Method and use of controlled separation of hydrogen gas from fuel cell systems and anode exhaust gases

Abstract

In a fuel cell system equipped with an HT-PEM fuel cell (2), hydrogen is separated from the anode exhaust gas to increase efficiency and recirculated to the anode (10A) of the fuel cell (10). Separation of H2 gas from the anode exhaust gas leaves the option of recovering residual CO2 after condensation of water, and an additional embodiment is to recirculate the anode exhaust gas, which does not contain dry residual CO2, to the reformer (7) and / or reformer heater (6). The latter is based on the fact that the electrochemical H2 separator (12) is used only to separate less than 90% of the available H2 in the anode exhaust gas, thereby extending the life of the H2 separator (12).

Description

Technical Field

[0001] The present invention relates to a fuel cell system including a hydrogen separation device for separating hydrogen gas H2 from anode exhaust gas and returning the separated H2 gas to the anode of a fuel cell by recirculation. In particular, the present invention relates to the system described in the preamble of the independent claims and its use.

Background Art

[0002] When supplying hydrogen gas H2 or hydrocarbons to a fuel cell, there are various options including pressurized H2, methane, or alcohols such as methanol or ethanol. Typical gas fuels for fuel cells include ethane, propane, and natural gas. Alcohol fuels for fuel cells are advantageous in that they can reuse most of the existing liquid fuel infrastructure for diesel and gasoline, which includes transportation to fuel supply stations and storage of fuel in vehicles.

[0003] When using methane or alcohol as fuel, it needs to be converted to H2 gas. The corresponding conversion reaction in a catalytic reformer is an endothermic reaction and requires energy. To supply such energy by heating the reformer, a reformer heater is provided within the fuel cell system. Usually, the reformer heater is a burner that burns fuel to provide thermal energy. The term "burner" is common in the technical field regardless of whether it uses a conventional flame to generate the required thermal energy or involves catalytic consumption of fuel. As an option, the burner consumes excess H2 gas from the anode exhaust gas of the fuel cell.

[0004] Synthesis gas is produced by the reforming of fuel, and it is a mixture of gases containing H2 gas, carbon dioxide (CO2), carbon monoxide (CO), and some residual water (H2O). In polymer electrolyte membrane (PEM) fuel cells that operate at low temperatures (below 100°C) (hence the names LT-PEM fuel cells or simply PEM fuel cells), the catalyst is sensitive to carbon monoxide (CO) gas in that temperature range, so CO is converted to carbon dioxide (CO2) in a shift reactor before the synthesis gas flows into the fuel cell. In HT-PEM fuel cells, which are typical high-temperature PEM fuel cells, the system is more robust to CO gas because it operates at high temperatures of 120°C or even 200°C, and can therefore avoid shift reactors and other gas purification processes.

[0005] Fuel cells are becoming increasingly attractive, particularly for power production in vehicles such as electric cars and ships, and the demand for improved power production efficiency is steadily increasing. In this field, even an improvement of just 1% can be of great value. Therefore, there is a demand for optimization in the technology field.

[0006] One way to improve the efficiency of fuel cell systems is to separate hydrogen gas (H2) from the anode exhaust gas and recirculate it within the system.

[0007] In the paper "Electrochemical hydrogen pumping using a high-temperature polybenzimidazole (PBI) membrane" published by Perry et al. in the Journal of Power Sources, 177(2008)478-484, the electrochemical separation of hydrogen from a mixture of gases containing N2, H2, CO, and CO2 was disclosed using a polybenzimidazole (PBI) membrane at high temperatures (>100°C). In particular, the electrochemical pump was operated at 160°C without external humidification under conditions of approximately 1.2 times the stoichiometric requirement for pure hydrogen.

[0008] Such hydrogen pumps are operated at temperatures equivalent to the coolant temperature of HT-PEM fuel cells due to their similarity, but it has been pointed out that sufficiently high temperatures cannot be obtained with the coolant from LT-PEM fuel cells.

[0009] US6475655B1 and EP1339125 A2 disclose a hydrogen separator positioned between a reformer and a stack of fuel cells.

[0010] U.S. Patent No. 8,293,412 discloses the use of an electrochemical H2 pump for H2 reuse, stating that the H2 pump may be voltage-controlled or current-controlled.

[0011] EP4151601 A1 discloses a fuel cell system including a solid oxide fuel cell (SOFC) and a hydrogen separator. A portion of the remaining hydrogen gas (AWG) in the anode exhaust gas is separated and recycled into the fuel cell. The remaining AWG is used in the combustor for heat generation, for example, to heat the reformer.

[0012] U.S. Patent No. 6,280,865 discloses a current-controlled electrochemical H2 pump for improving fuel cell efficiency, in which the separated H2 is mixed with fuel in the inflow line from the methanol reformer and recirculated to the stack of PEM fuel cells. Unreacted gases containing H2O and CO2 are discarded by exiting the H2 pump through a vent. It is clear from the disclosure of U.S. Patent No. 6,280,865 that the higher the current in the H2 separator, the larger the fraction of H2 separated from the anode exhaust gas, and the more residual gas is discarded, resulting in a greater energy gain from the fuel cells. However, this means that the H2 separator must be operated at high power and high separation efficiency.

