Patent Application
20190393525
The field of invention relates to a method and a system for producing hydrogen and electricity from a reformed process gas feed using a solid oxide fuel cell unit.
A combined hydrogen and electricity supply system using a solid oxide fuel cell unit allows simultaneous production of electrical power, hydrogen and heat. Such a system is also referred to as polygeneration or combined hydrogen, heat and power system, often abbreviated as CH2P system. Such a CH2P system is particularly interesting if the production of hydrogen, heat and electrical power can be modulated, i.e. the ratio between electrical power and hydrogen can be adjusted according to specific needs, for instance of a H2 fuelling station.
Document WO2005/041325A2 discloses a CH2P-system that utilizes a fuel cell for producing hydrogen, electrical power, or a combination of both hydrogen and electrical power. In a first mode, the fuel cell performs an electrochemical reaction by reacting a hydrogen-containing fuel with oxygen to produce electricity, water and heat. In a second mode, the fuel cell utilizes heat released by the electrochemical reaction of the fuel cell to reform a carbonaceous fuel to produce a hydrogen rich gas. In a third mode, both hydrogen and electricity are co-produced by the fuel cell. The CH2P-system can control the amount of hydrogen and/or electrical power produced and can switch between modes by varying an external electrical load and/or by acting on fuel feed flow through a mass flow regulator. Document WO2005/041325A2 discloses a SOFC system with internal reforming only, which is made possible by the presence of Nickel (Ni) in a state of the art SOFC. One disadvantage of the CH2P-system disclosed is that the modulation between hydrogen and electrical power production is strongly limited. Furthermore, the option of operating the SOFC stack close to short-circuit is not viable and will lead to strong degradation of the SOFC electrodes.
Document U.S. Pat. No. 8,071,241B2 discloses a CH2P-system comprising a SOFC system coupled with a fuel processing and a H2 separation unit to produce hydrogen and electrical power. This CH2P-system does not allow modulation between hydrogen and electrical power.
The objective of the present invention is thus to improve the modulation between hydrogen and electrical power production in a combined hydrogen and electricity supply system. A further objective of the present invention is to expand the use of a combined hydrogen and electricity supply system.
The above-identified objectives are solved by a method comprising the features of claim 1 and more particular by a method comprising the features of claims 2 to 13. The above-identified objectives are further solved by a combined hydrogen and electricity supply system comprising the features of claim 14 and more particular by a system comprising the features of claims 15 to 16.
The objective is in particular solved by a method for producing hydrogen and electrical power in a combined hydrogen and electricity supply system, the method comprising the steps of:
The objective is further in particular solved by a combined hydrogen and electricity supply system for producing hydrogen, electrical power and co-production, the system comprising:
The objective is further in particular solved by a combined hydrogen and electricity supply system for producing hydrogen, electrical power and co-production, the system comprising a solid oxide fuel cell stack, a pre-reformer, an electrical load consuming the electrical Power, a hydrogen separation unit, a control unit, a carbonaceous fuel source, and a steam source, the pre-reformer being connected to a stream of the carbonaceous fuel source and a stream of the steam source, wherein said pre-reformer produces a first reformate gas comprising at least hydrogen, carbon monoxide and unconverted carbonaceous fuel, the solid oxide fuel cell stack being coupled to the electrical load, being coupled to the pre-reformer to receive the first reformate gas and the unconverted carbonaceous fuel and being coupled to the hydrogen separation unit, wherein the pre-reformer is located outside of the solid oxide fuel cell stack to perform external reforming, wherein the electrical load is a controllable, variable electrical load, wherein the pre-reformer is thermally coupled to a controllable heating source, and wherein the control unit is adapted to at least control the variable electrical load and the heat provided by the heating source to thereby control the ratio between electrical power and amount of hydrogen being produced.
The present invention provides a hydrogen and electricity co-production system that is efficient, cost-effective and flexible.
