Patent Application
20200119373
Most of the energy of the world is produced using oil, coal, natural gas or nuclear power. All these production methods have their specific issues as far as, for example, availability and friendliness to environment are concerned. As far as the environment is concerned, especially oil and coal cause pollution when they are combusted. An issue with nuclear power is, at least, storage of used fuel.
Especially because of the environmental problems, new energy sources, more environmentally friendly and, for example, having a better efficiency than the above-mentioned energy sources, have been developed. Fuel cell's, by means of which energy of fuel, for example biogas, is directly converted to electricity via a chemical reaction in an environmentally friendly process and electrolysers, in which electricity is converted to a fuel, are promising future energy conversion devices.
A fuel cell, as presented in
Solid oxide electrolyser cells operate at temperatures which allow high temperature electrolysis reaction to take place, the temperatures being for example between 500-1000° C., but even over 1000° C. temperatures may be useful. These operating temperatures are similar to those conditions of the SOFCs. The net cell reaction produces hydrogen and oxygen gases. The reactions for one mole of water are shown below, with reduction of water occurring at the anode:
H2O+2e−--->2H2+O2− Anode:
O2−--->½O2+2e− Cathode:
H2O--->H2+½O2 Net Reaction:
By arranging anode exhaust gas recirculation at high temperature, it is also possible to omit at least one heat exchanger. While the primary purpose of anode gas recirculation in power generating mode is to ensure favorable gas composition at reformer inlet for facilitating desired reformation reactions, it also has a benefit of increasing overall fuel utilization within the bounds of desired level of single pass reactant utilization compared to a single pass operation alone. In an electrolysis operating mode, exhaust gas recirculation may also be used for conditioning cell inlet composition for optimal electrolysis performance at desired reactant utilization rate. In both operating modes, anode gas recirculation increases overall fluid flow through stacks, improving gas flow distribution and consequently temperature and composition distribution.
During start-up of a SOFC or SOEC system, recirculation of anode gas can serve a purpose of improving distribution of heat from heat sources throughout the system or, during shut-down, means for gas circulation may assist in flush dilution of the system. In case of partial oxidation (POX) of fuel for maintaining desired, or required, oxygen-to-carbon ratio, anode gas recirculation may serve a purpose of restraining otherwise high temperature increase in POX reactor.
Accomplishing anode gas recirculation involves recovering a fraction of anode exhaust gas flow and re-pressurising it in essence to boost its pressure enough to overcome pressure losses in the recirculation loop at a given flow. In one known embodiment of an arrangement, a high pressure fuel feed is used as a motive stream in a jet-ejector to entrain anode exhaust gas and to increase pressure of the entrained gas to the level of the fuel feed-in. Due to fixed geometry of the jet-ejector, these system topologies have a limited capability for controlling the re-circulation ratio and the resultant Oxygen-to Carbon (O/C) ratio at reformer or stack inlet and therefore can include compensating means such as an external water feed and a steam generator in the system to ensure adequate but not excessive steam content at fuel cell stacks. Insufficient steam flow rate can lead to disadvantageous and potentially irreversible formation of soot in the components throughout the fuel side of the system. On the other hand, excessive recirculation and consequent fuel gas dilution, while not as catastrophic to the system as insufficient recirculation, would lower fuel cell voltages and efficiency.
Another known embodiment of an arrangement is to accomplish desired recirculation pressure boosting by a fan or a compressor. Recirculation carried out by a fan or a compressor provides added flexibility and controllability to the system operation but involves sophisticated, complex and potentially unreliable machinery. Particularly accomplishing anode gas recirculation at very high temperature, which could simplify the thermal integration scheme, but sets difficult design requirements and potentially adds complexity to the fan or to the compressor.
A recirculation arrangement is disclosed for a high temperature fuel cell system, each cell in the system having an anode, a cathode, and an electrolyte between the anode and the cathode, the recirculation arrangement comprising: at least one supersonic ejector configured for recirculating a fraction of gas exhausted from an anode side of each cell and for providing a desired flow recirculation rate of recirculated flow, the ejector having at least one nozzle; means for providing at least one primary feedstock fuel fluid to said nozzle of the ejector, which nozzle has a convergent-divergent flow channel through which the fluid will expand from an initial higher pressure to a lower pressure; means for providing at least one secondary feedstock air fluid to said nozzle of the ejector; means for operating heat removal from anode recirculation when lambda exceeds 0.55; means for cutting off the secondary feedstock air fluid at a nominal operation; and means for cutting off the secondary feedstock air fluid to said nozzle of the ejector when a pressure ratio exceeds a designated critical pressure limit while fuel is being supplied, and wherein: when fuel cells are not loaded, an oxygen to carbon ratio of feedstocks alone is above a carbon formation limit and the recirculation rate exceeds 70%.
