Fuel cell system and method for controlled separation of hydrogen gas from anode exhaust gas and use thereof

Abstract

In a fuel cell system with a HT-PEM fuel cell (2), hydrogen is separated from the anode exhaust gas and recycled into the anode (10A) of the fuel cell (10) in order to increase efficiency. Separation of H2 gas from the anode exhaust gas leaves an option for collecting the remaining CO2 after condensing the water, with the additional aspect of using the dried remaining CO2-free anode exhaust gas for recirculation into the reformer (7) and / or the reformer heater (6). The latter is motivated by the fact that the electrochemical H2 separator (12) is only used for separating less than 90% of the H2 available H2 in the anode exhaust gas, which prolongs the lifetime of the H2 separator (12).

Description

[0001] Fuel cell system and method for controlled separation of hydrogen gas from anode exhaust gas and use thereof

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a fuel cell system comprising a hydrogen separator for separating hydrogen gas, H2, from the anode exhaust gas and recirculation of the separated H2 gas back into the anode of the fuel cell. In particular, it relates to a system as described in the preamble of the independent claims and use thereof.

[0004] BACKGROUND OF THE INVENTION

[0005] For feeding fuel cells with hydrogen gas, H2, or hydrocarbons, various options exist, including pressurised H2, methane, or alcohols, for example methanol or ethanol. Typical gas fuels for fuel cells include ethane, propane, and natural gas. Alcohol fuel for fuel cells is advantageous in that existing liquid fuel infrastructure for diesel and gasoline to a large extent can be re-used, which includes also transport of the fuel to supply stations and storage of the fuel in a vehicle.

[0006] When using methane or alcohols as fuel, it has to be converted into H2 gas. The corresponding conversion reaction in a catalytic reformer is endothermic and requires energy. For supplying such energy by heating the reformer, a reformer-heater is provided in the fuel cell system. Typically, the reformer-heater is a burner that bums fuel for providing thermal energy. The term burner is common in the technical field, irrespective of the burner using a traditional flame or a catalytic consumption of the fuel for producing the necessary thermal energy. As an option, the burner consumes excess H2 gas from the anode exhaust gas of the fuel cell.

[0007] The reformation of the fuel produces syngas, which is a mix of gases, including H2 gas, carbon dioxide, CO2, carbon monoxide, CO, and some remains of water, H2O. For fuel cells with Polymer Electrolyte Membranes (PEM) at low temperature, which is below 100°C, which is why this type of fuel cells is called LT-PEM fuel cells or just PEM fuel cells, the catalysts are sensitive to CO gas at these temperatures so that this is converted into CO2 in a shift reactor prior to the syngas entering the fuel cell. For typical High-Temperature PEM fuel cells, HT-PEM fuel cells, operating at higher temperatures, above 120°C and even up to 200°C, the corresponding systems are more robust against CO gas, and a shift reactor can be avoided as well as other clean-up processes for the gas.

[0008] As fuel cells are becoming increasingly attractive for production of electrical power, in particular in vehicles, such as electrically driven automobiles and marine vessels, there is a steady urge to increase the efficiency for power production, where even improvement in the order of a single percentage is attractive. Accordingly, there is a demand for optimization in the technical field.

[0009] A possibility for increasing the efficiency of fuel cell systems is to separate hydrogen gas H2 from the anode exhaust gas and recirculate it in the system.

[0010] In the article, “Electrochemical hydrogen pumping using a high-temperature polybenzimidazole (PBI) membrane” published by Perry et al. in Journal of Power Sources 177 (2008) 478-484, electrochemical hydrogen separation from a mix of gases, including N2, H2, CO, and CO2, was disclosed using a high-temperature (>100°C) polybenzimidazole (PBI) membrane. In particular, the electrochemical pump was operated at 160°C on approximately 1.2 times that of the stoichiometric requirements of pure hydrogen without external humidification.

[0011] It is pointed out that such hydrogen pump due to its similarity is operated at a temperature similar to the coolant temperature of HT-PEM fuel cell, whereas the coolant from a LT-PEM fuel cell would not deliver a sufficiently high temperature.

[0012] US 6475 655 Bl and EP 1339125 A2 disclose a hydrogen separator arranged between a reformer and a fuel cell stack.

[0013] US 8 293 412 discloses use of an electrochemical H2 pump for recycling of H2 and states that the H2 pump can be voltage controlled or current controlled. EP 4151601 Al discloses a fuel cell system comprising a solid oxide fuel cell, SOFC, and a hydrogen separator. A portion of the remaining hydrogen gas in anode waste gas, AWG, is separated and recycled into the fuel cell. The remaining AWG is used in a combustor for heat production, for example for heating the reformer.

[0014] A current-controlled electrochemical H2 pump for increased fuel cell efficiency is disclosed in US 6280 865, where the separated H2 is recirculated into the PEM fuel cell stack by mixing it with fuel in the inlet line coming from the methanol reformer. The rest gas, which contains H2O and CO2, leaves the H2 pump through a vent for being discarded. It is clear from the disclosure US 6 280 865 that the higher the current in the H2 separator, the higher the fraction of H2 separated from the anode exhaust gas, and the higher is the gain in energy from the fuel cell, as the remaining gas is discarded. However, this implies that the H2 separator is operated at high separation efficiency with high power.

[0015] A voltage-controlled electrochemical H2 pump is disclosed in US 2010 0266923 Al. The H2 is separated with high separation efficiency of 99% using an electrochemical H2 separator at fixed current and by varying the voltage. As the H2 separator is used for diagnosis of the performance of the solid oxide fuel cell (SOFC) system, the high separation efficiency is useful because the H2 pump is more sensitive on changes of the composition of the anode exhaust gas when the separation efficiency is high. It reads that at least 75% of the H2 quantity is separated from the anode exhaust gas. However, for monitoring the performance of the SOFC at desired fuel utilization rates, the H2 pump must be operated in excess of 90% separation efficiency, such as greater than 95%, preferably about 99%. In order to reach such high separation rates, the current for the H2 pump is fixed and the voltage adjusted as necessary above 0.1 Volts and up to 0.2 Volts, mostly operating at the upper end of this range.