The method and the combined hydrogen and electricity supply system according to the invention produces electrical power and hydrogen from a carbonaceous fuel. It includes a fuel processor respectively a pre-reformer that partially converts the carbonaceous fuel into hydrogen and carbon monoxide before feeding the SOFC stack. When referring to a SOFC stack herein, such a SOFC stack may consist of one SOFC stack or of multiple SOFC stacks. The converted carbonaceous fuel is also referred to as reformate or reformate gas. The method and system according to the invention uses an endothermic reaction in the pre-reformer. Therefore the steam and carbonaceous fuel entering the pre-reformer does not comprise added air or oxygen. The preferred fuel processing technique is steam reforming, since, being an endothermic reaction, waste heat can be valorised in the reforming reaction, thereby increasing the efficiency of the whole process. In the method and system according to the invention, part of the reforming of the fuel takes place in the pre-reformer by external reforming, and part of the reforming of the fuel takes place directly in the SOFC stack by internal reforming, taking advantage from the heat produced within the SOFC stack during the electrochemical conversion of reformate gas into electrical power. Thus the electrical power production in the SOFC stack provides heat for the internal steam reforming reaction. The preferred fuel processing technique for the external reforming as well as the internal reforming is steam reforming.
The terms “internal reforming” and “external reforming” in this context have the following meaning: As used herein, the term “internal reforming” refers to fuel reforming occurring within the body of a SOFC cell, a SOFC stack, or otherwise within a fuel cell assembly. External reforming, which is often used in conjunction with a fuel cell, occurs in a separate piece of equipment that is located outside of the SOFC stack. In other words, the body of the external reformer, in the previous paragraph referred to as “pre-reformer”, is not in direct physical contact with the body of a SOFC or SOFC stack. This thermal separation of pre-reformer and SOFC stack allows independent thermal control of the pre-reformer and the SOFC stack, which is essential to allow control of the generation of electrical power and hydrogen in a wide range. The method and system according to the invention therefore allows modulating the production of hydrogen and electrical power in a wide range, i.e. the ratio between purified hydrogen and electrical power can be adjusted according to the needs. Thus, remaining unconverted hydrogen leaves the SOFC stack and can be recovered because the SOFC stack is coupled with a hydrogen separation system. State of the art SOFC systems that modulate between electrical power and hydrogen production only use internal reforming within the SOFC stack. The method and system according to the invention has the advantage that external reforming in a pre-reformer as well as internal reforming in the SOFC stack is used to reform the carbonaceous fuel. One disadvantage of the method disclosed in the state of the art is that the control between electrical power and hydrogen production strongly restrains the range of operating points because of system heat balance requirements. State of the art SOFC stacks contain Ni in the anode side of the SOFC stack, which means the fuel electrode, which is a good catalyst for the steam reforming reaction, also favours methane cracking. Steam should therefore simultaneously be fed with the carbonaceous fuel. Internal reforming, i.e. the conversion of carbonaceous fuel and steam inside the fuel electrode, is thereby used in state of the art SOFCs. This endothermic reaction tends to cool down the SOFC stack if not sufficient heat is locally generated to compensate for the heat demand. It is therefore necessary in state of the art SOFCs that a minimal electrical power to hydrogen ratio has to be fulfilled to provide sufficient local heat balance, e.g. >68% fuel conversion rate in case of internal steam reforming. The method and system according to the invention has therefore the advantage that the fuel conversion in the SOFC can be lower, thereby the ratio between electrical power and hydrogen can be lower. In one exemplary method, most or all of the fuel is processed in the pre-reformer by external reforming, using steam reforming, so that minimal or no unconverted carbonaceous fuel is fed to the SOFC stack, so that there is minimal or no internal reforming in the SOFC stack. This may be achieved, for example, by using electricity to heat the pre-reformer. The method and system according to the invention expands control options and therefore adds one degree of freedom by tuning fuel flow rate, fuel utilization and heat supply to pre-reformer to manage the thermal sustainability of the system in all conditions in the modulation range of hydrogen and electrical power generation.
In an advantageous method, external heat, most preferably electrical heat, for example by using an electrical heating element such as a resistor, is provided to the SOFC stack, so that sufficient heat is locally available in the SOFC stack, even though little or no electrical power is produced in the SOFC stack, thus allowing all electrical heat to be used for internal reforming, which results in a high yield of hydrogen.