[0013] A voltage-controlled electrochemical H2 pump is disclosed in US20100266923A1. Using an electrochemical H2 separator with a fixed current, H2 is separated with a high separation efficiency of 99% by changing the voltage. When an H2 separator is used for performance diagnostics of a solid oxide fuel cell (SOFC) system, a higher separation efficiency is useful because the higher the separation efficiency, the more sensitive the H2 pump is to changes in the composition of the anode flue gas. It can be read that at least 75% of the amount of H2 is separated from the anode flue gas. However, to monitor the performance of the SOFC at the desired fuel utilization rate, the H2 pump needs to be operated with a separation efficiency beyond 90%, for example, higher than 95%, preferably higher than about 99%. To achieve such a high separation rate, the current of the H2 pump is fixed, and the voltage is adjusted as needed from over 0.1 volts to 0.2 volts, operating almost at the upper limit of this range.

[0014] Instead of using an H2 pump to separate and recirculate H2 gas from the anode exhaust gas, it is known to separate H2O and CO2 from the anode exhaust gas and recirculate the remaining dry anode exhaust gas, which contains not only H2 but also CO.

[0015] In this case, CO-sensitive LT-PEM fuel cells require attention to residual CO, which is why recirculation cannot be directly introduced into the fuel cell, in contrast to the system in U.S. Patent No. 6,280,865, which separates H2 and reintroduces it to the anode.

[0016] However, in certain fuel cell types, it is known that the dried exhaust gas is recirculated after H2O and CO2 are returned to the reformer.

[0017] For example, U.S. Patent No. 9,502,728 discloses a molten carbonate fuel cell system comprising an H2O separator and a CO2 separator downstream of the anode for separating H2O and CO2 from the anode exhaust gas. The remaining unreacted gas containing H2 and CO is mixed with a methane / water fuel and supplied as an auxiliary fuel to a preconverter and / or reforming unit.

[0018] CO2 capture is also disclosed in US2021299609 A1.

[0019] As is clear from the above discussion, there are various options for recirculating H2 from anode exhaust gas. These include separating H2 from the anode exhaust gas and using that portion for recirculation, or, as an alternative, separating H2O and CO2 from the anode exhaust gas and recirculating the remaining portion.

[0020] There are ongoing attempts to improve the efficiency of fuel cell systems, and there is still room for improvement compared to existing technologies. [Overview of the project]

[0021] The object of the present invention is to provide improvements in the art. In particular, it is to provide a fuel cell system, especially an HT-PEM fuel cell system, that includes an H2 gas separator and reformer, and optionally a reformer heater. This object and further advantages are achieved by the fuel cell system and method of operation described below and in the claims.

[0022] In the following, the abbreviations H2 for hydrogen gas, H2O for water, CO2 for carbon dioxide, and CO for carbon monoxide will be used. The abbreviation HT PEM is used for high-temperature polymer electrolyte membranes.

[0023] In short, in fuel cell systems using HT-PEM fuel cells, hydrogen is separated from the anode exhaust gas and recirculated to the anode to improve efficiency. Separating H2 gas from the anode exhaust gas leaves the option of recovering residual CO2 after condensation of water, and if an H2 combustion burner is provided, there is an additional means of using the dried, CO2-free anode exhaust gas for recirculation to the reformer and / or reformer heater. The motivation for the latter is the fact that the electrochemical H2 separator is only used to separate less than 90%, rather less than 84%, of the available H2 in the anode exhaust gas, thereby extending the lifespan of the H2 separator.

[0024] Details are explained below.

[0025] Fuel cells, particularly HT-PEM fuel cells, are typically supplied as part of a fuel cell stack, a common practice. A fuel cell has a membrane and an anode on one side of the membrane and a cathode on the opposite side. Hereafter, the terms "anode side" and "cathode side" will be used as simplified terms for fuel cells.

[0026] A fuel cell system includes a fuel supply unit. The choice of fuel is methane. However, in HT-PEM fuel cells in particular, the fuel supply unit provides alcohols such as ethanol, but especially methanol, which is usually mixed with water and then evaporated in a reformer, so that synthesis gas containing H2 can be supplied to the fuel cell after fuel reforming. Catalytic reforming of the fuel produces not only H2 but also other gaseous byproducts such as CO2, H2O, and CO. As discussed above, CO removal is usually not necessary for HT-PEM fuel cells.

[0027] Heat for evaporation is advantageously removed from the coolant in the coolant circuit. For example, the required thermal energy can be removed from the low-temperature side branch of the cooling circuit, the temperature of this branch being close to the coolant temperature supplied to the fuel cell, for example 160 °C, or the required thermal energy can also be removed from the high-temperature side branch of the cooling circuit, the temperature of this branch being the coolant temperature at the fuel cell outlet, for example 170 °C.