[0016] As an alternative to using an H2 pump for separating the H2 gas from the anode exhaust gas for recirculation, it is known, instead, to separate H20 and CO2 from the anode exhaust gas and use the remaining dried anode exhaust gas, which contains not only H2 but also CO, for recirculation. In this case, attention must be given to the remaining CO in case of LT-PEM fuel cells, which are sensitive to CO, why the recirculation cannot be directly into the fuel cell, which is in contrast to the system of US 6280 865 in which the H2 is separated for re- introduction into the anode.

[0017] However, it is also known for certain fuel cell types to recycle the dried exhaust gas after H2O and CO2 back into a reformer.

[0018] For example, US 9 502 728 discloses a molten carbonate fuel cell system with aH2O separator and a CO2 separator downstream of the anode in order to separate H2O and CO2 from the anode exhaust gas. The remaining rest gas that contains H2 and CO is mixed with methane / water fuel and fed into a pre-converter and / or reforming unit as a fuel supplement.

[0019] CO2 recovery is also disclosed in US 2021 299609 Al .

[0020] As it appears from the discussion above, there are various alternatives for recirculating H2 from the anode exhaust gas, be it by separating H2 from the anode exhaust gas and use that portion for recirculation, or as an alternative separating H2O and CO2 from the anode exhaust gas and recirculate the remaining portion.

[0021] There is an ongoing attempt for improving the efficiency of fuel cell systems, and there is still room for improvements relative to the prior art.

[0022] DESCRIPTION / SUMMARY OF THE INVENTION

[0023] It is an objective of the invention to provide an improvement in the art. In particular, it is an objective to provide a higher efficiency of a fuel cell system, especially a HT- PEM fuel cell system, that comprises a hydrogen gas separator and a reformer, optionally a reformer heater. This objective and further advantages are achieved with a fuel cell system and a method of its operation as described below and in the claims. In the following, the short terminology is used as follows: H2 for hydrogen gas, H2O for water, CO2 for carbon dioxide, and CO for carbon monoxide. The abbreviation of HT PEM is used for High-Temperature Polymer Electrolyte Membrane.

[0024] In short, in a fuel cell system with HT-PEM fuel cells, hydrogen is separated from the anode exhaust gas and recycled into the anode in order to increase efficiency. Separation of H2 gas from the anode exhaust gas leaves an option for collecting the remaining CO2 after condensing the water, with the addition of using the dried remaining CO2- firee anode exhaust gas for recirculation into the reformer and / or the reformer heater if it comprises a burner for burning H2. The latter is motivated by the fact that the electrochemical H2 separator is only used for separating less than 90%, or rather no more than 84%, of the available H2 in the anode exhaust gas, which prolongs the lifetime of the H2 separator.

[0025] Details are explained in the following.

[0026] A fuel cell, in particular a HT-PEM fuel cell, is provided, typically as part of a stack of fuel cells, which is common practice. The fuel cell has a membrane and an anode on one side of the membrane and a cathode on the opposite side of the membrane. In the following, instead of the terms anode side and cathode side, the short terminology of anode and cathode will be used for the fuel cell.

[0027] The fuel cell system comprises a fuel supply. An option is methane as fuel. However, especially for HT-PEM fuel cells, the fuel supply is providing alcohol, such as ethanol but especially methanol, after evaporation into a reformer, typically after mixing with water, so that the fuel cell can be fed with syngas containing H2 after reformation of the fuel. The catalytic reformation of the fuel produces not only H2 but also other gas by-products, such as CO2, H2O, and CO. As discussed above, for HT-PEM fuel cells, the removal of CO is typically not necessary.

[0028] The heat for the evaporation is advantageously taken from the coolant in the coolant circuit. For example, the necessary thermal energy can be taken at the low-temperature branch of the cooling circuit, where the temperature is closer to the coolant temperature that is fed into the fuel cell, for example 160°C, or the necessary thermal energy can be taken at the high temperature branch in the cooling circuit with the coolant temperature at the exit of the fuel cell, for example 170°C.

[0029] A reformer-heater is used for heating the reformer to the necessary operation temperature for the catalytic reformation of the fuel into the syngas.

[0030] The anode of the fuel cell has an anode inlet that is flow-connected by a syngas -conduit to the reformate-outlet of the reformer and receives syngas from the reformer through the syngas-conduit for the reaction in the fuel cell. The cathode of the fuel cell receives oxygen gas, for example as part of air. In HT-PEM fuel cells, hydrogen ions traverses the ion-conducting polymer electrolyte membrane from the anode side to the cathode side and form water in the cathode when the hydrogen ions combine with the oxygen. The water leaves the cathode as steam together with other gaseous components, such as nitrogen from supplied air.

[0031] The operation temperature of the HT-PEM fuel cell is in the range of 120°C-200°C, typically in the range of 150°C-180°C. This is in contrast to LT-PEM fuel cells, which operate below 100°C.

[0032] The anode consumes a dominant first portion of H2 from the received syngas for producing electricity, and a minor second portion, for example in the order of 20% of the initial H2 quantity in the anode, is released in the exhaust gas from the anode. The anode exhaust gas further comprises also water steam H2O, carbon dioxide CO2, and carbon monoxide CO.

[0033] Accordingly, for the separation, an electrochemical hydrogen separator, H2 separator, is used. It has an upstream-inlet that is conduit-connected to an anode outlet at a downstream side of the anode. The H2 separator is receiving the anode exhaust gas and separating a first fraction of the H2 gas from the anode exhaust gas into an H2 conduit. The hydrogen separator is on its downstream side connected to the anode inlet for recycling the separated H2 gas into the anode. For example, the H2 conduit merges with the syngas conduit upstream of the anode so that the H2 mixes with the syngas prior to entering the anode. By thorough and detailed studies of electrochemical H2 separators, it has turned out that high separation efficiency at high voltage reduces the lifetime of the H2 separator substantially. For example, the aforementioned separation of 99%, as disclosed in US patent application US 2010 0266923 Al is problematic for long term lifetime of the H2 separator. Thorough and long-term in-depth studies leading to the invention have revealed that the H2 separation should have an upper limit of 90% H2 separation relatively to the total quantity of H2 in the anode exhaust gas, and rather less than 90%, for example less than 85% or no more than 84%.