The method and system according to the invention proposes an alternative way of performing the heat management for the modulation between hydrogen and electrical power production, in particular by separately control external reforming and internal reforming. One aspect of the invention is to consider the pre-reformer and the SOFC cell as forming together the complete steam reforming unit. The steam reforming in the pre-reformer and the SOFC cell may be controlled independently. The heat required to complete the carbonaceous fuel steam reforming reaction is thereby provided from outside to the pre-reformer, in order to convert part of the carbonaceous fuel to synthesis gas, and is partially provided by the SOFC stack either through internal heat generated by electrical losses or through external heat, preferably through heat produced using external electrical energy, and most preferably heat produced using external excess electrical energy. In the electrical power production mode of the SOFC stack, a lot of heat is generated in the SOFC stack. The heat balance is managed by allowing internal reforming of up to 90% to take place in the SOFC stack. This is achieved by providing little heat to the pre-reformer, or in other words keeping the pre-reformer outlet temperature below 450° C. If required, excess heat can also be removed from the SOFC stack by increasing the air flow to the cathode side of the SOFC stack. On the other hand, in hydrogen production mode, electrical power production remains low, i.e. sufficient to cover the system power requirements, thereby little heat is generated in the SOFC. Therefore, most of the heat required for the carbonaceous fuel steam reforming will be provided to the pre-reformer, which will operate at a higher temperature of up to 700° C. The pre-reformer is preferably operated at variable temperature, and the temperature is controlled such that the prereforming rate of the pre-reformer is fixed by its outlet temperature, preferably a given outlet temperature. In further advantageous embodiments, the prereforming rate may also be controlled by other means, for example by bypassing the pre-reformer with part of the carbonaceous fuel and steam.
In an advantageous embodiment heat required by the pre-reformer is provided by burning off-gas exiting the H2 separation unit, which is composed of the remaining H2, CO and CO2. The caloric value of the off-gas can either be controlled by varying the degree of H2 separation in the H2 separation unit or by feeding additional carbonaceous fuel to the burner as a make-up gas.
One advantage of the method and system according to the invention is that the operation strategy enables to cover the full range of hydrogen and electrical power production, i.e. hydrogen production only to electrical power production only, in the most efficient way.
In a further advantageous embodiment at least one of the steam generator and the pre-reformer may be electrically heated, thus allowing producing hydrogen by dissipating excess electrical power, in particular electrical power from the grid.
During operation of the SOFC cell the electrochemical reaction generates electrical voltage across the electrodes and electrical current flow from the oxidizer electrode to the fuel electrode through an external electrical load. It also produces heat according to electrochemical laws.
When the SOFC stack performs fuel-to-electricity conversion the SOFC operating parameters can be adjusted to achieve high electrical efficiency, e.g. by increasing the fuel utilisation.
The pre-reformer used in the embodiment according to the invention reforms hydrocarbon fuel into hydrogen-rich reformate. Preferably a steam methane reformer is used to produce hydrogen. For steam reforming, hydrogen-rich gas is produced according to the following endothermic reaction:
CH4+H2O<→CO+3H2 ΔH=−206.16 kJ/mol CH4
Consequently, heat needs to be provided to drive the reaction. The heat is provided indirectly by heat transfer, preferably through a heat exchanger. The heat provided indirectly by heat transfer may be provided by the combustion of a fraction of the incoming natural gas feedstock or by burning waste gases, such as purge gas from a hydrogen purification system, or by using electrical power.
The expression “a reformed process gas feed” herein refers to the output of a conversion of a fuel, for example hydrocarbon or alcohol, into another fuel usually with a higher heating value using a reforming reaction, preferentially steam reforming.
Steam reforming is a method for producing hydrogen or other useful products from carbonaceous fuels such as hydrocarbon fuels, for example natural gas. This is achieved in a processing device called a reformer which reacts steam at high temperature with the fuel so that a reformed process gas feed is produced.
The reforming of any hydrocarbon is as follows:
CnH2n+2+nH2O→nCO+(2n+1)H2
Such a steam reforming can be performed for a wide range of fuels, but the process itself is similar in all cases.
The present invention provides for a hydrogen and electricity co-production system for producing hydrogen, electricity, or a combination of both hydrogen and electricity. Specifically, the invention provides for using a SOFC system comprising a SOFC stack, to perform multiple functions, such as reforming fuel to produce hydrogen, consuming reactants to produce electricity, and performing a combination of both, depending upon the condition of an electrical load, such as a variable electrical load, that is attached to the SOFC stack.
In a typical electricity-generating mode, the SOCF stack performs an electrochemical reaction by reacting a hydrogen-containing fuel with oxygen to produce electricity, water and heat. In an alternative or reformer mode, the SOFC stack can be adapted to utilize heat, preferably released by an electrochemical reaction of the SOFC stack to reform a hydrocarbon fuel to produce hydrogen. Furthermore, in a co-production mode, both hydrogen and electricity are co-produced by the fuel cell. The system according to the invention can control an amount of hydrogen and/or electricity produced and can switch between modes by in particular varying, adjusting or controlling an electric load on the system.