[0028] The reformer-heater is used to heat the reformer to the operating temperature required for the catalytic reforming of the fuel into synthesis gas.

[0029] The anode of the fuel cell has an anode inlet fluidly connected to the reformed gas outlet of the reformer by a synthesis gas conduit and receives the synthesis gas from the reformer via the synthesis gas conduit for the reaction within the fuel cell. The cathode of the fuel cell receives oxygen gas, for example, as part of the air. In an HT-PEM fuel cell, hydrogen ions move from the anode side to the cathode side through the ion-conductive polymer electrolyte membrane, and water is formed at the cathode when the hydrogen ions combine with oxygen. The water leaves the cathode as vapor, together with other gaseous components such as nitrogen from the supply air.

[0030] The operating temperature of the HT-PEM fuel cell is in the range of 120 °C to 200 °C, usually in the range of 150 °C to 180 °C. This is in contrast to the LT-PEM fuel cell, which operates below 100 °C.

[0031] The anode consumes the dominant first part of the H2 in the received synthesis gas for power production, and the remaining small second part, for example about 20% of the initial amount of H2 in the anode, is released into the exhaust gas from the anode. The anode exhaust gas further contains water vapor H2O, carbon dioxide CO2, and carbon monoxide CO.

[0032] Therefore, for separation, an electrochemical hydrogen separator, an H2 separator, is used. It has an upstream inlet that is connected to the anode outlet via a conduit downstream of the anode. The H2 separator receives the anode exhaust gas, separates the first fraction of H2 gas from the anode exhaust gas, and leads it to the H2 conduit. The hydrogen separator has an anode inlet connected to its downstream side, and recirculates the separated H2 gas back to the anode. For example, since the H2 conduit merges with the synthesis gas conduit upstream of the anode, the H2 mixes with the synthesis gas before entering the anode.

[0033] Complete and detailed studies of electrochemical H2 separators have revealed that high separation efficiency at high voltages significantly shortens the lifespan of the H2 separator. For example, as disclosed in U.S. Patent Application No. US20100266923A1, the aforementioned 99% separation is problematic for the long lifespan of the H2 separator. Through meticulous and long-term detailed research leading to the present invention, it has become clear that H2 separation should have an upper limit of 90%, or rather less than 90%, for example, less than 85%, or 84% or less, relative to the total amount of H2 in the anode flue gas.

[0034] The choice of electrochemical H2 separator is one that has an anode and cathode equipped with an ion-conducting membrane. For example, the structure of the H2 separator is similar to that of a PEM fuel cell. For instance, the H2 separator is of the type discussed by Perry et al. in the paper, “Electrochemical hydrogen pumping using a high-temperature polybenzimidazole (PBI) membrane,” published in the Journal of Power Sources, 177 (2008), 478-484, and in the references within that paper. As already mentioned in the introduction, this paper discloses the electrochemical separation of hydrogen from a mixture of gases containing N2, H2, CO, and CO2 using a high-temperature (>100°C) polybenzimidazole (PBI) membrane. In particular, the electrochemical pump was operated at 160°C without external humidification at approximately 1.2 times the stoichiometric requirement for pure hydrogen. Relatively low voltages of less than 1V were required to operate the hydrogen pump over a wide range of hydrogen flow rates.

[0035] Such H2 pumps are operated at a temperature of 160°C, which is similar to the coolant temperature of HT-PEM fuel cells. It has been noted that this is useful because the coolant from HT-PEM fuel cell systems can be used to supply the operating temperature required for H2 separators. Note that the coolant from LT-PEM fuel cells is below 100°C and therefore does not supply sufficiently high temperatures. Thus, when operating this type of electrochemical H2 separator, HT-PEM fuel cells are more advantageous than LT-PEM fuel cells.

[0036] HT-PEM fuel cells also have the advantage of being resistant to CO in the gas from the reformer, which allows for the avoidance of shift gas reactors. As a result, relatively small and lightweight reformers can be used, and accordingly, small reformer-heaters can be used.

[0037] The H2 separator includes at least one electrochemical cell, but potentially multiple electrochemical cells. Each electrochemical cell receives power for H2 separation. As described above, it has been found useful to adjust the voltage of the electrochemical cells, or each cell if multiple cells are present, to a predetermined maximum voltage level below, for example, 50mV to 125mV, and optionally within the range of 50mV to 100mV, in order to separate the H2 into a first fraction. In comparison, note that the voltage of US2010 0266923A1 is maintained above 100mV, reaching a maximum of approximately 200mV, and operation near this upper limit is preferred.

[0038] In some embodiments, the voltage of the electrochemical cell during power production operation of the fuel cell system is set to a constant value equal to the maximum voltage level. This is a simple and straightforward embodiment of H2 isolation and is particularly useful as an add-on kit for existing fuel cell systems.