[0034] An option for electrochemical H2 separator is of the type having an anode and cathode with an ion-conducting membrane. For example, the construction of the H2 separator is similar to PEM fuel cells. For example, the H2 separator is of the type as discussed in the article, “Electrochemical hydrogen pumping using a high-temperature polybenzimidazole (PBI) membrane” published by Perry et al. in Journal of Power Sources 177 (2008) 478-484 and references therein. As already mentioned in the introduction, this article discloses electrochemical hydrogen separation from a mix of gases, including N2, H2, CO, and CO2, was disclosed using a high-temperature (>100°C) poly benzimidazole (PBI) membrane. In particular, the electrochemical pump was operated at 160°C on approximately 1.2 times that of the stoichiometric requirements of pure hydrogen without external humidification. Relatively low voltages, less than 1 V, were required to operate the hydrogen pump over a wide range of hydrogen flow rates.

[0035] It is pointed out that such H2 pump is operated at a temperature of 160°C, which is similar to the coolant temperature of HT-PEM fuel cell, which is useful, as the coolant of the HT-PEM fuel cell system can be used for providing the necessary operational temperature of the H2 separator. Notice that the coolant from a LT-PEM fuel cell would be below 100°C and, thus, not deliver a sufficiently high temperature. Accordingly, a HT-PEM fuel cell is advantageous over the LT-PEM fuel cells when operating such type of electrochemical H2 separator.

[0036] HT-PEM fuel cells are also advantageous in that they are robust against CO in the gas from the reformer so that a shift gas reactor can be avoided. As a consequence, relatively small and lightweight reformers can be used and a correspondingly small-dimensioned reformer-heater. The H2 separator comprises at least one, but potentially multiple, electrochemical cells. Each electrochemical cell receives electrical power for the H2 separation. In order to delimit the H2 separation into the first fraction, as described above, it has been found useful to adjust the voltage for the electrochemical cell, or each cell if there are more than one cell, to no more than a predetermined maximum voltage level, for example in the range of 50 mV to 125 mV, optionally 50 mV to 100 mV. In comparison, notice that the voltage in US 2010 0266923 Al is kept above 100 mV and reaches up to about 200 mV, with a preference for operation near this upper limit.

[0037] In some embodiments, the voltage for the electrochemical cell during power-producing operation of the fuel cell system is set at a constant value equal to the maximum voltage level. This is a simple and easy implementation of H2 separation and especially useful for add-on kits for existing fuel cell systems.

[0038] In more advanced embodiments, the voltage for the electrochemical cell is varied during power-producing operation of the fuel cell system in dependence of the production rate of the anode exhaust gas, and adjusted to separate no more than 90%, optionally 84%, of the H2 in the anode exhaust gas.

[0039] If the fuel cell system comprises a reformer heater that comprises a burner for heating the reformer, a second fraction of H2 can be used for feeding the burner, the second fraction of H2 corresponding to the H2 remaining in the anode exhaust gas after removal of the first fraction of H2 by the H2 separator. The efficiency of the reformer heater is then regulated by varying the second fraction of H2 due to regulation of the separation efficiency of the H2 separator.

[0040] In more advanced embodiments, a correspondingly programmed computer system is provided for not only controlling operation of the fuel cell system but also including control of electrical power supplied to the H2 separator. This way, the separation of H2 into the first fraction and the second fraction can be controlled.

[0041] In advantageous embodiments, the computer system is measuring and controlling power output from the fuel cell system and adjusting, for example iteratively adjusting, the first fraction of H2 from the H2 separator repeatedly during operation of the fuel cell system and measuring increase or decrease of power output from the fuel cell system as caused by the adjustment. A repeated increase and decrease the first fraction is used for the control to maximize power output.

[0042] In some of the advanced embodiments, the method comprises comparing a change in power consumption by the H2 separator with a change of power production by the fuel cell system as caused by the change in power consumption by the H2 separator and adjusting the first fraction accordingly in order to optimize power production. In practice, the power consumption by the H2 separator is increased, however to not more than the predetermined upper separation limit, if an immediately previous increase of the power consumption by the H2 separator caused an increased power production by the fuel cell system or if an immediately previous decrease of the power consumption by the H2 separator caused a reduction in the power production by the fuel cell system. Correspondingly, the power consumption by the H2 separator is decreased if an immediately previous decrease of the power consumption by the H2 separator caused an increased power production by the fuel cell system or if an immediately previous increase of the power consumption by the H2 separator caused a reduction in the power production by the fuel cell system. By continuously adjusting the first fraction, the power production of the system can be continuously optimised.

[0043] A pronounced advantage of electrochemical H2 separation, in contrast to a passive membrane H2 separation technologies, as disclosed in the prior art, is the possibility of monitoring the performance of the H2-separator, in particular the electricity consumption, as an indication for the fuel cell system performance, while at the same time allowing use of the H2 separator for optimisation. In some embodiments of this invention described herein, such monitoring of the performance of the fuel cell system is done. For example, the H2 production can be monitored and the fuel feed lambda value determined.

[0044] As the first fraction of H2, separated from the anode exhaust gas, is no more than 90% of the quantity of the available H2 gas in the anode exhaust gas, there is still at least 10% of the H2 left in the remaining gas. Optionally, this can be used in the reformer, in particular after carbon capture, and / or in a reformer heater, which consumes the H2 as well as the CO in the remaining gas after the H2 separator. For example, H2O is removed from the remaining anode exhausts gas, and a portion of the dried remaining exhaust gas recirculated into the reformer or into a reformer heater with a burner. The separated H2O can be re-used for mixing with the fuel.

[0045] Advantageously, removal ofH2O from the anode exhaust gas is combined with carbon capture options, as the latter has received increased attention recently for environmental reasons.

[0046] Having separated H2 from the anode exhaust gas by the H2-separator, and after removal of water, typically by condensation, the remaining gas contains by far mostly CO2, for example more than 80%. As an option for carbon capture, this CO2 is liquefied and stored in tanks.