A recirculation arrangement is also disclosed for a high temperature electrolysis cell system, each cell in the system having an anode, a cathode, and an electrolyte between the anode and the cathode, the recirculation arrangement comprising: at least one supersonic ejector configured for recirculating a fraction of gas exhausted from an anode side of each cell and for providing a desired flow recirculation rate of recirculated flow, the ejector having at least one nozzle; means for providing at least one primary feedstock air fluid to said nozzle of the ejector, which nozzle has a convergent-divergent flow channel through which the fluid will expand from an initial higher pressure to a lower pressure; means for providing at least one secondary feedstock fuel fluid to said nozzle of the ejector; means for operating heat removal from cathode recirculation when lambda exceeds 0.55; means for cutting off the secondary feedstock fuel fluid at a nominal operation; and means for cutting off the secondary feedstock fuel fluid to said nozzle of the ejector when a pressure ratio exceeds a designated critical pressure limit while air is being supplied, and wherein: when electrolysis cells are not loaded, an oxygen to carbon ratio of feedstocks alone is above a carbon formation limit and the recirculation rate exceeds 70%.
A recirculation method for a high temperature fuel cell system or electrolysis cell system is disclosed, the method comprising: recirculating a fraction of gas exhausted from at least one of an anode side and a cathode side; providing a desired recirculated flow rate of the recirculated flow by using an ejector supplied with at least one primary feedstock fluid to a nozzle of the ejector, which nozzle has a convergent-divergent flow channel through which the fluid is expanded from an initial higher pressure to a lower pressure; providing at least one supplementary fluid to said nozzle of the ejector; regulating a respective ratio of the primary and supplementary fluids of the ejector to maintain a desired motive flow and pressure at the nozzle of said ejector in order to accomplish said desired recirculated flow rate; and cutting off the supplementary fluid when a level of system loading is such that the primary feedstock fluid alone maintains the desired motive flow and pressure at an ejector inlet.
Features and advantages of the present invention disclosed herein will be better understood by reading the following detailed description of exemplary embodiments in combination with the drawings, wherein:
With reference to the figures, a recirculation arrangement for an SOFC or an SOEC system is disclosed which can enable effective reactant utilization in the system and thus result in a high system performance, and ensure optimized thermal and compositional conditioning in the anode side in all modes of operation, which can result in an improved feasibility and lowered complexity of a SOFC or SOEC system. This can be achieved by a recirculation arrangement for a high temperature fuel cell system or electrolysis cell system, each cell in the system having an anode, a cathode, and an electrolyte between the anode and the cathode, the recirculation arrangement having at least one ejector for recirculating a fraction of gas exhausted from the anode side. The recirculation arrangement includes the ejector for accomplishing desired flow rate of the recirculated flow, the ejector having at least one nozzle, and the recirculation arrangement having means (e.g., fluid supply and conduit) for providing at least one primary feedstock fluid to the nozzle of the ejector, and means (e.g., fluid supply and conduit) for providing at least one supplementary fluid to the nozzle of the ejector and the recirculation arrangement includes means (e.g., fluid regulator or valve, including mechanical and/or processor controlled valve system) for regulating a respective ratio of at least part of the fluids of the ejector to maintain required motive flow and pressure at the nozzle of the ejector in order to accomplish the desired recirculated flow rate.
A recirculation method is provided for a high temperature fuel cell system or electrolysis cell system, in which is recirculated a fraction of gas exhausted from at least one of sides an anode side and a cathode side. The method can accomplish a desired flow rate of the recirculated flow by using an ejector, with at least one primary feedstock fluid to a nozzle of the ejector, with at least one supplementary fluid to said nozzle of the ejector, and a regulating of a respective ratio of the fluids of the ejector to maintain a desired motive flow and pressure at the nozzle of the ejector in order to accomplish a desired recirculated flow rate.
Exemplary embodiments are based on utilization of a recirculation arrangement by providing at least one primary feedstock fluid to a nozzle of the ejector, and by providing at least one supplementary fluid to a nozzle of the ejector, and by controlling a respective ratio of primary feedstock fluid and supplementary fluid of the ejector to maintain a desired motive flow and pressure at the nozzle of the ejector in order to accomplish a desired recirculated flow rate.