[0039] In more advanced embodiments, the voltage of the electrochemical cell is changed during the power production operation of the fuel cell system according to the generation rate of the anode exhaust gas, and is adjusted to separate 90% or less, optionally 84%, of H2 in the anode exhaust gas.

[0040] If the fuel cell system includes a reformer heater containing a burner for heating the reformer, a second fraction of H2 can be used to supply the burner, and this second fraction of H2 corresponds to the H2 remaining in the anode exhaust gas after the first fraction of H2 has been removed by the H2 separator. The efficiency of the reformer heater is then adjusted by changing the second fraction of H2 to adjust the separation efficiency of the H2 separator.

[0041] In more advanced embodiments, a correspondingly programmed computer system is provided that controls not only the operation of the fuel cell system but also the power supplied to the H2 separator. In this way, the separation of H2 into a first and a second fraction can be controlled.

[0042] In an advantageous embodiment, the computer system measures and controls the output power from the fuel cell system, repeatedly adjusts, for example, iteratively adjusts, a first fraction of H2 from the H2 separator during operation of the fuel cell system, and measures the increase or decrease in output power from the fuel cell system resulting from such adjustment. Repeatedly increasing and decreasing the first fraction is used for control to maximize output power.

[0043] In some advanced embodiments, the method includes comparing a change in the power consumption of the H2 separator with a change in the power output of the fuel cell system resulting from the change in the power consumption of the H2 separator, and adjusting a first fraction accordingly to optimize power output. In practice, if an immediate increase in the power consumption of the H2 separator leads to an increase in the power output of the fuel cell system, or if an immediate decrease in the power consumption of the H2 separator leads to a decrease in the power output of the fuel cell system, the power consumption of the H2 separator increases, but does not exceed a predetermined separation limit. Correspondingly, if an immediate decrease in the power consumption of the H2 separator leads to an increase in the power output of the fuel cell system, or if an immediate increase in the power consumption of the H2 separator leads to a decrease in the power output of the fuel cell system, the power consumption of the H2 separator decreases. By continuously adjusting the first fraction, the power output of the system can be continuously optimized.

[0044] In contrast to passive membrane H2 separation technologies disclosed in the prior art, a significant advantage of electrochemical H2 separation is the possibility of monitoring the performance of the H2 separator, particularly its power consumption, as a performance indicator of the fuel cell system, and simultaneously utilizing the H2 separator for optimization. In some embodiments of the present invention described herein, such monitoring of the fuel cell system performance is performed. For example, the amount of H2 produced can be monitored to determine the fuel supply lambda value.

[0045] Since the first fraction of H2 separated from the anode flue gas is less than 90% of the amount of available H2 gas in the anode flue gas, at least 10% of H2 remains in the residual gas. Optionally, this can be used in a reformer, particularly after carbon recovery, and / or in a reformer heater, which consumes CO as well as H2 in the residual gas after the H2 separation. For example, H2O is removed from the residual anode flue gas, and a portion of the dry residual flue gas is recirculated to a reformer or a reformer heater with a burner. The separated H2O can be reused for mixing with fuel.

[0046] An advantage is that H2O removal from anode exhaust gases can be combined with carbon capture options, the latter of which has been attracting attention in recent years for environmental reasons.

[0047] H2 is separated from the anode exhaust gas by an H2 separation unit, and after removing water by condensation, the residual gas contains almost entirely CO2, for example, more than 80% CO2. As an option for carbon recovery, this CO2 is liquefied and stored in a tank.

[0048] After separating H2, removing H2O, and recovering CO2 from the anode exhaust gas, the residual gas contains H2 and CO, as well as some CO2, because CO2 recovery is not 100% effective. Having such unreacted gas released into the atmosphere as exhaust gas with high CO and H2 content is generally undesirable. Therefore, it is advantageous to convert such unreacted gas into something safer for the environment. However, since the unreacted gas contains H2 and CO, it can be recycled to a reformer or used for heating by the reformer's burner, where CO is converted to CO2, making it useful for total power production in a fuel cell system.

[0049] Therefore, by combining an H2 separator for recirculating the first fraction of H2 obtained from the anode exhaust gas back to the anode with a system that separates H2O and CO2 and recirculates the second fraction of H2 together with CO to a reformer or reformer burner, optimal utilization of the entire H2 in the fuel cell system is provided, especially when CO2 recovery is one of the objectives. Although the remaining second fraction of H2 is small, it is only about 4% of the H2 initially provided by the synthesis gas, and it should be noted that this accordingly enhances the versatility of the system.

[0050] Although the second fraction of H2, which makes up only 2%, provides less additional power production in fuel cell systems than is typically required for carbon capture, it is still particularly useful in systems where carbon capture is necessary for other reasons. For example, carbon capture may be required on ships or in power plants for environmental and political reasons, in which case the use of residual H2 and CO is advantageous as it adds about 2-4% of H2 in addition to the energy from the conversion of CO to CO2.

[0051] However, as already mentioned above, carbon recovery is not a required feature of the systems and methods described herein, and the residual dry anode exhaust gas after water separation can be added to the reformer burner without first removing CO2. In either case, the final use of the second fraction of H2 is an advantage.