[0047] After H2 separation, H2O removal and CO2 capture from the anode exhaust gas, the remaining gas contains H2 and CO as well as some CO2, as the CO2 capture is not 100% effective. Having such rest gas released to the atmosphere as a waste gas with a high content of CO and H2 is typically not desired. Accordingly, it is advantageous to also convert such rest gas into something safer for the surroundings. However, due to the rest gas containing H2 and CO, it is useful for the total power production in the fuel cell system, as these can be recirculated into the reformer or used for heating by the reformer-burner, where the CO is converted into CO2.

[0048] Accordingly, the combination of an H2 separator for recirculating a first fraction of the H2 from the anode exhaust gas into the anode, and separation of H20 and CO2 and recirculating a second fraction of the H2 into the reformer or reformer-burner, together with CO, provides an optimise utilization of the entire H2 in the fuel cell system, in particular when CO2 capture is one of the objectives. Although, the remaining second fraction of H2 is small, it must be pointed out that it is in the order of 4% of the initially provided H2 by the syngas and, correspondingly, adds to the versatility of the system.

[0049] Although, the second fraction of H2 of only 2% provides less additional power production in the fuel cell system than typically necessary for the carbon capture, it is nevertheless particularly useful in systems where carbon capture is required due to other reasons. For example, environmental and political reasons may require carbon capture on marine vessels or in power plants, and in such cases, the use of the remaining H2 and CO is advantageous as it adds in the order of 2-4% H2 in addition to energy from conversion of CO to CO2.

[0050] However, as already mentioned above, the carbon capture is not a necessary feature for the system and method described herein, and the remaining dried anode exhaust gas after water separation can be added to the reformer-burner without removing the CO2 first. In either case, the final use of the second fraction of H2 is an advantage.

[0051] The less H2 is separated from the anode exhaust gas into the first fraction, the more is available for the reformer-burner from the second fraction. This implies that a regulation of the first fraction of H2, which is separated, also regulates the amount of H2 in the remaining gas to the reformer-burner and, thus, influences the temperature of the reformer. In this respect, the regulation of efficiency of the H2 separator and size of the separated first fraction of H2 can be used to regulate the reformer temperature. By separating less H2 into the first fraction that is recycled to the anode, the power consumption of the H2 separator is decreased, which is beneficial if a larger fraction of H2 in the remaining gas after the H2 separator is used in the burner.

[0052] In particular interesting is the use for larger fuel cell systems, such as designed for marine vessels. This use is also applicable for carbon capture, which can be stored in tanks on the vessel.

[0053] As it appears from the discussion above, the invention is used to combine two typically alternative technologies of, on the one hand, separating H2 from the anode exhaust gas and use that portion for recirculation into the anode, and on the other hand, removing H2O and potentially also CO2 from the anode exhaust gas and recirculating the remaining portion into the reformer or reformerheater. This combination of technologies is motivated by the fact that the H2 separation is not performed with high utilization but leaves a substantial amount for use in a reformer or reformer heater with a burner. This combination is especially justified from energy-considerations when carbon capture has to be included due to other reasons, such as environmental reasons as well as commercial value in captured CO2. SHORT DESCRIPTION OF THE DRAWINGS

[0054] The invention will be explained in more detail with reference to the drawing, where FIG. 1 is an overview sketch of the fuel cell system,

[0055] FIG. 2 is a graph showing Voltage- Amp relationship for the H2 separator; FIG. 3 is an extended graph similar to FIG. 2.

[0056] DETAILED DESCRIPTION / PREFERRED EMBODIMENT

[0057] FIG. 1 illustrates a fuel cell system 1 according to the invention. The fuel cell system comprises a fuel cell 10. Typically, there are provided a plurality of fuel cells 10, for example provided as a fuel cell stack. For simplicity, however, only a single fuel cell 10 is illustrated. The fuel cell comprises a membrane 10C and an anode 10A on an anode side of the membrane 10C and a cathode 10B on the opposite cathode side of the membrane 10C.

[0058] In the following, the fuel cell system 1 is exemplified with a HT-PEM fuel cell 10. A HT-PEM fuel cell 10 is not as sensitive to CO and does not need a shift reactor and neither a high water steam content in the syngas, in contrast to LT-PEM cells.

[0059] Fuel, for example methanol, is provided from a fuel tank 2 in combination with water from a water tank 3, mixed and evaporated in an evaporator 4. For example, the evaporator is heated by coolant in a coolant circuit that removes heat from the fuel cell 10. For a HT PEM fuel cell, the temperature of the coolant is above 120°C, which makes it useful for transfer of thermal energy to the evaporator 4 for evaporating the fuel and water mixture. However, the evaporator 4 could also be heated in other ways, for example electrically.

[0060] As an alternative to methanol, other alcohols can be used, for which the reformer temperature and catalyst would have to be correspondingly adjusted.

[0061] The evaporated mix of fuel and water flows, as indicated by arrow 4A, into a flow control device 5, which controls the flow speed of the fuel mix through a reformer inlet 7A into a reformer 7. It is pointed out that the flow control device 5 is only exemplified as a single unit but could be provided as a number of separate gauges and valves operated separately from each other. It is also possible to insert valves downstream of the reformer, between the reformer outlet and the fuel cell.

[0062] In the reformer 7, the fuel mix is catalytically converted into syngas as a reformate, which leaves the reformer 7 through the reformate outlet 7B into syngas conduit 8. Such syngas contains carbon dioxide, CO2, carbon monoxide, CO, and hydrogen gas, H2, as well as some remains of H2O. The syngas is fed from the reformer outlet 7B, as indicated by arrow 8A, into an inlet 9A of the anode 10A of the fuel cell 10, and the H2 of the syngas is used for production of electricity by the fuel cell 10. Oxygen gas, typically as part of air, is fed into the cathode 10B of the fuel cell 10.

[0063] The HT-PEM fuel cell 10 is by example driven at a temperature in the range of 160°C- 170°C, defined by coolant control, which is common for such fuel cells, and the coolant circuit, which is not shown for simplicity. It is possible to drive the fuel HT-PEM fuel cells 10 at slightly different temperature in the range of 120°C-200°C, but typically in the range of 150°C-180°C. Advantageously, the polymer electrolyte membrane, PEM, 10C in the HT-PEM fuel cell 10 is mineral acid based, typically a polymer film, for example polybenzimidazole, PBI, doped with phosphoric acid.