An exemplary benefit achieved by disclosed embodiments is that high system performance can be achieved together with improved feasibility and substantially low complexity of a high temperature cell system.
An arrangement as disclosed can accomplish feed stock feed and anode gas recirculation for an SOFC or an SOEC system, exploiting the inherent advantages of an ejector based recirculation topology—namely simple and reliable mechanical construction, omission of a heat exchanger and compact and cost effective anode side balance of plant sub-system—yet overcoming the known limitations and complications relating to constrained operability and controllability of ejectors—particularly unmanageability of rate of induced recirculation flow and insufficiency of motive flow and consequent motive pressure at low partial load modes of operation in relation to required recirculation flow at a given counter pressure.
Arrangements disclosed herein can include parallel, separately controlled, sufficiently pressurized and temperature controlled feed stock flows which can separately or with a supplementary feed stock fulfil the respective requirement for a motive flow passed through the ejector by which sufficient amount of anode gas suction and recirculation are induced at any given operating mode.
An ejector can have a single optimal operating point or narrow optimal operating range, outside of which their performance is impaired or at least constrained. Although ejectors with movable or adjustable parts have been made, induced flow may not be managed independent of motive flow. Additionally, such additions of complexity would likely compromise a key advantage of using an ejector at particularly high temperatures, the simplicity and reliability of a single part, solid piece of hardware. Therefore, control of recirculation flow or composition on the anode side of the system should be done elsewhere in the system, bearing in mind ejectors inherent limitations compared to an arrangement with a fan or a compressor.
Ejector performance is sensitive to counter pressure. Up to a point, increasing discharge pressure leads to diminishing entrainment capacity, after which operation becomes unstable and may switch to back-flow condition in which flow direction in suction may reverse. Since pressure losses in anode recycle loop can depend on flow restrictions in the loop and ejector counter pressure is therefore a function of recycle flow itself, back-flow condition is often not possible. However, an ejector does not inherently provide means for controlling entrainment—lowering of motive fluid flow will lower suction flow. Lowering motive flow in a fixed geometry primary nozzle will also lower motive flow pressure at the said nozzle and affect pressure boosting capability of the device. An ejector geometry has to be designed and configured with maximum operating point in mind as volumetric flow through a critical orifice of the nozzle is passively limited to a maximum when, at the given pressure and temperature, the velocity of the fluid reaches the speed of sound (Mach=1). All other modes of operation have to be covered with resulting, non-optimized performance and possible other compensatory means of any known configuration.
The condition for reaching choking conditions, i.e. the speed of sound at the nozzle throat is known as the critical pressure ratio, which is given by a designated critical pressure ratio equation:
P
1
/P
2=(2/n)n/(n+1)
where P1 and P2 are the absolute pressures upstream and downstream of the nozzle and n is the heat capacity ratio Cp/Cv if the fluid. For dry air the heat capacity ratio is 1.4 whereas for methane it is 1.307 at room temperature. This yields exemplary critical pressure ratios which can be designated 1.89 and 1.84, respectively. Thus, as a rule of thumb and with adequate control margins, choking is guaranteed if the primary pressure is roughly double to the suction pressure.
When the pressure ratio threshold for achieving choking conditions is achieved, the flow expansion at the divergent nozzle section gives rise to supersonic shocks. These shocks facilitate an efficient transfer of kinetic energy between the primary jet and fluid to be entrained in the mixing section of the ejector. This efficient mixing, as shown by document “Anode gas recirculation behavior of a fuel ejector in hybrid solid oxide fuel cell systems: Performance evaluation in three operational modes” (Zhu et al), incorporated herein by reference in its entirety, can facilitate superior entrainment ratios compared to ejectors operating in the subsonic regime. The favorable operating range is, however, rather narrow. At higher pressure ratios, the entrainment capability reduces, ejector efficiency reduces whereas the external work and equipment required to supply the feedstocks can become more complex. Therefore, practical implementations limit the maximum primary pressure to two, or at maximum three times the choking pressure in exemplary embodiments.
Off-nominal modes of operation may be but are not limited to system heat-up, unloaded hot idle operation, partial load operation, system purge situation as well as situations emerging due changing composition or relative ratios of feed stocks—due to external factors or deliberate consideration.
