-based fuel supplied to the reformer is set to FkE, and the method goes to step D2.
Step C24
When FkCALC>FkE in step C22, the reforming catalyst layer temperature T measured in step C22 is compared with Tr(J).
Step C25
When T>Tr(J) in step C24, the method returns to step C22. In other words, when FkCALC>FkE in step C22 and T>Tr(J) in step C24, steps C22, C24 and C25 are repeatedly performed. During this time, the reforming catalyst layer temperature decreases. Therefore, eventually, T≦Tr(J) is satisfied.
Step C26
When T≦Tr(J) in step C24, the flow rate Fk of the hydrocarbon-based fuel supplied to the reformer is increased from Fk(J) to Fk(J+1), and J is increased by 1.
Step C27
After step C26, J is compared with M, and if J≠M, then the method returns to step C22, and if J=M, then the method goes to step D2.
In step C2 and the subsequent steps, from a state in which FkCALC≧FkE, it is possible to set operation conditions to the operation conditions in the reforming-stoppable state, and bring the internal reforming solid oxide fuel cell to the reforming-stoppable state, without allowing the unreformed hydrocarbon-based fuel to flow into the anode. But, generally, within a temperature range preferred for reforming, as the reforming catalyst layer temperature T becomes higher, the reformed gas flow rate becomes larger. Therefore, while FkCALC≧FkE, the reformed gas flow rate is equal to or more than FrMin, and this means that an excessive hydrocarbon-based fuel is supplied.
On the other hand, in step C2 and the subsequent steps, by supplying the hydrocarbon-based fuel at a flow rate as small as possible, among the flow rates of the hydrocarbon-based fuel at which the reformed gas at a flow rate that is equal to or more than FrMin can be produced and which are smaller than FkE, to the reformer, it is possible to make the hydrocarbon-based fuel supplied to the reformer as small as possible, while producing a reformed gas at a flow rate that is equal to or more than FrMin in the reformer. But, if the supply of the hydrocarbon-based fuel at a flow rate as small as possible, among the flow rates of the hydrocarbon-based fuel at which the reformed gas at a flow rate that is equal to or more than FrMin can be produced, is continued, then FkCALC<FkE may be satisfied due to the decrease of the reforming catalyst layer temperature, and there may be a case where the hydrocarbon-based fuel at the flow rate as small as possible, among the flow rates of the hydrocarbon-based fuel at which the reformed gas at a flow rate that is equal to or more than FrMin can be produced, cannot be reformed. Further, there may be a case where it is impossible to set operation conditions to the operation conditions in the reforming-stoppable state, and bring the internal reforming solid oxide fuel cell to the reforming-stoppable state, without allowing the unreformed hydrocarbon-based fuel to flow into the anode.
Therefore, when FkCALC≦FkE is satisfied, by setting operation conditions to the operation conditions in the reforming-stoppable state, it is possible to bring the internal reforming solid oxide fuel cell to the reforming-stoppable state, without allowing the unreformed hydrocarbon-based fuel to flow into the anode.
Also, when FkCALC>FkE, by supplying the hydrocarbon-based fuel at a flow rate as small as possible, among the flow rates of the hydrocarbon-based fuel at which the reformed gas at FrMin or more can be produced, to the reformer, it is possible to make the hydrocarbon-based fuel supplied to the reformer as small as possible, while producing a reformed gas at a flow rate that is equal to or more than FrMin in the reformer.
In step D2, the method waits for the anode temperature to fall below the oxidative degradation temperature. During this time, the flow rate of the hydrocarbon-based fuel is maintained at FkE, and the flow rates of fluids supplied to the indirect internal reforming SOFC, such as the flow rate of water (including steam) for steam reforming or autothermal reforming and the flow rate of air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, the cathode air flow rate, the flow rates of the fuel and air supplied to the burner, and the flow rates of fluids, such as water and air, supplied to the heat exchanger; and the input and output of electricity to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, the water and liquid fuel evaporators, the cell stack, the fluid supply piping, and the like, and electrical input taken out from the thermoelectric conversion module and the like, are maintained in the operation conditions in the reforming-stoppable state determined beforehand. In other words, the operation conditions of the indirect internal reforming SOFC are maintained in the operation conditions of the indirect internal reforming SOFC in the reforming-stoppable state determined beforehand. The anode temperature decreases with time, and therefore, eventually, the anode temperature falls below the oxidative degradation temperature. The anode temperature may be appropriately monitored (continuously measured) using a temperature sensor, such as a thermocouple.
The monitoring of the anode temperature is preferably started immediately after the shutdown method is started. If the temperature monitoring has been performed since before the start of the shutdown method, then the temperature monitoring may be continued as it has been, also when the shutdown method is performed.
When the anode temperature falls below the oxidative degradation temperature, the supply of the hydrocarbon-based fuel to the reformer can be stopped to complete the shutdown method.
One example of the second embodiment will be described using
Stepwise flow rates Fk(1) and Fk(2)=FkE are determined beforehand. In this case, M=2.
The monitoring of the reforming catalyst layer temperature and the monitoring of the anode temperature have been continuously performed since before the point of time of the start of the shutdown method (the same applies to the subsequent cases).
Immediately after the shutdown method is started, step A2 is performed. In other words, the reforming catalyst layer temperature T is measured, the reformable flow rate FkCALC is calculated using this T, and the values of this FkCALC and FkE are compared.
In this case, FkCALC≧FkE, and therefore, step C2 is performed.
Tr(1) that corresponds to Fk(1) is equal to or less than the reforming catalyst layer temperature T measured in step A2 (
In step C21, the smallest Fk(j) among the selective flow rates is Fk(1), and j that gives Fk(1) is 1, and therefore, J=1. And the flow rate of the hydrocarbon fuel is adjusted from Fk0 to Fk(1). When the reforming method should be changed before and after the start of the shutdown method, the flow rate of the hydrocarbon-based fuel is adjusted from Fk0 to Fk(1), and the reforming method is changed, in step C21.
In step C22, the reforming catalyst layer temperature T is measured, FkCALC is calculated based on this T, and the values of this FkCALC and FkE are compared.
In this case, FkCALC>FkE (
For a while, steps C22, C24, and C25 are repeated, and during this time, the reforming catalyst layer temperature decreases with time. Also, during this time, FkCALC decreases with time.
While FkCALC≧FkE, the hydrocarbon-based fuel at the flow rate Fk(1) that is a flow rate that is equal to or less than FkE can be reformed. Also, while T≧Tr(1), by supplying the hydrocarbon-based fuel at the flow rate Fk(1) to the reformer, the reformed gas at a flow rate that is equal to or more than FrMin continues to be supplied to the anode.
In the case shown in
In step D2, the method waits for the anode temperature to fall below the oxidative degradation temperature.
If the anode temperature becomes less than the oxidative degradation temperature, then the supply of the hydrocarbon-based fuel to the reformer can be stopped to complete the shutdown method.
If the anode temperature falls below the oxidative degradation temperature before FkCALC becomes equal to or less than FkE in step C22, then step C23 need not be performed.
By operating in this manner, it is possible to supply the reformed gas at the requisite minimum flow rate or more to the anode, while reliably performing reforming.
In the above case, FkCALC becomes equal to or less than FkE before the reforming catalyst layer temperature T becomes equal to or less than Tr(1), and therefore, in step C23, at a point of time when FkCALC becomes equal to or less than FkE, Fk is set to FkE. In this case, T becomes equal to or less than Tr(1) before FkCALC becomes equal to or less than FkE. Therefore, at a point of time when T becomes equal to or less than Tr(1) in step C24, Fk is set to Fk(2)=FkE (step C26). This case will be described using
Until steps C22, C24, and C25 are repeatedly performed after the shutdown method is started, the operation is similar to that of case 2-1. In other words, after the method goes from step A2 to step C2, and step C21 is performed, steps C22, C24, and C25 are repeated. While FkCALC≧FkE, the hydrocarbon-based fuel at the flow rate Fk(1) that is a flow rate equal to or less than FkE can be reformed. Also, while T≧Tr(1), by supplying the hydrocarbon-based fuel at the flow rate Fk(1) to the reformer, the reformed gas at a flow rate that is equal to or more than FrMin continues to be supplied to the anode.
At a point of time when T becomes equal to or less than Tr(1) in step C24, immediately, Fk is increased from Fk(1) to Fk(2)=FkE and J is increased by 1 to become 2 (step C26). In step C27, J is compared with M, and J=M=2, and therefore, the method goes to step D2.
Step D2 and the subsequent steps are similar to those of case 2-1.
If the anode temperature falls below the oxidative degradation temperature before T becomes equal to or less than Tr(1), then step C26 need not be performed.
By operating in this manner, it is possible to supply the reformed gas at the requisite minimum flow rate or more to the anode, while reliably performing reforming.
The case of M=3 will be described using
Until a point of time when the first-time step C26 is performed after the shutdown method is started, the operation is similar to that of case 2-2. In other words, after the method goes from step A2 to step C2, and step C21 is performed, steps C22, C24, and C25 are repeated until a point of time when T≦Tr(1) is satisfied. While FkCALC≧FkE, the hydrocarbon-based fuel at the flow rates Fk(1) and Fk(2) that are flow rates equal to or less than FkE can be reformed. Also, while T≧Tr(1), by supplying the hydrocarbon-based fuel at the flow rate Fk(1) to the reformer, the reformed gas at a flow rate that is equal to or more than FrMin continues to be supplied to the anode. At a point of time when T becomes equal to or less than Tr(1) in step C24, immediately, Fk is increased from Fk(1) to Fk(2) and J is increased by 1 to become 2 (step C26).
In step C27, J is compared with M, and J≠M=3, and therefore, the method returns to step C22, and for a while, steps C22, C24, and C25 are repeated. While T≧Tr(2), by supplying the hydrocarbon-based fuel at the flow rate Fk(2) to the reformer, the reformed gas at a flow rate that is equal to or more than FrMin continues to be supplied to the anode.
At a point of time when the reforming catalyst layer temperature decreases and T becomes equal to or less than Tr(2) in step C24, immediately, Fk is increased from Fk(2) to Fk(3)=FkE and J is increased by 1 to become 3 (step C26). In step C27, J is compared with M, and J=M=3, and therefore, the method goes to step D2.
Step D2 and the subsequent steps are similar to those of case 2-1.
Of course, also in this case, if the anode temperature falls below the oxidative degradation temperature, then the supply of the hydrocarbon-based fuel to the reformer can be stopped at this point of time to complete the shutdown method.
In case 2-3, it is possible to reduce the amount of the hydrocarbon-based fuel supplied until the stop of reforming and shorten shutdown time (time from the start of the shutdown method until the anode temperature falls below the oxidative degradation temperature) compared with in case 2-2.
A case where FkCALC calculated in step A2 is smaller than FkE (the flow rate of the hydrocarbon-based fuel supplied to the reformer in the reforming-stoppable state), that is, a case where step B2 is performed, will be described using
After the start of the shutdown method, step A2 is immediately performed, and the measurement of the reforming catalyst layer temperature T, the calculation of FkCALC based on this T, and the comparison of this FkCALC with FkE are performed. FkCALC<FkE (
In this case, the temperature of the reforming catalyst layer is increased by an appropriate heat source, such as a burner and a heater annexed to the reformer, until FkCALC≧FkE is satisfied, so that the hydrocarbon-based fuel at the flow rate FkE can be reformed, as shown in
When FkCALC≧FkE is satisfied in step B22, the flow rate Fk of the hydrocarbon-based fuel supplied to the reformer is adjusted from Fk0 to FkE (step B24). When the reforming method should be changed before and after the start of the shutdown method, the fuel flow rate is adjusted from Fk0 to FkE, and the reforming method is changed. Then, the method goes to step D2 (step B24).
Step D2 and the subsequent steps are similar to those of case 2-1.
“The hydrocarbon-based fuel at a certain flow rate can be reformed (or is capable of being reformed) in the reforming catalyst layer” refers to that when the hydrocarbon-based fuel at this flow rate is supplied to the reforming catalyst layer, the composition of the gas discharged from the reforming catalyst layer becomes a composition suitable to be supplied to the anode of the SOFC.
For example, “can be reformed in the reforming catalyst layer” may be that the supplied hydrocarbon-based fuel can be decomposed to a C1 compound(s) (a compound(s) having a carbon number of 1). In other words, “can be reformed in the reforming catalyst layer” means a case where reforming can proceed in the reforming catalyst layer until a composition is obtained in which a C2+ component(s) (a component(s) having a carbon number of 2 or more) in the gas at the outlet of the reforming catalyst layer has a concentration, which does not cause the problems of anode degradation and flow blockage due to carbon deposition, or less. The concentration of the C2+ component(s) in this case is preferably 50 ppb or less as a mass fraction in the reformed gas. And in this case, it is enough that the gas at the outlet of the reforming catalyst layer is reducing gas. Methane is permitted to be contained in the gas at the outlet of the reforming catalyst layer. In the reforming of the hydrocarbon-based fuel, usually, methane remains in the equilibrium theory. Even if carbon is contained in the gas at the outlet of the reforming catalyst layer in the form of methane, CO, or CO2, carbon deposition can be prevented by adding steam as required. When methane is used as the hydrocarbon-based fuel, it is enough that reforming proceeds so that the gas at the outlet of the reforming catalyst layer becomes reducing.