[0064] Operating the fuel cell 10 in this temperature range is advantageous in that the coolant at such temperatures can be used to heat the H2 separator 12 to the desired operation temperature. For this reason, in some embodiments, the fuel cell coolant circuit (not shown) is passing through or along the H2 separator 12 for controlling its temperature.

[0065] The reformer 7 is heated by reformer-heater 6, which transfers heat to the catalyst inside the reformer 7. For example, the reformer heater 6 receives fuel from the flow control device 5, although, also a separate conduit and valve system can be used, instead. The anode exhaust gas that is leaving the anode 10A through an anode outlet 9B and into anode exhaust conduit 11 flows, as indicated by arrow 11 A, into a hydrogen (H2) separator 12, in which most of the hydrogen H2 is separated from the anode exhaust gas stream and fed into H2 conduit 13. The H2 conduit 13 is connected to the syngas conduit 8, and the H2 that flows in the H2 conduit 13, as indicated by arrow 13 A, is added to the syngas flow 8 A in the syngas conduit 8 between the reformer outlet 7B and the anode inlet 9A. The H2 from the H2 conduit 13 could also be added directly to the anode 10A, however, the addition to the syngas in the syngas conduit 8 leads to advantageous mixing.

[0066] As the H2 has been separated from the anode exhaust gas by the H2-separator 12, the remaining gas flowing out of the H2 separator 12 and into gas conduit 14, primarily water steam and CO2 gas can, in principle, be discarded. Advantageously, however, the water is separated in a H2O separator, typically water condenser 15, from the remaining anode exhaust gas in gas conduit 14 and collected in water reservoir 16, potentially for being re-used for mixing with fuel in evaporator 4.

[0067] Optionally, for carbon capture, the remaining CO2 in the CO2-conduit 16 downstream of the water condenser 15 is collected as liquid in CO2-tank 19, for example after compression in compressor 17 and condensation in heat exchanger 18.

[0068] After removal of H2O and CO2, there is a minor volume of rest gas remaining, which contains CO and H2 as well as some remaining CO2, as the carbon capture is not 100% efficient. This rest gas enters rest gas conduit 20 and is guided, as indicated by arrow 20A to the flow control device 5 and from there to the reformer 7, as it can be used for the reformation, and / or for a burner in the reformer heater 6. It is pointed out that the flow control device 5 is only optionally a unit, and that it is further possible that the rest gas by-passes the flow control device 5 and enters the reformer 7 or burner of the reformer heater 6 directly.

[0069] In the case that the anode exhaust gas contains more H2 and CO than needed for the burner in the reformer heater 6 to heat the reformer 7, the remaining gas can be led into the reformer 7, preventing H2 and CO from being released to the atmosphere. In this case the remaining anode exhaust gas is split into two portions, one for the burner in the reformer heater 6 and one for the reformer 7.

[0070] If, however, all of the remaining anode exhaust gas is entering the burner of the reformer heater 6, the remaining CO2 and H2O can be released to atmosphere without risk that otherwise would be caused by H2 and CO in the gas. On the other hand, if there is no burner in the reformer heater 6, the CO2 downstream of the reformer 7 can be recirculated and captured in CO2-tank 19 downstream of the H2 separator 12.

[0071] The system comprises a computer system 21 that is connected to various components in the system by corresponding signal buses, 21A, 21B, 21C, 21D, and 21E, typically cable buses, although wireless communication is also possible. The term bus is not delimited to a single bus and should be understood as a general term and also describe a collection of buses to corresponding components that are used for the specific purpose.

[0072] A first signal bus 21A is used for monitoring and operating the fuel cell. It comprises data communication lines for receiving and / or sending data signals to various components, for example including thermometers, pressure gauges, and valves. It may also be connected to gas analysing devices, for example for analysing the fuel cell exhaust gases.

[0073] A second signal bus 2 IB is used for monitoring and controlling the H2 separator. For example, the second signal bus 21B is used for monitoring and controlling voltage and / or current of the H2 separator in order to monitor H2 separation yields. For this purpose, the second signal bus 21B is connected to a voltmeter, amp-meter, powersource, and possibly flow meters and concentration sensors for monitoring the H2 flow in H2 conduit 13 and / or gas flow in gas conduit 14.

[0074] A third signal bus 21C is used for monitoring the reformer 7 performance, including flows, pressures and temperatures.

[0075] A fourth signal bus 21D is used for monitoring and controlling the reformer heater 6, for example burner of the reformer heater 6.

[0076] A fifth signal bus 21E is used for controlling the flow control device 5. Recalling that the flow control device 5 is not necessarily a single unit, the fifth data bus 21E is optionally connected to various flow meters and / or valves in order to monitor and control the flow of fuel into the reformer 7. The fifth signal bus 21E is potentially used for the mixing and directing of the fuel flow 4A and the rest gas flow 20A into the reformer 7 and the optional burner in reformer heater 6.

[0077] The list of signal buses is by example only and not exhaustive, as there would typically be further buses, for example controlling the evaporator 4, the compressor 17, the flow through the heat exchanger 18, and communication lines to various other components in the fuel cell system 1, in particular various gauges and valves that are used to control the functioning fuel cell system 1.

[0078] The computer system 21 evaluates the signals from the various monitoring devices and controls the functioning of the fuel cell system 1 by controlling various valves and adjusting power to the H2 separator 12.

[0079] In particular, the computer system 21 is programmed to optimise the H2 separation efficacy in relation to the overall functioning of the fuel cell system 1. For example, the computer system 21 calculates on the basis of predetermined adjustment parameters, including threshold levels, whether the H2 separation should be increased or decreased.

[0080] For example, if for a specific power production state of the fuel cell system 1, additional separation of H2 by the H2 separator 12 turns out consume more power than being created by the additional separation, it is not power-economic, and the power for the H2 separation would be reduced, until the reduction of the H2 separation reduces the overall power by the system. A useful method is an iterative process where the power for the H2 separator 12 is iterated up and down and a corresponding power production by the fuel cell system 1 is monitored as a result of the iteration in order to optimise the power production. This can be done in real time during operation.