According to the teachings of the present invention, a co-production energy supply system capable of producing hydrogen and electricity is contemplated. The system includes a heat controlled pre-reformer and a variable electric load for varying the amount of impedance on the system, and an SOFC stack coupled to the variable electric load. During use, the pre-reformer produces hydrogen and the SOFC stack produces hydrogen, electricity or both responsive to the amount of heat provided to the pre-reformer and the amount of impedance introduced to the system by the variable load.
According to still another aspect, the impedance of the variable electric load can be varied to vary the relative amount of or the ratio of electricity and hydrogen generated by the SOFC stack.
According to another aspect, the system can include structure or means for varying the impedance of the variable load so as to control the relative amount of hydrogen and electricity produced by the SOFC stack. The means for varying can include a controller coupled to the variable load. The controller varies the amount of impedance of the variable load to control the relative amount of hydrogen and electricity produced by the SOCF stack. In addition the controller controls the pre-reforming rate of the pre-reformer by controlling heat provided to the pre-reformer or by controlling the exit temperature of the pre-reformer. Optionally, the controller can operate one or more fluid regulating devices for regulating the flow of one or more input reactants to the pre-reformer and the SOFC stack to control the overall amount of hydrogen and/or electricity produced thereby.
The present invention also contemplates a method of co-producing hydrogen and electricity, that comprises the steps of providing a variable load for varying the amount of impedance on a system, providing a pre-reformer capable of producing a reformate gas, providing an SOFC stack capable of producing both hydrogen and electricity, and varying the impedance of the variable load to vary the relative amount of hydrogen and electricity generated by the SOFC stack.
According to one aspect, the method can include the additional step of configuring the variable load to be able to introduce, in a reformer operational mode, at least a minimum impedance amount, where the SOFC stack is adapted to reform any unspent input fuel reactant into hydrogen when the variable load is set to the minimum impedance amount. The minimum impedance amount can be about zero, and which corresponds to a short circuit electrical arrangement.
According to another aspect, the variable load can be configured to be set to a maximum impedance amount which corresponds to an open circuit electrical arrangement across the SOFC stack.
According to still another aspect, the method includes the step of configuring the variable load to be able to introduce, in a co-production operational mode, an impedance amount that is between the maximum impedance amount and the minimum impedance amount so that the pre-reformer produces a reformate gas, and the SOFC stack produces both hydrogen and electricity, where the amounts of the hydrogen and electricity produced by the SOFC stack correspond to the amount of impedance of the variable load.
In an advantageous embodiment electricity is used to heat at least some parts and preferably all parts of the system according to the invention that need heat for the reaction to provide hydrogen and electricity. It is advantageous to electrically heating at least one of the pre-reformer, the steam generator, the SOFC stack, and fluid flowing such as carbonaceous feed or oxidant flow. Most advantageously, such a system and method allows producing hydrogen out of electricity. Most advantageously CO2-free electricity such as solar energy or wind energy is used to feed the system according to the invention with electrical power. This allows producing hydrogen with a low carbon footprint. Most advantageously surplus CO2-free electricity is used in the system, when there is an excess of electrical power in the grid. Such surplus of electrical power may arise on a very sunny day during summer, or on a very windy day when electricity is produced by wind energy. The system according to the invention allows using such surplus energy to produce hydrogen. Hydrogen can therefore be produced very cheap, in fact, it is even possible to offering the service for using excess electrical energy from the grid. In addition, hydrogen can be stored, for a short period of time as well as for a long period of time. The system according to the invention allows producing hydrogen, electricity and co-production. Therefore the hydrogen produced and thereafter stored may later on be used to produce electricity, and most preferably to feed electrical power back into the electric grid. In a preferred embodiment, the system according to the invention may be used to stabilize the electric grid, in that electricity from the grid is used during surplus of electricity, and in the electricity is fed to the grid during lack of electrical power.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawings in which like numerals represent like components.
The present invention provides hydrogen, electricity and co-production system. The invention will be described below relative to illustrative embodiments. Those skilled in the art will appreciate that the present invention may be implemented in a number of different applications and embodiments and is not specifically limited in its application to the particular embodiment depicted herein.