According to an exemplary arrangement disclosed herein, alternative and supplementary feed stocks can be utilized, overcoming the challenge of maintaining sufficient motive fluid flow and pressure at the primary nozzle of the ejector for achieving a required level of recirculation at different operating modes. The arrangement can include parallel sources of sufficiently high pressure feed stocks, parallel or common means for controlling temperatures of the feed stocks and connecting pipelines bringing the feed stocks together and passing them to the primary nozzle inlet of the ejector. Additionally, the arrangement can include means (e.g., appropriate conduits) for by-passing desired amount of one or several feed stocks to the discharge side of the ejector. An exemplary embodiment can be arranged for heat transfer by a heat exchanger 125 (
An exemplary embodiment as illustrated in
Temperature of the mixture fluid can be controlled for adjusting the desired ratio of motive and induced flows as long as fluid temperature is maintained below reaction threshold temperature of said mixture. At this mode, oxygen-to-carbon ratio is solely determined by the feed stocks as fuel cells are not loaded and do not produce water.
Thus, in the presence of carbonaceous fluid in feedstocks, the preventing of formation of solid carbon in such operating mode can require that the oxygen-to-carbon rate of the feedstock mixture is sufficiently high not to promote the Boudouard reaction
2CO(g)<=>CO2(g)+C(s)
This implies that conditions across the complete recycle loop shall be kept outside the regime where solid carbon can be formed. As shown by the document U.S. Pat. No. 9,496,567 (Ruokomaki), the disclosure of which is incorporated by reference, in its entirety, meeting this criteria with a mixture of fuel and air undergoing partial oxidation in the recycle loop requires unconventionally high oxygen to carbon ratios for the partial oxidation reaction. Lambda designates, or denotes, a stoichiometric air-fuel equivalence ratio. A lambda value of 1.0 implies stoichiometry, i.e. an amount of oxidizing species matches exactly an amount required to fully oxidize a fuel. For methane, each mole of CH4 requires two moles of O2: CH4+2O2=>CO2+2H2O. As shown by U.S. Pat. No. 9,496,567, an air to fuel ratio (lambda) of no less than 0.55 is required to prevent favorable thermodynamic conditions for carbon formation throughout a system heat-up/cooldown cycle. This corresponds to an O/C ratio of 2.2 for methane and air as feedstocks. However, such a high air fraction is not applicable for an ordinary single-pass partial oxidation reactor, as the outlet temperature would way in excess of 1000° C. due to the high exotherm of the reaction.
By inducing adequate level of recirculation, temperature increasing effect of partial oxidation can be lessened. As shown by U.S. Pat. No. 9,496,567 (Ruokomaki), a recirculation ratio of for example, at least 70%, preferably 80-90% is required to keep reaction temperatures sufficiently low for practical catalysts. As soon as fuel cell loading and water formation begins, ratio of air in the primary fluid mixture may be lowered while overall fuel feed is ramped up for maintaining sufficient primary flow. Once level of system loading is such that fuel feed alone maintains desired flow and pressure at ejector inlet and fuel cell reactions provide enough of water for pre-reforming, air feed for partial oxidation may be cut off entirely while further system ramp-up to the nominal fuel feed and nominal load may take place purely by increasing primary fuel feed and consequent motive pressure. The described arrangement solves a problem of maintaining recirculation in such modes of operation when fuel cell stacks do not consume fuel but only require a certain atmosphere in terms of temperature and composition, by supplementing a fraction of fuel by air as motive flows in the ejector and the two fluids forming a suitable mixture for partial oxidation as means for achieving a desired hydrogen containing composition.
The high entrainment capability of a supersonic ejector, when operated in its favorable operating window, as shown by Zhu et al, allows for designing a system for nominal operation with fuel as the sole feedstock, providing sufficient recirculation (typically 50% or higher) for steam oxidation. Such high recirculation rates are, as explained by prior art document (Kushibiki et al), not practically achievable with subsonic ejectors.
For supersonic ejectors, optimal entrainment however occurs only in a narrow pressure ratio window. In low-load operating points, sufficient recirculation, in turn, would hardly be achieved. By supplementing the fuel feed with air, favorable conditions for circulations can be maintained. For no-load conditions, however, safe operation is possible only if the feedstocks alone have sufficiently high oxygen-to-carbon ratio to prevent carbon formation. This in turn implies a high exotherm in the partial oxidation reactions occurring in the anode recycle loop, which can require high recirculation rate together with thermal management means to transfer the heat out from the recirculation loop.