[0052] The less H2 separated from the anode exhaust gas into the first fraction, the more H2 becomes available from the second fraction for the reformer burner. This means that adjusting the first fraction of separated H2 also adjusts the amount of H2 in the residual gas going to the reformer burner, and therefore affects the reformer temperature. In this regard, the reformer temperature can be adjusted by adjusting the efficiency of the H2 separator and the size of the first fraction of separated H2. By separating even less H2 into the first fraction that is recycled back to the anode, the power consumption of the H2 separator is reduced, which is beneficial when the H2 separator has a large fraction of H2 in the residual gas after it has been used by the burner.

[0053] Of particular interest is its use in large-scale fuel cell systems designed for ships. This application could also be applied to carbon capture, which can be stored in tanks on board vessels.

[0054] As is evident from the above discussion, the present invention is used to combine two techniques that are normally substitutable: on the one hand, a technique for separating H2 from anode exhaust gas and using that portion for recirculation back to the anode; and on the other hand, a technique for removing H2O and possibly CO2 from anode exhaust gas and recirculating the remaining portion to a reformer or reformer heater. This combination of techniques is motivated by the fact that H2 separation is not performed with high utilization rates, but leaves a substantial amount for use in a reformer or a reformer heater with a burner. This combination is particularly justified from an energy perspective when carbon capture must be included for environmental reasons and other reasons such as the commercial value of the recovered CO2.

[0055] The present invention will be described in more detail with reference to the drawings. [Brief explanation of the drawing]

[0056] [Figure 1] This is a schematic diagram of a fuel cell system. [Figure 2] This graph shows the voltage-ampere relationship of the H2 isolation device. [Figure 3] This is an enlarged graph similar to Figure 2. [Modes for carrying out the invention]

[0057] Figure 1 shows a fuel cell system 1 according to the present invention. The fuel cell system includes a fuel cell 10. Generally, multiple fuel cells 10 are provided, for example, as a stack of fuel cells. However, for simplicity, only a single fuel cell 10 is illustrated. The fuel cell includes a membrane 10C, an anode 10A on the anode side of the membrane 10C, and a cathode 10B on the opposite cathode side of the membrane 10C.

[0058] In the following, fuel cell system 1 is illustrated with an HT-PEM fuel cell 10. The HT-PEM fuel cell 10 is insensitive to CO, does not require a shift reactor, and does not require a high water vapor content in the synthesis gas, in contrast to the LT-PEM cell.

[0059] Fuel, such as methanol, is supplied from the fuel tank 2 in combination with water from the water tank 3, mixed in the evaporator 4, and evaporated. For example, the evaporator is heated by a coolant in a coolant circuit that removes heat from the fuel cell 10. In the case of an HT-PEM fuel cell, the temperature of the coolant exceeds 120°C, which is useful for transferring thermal energy to the evaporator 4 for evaporating the fuel-water mixture. However, the evaporator 4 may also be heated in other ways, for example, electrically.

[0060] Other alcohols can be used instead of methanol, but the temperature of the reforming apparatus and the catalyst must be adjusted accordingly.

[0061] The evaporated fuel-water mixture flows into the flow control device 5, as indicated by arrow 4A, which controls the flow rate of the fuel mixture as it flows into the reformer 7 through the reformer inlet 7A. Although the flow control device 5 is illustrated as a single unit, it has been noted that it may be provided as a plurality of separate instruments and valves operated independently of each other. It is also possible to insert a valve downstream of the reformer, between the reformer outlet and the fuel cell.

[0062] In the reformer 7, the fuel mixture is converted into synthesis gas as a reformed gas by a catalyst, which leaves the reformer 7 through the reformed gas outlet 7B and is sent to the synthesis gas conduit 8. This synthesis gas contains carbon dioxide (CO2), carbon monoxide (CO), hydrogen gas (H2), and a small amount of residual water (H2O). The synthesis gas is supplied from the reformer outlet 7B to the inlet 9A of the anode 10A of the fuel cell 10, as indicated by arrow 8A, and the H2 in the synthesis gas is used for power generation by the fuel cell 10. Normally, oxygen gas is supplied to the cathode 10B of the fuel cell 10 as part of the air.

[0063] The HT-PEM fuel cell 10 is driven at a temperature in the range of 160°C to 170°C, defined by coolant control and coolant circuitry (not shown for simplicity), which are common in such fuel cells. While it is possible to drive the HT-PEM fuel cell 10 at slightly different temperatures in the range of 120°C to 200°C, it is typically driven at temperatures in the range of 150°C to 180°C. Advantageously, the polymer electrolyte membrane PEM 10C of the HT-PEM fuel cell is based on mineral acids and is typically a polymer membrane, such as phosphate-doped polybenzimidazole (PBI).