An oxygen depleted air stream 105 is fed from the cathode side 21 to the burner 9. An anode off gas 208 is fed from the anode side 23 to the hydrogen separation unit 8. The hydrogen separation unit 8 is adapted to separate at least a portion of hydrogen from the anode off gas 208 and is adapted to generate purified hydrogen 80 and an off gas 215 that is fed to the burner 9 where it is burnt. In an advantageous embodiment, disclosed in
Suitable techniques for the hydrogen separation unit 8 may be adsorbtion based, for example pressure-swing adsorption, or membrane base, for example palladium-based, or protonic, or electrochemical base, for example electrochemical pumps based on protonic conductors.
The input fuel reactant 20 can be any suitable hydrocarbon fuel known to those of ordinary skill in the art. The input oxidant reactant 100 can comprise any suitable oxygen-containing fluid.
The system 1 can be operated in a number of select modes to produce and modulate electrical power P, purified hydrogen 80 and a combination thereof. The main purpose of the method and system according to the invention is the production of electrical power P, hydrogen in the form of purified hydrogen 80 and a combination thereof from a carbonaceous fuel feed. The present description distinguishes between hydrogen produced within the system, and the output of the system, which are electrical power P and/or hydrogen in the form of purified hydrogen 80.
The illustrated system 1 is a multi-function system. In addition to the traditional function of electrical generation, the SOFC stack 2 can be utilized to perform reforming for hydrogen production or the co-production of hydrogen and electricity. In a traditional mode of operation, the SOFC stack 2 generates electricity by electrochemically reacting the input fuel reactant with the input oxidant reactant to generate power, waste heat and exhaust, which includes carbon dioxide and water. In an alternative mode of reformer operation, the SOFC stack 2 reforms the input fuel reactant to generate a hydrogen exhaust without simultaneously generating electricity. Additionally reactant by-products that can be included in the exhaust include carbon monoxide, carbon dioxide, and water. In a combined or co-production mode of operation, the SOFC stack 2 simultaneously generates both a hydrogen exhaust and electricity. The exhaust can include additional reaction species such as carbon monoxide, carbon dioxide and water respectively steam.
As used herein, the term “reforming” and the like refers to a chemical process performed by the pre-reformer 3 or the SOFC stack 2 that reacts hydrocarbon fuels, at an elevated temperature, such as above 250° C., and preferably between about 400° C. and about 1000° C., in the presence of steam and without oxygen to generate a reformate. In the present invention, the pre-reformer 3 and the SOFC stack 2 reforms hydrocarbon fuels to produce hydrogen by reacting the hydrocarbon fuel with water.
The combined hydrogen and electricity supply system 1 disclosed in
The combined hydrogen and electricity supply system 1 disclosed in
The modulation between electrical power P and output hydrogen production is managed by a control unit 17 as shown in
The fuel utilisation (FU) relates to the total electrical current (I) and the fuel flow per cell (f) through the relation:
FU=I/(nF*f)
Where n is the number of electrons involved in the oxidation of 1 molecule of fuel (e.g. 8 for CH4), F is the Faraday constant (=96485 C/mol). It can therefore be varied either by changing the total current in the SOFC by varying the external electrical load 6, or by changing the fuel flow.
A hydrocarbon fuel 20, typically biogas or natural gas, is fed to a controllable valve 14 and optionally to a fuel pretreatment unit 13, typically a desulphurization unit, to get a poison-free carbonaceous fuel. The carbonaceous fuel feed 200 is mixed with steam 40a, preheated in a heat exchanger 203, and fed through conduit 204 into a pre-reformer 3 to produce the reformed process gas feed 205. Steam 40a is generated in a steam generator 11. The pre-reformer 3 is provided with heat by heat exchanger 3a. The reaction in the pre-reformer 3 preferably takes place in the presence of a reforming catalyst in a temperature range of 500 to 800° C. The reformed process gas 205 is heated in heat exchanger 206 and is fed to the anode side 23 of the SOFC stack 2. The anode off-gas 208 leaving the SOFC stack 2 is cooled in heat exchanger 209 to for example about 300° C., and is first fed into a water gas shift reactor 4, is then cooled in a heat exchanger 212, and is fed to a water separator 15, a condenser, which at least separates water 40 from the gas stream 213, so that a water depleted stream 214 results. Water 40 is stored in a water tank 402, and may then be fed through conduits 403 and 405 and a water pump 404 to the steam generator 11. The water depleted stream 214 is fed to the hydrogen separation unit 8 to generate purified hydrogen 80 and a carbon dioxide rich gas stream 215 comprising also unseparated H2 and some CO, which is fed to the burner 9.