With respect to the reducing property of the gas at the outlet of the reforming catalyst layer, it is enough that the property is to the extent that if this gas is supplied to the anode, the oxidative degradation of the anode is suppressed. In order to do this, for example, the partial pressures of oxidizing O2, H2O, CO2, and the like contained in the gas at the outlet of the reforming catalyst layer may be lower than their equilibrium partial pressures of oxidation reactions of the anode electrode. For example, when the anode electrode material is Ni, and the anode temperature is 800° C., the partial pressure of O2 contained in the gas at the outlet of the reforming catalyst layer may be less than 1.2×10−14 atm (1.2×10−9 Pa), the partial pressure ratio of H2O to H2 may be less than 1.7×102, and the partial pressure ratio of CO2 to CO may be less than 1.8×102.
The method for calculating the flow rate of the hydrocarbon-based fuel capable of being reformed in the reforming catalyst layer, based on the measured temperature of the reforming catalyst layer, will be described below.
The meaning of “capable of being reformed (can be reformed)” is as described above, and the flow rate of the hydrocarbon-based fuel capable of being reformed in the reforming catalyst layer (reformable flow rate) refers to a flow rate such that when the hydrocarbon-based fuel at this flow rate is supplied to the reforming catalyst layer, the composition of the gas discharged from the reforming catalyst layer becomes a composition suitable to be supplied to the anode of the SOFC.
For example, the reformable flow rate in the reforming catalyst layer may be any flow rate that is equal to or less than the maximum value of flow rates at which the supplied hydrocarbon-based fuel can be decomposed to a C1 compound(s) (a compound(s) having a carbon number of 1). The reformable flow rate may be this maximum value, or may be a value obtained by dividing this maximum value by a safety factor (a value that exceeds 1, for example 1.4).
The reformable flow rate depends on the temperature of the reforming catalyst layer. Therefore, the calculation of the reformable flow rate in the reforming catalyst layer is performed based on the measured temperature of the reforming catalyst layer.
The reformable flow rate FkCALC in the reforming catalyst layer may be obtained beforehand as a function of the temperature T of the reforming catalyst layer by experiment (FkCALC is represented also as FkCALC(T) to explicitly show that it is a function of temperature). Also, it is possible to determine the reformable flow rate by dividing the function obtained by experiment by a safety factor, or offsetting the temperature to the safe side. The unit of FkCALC(T) is, for example, mol/s.
The reformable flow rate FkCALC(T) may be a function of only the temperature T. But, this is not limiting, and the reformable flow rate FkCALC may be a function having, in addition to the temperature T, a variable other than T, such as the volume of the catalyst layer, the concentration of the gas component, or time. In this case, when the reformable flow rate FkCALC(T) is calculated, it is possible to appropriately obtain a variable other than T, and calculate the reformable flow rate FkCALC(T) from the variable other than T and the measured T.
A position for the measurement of the reforming catalyst layer temperature will be described in detail below. It is possible to use this measurement position when finding beforehand Tr(j), and TrE and TR(j) used when the values of at least one or more of the steam/carbon ratio, the oxygen/carbon ratio, and the flow rate of the gas not contributing to a reaction, which are set correspondingly to each Fk(j), are different from the values in the reforming-stoppable state, and when measuring the temperature of the reforming catalyst layer in steps A2 to C2.
<Case where there is One Temperature Measurement Point>
Temperature Measurement Position
When there is a single temperature measurement point in the reforming catalyst layer, it is preferred to use preferably a position where the temperature becomes relatively low in the reforming catalyst layer, more preferably a position where the temperature becomes the lowest in the reforming catalyst layer, as the position for the measurement of temperature, in terms of safe side control. When the reaction heat in the reforming catalyst layer is endothermic, the vicinity of the center of the catalyst layer may be selected as the temperature measurement position. When the reaction heat in the reforming catalyst layer is exothermic, and the temperatures of the end positions are lower than that of the center portion due to heat release, an end of the catalyst layer may be selected as the temperature measurement position. A location where the temperature becomes low may be found by preliminary experiment or simulation.
<Case where there are Plurality of Temperature Measurement Points>
The point for the measurement of temperature need not be one. Two or more temperature measurement points are preferred in terms of more accurate control. For example, it is possible to measure the inlet temperature and outlet temperature of the reforming catalyst layer and use their average temperature as the above-described reforming catalyst layer temperature T. However, in a case where the rate of a reaction other than a reaction accompanied by the decrease of the hydrocarbon-based fuel (raw fuel) supplied to the reforming catalyst layer is much faster than that of the reaction accompanied by the decrease of the raw fuel, and it can be considered that components other than the raw fuel instantaneously reach an equilibrium composition, even if there are a plurality of temperature measurement points in the reforming catalyst layer, it is preferred to use the temperature of a point nearest to the outlet of the reforming catalyst layer, among the temperatures measured at the plurality of points, as the temperature to be compared with Tr(j) in step C2. When there are a plurality of temperatures of points nearest to the outlet of the reforming catalyst layer, a calculated value, such as the lowest value of them or their average value, may be appropriately used as a representative value.
Alternatively, for example, it is possible to consider regions Zi obtained by dividing the reforming catalyst layer into N (N is an integer of 2 or more, and i is an integer of 1 or more and N or less), find the temperature Ti of each divided region Zi, calculate a reformable flow rate FkCALCi(Ti) in each divided region from each temperature Ti, and calculate a value obtained by summing the reformable flow rates FkCALCi(Ti), as the reformable flow rate FkCALC in the reforming catalyst layer. Also, it is possible to find Tr(j) (={Tr(j)1, Tr(j)2, . . . , Tr(j)N}) for each divided region beforehand. In this case, when any of Ti becomes equal to or less than Tr(j)i, Fk(j) may be increased to Fk(j+1). Alternatively, in a case where the rate of a reaction other than a reaction accompanied by the decrease of the hydrocarbon-based fuel (raw fuel) supplied to the reforming catalyst layer is much faster than that of the reaction accompanied by the decrease of the raw fuel, and it can be considered that components other than the raw fuel instantaneously reach an equilibrium composition, when the temperature of a point nearest to the outlet of the reforming catalyst layer, among T1, becomes equal to or less than Tr(j), Fk(j) may be increased to Fk(j+1). Also, when the values of at least one or more of the steam/carbon ratio, the oxygen/carbon ratio, and the flow rate of the gas not contributing to a reaction, which are set correspondingly to each Fk(j), are different from values in the operation conditions in the reforming-stoppable state determined beforehand, it is possible to find TrE(j) (={TrE1, TrE2, . . . , TrEN}) and TR(j) (={TR1, TR2, . . . , TRN}) for each divided region beforehand.
When N divided regions Zi are considered, reformable flow rates of all divided regions may be summed, or a total of reformable flow rates of only one or some (not all) regions among the N divided regions may be used as the reformable flow rate FkCALC in the reforming catalyst layer. The catalyst layer regions for summation may be appropriately changed according to the feed rate of the hydrocarbon-based fuel. Also, temperatures of all divided regions may be set as the temperature condition, or temperature(s) of one or some (not all) regions among the N divided regions may be set as the temperature condition. The catalyst layer regions for the temperature condition may be appropriately changed according to the feed rate of the hydrocarbon-based fuel.
As the temperature of the divided region Zi, actually measured temperature may be used as it is, but a calculated value, such as the average value of the inlet temperature and outlet temperature of the divided region, may be appropriately used as a representative value.
Also, it is not necessary to measure temperatures for all divided regions Zi. Also, the number of divisions of the catalyst layer, N, and the number of temperature measurement point(s) may be independently set.
It is also possible to measure temperature(s) of one or some (not all) of the N divided regions and find temperature(s) of the remaining divided region(s) by appropriate interpolation from the measured temperature(s).
For example, as a temperature of a divided region where no temperature sensor is installed, a temperature of a divided region nearest to this divided region may be used. When there are two nearest divided regions, a temperature of either of the two divided regions may be used, or the average value of temperatures of the two divided regions may be used.
It is also possible to measure temperatures at a plurality of points in the reforming catalyst layer (at different positions along the gas flow direction), independently of the divided regions, and find a temperature of each divided region from the measured temperatures at the plurality of points. For example, it is possible to measure temperatures of the inlet and outlet of the reforming catalyst layer (a temperature of any position in the middle portion may be further measured), interpolate the temperature of the reforming catalyst layer from these measured temperatures by an approximation method, such as a least squares method, and find temperatures of the divided regions from the interpolation curve.
When reforming catalyst layer temperatures at a plurality of positions are measured in step C22, the calculation of FkCALC (step C22), and comparison with Tr(J) may be performed using a temperature at the same position. Alternatively, the calculation of FkCALC, and comparison with Tr(J) may be performed using temperatures at different positions.
In order to find temperatures of all divided regions, temperatures of the following positions may be measured.
The inlet and outlet of each divided region.
The interior (one point or a plurality of points) of each divided region (inner side of the inlet and the outlet).
The inlet, outlet, and interior (one point or a plurality of points for one divided region) of each divided region.
In order to find a temperature of one or some (not all) of the divided regions, temperatures of the following positions may be measured.
The inlet and outlet of one or some (not all) of the divided regions.
The interior (one point or a plurality of points) of one or some (not all) of the divided regions (inner side of the inlet and the outlet).
The inlet, outlet, and interior (one point or a plurality of points for one divided region) of one or some (not all) of the divided regions.
When the flow rate Fk of the hydrocarbon-based fuel is set to FkE, the flow rates of fluids supplied to the indirect internal reforming SOFC, such as the flow rate of water (including steam) for steam reforming or autothermal reforming and the flow rate of air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, the cathode air flow rate, the flow rates of the fuel and air supplied to the burner, and the flow rates of fluids, such as water and air, supplied to the heat exchanger; and the input and output of electricity to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, the water and liquid fuel evaporators, the cell stack, the fluid supply piping, and the like, and electrical input taken out from the thermoelectric conversion module and the like, are accordingly set, as required, to the operation conditions in the reforming-stoppable state determined beforehand. In other words, the operation conditions of the indirect internal reforming SOFC are set to the operation conditions of the indirect internal reforming SOFC in the reforming-stoppable state determined beforehand.
When Fk is set to a value other than FkE, that is, when the flow rate of the hydrocarbon-based fuel supplied to the reformer is changed in step C21 and step C26, the flow rates of fluids supplied to the indirect internal reforming SOFC, and the input and output of electricity to and from the indirect internal reforming SOFC may be accordingly set to operation conditions determined beforehand, as required, as in the above. For example, with respect to the flow rate of water supplied to the reformer, in order to suppress carbon deposition, the water flow rate may be changed with the change of the fuel flow rate, so that a predetermined value of the steam/carbon ratio is maintained. With respect to the flow rate of air supplied to the reformer, the air flow rate may be changed with the change of the fuel flow rate, so that a predetermined value of the oxygen/carbon ratio is maintained. The flow rates of fluids supplied to the indirect internal reforming SOFC, other than the water and air supplied to the reformer, and the input and output of electricity to and from the indirect internal reforming SOFC may be set to the operation conditions in the reforming-stoppable state determined beforehand, or may be set to operation conditions determined beforehand as functions of the fuel flow rate.
When the reforming method is changed, the flow rates of fluids supplied to the indirect internal reforming SOFC, and the input and output of electricity to and from the indirect internal reforming SOFC may be accordingly set to operation conditions determined beforehand, as required, as in the above. For example, in order to suppress carbon deposition, the flow rate of water supplied to the reformer may be changed to a flow rate at which a steam/carbon ratio determined beforehand is obtained. The flow rate of air supplied to the reformer may be changed to a flow rate at which an oxygen/carbon ratio determined beforehand is obtained. The flow rates of fluids supplied to the indirect internal reforming SOFC, other than the water and air supplied to the reformer, and the input and output of electricity to and from the indirect internal reforming SOFC may be set to the operation conditions in the reforming-stoppable state determined beforehand, or may be set to operation conditions determined beforehand as functions of the fuel flow rate.
When a steam reforming reaction is performed, that is, steam reforming or autothermal reforming is performed, steam is supplied to the reforming catalyst layer. When a partial oxidation reforming reaction is performed, that is, partial oxidation reforming or autothermal reforming is performed, an oxygen-containing gas is supplied to the reforming catalyst layer. As the oxygen-containing gas, a gas containing oxygen may be appropriately used, but in terms of the ease of availability, air is preferred.
The present invention is particularly effective when the hydrocarbon-based fuel has a carbon number of 2 or more, because in the case of such a fuel, particularly, reliable reforming is required.
In order to perform the method of the present invention, appropriate instrumentation and controlling equipment, including a computing means, such as a computer, may be used.
Next, the third embodiment of the shutdown method of the present invention will be described.
The flow rate of the hydrocarbon-based fuel supplied to the reformer (particularly, the reforming catalyst layer) in the reforming-stoppable state is represented as FkE.