[0064] Operating the fuel cell 10 within this temperature range is advantageous in that the H2 separator 12 can be heated to a desired operating temperature using a coolant at such temperatures. For this reason, in some embodiments, a fuel cell coolant circuit (not shown) passes through or along the H2 separator 12 to control its temperature.

[0065] The reformer 7 is heated by the reformer-heater 6, which transfers heat to the catalyst within the reformer 7. For example, the reformer-heater 6 receives fuel from the flow control device 5, but a separate conduit and valve system can also be used instead. The anode exhaust gas, which leaves the anode 10A and enters the anode exhaust conduit 11 through the anode outlet 9B, flows into the hydrogen (H2) separator 12, as indicated by arrow 11A, where most of the hydrogen H2 is separated from the anode exhaust gas flow and supplied to the H2 conduit 13. The H2 conduit 13 is connected to the synthesis gas conduit 8, and the H2 flowing through the H2 conduit 13, as indicated by arrow 13A, is added to the synthesis gas flow 8A in the synthesis gas conduit 8 between the reformer outlet 7B and the anode inlet 9A. Alternatively, H2 from the H2 conduit 13 can also be added directly to the anode 10A, but adding it to the synthesis gas in the synthesis gas conduit 8 leads to a favorable mixture.

[0066] Since H2 is separated from the anode exhaust gas by the H2 separator 12, the residual gases, mainly water vapor and CO2 gas, that leave the H2 separator 12 and flow into the gas conduit 14 can, in principle, be discarded. However, advantageously, water is separated from the residual anode exhaust gas in the gas conduit 14 in the H2O separator, usually in the water condenser 15, and is recovered in the water reservoir 16 for reuse, possibly in mixing with fuel in the evaporator 4.

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

[0068] After the removal of H2O and CO2, carbon recovery is not 100% efficient, so a small amount of unreacted gas remains, containing CO and H2, and similarly some residual CO2. This unreacted gas enters the unreacted gas conduit 20, which is led to the flow control device 5 as indicated by arrow 20A, and from there to the reformer 7, as it can be used for reforming and / or for the burner in the reformer heater 6. It has been pointed out that the flow control device 5 is merely an optional unit, and furthermore, the unreacted gas may bypass the flow control device 5 and flow directly into the burner of the reformer 7 or the reformer heater 6.

[0069] If the anode exhaust gas contains more H2 and CO than is necessary for the burner in the reformer heater 6 to heat the reformer 7, the residual gas can be directed to the reformer 7 to prevent the H2 and CO from being released into the atmosphere. In this case, the residual anode exhaust gas is divided into two parts: one for the burner in the reformer heater 6 and the other for the reformer 7.

[0070] However, if all of the remaining anode exhaust gas enters the burner of the reformer heater 6, the remaining CO2 and H2O can be released into the atmosphere without the risks that would otherwise be caused by H2 and CO in the gas.

[0071] 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 recovered in the CO2 tank 19 downstream of the H2 separator 12.

[0072] The system includes a computer system 21 that is capable of wireless communication but is connected to various components within the system by corresponding signal buses 21A, 21B, 21C, 21D, and 21E, typically by cable buses. The term "bus" should not be limited to a single bus but should be understood as a general term and to describe a collection of buses to corresponding components used for a particular purpose.

[0073] The first signal bus 21A is used for monitoring and operating the fuel cell. It includes data communication lines for receiving and / or transmitting data signals to various components, including thermometers, pressure gauges, and valves. It may also be connected to a gas analysis device, for example, for analyzing the exhaust gas of the fuel cell.

[0074] The second signal bus 21B is used for monitoring and controlling the H2 separator. For example, the second signal bus 21B is used to monitor and control the voltage and / or current of the H2 separator in order to monitor the H2 separation yield. For this purpose, the second signal bus 21B is connected to a voltmeter, an ammeter, a power supply, and optionally, a flow meter and concentration sensor for monitoring the H2 flow rate in the H2 conduit 13 and / or the gas flow rate in the gas conduit 14.

[0075] The third signal bus 21C is used to monitor the performance of the reformer 7, including flow rate, pressure, and temperature.

[0076] The fourth signal bus 21D is used to monitor and control the burners of the reformer heater 6, for example.

[0077] A fifth signal bus 21E is used to control 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 to monitor and control the fuel flow to the reformer 7. The fifth signal bus 21E may be used to mix and direct the fuel flow 4A and the unreacted gas flow 20A to any burner in the reformer 7 and the reformer heater 6.

[0078] Typically, there are many more buses, so the list of signal buses is merely illustrative and not exhaustive. For example, there are buses that control the flow rates through the evaporator 4, compressor 17, and heat exchanger 18, as well as communication lines to various other components within the fuel cell system 1, particularly the various instruments and valves used to control the functioning fuel cell system 1.

[0079] The computer system 21 controls the functions of the fuel cell system 1 by evaluating signals from various monitoring devices, controlling various valves, and adjusting the power supplied to the H2 separator 12.