[0081] As an option, when evaluating the total power consumption and adjusting the operation of the various components, the consumption by the CO2 capture facility, including one or more compressors, represented in the example by the compressor 17, such as turbine compressor, the heat exchanger 18 and the tank 19, is also taken into account in the optimization of the net production of power by the fuel cell system. The H2 separator 12 is optionally also used as a performance monitor for the fuel cell system 1 and / or optionally for controlling the correct flow amounts of hydrogen into the fuel cell anode. If large portions of H2 can be produced at low power consumption, it indicates a low H2 conversion efficacy of the fuel cell and vice-versa. Correspondingly, the fuel cell performance can be adjusted by control of gas flow through the various components of the system and corresponding control of temperatures in the various system components.

[0082] An option for electrochemical H2 separator 12 is of the type having an anode and cathode with an ion-conducting membrane. For example, the construction of the H2 separator is similar to PEM fuel cells. For example, the H2 separator is of the type as discussed in the article, “Electrochemical hydrogen pumping using a high-temperature polybenzimidazole (PBI) membrane” published by Perry et al. in Journal of Power Sources 177 (2008) 478-484 and references therein. As already mentioned in the introduction, this article discloses electrochemical hydrogen separation from a mix of gases, including N2, H2, CO, and CO2, was disclosed using a high-temperature (>100°C) polybenzimidazole (PBI) membrane.

[0083] FIG. 2 illustrates a current-voltage relationship for separation of H2 by the H2 separator of the type as explained above. The two vertical lines represent levels of 84% and 100%, respectively, of separation ofH2 from the supplied gas. Two curves are shown. The right curve was measured when supplying a model gas mix containing 70% H2 and 30% H2O. The entire H2 gas could be separated from the H2O steam at a voltage of 60 mV and a current density of 315 mA / cm2.

[0084] However, when a portion of the H2 was exchanged with CO2 and CO in order for the gas mix to better resemble the anode exhaust gas with 49% H2, 30% H2O, 19.5% CO2, and 1.5% CO, all percentages by volume, the voltage had to be increased drastically to slightly above 100 mV in order to separate more than 84% of the H2. The increase was even more pronounced to above 150 mV when separating 90% or more.

[0085] This is in agreement with the observations in the aforementioned reference

[0086] US 2010266923 Al, where little voltage increase was observed until 90% H2 separation and a drastic voltage increase for higher separation rates. In US 2010266923 Al, the high voltage increase for constant current at separation rates above 90% and up to 99% is desired, as it makes the diagnostic function of the H2 pump more sensitive with respect to H2 concentration changes in the gas supplied to the H2 pump, which is the objective in US 2010 266923 Al. As will be explained in the following, the present invention is using the H2 separator differently.

[0087] For the H2 separator 12 in the fuel cell system 1 described herein, it has been observed that high voltage is detrimental to the lifetime of the H2 separator 12. It is therefore useful to delimit the voltage. This is equivalent to delimiting the first fraction of separated H2.

[0088] In practical embodiments, in order to have a more general approach to different types of separators, the H2 separation has been delimited such that less than 90% of the H2 of the anode exhaust gas is separated, rather no more or even less than 85%, or as shown with the left vertical line in FIG. 2, no more than 84%.

[0089] This implies that a substantial rest of H2 remains in the anode exhaust gas after the H2 separator 12. As explained above in connection with FIG. 1, this fraction of H2 can be re-used in the reformer 7 and / or in a burner of reformer heater 6 after separation of H2O and CO2 from the anode exhaust gas.

[0090] In some practical embodiments, the voltage for the H2 separator is variable but delimited by an upper value, for example a value less than 125 mV, such as less than 100 mV, per cell in the H2 separator. This voltage level for a certain amount of produced node exhaust gas is indicated by stippled horizontal lines in FIG. 2. In the above-mentioned embodiments with a real time adjustment of the H2 separation in dependence on increased power production of the entire system, the voltage is varied, and the corresponding current density correspondingly, dependent on the composition of the anode exhaust gas. For example, the current density for a certain voltage can be regulated by controlling the supply of syngas from the reformer to the anode, which affects also the anode exhaust gas.

[0091] By delimiting the voltage by an upper limit, the maximum H2 separation is also delimited, for example by a max level of less than 90%, such as less than 85%, the limit, however, depending on the specific composition of the anode exhaust gas. By such delimitation, it is assured that the H2 separator experiences no overload, which safeguards a long lifetime.

[0092] In some more simple practical embodiments, the voltage for the H2 separator is fixed, for example at 125 mV, or even at a lower level of 100 mV, for the cell or cells in the H2 separator 12. This type of control is of more simple character that the aforementioned real-time adjustment of the voltage for optimization of the H2 separation in relation to optimisation of the overall power production of the system. For example, for constant voltage, the current density can be regulated by controlling the supply of syngas from the reformer to the anode, which affects also the anode exhaust gas.

[0093] FIG 3 shows the same lines as in FIG. 2 but supplemented by a series of curves for different production rates of anode exhaust gas, corresponding to different settings of the fuel cell production rate. For each of these separation curves, a separation level of 84% and of 90% has been determined, and the corresponding points connected by a weakly upward sloping curve, resulting in the two curves at which the numbers of 84% and 90% are indicated. In order to optimise the lifetime of the separator, it is better to use these curves for determining the applicable voltage, instead of a fixed voltage. For example, at low production of anode exhaust gases, when the electricity production is low, the separation voltage is advantageously below 50 mA / cm2, minimizing the current density, while maintaining a sufficient H2 separation.

[0094] In agreement with the conclusion above when discussing FIG. 2, also in this more generalised illustration for different anode exhaust gas production rates, the H2 separation is advantageously delimited to less than 90% or even better, for prolonged lifetime to no more than 84%.

[0095] As an example, the gas that is entering the anode is as follows, when using a normalisation to the amount of CO2, the latter therefore being set to unity. The right column shows exemplary but not limiting figures for the correspondingly adjusted percentages by volume of the syngas and the anode exhaust gas.

[0096] It is observed that the exhaust gas contains in the order of 30% H2, similar to the illustration of FIG. 2.

[0097] Whereas, there was 3 times as much H2 than CO2 in the syngas, the anode exhaust gas contains only an amount of H2 that makes up a fraction of 0.6 relative to the CO2 content. Accordingly, it expresses that consumption of H2 by the fuel cell is about 80% of the H2 and 20% (0.6 : 3) of the H2 leave the fuel cell in this example.