FkE may be obtained beforehand by experiment or simulation. FkE may be found by performing an experiment or a simulation, while varying flow rates of fluids supplied to the indirect internal reforming SOFC, such as the flow rate of water (including steam) for steam reforming or autothermal reforming and the flow rate of air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, a cathode air flow rate, the flow rates of a fuel and air supplied to a burner, and flow rates of fluids, such as water and air, supplied to a heat exchanger; and electrical input and output to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, water and liquid fuel evaporators, the SOFC, fluid supply piping, and the like, and electrical input taken out from a thermoelectric conversion module and the like, that is, varying the operation conditions of the indirect internal reforming SOFC, and searching for FkE that steadily satisfies the conditions i to iv. FkE may be any value as long as the conditions i to iv are satisfied, but in terms of thermal efficiency, the smallest FkE is preferably used. The operation conditions of the indirect internal reforming SOFC, including the FkE, are determined beforehand as operation conditions in the reforming-stoppable state.
The temperature condition of the reforming catalyst layer included in the above operation conditions in the reforming-stoppable state determined beforehand, is represented as TrE. TrE may be found together with FkE in the process of search for FkE, and TrE is a temperature condition of the reforming catalyst layer that corresponds to the single FkE used.
The flow rate of the hydrocarbon-based fuel supplied to the reformer at the point of time of the start of the shutdown method is represented as Fk0.
In order to determine a flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer, the measured value of the reforming catalyst layer temperature is compared with the above TrE. Also, the measured value of the reforming catalyst layer temperature is used for the calculation of FkMinCALC described later. In order to do this, the reforming catalyst layer temperature is measured. For example, the reforming catalyst layer temperature may be monitored (continuously measured).
When the monitoring of the temperature of the reforming catalyst layer has been performed since before the start of the shutdown method, the temperature monitoring may be continuously performed as it has been.
If the anode temperature falls below the oxidative degradation temperature, then the reducing gas becomes unnecessary, and therefore, the supply of the hydrocarbon-based fuel to the reformer can be stopped to complete the shutdown method. Therefore, the monitoring of the temperature of the reforming catalyst layer may be continuously performed until the anode temperature falls below the oxidative degradation temperature.
An appropriate temperature sensor, such as a thermocouple, may be used for the measurement of the reforming catalyst layer temperature.
[Case where Reforming Method is Changed Before and after Start of Shutdown Method]
When the reforming method is changed before and after the start of the shutdown method, the above-described FkE, TrE, and FrMin are determined for a reforming-stoppable state, when reforming after the change of the reforming method is performed. Also, FkMinCALC described later is calculated for a case where reforming after the change of the reforming method is performed.
While the anode temperature does not fall below the oxidative degradation temperature, the following steps A3 to D3 are performed. When the anode temperature falls below the oxidative degradation temperature, the supply of the hydrocarbon-based fuel to the reformer can be stopped, regardless of the status of the implementation of steps A3 to D3, to complete the shutdown method. It is possible to stop the supply of fluids supplied to the indirect internal reforming SOFC, such as water (including steam) for steam reforming or autothermal reforming and air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, cathode air, the fuel and air supplied to the burner, and fluids, such as water and air, supplied to the heat exchanger; and the input and output of electricity to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, the water and liquid fuel evaporators, the cell stack, the fluid supply piping, and the like, and electrical input taken out from the thermoelectric conversion module and the like, according to the stop of the supply of the hydrocarbon-based fuel to the reformer.
The shutdown method includes steps A3 to D3, but it is not necessary to actually perform all of steps A3 to D3, and only part of steps A3 to D3 may be performed according to the circumstances.
First, a reforming catalyst layer temperature T is measured. Then, the magnitude relationship between this temperature T and the above-described TrE is checked.
When T<TrE in step A3, the following steps B31 to B34 are performed in order. “T<TrE” is considered to mean that the hydrocarbon-based fuel at the flow rate FkE cannot be reformed.
Step B31
First, step B31 is performed. In other words, the step of increasing the temperature of the reforming catalyst layer is performed.
For example, the temperature of the reforming catalyst layer is increased using an appropriate heat source, such as a heater or a burner annexed to the reformer.
Step B32
Then, step B32 is performed. In other words, the step of measuring the reforming catalyst layer temperature T and comparing the values of this T and TrE is performed.
Step B33
When T<TrE in step B32, the method returns to step B31. In other words, while T<TrE, steps B31 to B33 are repeatedly performed. During this time, the temperature of the reforming catalyst layer increases.
In performing steps B32 and B33, the temperature increase in step B31 may be stopped once, but while steps B32 and B33 are performed, step B31 may be continued.
Step B34
When T≧TrE in step B32, the step of adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer (represented as Fk) from Fk0 to FkE and going to step D3 is performed. “T≧TrE” is considered to mean that the hydrocarbon-based fuel at a flow rate that is equal to or less than FkE can be reformed.
At this time, in a case where the reforming method should be changed before and after the start of the shutdown method, the fuel flow rate is adjusted from Fk0 to FkE, and the reforming method is changed. By this method, it is possible to prevent the oxidative degradation of the anode with the reformed gas, while reliably reforming the hydrocarbon-based fuel.
When T≧TrE in step A3, step C3 is performed. “T≧TrE” is considered to mean that the hydrocarbon-based fuel at a flow rate that is equal to or less than FkE can be reformed.
Step C31
The reforming catalyst layer temperature is measured, and this measured temperature T is compared with TrE.
Step C32
When T≦TrE in step C31, the flow rate of the hydrocarbon-based fuel supplied to the reformer is set to FkE, and the method goes to step D3.
In a case where the reforming method is changed before and after the start of the shutdown method, and where step C32 is performed without performing step C33 even once, that is, when T≦TrE in the first-time step C31, the fuel flow rate is adjusted from Fk0 to FkE, and the reforming method is changed.
Step C33
When T>TrE in step C31, FkMinCALC is calculated based on this T, and the values of this FkMinCALC and FkE are compared.
Here, the calculated value of the flow rate of the hydrocarbon-based fuel at which the reformed gas at the flow rate FrMin can be produced in the reformer at a measured reforming catalyst layer temperature by a reforming method of a type performed after the start of the shutdown method is represented as FkMinCALC. In other words, FkMinCALC may be obtained by measuring the temperature of the reforming catalyst layer, and calculating the flow rate of the hydrocarbon-based fuel at which the reformed gas at the flow rate FrMin can be produced in the reformer when the reforming catalyst layer has this temperature. At this time, it is assumed that the reforming method of the type performed after the start of the shutdown method is performed in the reforming catalyst layer. The reforming type is, for example, steam reforming, autothermal reforming, or partial oxidation reforming.
Specifically, when a certain type of reforming is performed before the start of the shutdown method, the same type of reforming as this may be performed after the start of the shutdown method. In this case, the flow rate (calculated value) of the hydrocarbon-based fuel at which the reformed gas at the flow rate FrMin can be produced in the reformer, when this type of reforming is performed in the reformer, is FkMinCALC. For example, when steam reforming is performed before the start of the shutdown method, steam reforming may also be continuously performed after the start of the shutdown method, and the flow rate (calculated value) of the hydrocarbon-based fuel at which the reformed gas at the flow rate FrMin can be produced in the reformer at the measured temperature of the reforming catalyst layer when steam reforming is performed in the reformer is FkMinCALC.
Alternatively, when a certain type of reforming (a first type of reforming) is performed before the start of the shutdown method, a different type of reforming from this (a second type of reforming) may be performed after the start of the shutdown method. In this case, the flow rate (calculated value) of the hydrocarbon-based fuel at which the reformed gas at the flow rate FrMin can be produced in the reformer, when the second type of reforming is performed in the reformer, is FkMinCALC. For example, when autothermal reforming is performed before the start of the shutdown method, the reforming may be switched to steam reforming after the start of the shutdown method. In this case, the flow rate (calculated value) of the hydrocarbon-based fuel at which the reformed gas at the flow rate FrMin can be produced in the reformer at the measured temperature of the reforming catalyst layer when steam reforming is performed is FkMinCALC.
Step C34
When FkMinCALC<FkE in step C33, the step of setting the flow rate of the hydrocarbon-based fuel supplied to the reformer to FkMinCALC and returning to step C31 is performed. In other words, when T>TrE in step 12 and FkMinCALC<FkE in step C33, steps C31, C33, and C34 are repeatedly performed. During this time, the reforming catalyst layer temperature decreases. Therefore, eventually, T≦TrE is satisfied.
When the reforming method is changed before and after the start of the shutdown method, the fuel flow rate is adjusted from Fk0 to FkMinCALC, and the reforming method is changed, in the first-time step C34.
C35
When FkMinCALC≧FkE in step C33, the step of setting the flow rate of the hydrocarbon-based fuel supplied to the reformer (represented as Fk) to FkE and going to step D3 is performed.
In step C35, Fk may be immediately set to FkE, or Fk may be gradually set to FkE (see case 3-3 described later).
In a case where the reforming method is changed before and after the start of the shutdown method, and where step C35 is performed without performing step C34 even once, that is, when FkMinCALC≧FkE in the first-time step C33, the fuel flow rate is changed from Fk0 to FkE, and the reforming method is changed. When the fuel flow rate is gradually adjusted from Fk0 to FkE via an intermediate flow rate FkM, as in case 3-3 described later, the fuel flow rate is changed from Fk0 to FkM, and the reforming method is changed.
In step C3 and the subsequent steps, from a state in which T≧TrE, it is possible to set operation conditions to the operation conditions in the reforming-stoppable state, and bring the internal reforming solid oxide fuel cell to the reforming-stoppable state, without allowing the unreformed hydrocarbon-based fuel to flow into the anode. But, generally, within a temperature range preferred for reforming, as the reforming catalyst layer temperature T becomes higher, the reformed gas flow rate becomes larger. Therefore, while T≧TrE, the reformed gas flow rate is equal to or more than FrMin, and this means that an excessive hydrocarbon-based fuel is supplied.
On the other hand, in step C3 and the subsequent steps, by supplying the hydrocarbon-based fuel at a flow rate as small as possible, among the flow rates of the hydrocarbon-based fuel at which the reformed gas at a flow rate that is equal to or more than FrMin can be produced and which are smaller than FkE, to the reformer, it is possible to make the hydrocarbon-based fuel supplied to the reformer as small as possible, while producing a reformed gas at a flow rate that is equal to or more than FrMin in the reformer. But, if the supply of the hydrocarbon-based fuel at a flow rate as small as possible, among the flow rates of the hydrocarbon-based fuel at which the reformed gas at a flow rate that is equal to or more than FrMin can be produced, is continued, then T<TrE may be satisfied due to the decrease of the reforming catalyst layer temperature, and there may be a case where the hydrocarbon-based fuel at the flow rate as small as possible, among the flow rates of the hydrocarbon-based fuel at which the reformed gas at a flow rate that is equal to or more than FrMin can be produced, cannot be reformed. Further, there may be a case where it is impossible to set operation conditions to the operation conditions in the reforming-stoppable state, and bring the internal reforming solid oxide fuel cell to the reforming-stoppable state, without allowing the unreformed hydrocarbon-based fuel to flow into the anode.
Therefore, when T≦TrE is satisfied, by setting operation conditions to the operation conditions in the reforming-stoppable state, it is possible to bring the internal reforming solid oxide fuel cell to the reforming-stoppable state, without allowing the unreformed hydrocarbon-based fuel to flow into the anode.
Also, when T>TrE, by supplying the hydrocarbon-based fuel at a flow rate as small as possible, among the flow rates of the hydrocarbon-based fuel at which the reformed gas at FrMin or more can be produced, to the reformer, it is possible to make the hydrocarbon-based fuel supplied to the reformer as small as possible, while producing a reformed gas at a flow rate that is equal to or more than FrMin in the reformer.
In step D3, the method waits for the anode temperature to fall below the oxidative degradation temperature. During this time, the flow rate of the hydrocarbon-based fuel is maintained at FkE, and the flow rates of fluids supplied to the indirect internal reforming SOFC, such as the flow rate of water (including steam) for steam reforming or autothermal reforming and the flow rate of air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, the cathode air flow rate, the flow rates of the fuel and air supplied to the burner, and the flow rates of fluids, such as water and air, supplied to the heat exchanger; and the input and output of electricity to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, the water and liquid fuel evaporators, the cell stack, the fluid supply piping, and the like, and electrical input taken out from the thermoelectric conversion module and the like, are maintained in the operation conditions in the reforming-stoppable state determined beforehand. In other words, the operation conditions of the indirect internal reforming SOFC are maintained in the operation conditions of the indirect internal reforming SOFC in the reforming-stoppable state determined beforehand. The anode temperature decreases with time, and therefore, eventually, the anode temperature falls below the oxidative degradation temperature. The anode temperature may be appropriately monitored (continuously measured) using a temperature sensor, such as a thermocouple.
The monitoring of the anode temperature is preferably started immediately after the shutdown method is started. If the temperature monitoring has been performed since before the start of the shutdown method, then the temperature monitoring may be continued as it has been, also when the shutdown method is performed.
When the anode temperature falls below the oxidative degradation temperature, the supply of the hydrocarbon-based fuel to the reformer can be stopped to complete the shutdown method.
One example of the third embodiment will be described using
The monitoring of the reforming catalyst layer temperature and the monitoring of the anode temperature have been continuously performed since before the point of time of the start of the shutdown method (the same applies to the subsequent cases).
As shown in
In this case, T≧TrE (
In step C31, the reforming catalyst layer temperature T is measured, and the values of this T and TrE are compared.