[0080] In particular, the computer system 21 is programmed to optimize the effectiveness of H2 separation in relation to the overall function of the fuel cell system 1. For example, the computer system 21 calculates whether H2 separation should be increased or decreased based on predetermined adjustment parameters, including threshold levels.

[0081] For example, if, for a particular power production state of the fuel cell system 1, it is found that the additional separation of H2 by the H2 separator 12 consumes more power than is generated by the additional separation, then this is not power-economical, and the power for H2 separation will be reduced until reducing H2 separation reduces the overall power consumption of the system. A useful method is an iterative process of repeatedly increasing or decreasing the power for the H2 separator 12 and monitoring the corresponding power production by the fuel cell system 1 as a result of the iterations to optimize power production. This can be done in real time during operation.

[0082] As an option, when evaluating total power consumption and adjusting the operation of various components, the consumption of CO2 capture equipment, including one or more compressors (examplely represented by a compressor 17 such as a turbine compressor), a heat exchanger 18, and a tank 19, may also be taken into consideration when optimizing the net power generation by the fuel cell system.

[0083] The H2 separator 12 is also optionally used as a performance monitor for the fuel cell system 1 and / or optionally to control the correct flow rate of hydrogen to the fuel cell anode. If the majority of H2 can be produced with low power consumption, it indicates that the H2 conversion efficiency of the fuel cell is low, and vice versa. Accordingly, fuel cell performance can be adjusted by controlling the gas flow through the various components of the system and, accordingly, by controlling the temperature within the various system components.

[0084] The options for the electrochemical H2 separator 12 include types that have an anode and cathode along with an ion-conducting membrane. For example, the structure of the H2 separator is similar to that of a PEM fuel cell. For example, the H2 separator is of the type discussed by Perry et al. in the paper, “Electrochemical hydrogen pumping using a high-temperature polybenzimidazole (PBI) membrane,” published in the Journal of Power Sources, 177 (2008), 478-484, and in the references within the same paper. As already mentioned in the introduction, this paper discloses the electrochemical separation of hydrogen from a mixture of gases containing N2, H2, CO, and CO2 using a high-temperature (>100°C) polybenzimidazole (PBI) membrane.

[0085] Figure 2 illustrates the current-voltage relationship for H2 separation using the type of H2 separator described above. The two vertical lines represent levels of H2 separation from the feed gas of 84% and 100%, respectively. Two curves are shown. The curve on the right was measured when a model gas mixture containing 70% H2 and 30% H2O was supplied. At a voltage of 60 mV and a current density of 315 mA / cm², the entire H2 gas could be separated from the H2O vapor.

[0086] However, when some of the H2 was replaced with CO2 and CO to bring the gas mixture closer to the anode exhaust gas, which was 49% H2, 30% H2O, 19.5% CO2, and 1.5% CO by volume percentage, the voltage had to be significantly increased to slightly over 100mV to separate more than 84% of the H2. When more than 90% was separated, the increase was even more pronounced, exceeding 150mV.

[0087] This is consistent with the observations in the aforementioned reference US2010266923A1, which show that there is almost no voltage increase until 90% H2 separation, and that the voltage increases significantly as the separation rate increases. In US2010-266923A1, a high voltage increase at constant current is desired in the range of separation rates from 90% to 99%, which is to make the characteristic function of the H2 pump more sensitive to changes in H2 concentration in the gas supplied to the H2 pump, and this is the objective of US2010-266923A1. As described below, the present invention uses the H2 separation device in a different way.

[0088] In the case of the H2 separator 12 in the fuel cell system 1 described herein, it has been observed that high voltage adversely affects the lifespan of the H2 separator 12. Therefore, it is useful to limit the voltage. This corresponds to limiting the first fraction of separated H2.

[0089] In practical embodiments, to have a more general approach for various types of separation devices, H2 separation is limited to separating less than 90% of the H2 in the anode exhaust gas, rather 85% or less, or even less than 85%, or 84% or less, as shown by the left vertical line in Figure 2.

[0090] This means that a substantial amount of unreacted H2 remains in the anode exhaust gas after the H2 separator 12. As described above in relation to Figure 1, this H2 fraction can be reused in the burners of the reformer 7 and / or reformer heater 6 after separating H2O and CO2 from the anode exhaust gas.

[0091] In some practical embodiments, the voltage of the H2 separator is variable, but is limited by an upper limit per cell in the H2 separator, e.g., less than 125mV, e.g., less than 100mV. In Figure 2, this voltage level for a given amount of generated anode flue gas is shown by a dotted horizontal line. In the embodiments described above, where H2 separation is adjusted in real time in response to an increase in overall system power production, the voltage changes depending on the composition of the anode flue gas, and the corresponding current density also changes. For example, the current density for a particular voltage can be adjusted by controlling the supply of synthesis gas from the reformer to the anode, which also affects the anode flue gas.

[0092] By limiting the voltage with an upper limit, the maximum H2 separation is also limited to a maximum level, for example, less than 90% or less than 85%, depending on the specific composition of the anode exhaust gas. Such limitations ensure that the H2 separator is not overloaded and ensure a long lifespan.