[0098] If 90% of the H2 in the anode exhaust gas is separated by the H2 separator 12, below called ePump, the remaining fraction is 10%, which corresponds to 0.06 relatively to the CO2 content. After the H2O separator 15, typically a condenser, and capture of the H2O in a tank 16, the CO2 amount remains, still normalised to unity, in the now dried anode exhaust gas. In the dried anode exhaust gas, there is H2 and CO left in a mutual ratio of 0.06:0. 1 equal to 6: 10. Although, the total amount of this rest gas relatively to the amount of initial syngas entering the anode is small, it is not negligible, seeing that this second fraction of 0.06 relatively to the original H2 content of 3 is 2%, thus, having a potential for increase efficiency of 2% in addition to the efficiency increase by the H2 separator and recycling of H2 from the H2 separator.

[0099] In a system where carbon capture is required and high power efficiency is critical, such 2% increase is highly desired. In particular, an increase of 2% by the last step of the recycling, also implies a fuel reduction at a corresponding level.

[0100] The CO that is recirculated into the reformer 7 may be oxidised into CO2 in a shift reaction, which is especially important for CO-sensitive LT-PEM fuel cells but less for HT-PEM fuel cells. The shift reaction of the CO with water may add to the energy balance due to production of H2 by this reaction. After traversing the fuel cell, the CO2 can be captured in the subsequent gas cycle by the CO2 separator 16, 17, 18.

[0101] Alternatively, the CO and the remaining H2 are used in a burner of the reformer heater 6 and converted to CO2 and H2O and released to atmosphere, together with remaining CO2 from the carbon capture process, which is not 100% efficient.

[0102] The less H2 is separated from the anode exhaust gas, the more is possible to feed into the burner of the reformer heater 6. This implies that a regulation of the first fraction of H2, which is separated, also regulates the amount of H2 in the remaining gas to the burner in the reformer heater 6 and, thus, influences the temperature of the reformer 7. In this respect, the regulation of efficiency of the H2 separator 12 and size of the separated first fraction of H2 can be used to regulate the temperature of the reformer 7. By separating less H2 into the first fraction that is recycled to the anode 10A, the power consumption of the H2 separator 12 is decreased, which is beneficial if a larger fraction of H2 in the remaining gas after the H2 separator 12 is used in the burner.

[0103] Although, HT-PEM fuel cells are generally robust, a certain reduction of CO concentration is desired, as high CO concentration affects the catalyser of the fuel cells 10. Similar considerations apply to the H2 separator 12, as an increased CO concentration in an H2 separator 12 stack has negative influence on the catalyser.

[0104] In a combination of carbon capture and hydrogen separation, one affects the other, as a low capture rate of CO2 increases the CO2 level in the circuit and lowers the relative level of H2 in the anode exhaust gas, making it more difficult to separate a certain amount of H2. This, in turn, leaves a higher H2 concentration in the anode exhaust gas downstream of the H2 separator 12, which in turn reduces the carbon capture rate. Circumventing this argument, a higher carbon capture rate leads to lower CO2 concentration in the circuit and increases the H2 separation efficiency, which in turn increases the carbon capture efficiency due to less H2 in the remaining gas downstream of the H2 separator and reduces the necessary power consumption therefore. Accordingly, up to a certain limit, a higher carbon capture efficiency is improving the overall system efficiency.

Claims

CLAIMS1. A method for operating a fuel cell system (1), the fuel cell system (1) comprising- a fuel supply (2) for supplying fuel,- a reformer (7) for catalytic reformation of the fuel into syngas that contains hydrogen, H2,- a fuel cell (10) comprising an anode (10A) and a cathode (10B) and a separator membrane (10C) in between, wherein an anode inlet (9 A) of the anode (10) is conduit- connected by a syngas conduit (8) to a reformate-outlet (7B) of the reformer (7) for receiving the H2 from the reformer (7) through the syngas conduit (8) and using the H2 for producing electricity,- a reformer heater (6) for heating the reformer (7) for the catalytic reformation of the fuel into the syngas,- an electrochemical H2 separator (12) having an upstream-inlet that is conduit-connected to an anode outlet (9B) of the anode (10A) by an anode-exhaust conduit (11) for receiving anode exhaust gas by the H2 separator (12) from the anode (10A) through the anode-exhaust conduit (11), wherein the H2 separator (12) is configured for separating H2 from the anode exhaust gas into an H2 conduit (13) that is flow-connected to the anode inlet (9A) for recycling the separated H2 gas into the anode (10A), wherein the method comprises,- receiving fuel from the fuel supply (2) by the reformer (8) and catalytically reforming the fuel into syngas and supplying the syngas to the anode (10A),- producing electrical power by the fuel cell (10) and producing anode exhaust gas due to the power production, the exhaust gas comprising hydrogen gas H2, water steam H2O, carbon dioxide CO2, and carbon monoxide CO;- receiving the anode exhaust gas by the H2 separator (12) and separating a first fraction of the H2 into the H2 conduit (13) by the H2 separator (12). characterised in that the method comprises delimiting the first fraction to less than 90% of the total quantity of H2 in the anode exhaust gas.

2. The method according to claim 1 , wherein the method comprises delimiting the first faction of the H2 to no more than 84% relatively to the total quantity of H2 in the anode exhaust gas.

3. The method according to claim 1 or 2, wherein the H2-separator (12) comprises at least one electrochemical cell (12a) for the H2 separation, the electrochemical cell (12A) receiving electrical power for the H2 separation, wherein the method comprises adjusting the voltage for the electrochemical cell (12A) to no more than a predetermined maximum voltage level.

4. The method according to claim 3, wherein the method comprises predetermining the maximum voltage level in the range of 50 mV to 100 mV.

5. The method according to any preceding claim, wherein method comprises varying the voltage for the electrochemical cell (12A) during power-producing operation of the fuel cell system (1) in dependence of the production rate of the anode exhaust gas, and adjusting the voltage to separate no more than 90%, optionally 84%, of the H2 in the anode exhaust gas.