An alternative exemplary embodiment for arranging sufficient motive flow in the ejector 110 nozzle 114 independent of rate of fuel feed-in flow for inducing desired level of suction flow is illustrated in
In a controlled system shut-down the above described stages proceed in reverse order. The difference compared to system start-up is that fuel cell stacks and balance of plant are already at their operational temperature, and in the fuel cell system means for removing heat from the stacks can be arranged, while maintaining required inlet temperature of the POX reactor. This can be accomplished for example by an additional cooling heat exchanger downstream of POX reactor or for example by utilizing the existing fuel pre-heater 105 upstream of stacks 103 and injecting and mixing cooling fluid flow in hot inlet stream of the said pre-heater 105. If such cooling fluid is air, it will participate in combustion of residual fuel in an after burner 152. Such an arrangement to facilitate cooling of anode exhaust streams can during other operation modes, such as start-up and nominal operation (e.g., below 50% or 25% capacity operation), also be utilized to increase temperatures of outgoing streams by allowing combustion to take place in relation to the mixing, prior to the afterburner. Such means (e.g., heater) for boosting anode exhaust temperatures can be located at multiple locations along the anode exhaust stream e.g., in relation to heat exchangers recuperating heat to anode or reformer inlet streams, possibly part of them being configured to be capable of also providing cooling in certain operation modes.
Next is explained more in detail exemplary embodiments according to the present disclosure. In
As presented already in
In an exemplary embodiment according to the present disclosure means 129 are for activating and promoting a decomposition reaction of at least one feed stock or a chemical reaction between at least two feed stocks upstream of the nozzle 114 of the ejector 110 for achieving desired temperature and desired volume of the primary feedstock flow. The means 129 are for example an element with catalytic properties and sufficient surface area and suitable geometry for bringing the gas mixture to a thermodynamic equilibrium or sufficiently close to equilibrium. The means 129 can also be for example a device such as an ignition element providing a hot surface igniting an otherwise exothermic reaction. The means 129 are presented in an exemplary embodiment of
Temperature of at least one of the feed stocks and supplementary fluids can be actively controlled by means 122, which are for example a heat exchanger utilizing secondary hot flows available in the system or from an outside heat source or an electrical heater, a gas temperature measurement device such as a thermocouple, means for controlling heat input by adjusting flow or temperature of a secondary heating flow or power of the electrical heater. In an exemplary embodiment the means 122 can perform active temperature controlling for example below reaction threshold temperature. This can be performed for example by keeping highest local temperature below the level. The means 122 performing active temperature controlling for example below reaction threshold temperature are for example a heat exchanger utilizing secondary hot flows available in the system or an electrical heater, gas temperature measurement device such as a thermocouple, means for controlling heat input by adjusting flow or temperature of a secondary heating flow or power of the electrical heater. The means 122 can also be accomplished by processor means utilizing information on reaction threshold temperature of known reactants at a known pressure range taking into account possible catalytic effects of materials which the fluid is in contact with. It is also possible in an exemplary embodiment according to the disclosure that an exothermic reaction is designed to take place between the primary feedstock fluid and the supplementary fluid downstream of the ejector 110.
In a further exemplary embodiment the recirculation arrangement can include means 124 for by-passing the nozzle 114 of the ejector 110 by a fraction of at least one feed stock to downstream of the ejector when amount of the feed stocks needed by the system is greater than amount required as a motive flow for inducing desired flow rate of recirculation. The means 124 are for example a by-pass pipe equipped with a control valve or a defined restriction equipped with a shut-off valve. The bypassing can be performed for example passively by means 124 for by-passing passively based on pressure level the nozzle 114 of the ejector 110 by the fraction of at least one feed stock. The means 124 for by-passing passively are for example an overflow or relief valve, which closes passively by means of a spring or pneumatic action when process pressure is below a set level, for example with known flow characteristics. The bypassing of the primary nozzle of the ejector by the fraction of at least one feed stock can be also performed actively controlled by fluid control means, which utilize measurement information in the bypass control.
The recirculation arrangement of
In an exemplary embodiment according to the present disclosure the ejector 110 presented in
Exemplary embodiments according to the present disclosure can use different kinds of ejector arrangements. There can be used parallel connected ejectors for example so that each group of cell stacks has an own ejector.
Although the invention has been presented in reference to the attached figures and specification, the invention is not limited to those as the invention is subject to fuel cell or electrolysis cell system and method variations within the scope allowed for by the claims.
Thus, it will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.
The present application is a Continuation-In-Part application of parent U.S. application Ser. No. 15/077,230, filed Mar. 22, 2016 and also claims priority under 35 U.S.C. § 120 to International Application PCT/FI2013/050918 which was filed on Sep. 23, 2013, designating the U.S. The content of the prior applications are hereby incorporated by reference in their entireies.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15077230 | Mar 2016 | US |
| Child | 16711535 | US |