In this case, T>TrE (
In this case, FkMinCALC<FkE, and therefore, the step of setting the flow rate of the hydrocarbon-based fuel supplied to the reformer to FkMinCALC and returning to step C31 is performed (step C34). When the reforming method should be changed before and after the start of the shutdown method, the flow rate of the hydrocarbon-based fuel is adjusted from Fk0 to FkMinCALC, and the reforming method is changed, in the first-time step C34.
While T>TrE, steps C31, C33, and C34 are repeatedly performed. For a while, steps C31, C33, and C34 are repeated, and during this time, the reforming catalyst layer temperature T decreases with time.
For a period from the point of time of the start of the shutdown method until T≦TrE is satisfied, the flow rate of the hydrocarbon-based fuel supplied to the reformer is set to FkMinCALC (Fk=FkMinCALC). Therefore, in
While T≧TrE, the hydrocarbon-based fuel at the flow rate FrMinCALC that is a flow rate equal to or less than FkE can be reformed. Also, by supplying the hydrocarbon-based fuel at FrMinCALC to the reformer, the reformed gas at a flow rate that is equal to or more than FrMin continues to be supplied to the anode.
In the case of
In step D3, the method waits for the anode temperature to fall below the oxidative degradation temperature.
If the anode temperature becomes less than the oxidative degradation temperature, then the supply of the hydrocarbon-based fuel to the reformer can be stopped to complete the shutdown method.
If the anode temperature falls below the oxidative degradation temperature before T becomes equal to or less than TrE in step C31, then step C32 need not be performed.
By operating in this manner, it is possible to supply the reformed gas at the requisite minimum flow rate or more to the anode, while reliably performing reforming.
In the above case, T becomes equal to or less than TrE before FkMinCALC becomes equal to or more than FkE, and therefore, in step C32, at a point of time when T becomes equal to or less than TrE, Fk is set to FkE. In this case, FkMinCALC becomes equal to or more than FkE before T becomes equal to or less than TrE, and therefore, in step C35, at a point of time when FkMinCALC becomes equal to or more than FkE, Fk is set to FkE. This case will be described using
Until steps C31, C33, and C34 are repeatedly performed after the shutdown method is started, the operation is similar to that of case 3-1. In other words, after the method goes from step A3 to step C3, and step C31 is performed, steps C31, C33, and C34 are repeated until a point of time when FkMinCALC≧FkE is satisfied. During this time, the reforming catalyst layer temperature decreases with time.
As in
At a point of time when FkMinCALC becomes equal to or more than FkE, immediately, Fk is set to FkE, and the method goes to step D3 (step C35).
Step D3 and the subsequent steps are similar to those of case 3-1.
If the anode temperature falls below the oxidative degradation temperature before FkMinCALC becomes equal to or more than FkE, then step C35 need not be performed.
By operating in this manner, it is possible to supply the reformed gas at the requisite minimum flow rate or more to the anode, while reliably performing reforming.
In case 3-1, in step C32, at a point of time when the reforming catalyst layer temperature T becomes equal to or less than TrE, Fk is immediately set to FkE. In this case, when Fk is smaller than FkE at a point of time when T becomes equal to or less than TrE, the increase in the flow rate from Fk to FkE is performed gradually, particularly, stepwise. This case will be described using
Until T≦TrE is satisfied, case 3-3 is similar to case 3-1. As in
As in the case of
After Fk is increased to FkM, the measurement of the reforming catalyst layer temperature T, and the comparison of this T with TrE are continued until T≦TrE is satisfied again. Immediately after Fk is increased to FkM, the reforming catalyst layer temperature T increases (due to the increase in heat input to the reformer), and T becomes a value that exceeds TrE, again. But, the heat input to the reformer is smaller than in the reforming-stoppable state, and therefore, the reforming catalyst layer temperature subsequently decreases. When T≦TrE is satisfied again, Fk is immediately increased to FkE. Then, the method waits until the anode temperature becomes less than the oxidative degradation temperature, and the supply of the hydrocarbon-based fuel to the reformer may be stopped. The comparison of T with TrE may be completed at a point of time when T≦TrE is satisfied for the second time.
In the above description, only a single intermediate flow rate is used, but this is not limiting, and a plurality of intermediate flow rates may be used.
In other words, it is possible to use one or a plurality (this number is represented as JM, JM is an integer of 2 or more) of intermediate flow rates FkM(j) (wherein, j is an integer that satisfies 1≦j≦JM), where FkM(j)<FkM(j+1), increase Fk to FkM(j) at a point of time when T≦TrE is satisfied for the j-th time, while increasing j by 1 at a time, such as increasing Fk to FkM(1) when T≦TrE is satisfied for the first time, and increasing Fk to FkM(2) when T≦TrE is satisfied for the second time, and set Fk to FkE when T≦TrE is satisfied for the last time (JM+1-th time). Then, the method waits until the anode temperature becomes less than the oxidative degradation temperature, and the supply of the hydrocarbon-based fuel to the reformer may be stopped. The comparison of T with TrE may be completed at a point of time when T≦TrE is satisfied for the last time. The intermediate flow rates FkM(j) may be determined, for example, by calculating a flow rate obtained by equally dividing the difference between FkMinCALC when T≦TrE is satisfied for the first time and FkE by JM+1. However, it is preferred to make JM as large as possible and make the interval between FkM(j) small, within the allowable range of the memory consumption of a flow rate controlling means, and within a range in which the interval exceeds the precision of a pressure increasing means and flow rate controlling and measuring means, in terms of the reduction of the integrated value of the flow rates of the hydrocarbon-based fuel, that is, thermal efficiency.
Of course, also in this case, if the anode temperature falls below the oxidative degradation temperature, then the supply of the hydrocarbon-based fuel to the reformer can be stopped at this point of time to complete the shutdown method.
In case 3-3, it is possible to reduce the amount of the hydrocarbon-based fuel supplied until the stop of reforming and shorten shutdown time (time from the start of the shutdown method until the anode temperature falls below the oxidative degradation temperature) compared with case 3-1.
A case where T<TrE in step A3, that is, a case where step B3 is performed, will be described using
After the start of the shutdown method, step A3 is immediately performed, and the measurement of the reforming catalyst layer temperature T, and the comparison of this T with TrE are performed. T<TrE (
In this case, the temperature of the reforming catalyst layer is increased by an appropriate heat source, such as a burner and a heater annexed to the reformer, until the reforming catalyst layer temperature becomes equal to or more than TrE, so that the hydrocarbon-based fuel at the flow rate FkE can be reformed, as shown in
When T≧TrE is satisfied in step B32, the flow rate Fk of the hydrocarbon-based fuel supplied to the reformer is adjusted from Fk0 to FkE (step B34). When the reforming method should be changed before and after the start of the shutdown method, the fuel flow rate is adjusted from Fk0 to FkE, and the reforming method is changed. Then, the method goes to step D3 (step B34).
Step D3 and the subsequent steps are similar to those of case 3-1.
“The hydrocarbon-based fuel at a certain flow rate can be reformed (or is capable of being reformed) in the reforming catalyst layer” refers to that when the hydrocarbon-based fuel at this flow rate is supplied to the reforming catalyst layer, the composition of the gas discharged from the reforming catalyst layer becomes a composition suitable to be supplied to the anode of the SOFC.
For example, “can be reformed in the reforming catalyst layer” may be that the supplied hydrocarbon-based fuel can be decomposed to a C1 compound(s) (a compound(s) having a carbon number of 1). In other words, “can be reformed in the reforming catalyst layer” means a case where reforming can proceed in the reforming catalyst layer until a composition is obtained in which a C2+ component(s) (a component(s) having a carbon number of 2 or more) in the gas at the outlet of the reforming catalyst layer has a concentration, which does not cause the problems of anode degradation and flow blockage due to carbon deposition, or less. The concentration of the C2+ component(s) in this case is preferably 50 ppb or less as a mass fraction in the reformed gas. And in this case, it is enough that the gas at the outlet of the reforming catalyst layer is reducing gas. Methane is permitted to be contained in the gas at the outlet of the reforming catalyst layer. In the reforming of the hydrocarbon-based fuel, usually, methane remains in the equilibrium theory. Even if carbon is contained in the gas at the outlet of the reforming catalyst layer in the form of methane, CO, or CO2, carbon deposition can be prevented by adding steam as required. When methane is used as the hydrocarbon-based fuel, it is enough that reforming proceeds so that the gas at the outlet of the reforming catalyst layer becomes reducing.
With respect to the reducing property of the gas at the outlet of the reforming catalyst layer, it is enough that the property is to the extent that if this gas is supplied to the anode, the oxidative degradation of the anode is suppressed. In order to do this, for example, the partial pressures of oxidizing O2, H2O, CO2, and the like contained in the gas at the outlet of the reforming catalyst layer may be lower than their equilibrium partial pressures of oxidation reactions of the anode electrode. For example, when the anode electrode material is Ni, and the anode temperature is 800° C., the partial pressure of O2 contained in the gas at the outlet of the reforming catalyst layer may be less than 1.2×10−14 atm (1.2×10−9 Pa), the partial pressure ratio of H2O to H2 may be less than 1.7×102, and the partial pressure ratio of CO2 to CO may be less than 1.8×102,
A position for the measurement of the reforming catalyst layer temperature will be described in detail below. This measurement position may be used when TrE is found beforehand, and when the temperature of the reforming catalyst layer is measured in steps A3 to C3.
<Case where there is One Temperature Measurement Point>
Temperature Measurement Position
When there is a single temperature measurement point in the reforming catalyst layer, it is preferred to use preferably a position where the temperature becomes relatively low in the reforming catalyst layer, more preferably a position where the temperature becomes the lowest in the reforming catalyst layer, as the position for the measurement of temperature, in terms of safe side control. When the reaction heat in the reforming catalyst layer is endothermic, the vicinity of the center of the catalyst layer may be selected as the temperature measurement position. When the reaction heat in the reforming catalyst layer is exothermic, and the temperatures of the end positions are lower than that of the center portion due to heat release, an end of the catalyst layer may be selected as the temperature measurement position. A location where the temperature becomes low may be found by preliminary experiment or simulation.
<Case where there are Plurality of Temperature Measurement Points>
The point for the measurement of temperature need not be one. Two or more temperature measurement points are preferred in terms of more accurate control. For example, it is possible to measure the inlet temperature and outlet temperature of the reforming catalyst layer and use their average temperature as the above-described reforming catalyst layer temperature T. However, in a case where the rate of a reaction other than a reaction accompanied by the decrease of the hydrocarbon-based fuel (raw fuel) supplied to the reforming catalyst layer is much faster than that of the reaction accompanied by the decrease of the raw fuel, and it can be considered that components other than the raw fuel instantaneously reach an equilibrium composition, even if there are a plurality of temperature measurement points in the reforming catalyst layer, it is preferred to use the temperature of a point nearest to the outlet of the reforming catalyst layer, among the temperatures measured at the plurality of points, as the temperature used for calculating FkMinCALC in step C3. When there are a plurality of temperatures of points nearest to the outlet of the reforming catalyst layer, a calculated value, such as the lowest value of them or their average value, may be appropriately used as a representative value.
Alternatively, for example, it is possible to consider regions Zi obtained by dividing the reforming catalyst layer into N (N is an integer of 2 or more, and i is an integer of 1 or more and N or less), find the temperature Ti of each divided region Zi, and find TrE(j) (={TrE1, TrE2, . . . , TrEN}) for each divided region beforehand. In this case, when any of Ti becomes equal to or less than TrEi, the flow rate of the hydrocarbon-based fuel may be set to FkE.
When N divided regions Zi are considered, temperatures of all divided regions may be set as the temperature condition, or temperature(s) of one or some (not all) regions among the N divided regions may be set as the temperature condition. The catalyst layer regions for the temperature condition may be appropriately changed according to the feed rate of the hydrocarbon-based fuel. FkMinCALC may be calculated for all divided regions, or a value(s) calculated for only one or some (not all) regions among the N divided regions may be used as FkMinCALC. The catalyst layer regions for the calculation may be appropriately changed according to the feed rate of the hydrocarbon-based fuel.
As the temperature of the divided region Zi, actually measured temperature may be used as it is, but a calculated value, such as the average value of the inlet temperature and outlet temperature of the divided region, may be appropriately used as a representative value.
Also, it is not necessary to measure temperatures for all divided regions Zi. Also, the number of divisions of the catalyst layer, N, and the number of temperature measurement point(s) may be independently set.
It is also possible to measure temperature(s) of one or some (not all) of the N divided regions and find temperature(s) of the remaining divided region(s) by appropriate interpolation from the measured temperature(s).
For example, as a temperature of a divided region where no temperature sensor is installed, a temperature of a divided region nearest to this divided region may be used. When there are two nearest divided regions, a temperature of either of the two divided regions may be used, or the average value of temperatures of the two divided regions may be used.
It is also possible to measure temperatures at a plurality of points in the reforming catalyst layer (at different positions along the gas flow direction), independently of the divided regions, and find a temperature of each divided region from the measured temperatures at the plurality of points. For example, it is possible to measure temperatures of the inlet and outlet of the reforming catalyst layer (a temperature of any position in the middle portion may be further measured), interpolate the temperature of the reforming catalyst layer from these measured temperatures by an approximation method, such as a least squares method, and find temperatures of the divided regions from the interpolation curve.