6. The method according to any preceding claim, wherein the reformer heater (6) comprises a burner for heating the reformer by burning H2 from a second fraction of H2, the second fraction of H2 corresponding to the H2 remaining in the anode exhaust gas after removal of the first fraction of H2 by the H2 separator, and wherein the method comprises regulating the efficiency of the reformer heater (6) by varying the second fraction of H2 due to regulation of the separation efficiency of the H2 separator.

7. The method according to any preceding claim, wherein method comprises iteratively adjusting the first fraction repeatedly during power-producing operation of the fuel cell system (1) and measuring increase or decrease of power output of the fuel cell system (1) as caused by the adjustment of the first fraction, and repeatedly increasing and decreasing the first fraction for maximizing the power output of the fuel cell system (1).

8. The method according to any preceding claim, wherein the method comprises comparing a change in power consumption by the H2 separator (12) with a change of power production by the fuel cell system (1) as caused by the change in power consumption by the H2 separator (12), and only increasing the power consumption by the H2separator (12) if an immediately previous increase of the power consumption by the H2 separator (12) caused an increased power production by the fuel cell system (1) or if an immediately previous decrease of the power consumption by the H2 separator (12) caused a reduction in the power production by the fuel cell system (1).

9. The method according to any preceding claim, wherein the method comprises separating H2O from the remaining anode exhaust gas after separation of the first fraction of H2 anode exhaust gas by the H2 separator, and leading at least a portion of the remaining dried anode exhaust gas after water separation into the reformer (7) or the reformer heater (6), the remaining dried exhaust gas comprising a second fraction of H2 and the reformer heater (6) comprises a burner for heating the reformer by burning H2 from the second fraction of H2.

10. The method according to claim 9, wherein the method comprises separating CO2 from the dried anode exhaust gas after the separation of H2O and before leading at least a portion of the remaining dried anode exhaust gas with the second fraction of H2 into the reformer (7) or into the reformer heater (6).

11. The method according to any preceding claim, wherein the fuel comprises alcohol and the fuel cell (2) is a HT-PEM fuel cell, and wherein the method comprises evaporating the fuel by an evaporator (4) upstream of the reformer (7) and operating the fuel cell at a temperature in the range of 120°C-200°C.

12. A fuel cell system (1) for a method according to any preceding claim, comprising- a fuel supply (2) for supplying fuel,- a reformer (7) for catalytic reformation of the fuel into syngas that contains hydrogen, H2,- a fuel cell (10) comprising an anode (10A) and a cathode (10B) and a separator membrane (10C) in between, wherein an anode inlet (9 A) of the anode (10) is conduit- connected by a syngas conduit (8) to a reformate-outlet (7B) of the reformer (7) for receiving the H2 from the reformer through the syngas conduit (8) and using the H2 for producing electricity,- a reformer-heater (6) for heating the reformer (7) for the catalytic reformation of the fuel into the syngas,- an electrochemical H2-separator (12) having an upstream-inlet that is conduit-connected to an anode outlet (9B) of the anode (10A) by an anode-exhaust conduit (11) for receiving anode exhaust gas by the H2 from the anode (10A) through the anodeexhaust conduit (11), wherein the H2 separator is configured for separating H2 from the anode exhaust gas into an H2 conduit (13) that is flow-connected to the anode inlet (9 A) for recycling the separated H2 gas into the anode (10A),- a water separator (15, 16), H2O separator, having an upstream side of the H2O separator (15, 16) conduit-connected to a downstream side of the H2 separator (12) for receiving remaining anode exhaust gas after removal of the first fraction of H2 by the H2 separator (12) and for removing H2O from the remaining anode exhaust gas;- a CO2 separator (17, 18, 19) having an upstream side of the CO2 separator (17, 18, 19) connected to a downstream side of the H2O separator (15, 16) for receiving dry remaining anode exhaust gas and for separating CO2 from it, wherein a downstream side of the CO2 separator (17, 18, 19) is provided with a conduit (20) for supply of at least a portion of a second fraction of H2 from the anode exhaust gas to the reformer (7) or the reformer heater (6) after removal of the first fraction of H2, the H2O, and the CO2 by the respective separators (12, 15-16, 17, 18, 19), characterised in that the system is configured for delimiting the first faction to no more than 90% relatively to the total content of H2 in the anode exhaust gas.

13. The system according to claim 12, wherein the system (1) comprises a computer system (21) configured for controlling operation of the fuel cell system (1) including control of electrical power supplied to the H2 separator (12) and configured for measuring and controlling power output from the fuel cell system (1), wherein the computer system (21) is programmed for iteratively adjusting the first fraction repeatedly during operation of the fuel cell system (1) and measuring increase or decrease of power output from the fuel cell system (1) as caused by the adjustment, and repeatedly increasing and decreasing the first fraction for maximizing power output.

14. The system according to claim 13, wherein the computer system (21) is programmed for comparing a change in power consumption by the H2 separator (12) witha change of power production by the fuel cell system (1) as caused by the change in power consumption by the H2 separator (12), and configured for only increasing the power consumption by the H2 separator (12) if an immediately previous increase of the power consumption by the H2 separator (12) caused an increased power production by the fuel cell system (1) or if an immediately previous decrease of the power consumption by the H2 separator (12) caused a reduction in the power production by the fuel cell system (1).

15. The system according to anyone of the claims 12-14, the system (1) comprises a computer system (21) configured for controlling operation of the fuel cell system (1) including control of electrical power supplied to the H2 separator (12) and wherein the computer system (21) is programmed for varying the voltage for the electrochemical cell (12A) during power-producing operation of the fuel cell system (1) in dependence of the production rate of the anode exhaust gas, and adjusting the voltage to separate no more than 90%, optionally 84%, of the H2 in the anode exhaust gas.

16. The system according to anyone of the claims 12-15, wherein the fuel (2) comprises methanol to be mixed with water, and wherein the system (1) comprises an evaporator (4) for receiving and evaporating the fuel and water mix for reformation in the reformer (7), wherein the fuel cell (10) is a HT-PEM fuel cell (2) configured for operating in the range of 120°C-200°C.

17. Use of a method according to anyone of the claims 1-11 or a system according to anyone of the claims 12-16 for producing electricity on an electrically driven marine vessel.

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