The SOFC stack 2 also comprises a cathode side 21 as well as an electrolyte 22. The SOFC stack 2 keeps the oxidant stream respectively air stream 100 and the reformed process gas 205 separated, so that they do not mix. No further details of the solid oxide fuel stack 2 are shown. Air 100 is slightly compressed in blower 101 to compressed cold air 102, is heated in heat exchanger 103 to pre-heated air 104 and is then fed to the cathode side 21 of the solid oxide fuel stack 2. The small overpressure of the compressed cold air 102, for example about 50 mbar, is required to overcome the pressure drops in the heat exchangers and the SOFC stack 2. A hot depleted air stream 105 leaving the cathode side 21 of the SOFC stack 2 is fed to the burner 9. Electricity produced by the SOFC stack 2 is converted from DC to AC and is forwarded to a variable electrical load 6 not shown in detail. The electrical load 6 consumes the electrical power P.
Control unit 17 preferably controls through control lines 17a to 17g at least one of the blower 101, the H2 separation unit 8, the SOFC stack 2, the valve 14, the valve 18, the water pump 404 and the electrical load 6. In addition, in a preferred embodiment, the exit temperature T1 of the depleted air 105 leaving the SOFC stack 2, the exit temperature T2 of the reformate 205 leaving the pre-reformer 3 and the exit temperature T3 of the heat leaving the heat exchanger 9a is measured by control unit 17.
The control of the exit temperature T1 has the advantage that at any operating conditions defined by the pre-reforming rate and the FU, the temperature of the SOFC stack 2 may be monitored using sensor T1, which measures the temperature of the hot depleted air stream 105 exiting the SOFC. The temperature T1 can be adjusted or can be fine-tuned by varying the air flow via the blower 101. Increasing the air feed flow will increase the SOFC stack cooling and thereby lower the SOFC stack temperature.
Depending on the required amount of heat 9b, make-up gas 19 may be fed to the burner 9 to provide heat 9b to heat exchanger 9a, which provides heat to heat exchanger 3a.
In addition to the pre-reforming rate and the fuel utilisation FU, also the fuel feed flow may be controlled. The different lines LD1, LD2, LD3, LD4, MD1, MD2, MD3, MD4 in
In the ratio H2/(H2+P) H2 refers to the H2 produced by the system 1 by unit time, which means the purified hydrogen 80 by unit time. P refers to the produced electrical Power, which means electrical energy per unit time. The figures show H2 and P in arbitrary units. The unit of electrical power is Watt. For H2 the equivalent power based on the LHV (low heating value) may be used, which means H2=molar flow (mol/s)*LHV(J/mol)=Watt.
In the example disclosed in
Any possible specific operating points L1, L2, L3, . . . . L100 in
1) controlling the fuel feed flow with valve 14 and the water flow with water pump 404 concurrently in order to maintain the correct steam-to-carbon ratio in the feed stream 204 and to maintain the correct fuel feed stream;
2) adjusting the pre-reforming rate in the pre-reformer 3 by controlling the heat provided to the pre-reformer 3, and
3) adjusting the fuel utilisation FU in the solid oxide fuel cell 2, as disclosed in
Of particular interest is an operation of system 1 with the ratio H2/(H2+P)=0, which means that no purified H2 is produced, but only electrical Power P is produced. Of particular interest is also an operation of the system 1 with the ratio H2/(H2+P)=1, which means that only purified H2 is produced, but no electrical Power P is produced. As indicated in
The modulation strategy is as follows. The modulation between the production of hydrogen and electrical power is achieved by varying the degree of pre-reforming and adjusting the fuel utilization FU in the SOFC stack 2 according to the relation given in
The pre-reforming rate R is the key control parameter in combination with FU.
In a first example the pre-reforming rate R can be adjusted by changing the pre-reformer outlet temperature T2 to 570° C., controlling the heat provided to the pre-reformer 3 by burning the hydrogen separation unit off-gas and/or additional make-up gas 19. In a second example the pre-reforming rate R can be adjusted by adjusting a by-pass as disclosed in
By way of example, the operating point shall now be changed to 50 kW equivalent H2 keeping the electrical power P at 80 kW, which means H2/(H2+P) becomes 0.385. As disclosed in
In addition any other part of the system 1 that need heat, for example fluids flowing, such as carbonaceous feed, oxidant flow, evaporator 11, steam superheating, make-up-gas may be electrically heated.