When reforming catalyst layer temperatures at a plurality of positions are measured in step C31, comparison with TrE, and the calculation of FkMinCALC (step C33) may be performed using a temperature at the same position. Alternatively, comparison with TrE, and the calculation of FkMinCALC may be performed using temperatures at different positions.
In order to find temperatures of all divided regions, temperatures of the following positions may be measured.
The inlet and outlet of each divided region.
The interior (one point or a plurality of points) of each divided region (inner side of the inlet and the outlet).
The inlet, outlet, and interior (one point or a plurality of points for one divided region) of each divided region.
In order to find a temperature of one or some (not all) of the divided regions, temperatures of the following positions may be measured.
The inlet and outlet of one or some (not all) of the divided regions.
The interior (one point or a plurality of points) of one or some (not all) of the divided regions (inner side of the inlet and the outlet).
The inlet, outlet, and interior (one point or a plurality of points for one divided region) of one or some (not all) of the divided regions.
The method for calculating the flow rate FkMinCALC of the hydrocarbon-based fuel at which the reformed gas at the flow rate FrMin can be produced in the reformer in the reforming catalyst layer, based on the measured temperature of the reforming catalyst layer, will be described below.
The flow rate of the hydrocarbon-based fuel at which the reformed gas at the flow rate FrMin can be produced in the reformer may be any flow rate that is equal to or more than a flow rate at which the flow rate of the reformed gas is exactly FrMin. The flow rate of the hydrocarbon-based fuel at which the reformed gas at the flow rate FrMin can be produced in the reformer may be the flow rate of the hydrocarbon-based fuel at which the reformed gas at a flow rate that is exactly FrMin can be produced in the reformer, or may be a value obtained by multiplying this flow rate by a safety factor (a value that exceeds 1, for example 1.4).
FkMinCALC depends on the temperature of the reforming catalyst layer. Therefore, FkMinCALC is performed based on the measured temperature of the reforming catalyst layer.
FkMinCALC may be calculated by finding a relation equation between the temperature of the reforming catalyst layer and FkMinCALC beforehand by equilibrium calculation or preliminary experiment, and substituting the measured temperature T of the reforming catalyst layer into this relation equation. Also, it is possible to determine FkMinCALC by multiplying the function obtained by experiment by a safety factor, or offsetting the temperature to the safe side. The unit of FkMinCALC is, for example, mol/s.
FkMinCALC may be a function of only the temperature T. But, this is not limiting, and FkMinCALC may be a function having, in addition to the temperature T, a variable other than T, such as pressure, the concentration of the gas component, or time. In this case, when FkMinCALC is calculated, it is possible to appropriately obtain a variable other than T, and calculate FkMinCALC from the variable other than T and the measured T.
The outlet of the reforming catalyst layer is preferred as the position for the measurement of the temperature used for the calculation of FkMinCALC, in terms of accuracy.
When the flow rate Fk of the hydrocarbon-based fuel is set to FkE, the flow rates of fluids supplied to the indirect internal reforming SOFC, such as the flow rate of water (including steam) for steam reforming or autothermal reforming and the flow rate of air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, the cathode air flow rate, the flow rates of the fuel and air supplied to the burner, and the flow rates of fluids, such as water and air, supplied to the heat exchanger; and the input and output of electricity to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, the water and liquid fuel evaporators, the cell stack, the fluid supply piping, and the like, and electrical input taken out from the thermoelectric conversion module and the like, are accordingly set, as required, to the operation conditions in the reforming-stoppable state determined beforehand. In other words, the operation conditions of the indirect internal reforming SOFC are set to the operation conditions of the indirect internal reforming SOFC in the reforming-stoppable state determined beforehand.
When Fk is set to a value other than FkE, that is, when the flow rate of the hydrocarbon-based fuel supplied to the reformer is changed in step C34 and the like, the flow rates of fluids supplied to the indirect internal reforming SOFC, and the input and output of electricity to and from the indirect internal reforming SOFC may be accordingly set to operation conditions determined beforehand, as required, as in the above. For example, with respect to the flow rate of water supplied to the reformer, in order to suppress carbon deposition, the water flow rate may be decreased with the decrease of the fuel flow rate, so that a predetermined value of the steam/carbon ratio is maintained. With respect to the flow rate of air supplied to the reformer, the air flow rate may be decreased with the decrease of the fuel flow rate, so that a predetermined value of the oxygen/carbon ratio is maintained. The flow rates of fluids supplied to the indirect internal reforming SOFC, other than the water and air supplied to the reformer, and the input and output of electricity to and from the indirect internal reforming SOFC may be set to the operation conditions in the reforming-stoppable state determined beforehand, or may be set to operation conditions determined beforehand as functions of the fuel flow rate.
When the reforming method is changed, the flow rates of fluids supplied to the indirect internal reforming SOFC, and the input and output of electricity to and from the indirect internal reforming SOFC may be accordingly set to operation conditions determined beforehand, as required, as in the above. For example, in order to suppress carbon deposition, the flow rate of water supplied to the reformer may be changed to a flow rate at which a steam/carbon ratio determined beforehand is obtained. The flow rate of air supplied to the reformer may be changed to a flow rate at which an oxygen/carbon ratio determined beforehand is obtained. The flow rates of fluids supplied to the indirect internal reforming SOFC, other than the water and air supplied to the reformer, and the input and output of electricity to and from the indirect internal reforming SOFC may be set to the operation conditions in the reforming-stoppable state determined beforehand, or may be set to operation conditions determined beforehand as functions of the fuel flow rate.
When a steam reforming reaction is performed, that is, steam reforming or autothermal reforming is performed, steam is supplied to the reforming catalyst layer. When a partial oxidation reforming reaction is performed, that is, partial oxidation reforming or autothermal reforming is performed, an oxygen-containing gas is supplied to the reforming catalyst layer. As the oxygen-containing gas, a gas containing oxygen may be appropriately used, but in terms of the ease of availability, air is preferred.
The present invention is particularly effective when the hydrocarbon-based fuel has a carbon number of 2 or more, because in the case of such a fuel, particularly, reliable reforming is required.
In order to perform the method of the present invention, appropriate instrumentation and controlling equipment, including a computing means, such as a computer, may be used.
Next, the fourth embodiment of the shutdown method of the present invention will be described.
The flow rate of the hydrocarbon-based fuel supplied to the reformer (particularly, the reforming catalyst layer) in the reforming-stoppable state is represented as FkE.
FkE may be obtained beforehand by experiment or simulation. FkE may be found by performing an experiment or a simulation, while varying flow rates of fluids supplied to the indirect internal reforming SOFC, such as the flow rate of water (including steam) for steam reforming or autothermal reforming and the flow rate of air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, a cathode air flow rate, the flow rates of a fuel and air supplied to a burner, and flow rates of fluids, such as water and air, supplied to a heat exchanger; and electrical input and output to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, water and liquid fuel evaporators, the SOFC, fluid supply piping, and the like, and electrical input taken out from a thermoelectric conversion module and the like, that is, varying the operation conditions of the indirect internal reforming SOFC, and searching for FkE that steadily satisfies the conditions i to iv. FkE may be any value as long as the conditions i to iv are satisfied, but in terms of thermal efficiency, the smallest FkE is preferably used. The operation conditions of the indirect internal reforming SOFC, including the FkE, are determined beforehand as operation conditions in the reforming-stoppable state.
The flow rate of the hydrocarbon-based fuel supplied to the reformer at the point of time of the start of the shutdown method is represented as Fk0.
The calculated value of the flow rate of the hydrocarbon-based fuel capable of being reformed at a measured reforming catalyst layer temperature by a reforming method of a type performed after the start of the shutdown method (this flow rate is hereinafter sometimes referred to as a “reformable flow rate”) is represented as FkCALC. In other words, FkCALC may be obtained by measuring the temperature of the reforming catalyst layer, and calculating the flow rate of the hydrocarbon-based fuel capable of being reformed in the reforming catalyst layer when the reforming catalyst layer has this temperature. At this time, it is assumed that the reforming method of the type performed after the start of the shutdown method is performed in the reforming catalyst layer (the type of the reforming method is hereinafter sometimes referred to as a reforming type). The reforming type is, for example, steam reforming, autothermal reforming, or partial oxidation reforming.
Specifically, when a certain type of reforming is performed before the start of the shutdown method, the same type of reforming as this may be performed after the start of the shutdown method. In this case, the flow rate (calculated value) of the hydrocarbon-based fuel capable of being reformed, when this type of reforming is performed in the reformer, is FkCALC. For example, when steam reforming is performed before the start of the shutdown method, steam reforming may also be continuously performed after the start of the shutdown method, and the flow rate of the hydrocarbon-based fuel capable of being reformed at the measured temperature of the reforming catalyst layer when steam reforming is performed in the reformer is FkCALC.
Alternatively, when a certain type of reforming (a first type of reforming) is performed before the start of the shutdown method, a different type of reforming from this (a second type of reforming) may be performed after the start of the shutdown method. In this case, the flow rate of the hydrocarbon-based fuel capable of being reformed, when the second type of reforming is performed in the reformer, is FkCALC. For example, when autothermal reforming is performed before the start of the shutdown method, the reforming may be switched to steam reforming after the start of the shutdown method. In this case, the flow rate (calculated value) of the hydrocarbon-based fuel capable of being reformed at the measured temperature of the reforming catalyst layer when steam reforming is performed is FkCALC.
A measured value of the reforming catalyst layer temperature is used for the calculation of FkCALC. In order to do this, the reforming catalyst layer temperature is measured. For example, the reforming catalyst layer temperature may be monitored (continuously measured).
When the monitoring of the temperature of the reforming catalyst layer has been performed since before the start of the shutdown method, the temperature monitoring may be continuously performed as it has been.
When the anode temperature falls below the oxidative degradation temperature, the reducing gas becomes unnecessary, and therefore, the supply of the hydrocarbon-based fuel to the reformer can be stopped to complete the shutdown method. Therefore, the monitoring of the temperature of the reforming catalyst layer may be continuously performed until the anode temperature falls below the oxidative degradation temperature.
An appropriate temperature sensor, such as a thermocouple, may be used for the measurement of the reforming catalyst layer temperature.
[Case where Reforming Method is Changed Before and after Start of Shutdown Method]
When the reforming method is changed before and after the start of the shutdown method, the above-described FkE and FrMin are determined for a reforming-stoppable state, when reforming after the change of the reforming method is performed.
While the anode temperature does not fall below the oxidative degradation temperature, the following steps A4 to D4 are performed. When the anode temperature falls below the oxidative degradation temperature, the supply of the hydrocarbon-based fuel to the reformer can be stopped, regardless of the status of the implementation of steps A4 to D4, to complete the shutdown method. It is possible to stop the supply of fluids supplied to the indirect internal reforming SOFC, such as water (including steam) for steam reforming or autothermal reforming and air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, cathode air, the fuel and air supplied to the burner, and fluids, such as water and air, supplied to the heat exchanger; and the input and output of electricity to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, the water and liquid fuel evaporators, the cell stack, the fluid supply piping, and the like, and electrical input taken out from the thermoelectric conversion module and the like, according to the stop of the supply of the hydrocarbon-based fuel to the reformer.
The shutdown method includes steps A4 to D4, but it is not necessary to actually perform all of steps A4 to D4, and only part of steps A4 to D4 may be performed according to the circumstances.
First, a reforming catalyst layer temperature T is measured. Then, a reformable flow rate FkCALC is calculated based on this temperature T. Further, the magnitude relationship between the flow rate FkE of the hydrocarbon-based fuel supplied to the reformer in the above-described reforming-stoppable state and this FkCALC is checked.
When FkCALC<FkE in step A4, the following steps B41 to B44 are performed in order. “FkCALC<FkE” is considered to mean that the hydrocarbon-based fuel at the flow rate FkE cannot be reformed in the reformer (by a reforming type after change, if the reforming type is changed).
Step B41
First, step B41 is performed. In other words, the step of increasing the temperature of the reforming catalyst layer is performed.
For example, the temperature of the reforming catalyst layer is increased using an appropriate heat source, such as a heater or a burner annexed to the reformer.
Step B42
Then, step B42 is performed. In other words, the step of measuring the reforming catalyst layer temperature T, calculating FkCALC using this T, and comparing the values of this FkCALC and FkE is performed.
Step B43
When FkCALC<FkE in step B42, the step of returning to step B41 is performed. In other words, while FkCALC<FkE, steps B41 to B43 are repeatedly performed. During this time, the temperature of the reforming catalyst layer increases.
In performing steps B42 and B43, the temperature increase in step B41 may be stopped once, but while steps B42 and B43 are performed, step B41 may be continued.
Step B44
When FkCALC≧FkE in step B42, the step of adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer (represented as Fk) from Fk0 to FkE and going to step D4 is performed. “FkCALC≧FkE” is considered to mean that the hydrocarbon-based fuel at the flow rate FkE can be reformed in the reforming catalyst layer (by a reforming type after change, if the reforming type is changed).
At this time, in a case where the reforming type should be changed before and after the start of the shutdown method, the fuel flow rate is adjusted from Fk0 to FkE, and the reforming type is changed. By this method, it is possible to prevent the oxidative degradation of the anode with the reformed gas, while reliably reforming the hydrocarbon-based fuel.
When FkCALC≧FkE in step A4, the flow rate of the hydrocarbon-based fuel supplied to the reformer is adjusted from Fk0 to FkE, and the method goes to step D4.
At this time, in a case where the reforming type should be changed before and after the start of the shutdown method, the fuel flow rate is adjusted from Fk0 to FkE, and the reforming type is changed. By this method, it is possible to prevent the oxidative degradation of the anode with the reformed gas, while reliably reforming the hydrocarbon-based fuel.
In step D4, the method waits for the anode temperature to fall below the oxidative degradation temperature. During this time, the flow rate of the hydrocarbon-based fuel is maintained at FkE, and the flow rates of fluids supplied to the indirect internal reforming SOFC, such as the flow rate of water (including steam) for steam reforming or autothermal reforming and the flow rate of air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, the cathode air flow rate, the flow rates of the fuel and air supplied to the burner, and the flow rates of fluids, such as water and air, supplied to the heat exchanger; and the input and output of electricity to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, the water and liquid fuel evaporators, the cell stack, the fluid supply piping, and the like, and electrical input taken out from the thermoelectric conversion module and the like, are maintained in the operation conditions in the reforming-stoppable state determined beforehand. In other words, the operation conditions of the indirect internal reforming SOFC are maintained in the operation conditions of the indirect internal reforming SOFC in the reforming-stoppable state determined beforehand. The anode temperature decreases with time, and therefore, eventually, the anode temperature falls below the oxidative degradation temperature. The anode temperature may be appropriately monitored (continuously measured) using a temperature sensor, such as a thermocouple.
The monitoring of the anode temperature is preferably started immediately after the shutdown method is started. If the temperature monitoring has been performed since before the start of the shutdown method, then the temperature monitoring may be continued as it has been also when the shutdown method is performed.
When the anode temperature falls below the oxidative degradation temperature, the supply of the hydrocarbon-based fuel to the reformer can be stopped to complete the shutdown method.
A case where FkCALC calculated in step A4 is equal to or more than the flow rate FkE of the hydrocarbon-based fuel supplied to the reformer in the reforming-stoppable state, that is, the case of FkCALC≧FkE, will be described using
In
The monitoring of the reforming catalyst layer temperature and the monitoring of the anode temperature have been continuously performed since before the point of time of the start of the shutdown method (the same applies to the subsequent cases).
Immediately after the shutdown method is started, step A4 is performed. In other words, the reforming catalyst layer temperature T is measured, the reformable flow rate FkCALC is calculated using this T, and the values of this FkCALC and FkE are compared.
In this case, FkCALC≧FkE (
Then, the method goes to step D4, and waits until the anode temperature falls below the oxidative degradation temperature.
If the anode temperature becomes less than the oxidative degradation temperature, then the supply of the hydrocarbon-based fuel to the reformer can be stopped to complete the shutdown method.
By operating in this manner, it is possible to supply the reformed gas at the requisite minimum flow rate or more to the anode, while reliably performing reforming.
A case where FkCALC calculated in step A4 is smaller than the flow rate FkE of the hydrocarbon-based fuel supplied to the reformer in the reforming-stoppable state, that is, the case of FkCALC<FkE, will be described using
After the start of the shutdown method, step A4 is immediately performed, and the measurement of the reforming catalyst layer temperature T, the calculation of FkCALC based on this T, and the comparison of this FkCALC with FkE are performed. FkCALC<FkE (
In this case, the temperature of the reforming catalyst layer is increased by an appropriate heat source, such as a burner and a heater annexed to the reformer, until FkCALC≧FkE is satisfied, so that the hydrocarbon-based fuel at the flow rate FkE can be reformed, as shown in
When FkCALC≧FkE is satisfied in step B42, the flow rate Fk of the hydrocarbon-based fuel supplied to the reformer is adjusted from Fk0 to FkE (step B44). When the reforming method should be changed before and after the start of the shutdown method, the fuel flow rate is adjusted from Fk0 to FkE, and the reforming method is changed. Then, the method goes to step D4 (step B44).
Step D4 and the subsequent steps are similar to those of case 4-1.
In this embodiment, control is simple, and a program when control by the program is performed is also simple.
“The hydrocarbon-based fuel at a certain flow rate can be reformed (or is capable of being reformed) in the reforming catalyst layer” refers to that when the hydrocarbon-based fuel at this flow rate is supplied to the reforming catalyst layer, the composition of the gas discharged from the reforming catalyst layer becomes a composition suitable to be supplied to the anode of the SOFC.
For example, “can be reformed in the reforming catalyst layer” may be that the supplied hydrocarbon-based fuel can be decomposed to a C1 compound(s) (a compound(s) having a carbon number of 1). In other words, “can be reformed in the reforming catalyst layer” means a case where reforming can proceed in the reforming catalyst layer until a composition is obtained in which a C2+ component(s) (a component(s) having a carbon number of 2 or more) in the gas at the outlet of the reforming catalyst layer has a concentration, which does not cause the problems of anode degradation and flow blockage due to carbon deposition, or less. The concentration of the C2+ component(s) in this case is preferably 50 ppb or less as a mass fraction in the reformed gas. And in this case, it is enough that the gas at the outlet of the reforming catalyst layer is reducing gas. Methane is permitted to be contained in the gas at the outlet of the reforming catalyst layer. In the reforming of the hydrocarbon-based fuel, usually, methane remains in the equilibrium theory. Even if carbon is contained in the gas at the outlet of the reforming catalyst layer in the form of methane, CO, or CO2, carbon deposition can be prevented by adding steam as required. When methane is used as the hydrocarbon-based fuel, it is enough that reforming proceeds so that the gas at the outlet of the reforming catalyst layer becomes reducing.
With respect to the reducing property of the gas at the outlet of the reforming catalyst layer, it is enough that the property is to the extent that if this gas is supplied to the anode, the oxidative degradation of the anode is suppressed. In order to do this, for example, the partial pressures of oxidizing O2, H2O, CO2, and the like contained in the gas at the outlet of the reforming catalyst layer may be lower than their equilibrium partial pressures of oxidation reactions of the anode electrode. For example, when the anode electrode material is Ni, and the anode temperature is 800° C., the partial pressure of O2 contained in the gas at the outlet of the reforming catalyst layer may be less than 1.2×10−14 atm (1.2×10−9 Pa), the partial pressure ratio of H2O to H2 may be less than 1.7×102, and the partial pressure ratio of CO2 to CO may be less than 1.8×102.
The method for calculating the flow rate of the hydrocarbon-based fuel capable of being reformed in the reforming catalyst layer, based on the measured temperature of the reforming catalyst layer, will be described below.
The meaning of “capable of being reformed (can be reformed)” is as described above, and the flow rate of the hydrocarbon-based fuel capable of being reformed in the reforming catalyst layer (reformable flow rate) refers to a flow rate such that when the hydrocarbon-based fuel at this flow rate is supplied to the reforming catalyst layer, the composition of the gas discharged from the reforming catalyst layer becomes a composition suitable to be supplied to the anode of the SOFC.
For example, the reformable flow rate in the reforming catalyst layer may be any flow rate that is equal to or less than the maximum value of flow rates at which the supplied hydrocarbon-based fuel can be decomposed to a C1 compound(s) (a compound(s) having a carbon number of 1). The reformable flow rate may be this maximum value, or may be a value obtained by dividing this maximum value by a safety factor (a value that exceeds 1, for example 1.4).
The reformable flow rate depends on the temperature of the reforming catalyst layer. Therefore, the calculation of the reformable flow rate in the reforming catalyst layer is performed based on the measured temperature of the reforming catalyst layer.
The reformable flow rate FkCALC in the reforming catalyst layer may be obtained beforehand as a function of the temperature T of the reforming catalyst layer by experiment (FkCALC is represented also as FkCALC(T) to explicitly show that it is a function of temperature). Also, it is possible to determine the reformable flow rate by multiplying the function obtained by experiment by a safety factor, or offsetting the temperature to the safe side. The unit of FkCALC(T) is, for example, mol/s.
The reformable flow rate FkCALC(T) may be a function of only the temperature T. But, this is not limiting, and the reformable flow rate FkCALC may be a function having, in addition to the temperature T, a variable other than T, such as the volume of the catalyst layer, the concentration of the gas component, or time. In this case, when the reformable flow rate FkCALC(T) is calculated, it is possible to appropriately obtain a variable other than T, and calculate the reformable flow rate FkCALC(T) from the variable other than T and the measured T.
A position for the measurement of the reforming catalyst layer temperature will be described in detail below. This measurement position may be used when the temperature of the reforming catalyst layer is measured in steps A4 to C4.
<Case where there is One Temperature Measurement Point>
Temperature Measurement Position
When there is a single temperature measurement point in the reforming catalyst layer, it is preferred to use preferably a position where the temperature becomes relatively low in the reforming catalyst layer, more preferably a position where the temperature becomes the lowest in the reforming catalyst layer, as the position for the measurement of temperature, in terms of safe side control. When the reaction heat in the reforming catalyst layer is endothermic, the vicinity of the center of the catalyst layer may be selected as the temperature measurement position. When the reaction heat in the reforming catalyst layer is exothermic, and the temperatures of the end positions are lower than that of the center portion due to heat release, an end of the catalyst layer may be selected as the temperature measurement position. A location where the temperature becomes low may be found by preliminary experiment or simulation.
<Case where there are Plurality of Temperature Measurement Points>
The point for the measurement of temperature need not be one. Two or more temperature measurement points are preferred in terms of more accurate control. For example, it is possible to measure the inlet temperature and outlet temperature of the reforming catalyst layer and use their average temperature as the above-described reforming catalyst layer temperature T.
Alternatively, for example, it is possible to consider regions Zi obtained by dividing the reforming catalyst layer into N (N is an integer of 2 or more, and i is an integer of 1 or more and N or less), find the temperature Ti of each divided region Zi, calculate a reformable flow rate FkCALCi(Ti) in each divided region from each temperature Ti, and calculate a value obtained by summing the reformable flow rates FkCALCi(Ti), as the reformable flow rate FkCALC in the reforming catalyst layer.
When N divided regions Zi are considered, reformable flow rates of all divided regions may be summed, or a total of reformable flow rates of only one or some (not all) regions among the N divided regions may be used as the reformable flow rate FkCALC in the reforming catalyst layer. The catalyst layer regions for summation may be appropriately changed according to the feed rate of the hydrocarbon-based fuel. Also, temperatures of all divided regions may be set as the temperature condition, or temperature(s) of one or some (not all) regions among the N divided regions may be set as the temperature condition. The catalyst layer regions for the temperature condition may be appropriately changed according to the feed rate of the hydrocarbon-based fuel.
As the temperature of the divided region Zi, actually measured temperature may be used as it is, but a calculated value, such as the average value of the inlet temperature and outlet temperature of the divided region, may be appropriately used as a representative value.
Also, it is not necessary to measure temperatures for all divided regions Zi. Also, the number of divisions of the catalyst layer, N, and the number of temperature measurement point(s) may be independently set.
It is also possible to measure temperature(s) of one or some (not all) of the N divided regions and find temperature(s) of the remaining divided region(s) by appropriate interpolation from the measured temperature(s).
For example, as a temperature of a divided region where no temperature sensor is installed, a temperature of a divided region nearest to this divided region may be used. When there are two nearest divided regions, a temperature of either of the two divided regions may be used, or the average value of temperatures of the two divided regions may be used.
It is also possible to measure temperatures at a plurality of points in the reforming catalyst layer (at different positions along the gas flow direction), independently of the divided regions, and find a temperature of each divided region from the measured temperatures at the plurality of points. For example, it is possible to measure temperatures of the inlet and outlet of the reforming catalyst layer (a temperature of any position in the middle portion may be further measured), interpolate the temperature of the reforming catalyst layer from these measured temperatures by an approximation method, such as a least squares method, and find temperatures of the divided regions from the interpolation curve.
In order to find temperatures of all divided regions, temperatures of the following positions may be measured.
The inlet and outlet of each divided region.
The interior (one point or a plurality of points) of each divided region (inner side of the inlet and the outlet).
The inlet, outlet, and interior (one point or a plurality of points for one divided region) of each divided region.
In order to find a temperature of one or some (not all) of the divided regions, temperatures of the following positions may be measured.
The inlet and outlet of one or some (not all) of the divided regions.
The interior (one point or a plurality of points) of one or some (not all) of the divided regions (inner side of the inlet and the outlet).
The inlet, outlet, and interior (one point or a plurality of points for one divided region) of one or some (not all) of the divided regions.
When the flow rate Fk of the hydrocarbon-based fuel is set to FkE, the flow rates of fluids supplied to the indirect internal reforming SOFC, such as the flow rate of water (including steam) for steam reforming or autothermal reforming and the flow rate of air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, the cathode air flow rate, the flow rates of the fuel and air supplied to the burner, and the flow rates of fluids, such as water and air, supplied to the heat exchanger; and the input and output of electricity to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, the water and liquid fuel evaporators, the cell stack, the fluid supply piping, and the like, and electrical input taken out from the thermoelectric conversion module and the like, are accordingly set, as required, to the operation conditions in the reforming-stoppable state determined beforehand. In other words, when the flow rate Fk of the hydrocarbon-based fuel is set to FkE, the operation conditions of the indirect internal reforming SOFC may be set to the operation conditions in the reforming-stoppable state determined beforehand.