In a further advantageous embodiment the hydrogen 80 may be stored in a hydrogen storage container 81. In a further advantageous embodiment the hydrogen 80 stored in the container 81 may be fed to the SOFC stack 2, to produce electrical power. The system according to the invention may therefore be used to withdraw electricity from the electrical grid, and to later on supply the electrical grid with electricity. In an advantageous embodiment the system according to the invention may be used for grid control, to control supply and demand of electrical energy.
The controlling device 17 providing control signals 17a-17g for controlling at least one or more of the fuel 20 being fed to the reformer 3, of the oxidant reactants 100 being introduced to the solid oxide fuel cell 2, of the water 40 respectively steam 40a being introduced to the reformer 3, of a make-up gas 19 introduced to the burner 9, and the electrical load 6 being controlled. In addition means such as heat exchangers 103, 203, 206, 209, 212, 3a, 11a, sensors and connecting lines, not shown in detail, are provided for controlling the heat exchange within the system 1.
The system 1 disclosed in
The control unit 17 can act on the fuel valve 14, air blower 101, water recirculation pump 404, hydrogen separation unit 8, the electrical load 6 and the SOFC cell 2. It monitors the pre-reformer outlet temperature T2, the SOFC outlet temperature T1 at the air side and the burner temperature T3. To change the operating point respectively the H2/power ratio, the H2 separation rate will be changed so as to reach the desired pre-reformer outlet temperature T2. The fuel utilisation (FU) in the SOFC stack 2 will be adjusted accordingly to
A hydrocarbon fuel 20, typically biogas or natural gas, is fed to a controllable valve 14 and to a fuel pretreatment unit 13 to get a poison-free carbonaceous fuel. The carbonaceous fuel feed 200 is mixed with steam 40a and fed through conduit 204 into a pre-reformer 3 to produce the reformed process gas feed 205. Steam 40a is generated in a steam generator 11. The pre-reformer 3 is provided with heat 9a by heat exchanger 3a. The reformed process gas 205 is heated in heat exchanger 206 and is fed to the anode side 23 of the solid oxide fuel cell stack 2. The anode off-gas 208 leaving the solid oxide fuel cell stack 2 is cooled in heat exchanger 206, and is first fed into the steam generator 11 and then into a water gas shift reactor 4, is then cooled in a heat exchanger 212, and is fed to a water separator 15, a condenser, which at least separates water 40 from the gas stream 213, so that a water depleted stream 214 results. Water 40 is stored in a water tank 402, and may then be fed through conduits 403 and 405 and a water pump 404 to the steam generator 11. The water depleted stream 214 is fed to the hydrogen separation unit 8 to generate purified hydrogen 80 and a carbon dioxide rich gas stream 215, which is fed to the burner 9.
The solid oxide fuel cell stack 2 also comprises a cathode side 21 as well as an electrolyte 22. The solid oxide fuel cell stack 2 keeps the air stream 100 and the reformed process gas 205 separated, so that they do not mix. Air 100 is slightly compressed in blower 101 to compressed cold air 102, is heated in heat exchanger 103 to pre-heated air 104 and is then fed to the cathode side 21 of the solid oxide fuel cell 2. A hot depleted air stream 105 leaving the cathode side 21 of the solid oxide fuel cell stack 2 is fed the heat exchanger 103 and then to the burner 9. Electricity produced by the solid oxide fuel cell stack 2 is converted from DC to AC and is forwarded to a variable electrical load 6. The electrical load 6 consumes the electrical power P.
Control unit 17 preferably controls through control lines 17a to 17g at least one of the blower 101, the H2 separation unit 8, the solid oxide fuel cell stack 2, the valve 14, the valve 18 and the water pump 404. In addition, in a preferred embodiment, the exit temperature T1 of the depleted air 105 leaving the solid oxide fuel cell stack 2, the exit temperature T2 of the reformate 205 leaving the pre-reformer 3 and the exit temperature T3 of the heat leaving the heat exchanger 9a is measured by control unit 17.
Depending on the required heat 9b, make-up gas 19 may be fed to the burner 9.
The combined hydrogen and electricity supply system 1 according to the invention has also the advantage that heat may be provided for external use. For example in
| Number | Date | Country | Kind |
|---|---|---|---|
| 17154005.7 | Jan 2017 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/052453 | 1/31/2018 | WO | 00 |