When a steam reforming reaction is performed, that is, steam reforming or autothermal reforming is performed, steam is supplied to the reforming catalyst layer. When a partial oxidation reforming reaction is performed, that is, partial oxidation reforming or autothermal reforming is performed, an oxygen-containing gas is supplied to the reforming catalyst layer. As the oxygen-containing gas, a gas containing oxygen may be appropriately used, but in terms of the ease of availability, air is preferred.
The present invention is particularly effective when the hydrocarbon-based fuel has a carbon number of 2 or more, because in the case of such a fuel, particularly, reliable reforming is required.
In order to perform the method of the present invention, appropriate instrumentation and controlling equipment, including a computing means, such as a computer, may be used.
Next, the fifth embodiment of the shutdown method of the present invention will be described.
The flow rate of the hydrocarbon-based fuel supplied to the reformer (particularly, the reforming catalyst layer) in the reforming-stoppable state is represented as FkE.
FkE may be obtained beforehand by experiment or simulation. FkE may be found by performing an experiment or a simulation, while varying flow rates of fluids supplied to the indirect internal reforming SOFC, such as the flow rate of water (including steam) for steam reforming or autothermal reforming and the flow rate of air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, a cathode air flow rate, the flow rates of a fuel and air supplied to a burner, and flow rates of fluids, such as water and air, supplied to a heat exchanger; and electrical input and output to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, water and liquid fuel evaporators, the SOFC, fluid supply piping, and the like, and electrical input taken out from a thermoelectric conversion module and the like, that is, varying the operation conditions of the indirect internal reforming SOFC, and searching for FkE that steadily satisfies the conditions i to iv. FkE may be any value as long as the conditions i to iv are satisfied, but in terms of thermal efficiency, the smallest FkE is preferably used. The operation conditions of the indirect internal reforming SOFC, including the FkE, are determined beforehand as operation conditions in the reforming-stoppable state.
The temperature condition of the reforming catalyst layer included in the above operation conditions in the reforming-stoppable state determined beforehand, is represented as TrE. TrE may be found together with FkE in the process of search for FkE, and TrE is a temperature condition of the reforming catalyst layer that corresponds to the single FkE used.
The flow rate of the hydrocarbon-based fuel supplied to the reformer at the point of time of the start of the shutdown method is represented as Fk0.
In order to determine a flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer, the measured value of the reforming catalyst layer temperature is compared with the above TrE. In order to do this, the reforming catalyst layer temperature is measured. For example, the reforming catalyst layer temperature may be monitored (continuously measured).
When the monitoring of the temperature of the reforming catalyst layer has been performed since before the start of the shutdown method, the temperature monitoring may be continuously performed as it has been.
If the anode temperature falls below the oxidative degradation temperature, then the reducing gas becomes unnecessary, and therefore, the supply of the hydrocarbon-based fuel to the reformer can be stopped to complete the shutdown method. Therefore, the monitoring of the temperature of the reforming catalyst layer may be continuously performed until the anode temperature falls below the oxidative degradation temperature.
An appropriate temperature sensor, such as a thermocouple, may be used for the measurement of the reforming catalyst layer temperature.
[Case where Reforming Method is Changed Before and after Start of Shutdown Method]
When the reforming method is changed before and after the start of the shutdown method, the above-described FkE, TrE, and FrMin are determined for a reforming-stoppable state, when reforming after the change of the reforming method is performed.
While the anode temperature does not fall below the oxidative degradation temperature, the following steps A5 to D5 are performed. When the anode temperature falls below the oxidative degradation temperature, the supply of the hydrocarbon-based fuel to the reformer can be stopped, regardless of the status of the implementation of steps A5 to D5, to complete the shutdown method. It is possible to stop the supply of fluids supplied to the indirect internal reforming SOFC, such as water (including steam) for steam reforming or autothermal reforming and air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, cathode air, the fuel and air supplied to the burner, and fluids, such as water and air, supplied to the heat exchanger; and the input and output of electricity to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, the water and liquid fuel evaporators, the cell stack, the fluid supply piping, and the like, and electrical input taken out from the thermoelectric conversion module and the like, according to the stop of the supply of the hydrocarbon-based fuel to the reformer.
The shutdown method includes steps A5 to D5, but it is not necessary to actually perform all of steps A5 to D5, and only part of steps A5 to D5 may be performed according to the circumstances.
First, a reforming catalyst layer temperature T is measured. Then, the magnitude relationship between this temperature T and the above-described TrE is checked.
When T<TrE in step A5, the following steps B51 to B54 are performed in order. “T<TrE” is considered to mean that the hydrocarbon-based fuel at the flow rate FkE cannot be reformed.
Step B51
First, step B51 is performed. In other words, the step of increasing the temperature of the reforming catalyst layer is performed.
For example, the temperature of the reforming catalyst layer is increased using an appropriate heat source, such as a heater or a burner annexed to the reformer.
Step B52
Then, step B52 is performed. In other words, the step of measuring a reforming catalyst layer temperature T and comparing the values of this T and TrE is performed.
Step B53
When T<TrE in step B52, the method returns to step B51. In other words, while T<TrE, steps B51 to B53 are repeatedly performed. During this time, the temperature of the reforming catalyst layer increases.
In performing steps B52 and B53, the temperature increase in step B51 may be stopped once, but while steps B52 and B53 are performed, step B51 may be continued.
Step B54
When T≧TrE in step B52, the step of adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer (represented as Fk) from Fk0 to FkE and going to step D5 is performed. “T≧TrE” is considered to mean that the hydrocarbon-based fuel at a flow rate that is equal to or less than FkE can be reformed.
At this time, in a case where the reforming method should be changed is before and after the start of the shutdown method, the fuel flow rate is adjusted from Fk0 to FkE, and the reforming method is changed. By this method, it is possible to prevent the oxidative degradation of the anode with the reformed gas, while reliably reforming the hydrocarbon-based fuel.
When T≧TrE in step A5, the flow rate of the hydrocarbon-based fuel supplied to the reformer is adjusted from Fk0 to FkE, and the method goes to step D5.
At this time, in a case where the reforming type should be changed before and after the start of the shutdown method, the fuel flow rate is adjusted from Fk0 to FkE, and the reforming type is changed. By this method, it is possible to prevent the oxidative degradation of the anode with the reformed gas, while reliably reforming the hydrocarbon-based fuel.
In step D5, the method waits for the anode temperature to fall below the oxidative degradation temperature. During this time, the flow rate of the hydrocarbon-based fuel is maintained at FkE, and the flow rates of fluids supplied to the indirect internal reforming SOFC, such as the flow rate of water (including steam) for steam reforming or autothermal reforming and the flow rate of air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, the cathode air flow rate, the flow rates of the fuel and air supplied to the burner, and the flow rates of fluids, such as water and air, supplied to the heat exchanger; and the input and output of electricity to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, the water and liquid fuel evaporators, the cell stack, the fluid supply piping, and the like, and electrical input taken out from the thermoelectric conversion module and the like, are maintained in the operation conditions in the reforming-stoppable state, determined beforehand. In other words, the operation conditions of the indirect internal reforming SOFC are maintained in the operation conditions of the indirect internal reforming SOFC in the reforming-stoppable state determined beforehand. The anode temperature decreases with time, and therefore, eventually, the anode temperature falls below the oxidative degradation temperature. The anode temperature may be appropriately monitored (continuously measured) using a temperature sensor, such as a thermocouple.
The monitoring of the anode temperature is preferably started immediately after the shutdown method is started. If the temperature monitoring has been performed since before the start of the shutdown method, then the temperature monitoring may be continued as it has been, also when the shutdown method is performed.
When the anode temperature falls below the oxidative degradation temperature, the supply of the hydrocarbon-based fuel to the reformer can be stopped to complete the shutdown method.
A case where T measured in step A5 is equal to or more than TrE, that is, the case of T≧TrE, will be described using
In
The monitoring of the reforming catalyst layer temperature and the monitoring of the anode temperature have been continuously performed since before the point of time of the start of the shutdown method (the same applies to the subsequent cases).
As shown in
In this case, T≧TrE (
Then, the method goes to step D5, and waits until the anode temperature falls below the oxidative degradation temperature.
When the anode temperature becomes less than the oxidative degradation temperature, the supply of the hydrocarbon-based fuel to the reformer can be stopped to complete the shutdown method.
By operating in this manner, it is possible to supply the reformed gas at the requisite minimum flow rate or more to the anode, while reliably performing reforming.
A case where T<TrE in step A5, that is, a case where step B5 is performed, will be described using
After the start of the shutdown method, step A5 is immediately performed, and the measurement of the reforming catalyst layer temperature T, and the comparison of this T with TrE are performed. T<TrE (
In this case, the temperature of the reforming catalyst layer is increased by an appropriate heat source, such as a burner and a heater annexed to the reformer, until T≧TrE is satisfied, so that the hydrocarbon-based fuel at the flow rate FkE can be reformed, as shown in
When T≧TrE is satisfied in step B52, the flow rate Fk of the hydrocarbon-based fuel supplied to the reformer is adjusted from Fk0 to FkE (step B54). When the reforming method should be changed before and after the start of the shutdown method, the fuel flow rate is adjusted from Fk0 to FkE, and the reforming method is changed. Then, the method goes to step D5 (step B54).
Step D5 and the subsequent steps are similar to those of case 5-1.
In this embodiment, control is simple, and a program when control by the program is performed is also simple.
“The hydrocarbon-based fuel at a certain flow rate can be reformed (or is capable of being reformed) in the reforming catalyst layer” refers to that when the hydrocarbon-based fuel at this flow rate is supplied to the reforming catalyst layer, the composition of the gas discharged from the reforming catalyst layer becomes a composition suitable to be supplied to the anode of the SOFC.
For example, “can be reformed in the reforming catalyst layer” may be that the supplied hydrocarbon-based fuel can be decomposed to a C1 compound(s) (a compound(s) having a carbon number of 1). In other words, “can be reformed in the reforming catalyst layer” means a case where reforming can proceed in the reforming catalyst layer until a composition is obtained in which a C2+ component(s) (a component(s) having a carbon number of 2 or more) in the gas at the outlet of the reforming catalyst layer has a concentration, which does not cause the problems of anode degradation and flow blockage due to carbon deposition, or less. The concentration of the C2+ component(s) in this case is preferably 50 ppb or less as a mass fraction in the reformed gas. And in this case, it is enough that the gas at the outlet of the reforming catalyst layer is reducing gas. Methane is permitted to be contained in the gas at the outlet of the reforming catalyst layer. In the reforming of the hydrocarbon-based fuel, usually, methane remains in the equilibrium theory. Even if carbon is contained in the gas at the outlet of the reforming catalyst layer in the form of methane, CO, or CO2, carbon deposition can be prevented by adding steam as required. When methane is used as the hydrocarbon-based fuel, it is enough that reforming proceeds so that the gas at the outlet of the reforming catalyst layer becomes reducing.
With respect to the reducing property of the gas at the outlet of the reforming catalyst layer, it is enough that the property is to the extent that if this gas is supplied to the anode, the oxidative degradation of the anode is suppressed. In order to do this, for example, the partial pressures of oxidizing O2, H2O, CO2, and the like contained in the gas at the outlet of the reforming catalyst layer may be lower than their equilibrium partial pressures of oxidation reactions of the anode electrode. For example, when the anode electrode material is Ni, and the anode temperature is 800° C., the partial pressure of O2 contained in the gas at the outlet of the reforming catalyst layer may be less than 1.2×10−14 atm (1.2×10−9 Pa), the partial pressure ratio of H2O to H2 may be less than 1.7×102, and the partial pressure ratio of CO2 to CO may be less than 1.8×102.
A position for the measurement of the reforming catalyst layer temperature will be described in detail below. This measurement position may be used when TrE is found beforehand, and when the temperature of the reforming catalyst layer is measured in steps A5 and B5.
<Case where there is One Temperature Measurement Point>
Temperature Measurement Position
When there is a single temperature measurement point in the reforming catalyst layer, it is preferred to use preferably a position where the temperature becomes relatively low in the reforming catalyst layer, more preferably a position where the temperature becomes the lowest in the reforming catalyst layer, as the position for the measurement of temperature, in terms of safe side control. When the reaction heat in the reforming catalyst layer is endothermic, the vicinity of the center of the catalyst layer may be selected as the temperature measurement position. When the reaction heat in the reforming catalyst layer is exothermic, and the temperatures of the end positions are lower than that of the center portion due to heat release, an end of the catalyst layer may be selected as the temperature measurement position. A location where the temperature becomes low may be found by preliminary experiment or simulation.
<Case where there are Plurality of Temperature Measurement Points>
The point for the measurement of temperature need not be one. Two or more temperature measurement points are preferred in terms of more accurate control. For example, it is possible to measure the inlet temperature and outlet temperature of the reforming catalyst layer and use their average temperature as the above-described reforming catalyst layer temperature T.
Alternatively, for example, it is possible to consider regions Zi obtained by dividing the reforming catalyst layer into N (N is an integer of 2 or more, and i is an integer of 1 or more and N or less), find the temperature Ti of each divided region Zi, and find TrE(j) (={TrE1, TrE2, . . . , TrEN}) for each divided region beforehand. In this case, when any of Ti becomes equal to or less than TrEi, the flow rate of the hydrocarbon-based fuel may be set to FkE.
When N divided regions Zi are considered, temperatures of all divided regions may be set as the temperature condition, or temperature(s) of one or some (not all) regions among the N divided regions may be set as the temperature condition. The catalyst layer regions for the temperature condition may be appropriately changed according to the feed rate of the hydrocarbon-based fuel.
As the temperature of the divided region Zi, actually measured temperature may be used as it is, but a calculated value, such as the average value of the inlet temperature and outlet temperature of the divided region, may be appropriately used as a representative value.
Also, it is not necessary to measure temperatures for all divided regions Zi. Also, the number of divisions of the catalyst layer, N, and the number of temperature measurement point(s) may be independently set.
It is also possible to measure temperature(s) of one or some (not all) of the N divided regions and find temperature(s) of the remaining divided region(s) by appropriate interpolation from the measured temperature(s).
For example, as a temperature of a divided region where no temperature sensor is installed, a temperature of a divided region nearest to this divided region may be used. When there are two nearest divided regions, a temperature of either of the two divided regions may be used, or the average value of temperatures of the two divided regions may be used.
It is also possible to measure temperatures at a plurality of points in the reforming catalyst layer (at different positions along the gas flow direction), independently of the divided regions, and find a temperature of each divided region from the measured temperatures at the plurality of points. For example, it is possible to measure temperatures of the inlet and outlet of the reforming catalyst layer (a temperature of any position in the middle portion may be further measured), interpolate the temperature of the reforming catalyst layer from these measured temperatures by an approximation method, such as a least squares method, and find temperatures of the divided regions from the interpolation curve.
In order to find temperatures of all divided regions, temperatures of the following positions may be measured.
The inlet and outlet of each divided region.
The interior (one point or a plurality of points) of each divided region (inner side of the inlet and the outlet).
The inlet, outlet, and interior (one point or a plurality of points for one divided region) of each divided region.
In order to find a temperature of one or some (not all) of the divided regions, temperatures of the following positions may be measured.
The inlet and outlet of one or some (not all) of the divided regions.
The interior (one point or a plurality of points) of one or some (not all) of the divided regions (inner side of the inlet and the outlet).
The inlet, outlet, and interior (one point or a plurality of points for one divided region) of one or some (not all) of the divided regions.
When the flow rate Fk of the hydrocarbon-based fuel is set to FkE, the flow rates of fluids supplied to the indirect internal reforming SOFC, such as the flow rate of water (including steam) for steam reforming or autothermal reforming and the flow rate of air for autothermal reforming or partial oxidation reforming, which are supplied to the reformer, the cathode air flow rate, the flow rates of the fuel and air supplied to the burner, and the flow rates of fluids, such as water and air, supplied to the heat exchanger; and the input and output of electricity to and from the indirect internal reforming SOFC, such as electrical heater output for heating the reformer, the water and liquid fuel evaporators, the cell stack, the fluid supply piping, and the like, and electrical input taken out from the thermoelectric conversion module and the like, are accordingly set, as required, to the operation conditions in the reforming-stoppable state determined beforehand. In other words, when the flow rate Fk of the hydrocarbon-based fuel is set to FkE, the operation conditions of the indirect internal reforming SOFC may be set to the operation conditions in the reforming-stoppable state determined beforehand.
When a steam reforming reaction is performed, that is, steam reforming or autothermal reforming is performed, steam is supplied to the reforming catalyst layer. When a partial oxidation reforming reaction is performed, that is, partial oxidation reforming or autothermal reforming is performed, an oxygen-containing gas is supplied to the reforming catalyst layer. As the oxygen-containing gas, a gas containing oxygen may be appropriately used, but in terms of the ease of availability, air is preferred.
The present invention is particularly effective when the hydrocarbon-based fuel has a carbon number of 2 or more, because in the case of such a fuel, particularly, reliable reforming is required.
In order to perform the method of the present invention, appropriate instrumentation and controlling equipment, including a computing means, such as a computer, may be used.
It is possible to use a hydrocarbon-based fuel appropriately selected from compounds of which molecules contain carbon and hydrogen (may also contain other elements, such as oxygen) or mixtures thereof that are known as raw materials of reformed gas in the field of SOFCs. It is possible to use compounds of which molecules contain carbon and hydrogen, such as hydrocarbons and alcohols. For example, hydrocarbon fuels, such as methane, ethane, propane, butane, natural gas, LPG (liquefied petroleum gas), city gas, gasoline, naphtha, kerosene and gas oil, alcohols, such as methanol and ethanol, ethers, such as dimethylether, and the like may be used.
Particularly, kerosene and LPG are preferred because they are readily available. In addition, they can be stored in a stand-alone manner, and therefore, they are useful in areas where the city gas pipeline is not built. Further, an SOFC power generating apparatus using kerosene or LPG is useful as an emergency power supply. Particularly, kerosene is preferred because it is easy to handle.
The reformer produces a reformed gas containing hydrogen from a hydrocarbon-based fuel.
In the reformer, any of steam reforming, partial oxidation reforming and autothermal reforming in which a steam reforming reaction is accompanied by a partial oxidation reaction may be performed.
In the reformer, a steam reforming catalyst having steam reforming activity, a partial oxidation reforming catalyst having partial oxidation reforming activity, or an autothermal reforming catalyst having both partial oxidation reforming activity and steam reforming activity may be appropriately used.
With respect to the structure of the reformer, a structure known as that of a reformer may be appropriately used. For example, the structure of the reformer may be a structure having a region for housing a reforming catalyst in a vessel which can be closed to the atmosphere, and having an introduction port for fluids required for reforming and a discharge port for a reformed gas.
The material of the reformer may be appropriately selected for use from materials known as those of reformers, considering resistance in the environment used.
The shape of the reformer may be an appropriate shape, such as a rectangular parallelepiped shape or a circular tube shape.
A hydrocarbon-based fuel (vaporized beforehand as required) and steam, and further an oxygen-containing gas, such as air, as required, may be supplied to the reformer (the reforming catalyst layer), each independently, or appropriately mixed beforehand. The reformed gas is supplied to the anode of the SOFC.
The reformed gas obtained from the reformer is supplied to the anode of the SOFC. On the other hand, an oxygen-containing gas, such as air, is supplied to the cathode of the SOFC. During electric power generation, the SOFC generates heat with electric power generation, and the heat is transferred from the SOFC to the reformer by radiation heat transfer and the like. In this manner, the exhaust heat of the SOFC is used to heat the reformer. Gas interfacing or the like is appropriately performed using piping and the like.
As the SOFC, a known SOFC may be appropriately selected for use. In the SOFC, generally, an oxygen-ion conductive ceramic or a proton-ion conductive ceramic is used as the electrolyte.
The SOFC may be a single cell, but practically, a stack in which a plurality of single cells are arrayed (the stack is sometimes referred to as a bundle in the case of a tubular type, and the stack in this specification includes a bundle) is preferably used. In this case, one stack or a plurality of stacks may be used.
The shape of the SOFC is also not limited to a cubic stack, and an appropriate shape may be used.
The oxidative degradation of the anode may occur, for example, at about 400° C.
The enclosure (module container) may be any appropriate container capable of housing the SOFC, the reformer, and the combustion region. An appropriate material having resistance to the environment used, for example, stainless steel, may be used as the material of the container. A connection port is appropriately provided for the container for gas interfacing or the like.
The module container is preferably hermetic in order to prevent communication between the interior of the module container and the surroundings (atmosphere).
The combustion region is a region where an anode off-gas discharged from the anode of the SOFC can be combusted. For example, the anode outlet is opened in the enclosure, and a space near the anode outlet may be the combustion region. This combustion may be performed using, for example, a cathode off-gas, as an oxygen-containing gas. In order to do this, a cathode outlet may be opened in the enclosure.
In order to combust a combustion fuel or the anode off-gas, an ignition means, such as an igniter, may be appropriately used.
A known catalyst may be used for each of the steam reforming catalyst, the partial oxidation reforming catalyst and the autothermal reforming catalyst used in the reformer. Examples of the steam reforming catalyst include ruthenium-based and nickel-based catalysts. Examples of the partial oxidation reforming catalyst include a platinum-based catalyst. Examples of the autothermal reforming catalyst include a rhodium-based catalyst. When steam reforming is performed, an autothermal reforming catalyst having steam reforming function may be used.
A temperature at which the partial oxidation reforming reaction can proceed is, for example, 200° C. or more. A temperature at which the steam reforming reaction or the autothermal reforming reaction can proceed is, for example, 400° C. or more.
The conditions during shutdown operation of the reformer for each of steam reforming, autothermal reforming, and partial oxidation reforming will be described below.
In steam reforming, steam is added to a reforming raw material, such as kerosene. The reaction temperature of the steam reforming may be in the range of, for example, 400° C. to 1000° C., preferably 500° C. to 850° C., and further preferably 550° C. to 800° C. An amount of the steam introduced into the reaction system is defined as a ratio of the number of moles of water molecules to the number of moles of carbon atoms contained in the hydrocarbon-based fuel (steam/carbon ratio). This value is preferably 1 to 10, more preferably 1.5 to 7, and further preferably 2 to 5. When the hydrocarbon-based fuel is liquid, a space velocity (LHSV) can be represented as A/B, wherein a flow velocity of the hydrocarbon-based fuel in a liquid state is represented as A (L/h), and a volume of the catalyst layer is represented as B (L). This value is set in the range of preferably 0.05 to 20 h−1, more preferably 0.1 to 10 h−1, and further preferably 0.2 to 5 h−1.
In autothermal reforming, in addition to the steam, an oxygen-containing gas is added to the reforming raw material. The oxygen-containing gas may be pure oxygen, but in terms of the ease of availability, air is preferred. It is possible to perform equilibrium calculation, and add the oxygen-containing gas so that an overall reaction heat is exothermic. With respect to the amount of the oxygen-containing gas added, a ratio of the number of moles of oxygen molecules to the number of moles of carbon atoms contained in the hydrocarbon-based fuel (oxygen/carbon ratio) is preferably 0.005 to 1, more preferably 0.01 to 0.75, and further preferably 0.02 to 0.6. A reaction temperature of the autothermal reforming reaction is set in the range of, for example, 400° C. to 1000° C., preferably 450° C. to 850° C., and further preferably 500° C. to 800° C. When the hydrocarbon-based fuel is liquid, the space velocity (LHSV) is selected from the range of preferably 0.05 to 20 h−1, more preferably 0.1 to 10 h−1, and further preferably 0.2 to 5 h−1. With respect to an amount of the steam introduced into the reaction system, the steam/carbon ratio is preferably 1 to 10, more preferably 1.5 to 7, and further preferably 2 to 5.
In partial oxidation reforming, an oxygen-containing gas is added to the reforming raw material. The oxygen-containing gas may be pure oxygen, but in terms of the ease of availability, air is preferred. An amount of the oxygen-containing gas added is appropriately determined in terms of heat loss and the like to ensure a temperature at which the reaction proceeds. With respect to this amount, the ratio of the number of moles of oxygen molecules to the number of moles of carbon atoms contained in the hydrocarbon-based fuel (oxygen/carbon ratio) is preferably 0.1 to 3 and more preferably 0.2 to 0.7. A reaction temperature of the partial oxidation reaction may be set in the range of, for example, 450° C. to 1000° C., preferably 500° C. to 850° C., and further preferably 550° C. to 800° C. When the hydrocarbon-based fuel is liquid, the space velocity (LHSV) is selected from the range of preferably 0.1 to 30 h−1. Steam can be introduced into the reaction system to suppress the generation of soot, and with respect to an amount of the steam, the steam/carbon ratio is preferably 0.1 to 5, more preferably 0.1 to 3, and further preferably 1 to 2.
Known components of an indirect internal reforming SOFC may be appropriately provided as required. Specific examples of the known components include a vaporizer for vaporizing a liquid; a pressure increasing means for pressurizing various fluids, such as a pump, a compressor, and a blower; a flow rate controlling means or a flow path blocking/switching means for controlling the flow rate of a fluid, or blocking/switching the flow of a fluid, such as a valve; a heat exchanger for performing heat exchange and heat recovery; a condenser for condensing a gas; a heating/warming means for externally heating various devices with steam or the like; a storage means of a hydrocarbon-based fuel (reforming raw material) or a combustion fuel; an air or electrical system for instrumentation; a signal system for control; a control apparatus; and an electrical system for output and powering; a desulfurizer for reducing a sulfur concentration in a fuel; and the like.
The present invention can be applied to an indirect internal reforming SOFC used for, for example, a stationary or mobile power generating apparatus and a cogeneration system.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-093781 | Apr 2009 | JP | national |
| 2009-136290 | Jun 2009 | JP | national |
| 2009-138191 | Jun 2009 | JP | national |
| 2009-140144 | Jun 2009 | JP | national |
| 2009-143402 | Jun 2009 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2010/056360 | 4/8/2010 | WO | 00 | 10/3/2011 |