[0093] In some simpler, more practical embodiments, the voltage of the H2 separator 12 is fixed for a single cell or multiple cells, for example, at 125 mV, or even lower at 100 mV. This type of control has the advantage of being simpler than the aforementioned real-time adjustment of voltage for optimizing H2 separation, which is related to optimizing the overall power production of the system. For example, the current density for a given voltage can be adjusted by controlling the supply of synthesis gas from the reformer to the anode, which also affects the anode exhaust gas.

[0094] Figure 3 shows the same lines as Figure 2, but is supplemented by a series of curves representing different anode exhaust gas production rates corresponding to different fuel cell production rate settings. For each of these separation curves, separation levels of 84% and 90% were determined, and the corresponding points were connected by a gently sloping upward curve, resulting in two curves showing the values ​​of 84% and 90%. To optimize the lifespan of the separation device, it is even better to use these curves to determine the applicable voltage instead of a fixed voltage. For example, at low anode exhaust gas productivity, when power output is low, the separation voltage is favorably 50 mA / cm². 2 It is less than this, minimizing the current density while maintaining sufficient H2 isolation.

[0095] Similar to the conclusions regarding Figure 2, in this more general figure showing various anode exhaust gas production rates, H2 separation is favorably limited to less than 90%, and even more desirable to keep it below 84% for extended lifespan.

[0096] For example, the gases flowing into the anode are as follows, and when using normalization for CO2 content, the latter is therefore set to 1. The right column shows illustrative (but not limited) values ​​indicating adjustment values ​​corresponding to the volume percentages of synthesis gas and anode exhaust gas. [Table 1] It has been observed that the exhaust gas contains approximately 30% H2, which is similar to the example in Figure 2.

[0097] On the other hand, while the synthesis gas contained three times the amount of H2 as CO2, the anode exhaust gas contained only 0.6 times the amount of H2 compared to the CO2 content. Therefore, in this example, the fuel cell consumes approximately 80% of the H2, indicating that 20% (0.6:3) of the H2 is emitted from the fuel cell.

[0098] If 90% of the H2 in the anode exhaust gas is separated by the H2 separator 12, referred to as ePump below, the remaining percentage is 10%, which corresponds to 0.06 of the CO2 content. [Table 2]

[0099] Even after H2O is recovered in tank 16 via the H2O separation unit 15 and, normally, the condenser, the amount of CO2 remains in the dry anode exhaust gas in a normalized state. In the dry anode exhaust gas, H2 and CO remain in a ratio of 0.06:0.1 = 6:10. Although the total amount of this unreacted gas is small compared to the amount of initial synthesis gas flowing into the anode, it is not negligible. This is because this second fraction of 0.06 corresponds to 2% of the original H2 content of 3, and in addition to the efficiency improvement by the H2 separation unit and the reuse of H2 from the H2 separation unit, there is a possibility of a further 2% efficiency improvement.

[0100] In systems where carbon capture is necessary and high power efficiency is crucial, such a 2% increase is highly desirable. In particular, a 2% increase from the final step of reuse also means a corresponding reduction in fuel consumption.

[0101] The CO recycled to the reformer 7 may be oxidized to CO2 by a shift reaction, which is particularly important for CO-sensitive LT-PEM fuel cells but less important for HT-PEM fuel cells. The shift reaction of CO with water may contribute to the energy balance through the production of H2 by this reaction. After passing through the fuel cell, the CO2 can be recovered in the subsequent gas cycle by CO2 separators 16, 17, and 18.

[0102] Alternatively, the CO and remaining H2 are used in the burner of the reformer heater 6 to convert them into CO2 and H2O, which are then released into the atmosphere along with residual CO2 from the carbon capture process, which is not 100% efficient.

[0103] The less H2 separated from the anode exhaust gas, the greater the amount that can be supplied to the burner of the reformer heater 6. This means that adjusting the amount of H2 in the first fraction that is separated also adjusts the amount of H2 in the residual gas sent to the burner of the reformer heater 6, and thus affects the temperature of the reformer 7. In this respect, the temperature of the reformer 7 can be adjusted by adjusting the efficiency of the H2 separator 12 and the amount of H2 in the first fraction that is separated. Reducing the separation of H2 into the first fraction that is recirculated to the anode 10A reduces the power consumption of the H2 separator 12, which is beneficial when more of the H2 fraction in the gas remaining after passing through the H2 separator 12 is to be used by the burner.

[0104] While HT-PEM fuel cells are generally robust, high CO concentrations affect the catalyst in fuel cell 10, so a certain reduction in CO concentration is desirable. Similar considerations apply to the H2 separator 12, as an increase in CO concentration within the H2 separator 12 stack negatively impacts the catalyst.

[0105] In a combination of carbon capture and hydrogen separation, the two processes influence each other. If the CO2 capture rate is low, the CO2 level in the circuit rises, and the relative level of H2 in the anode exhaust gas decreases, making it difficult to separate a certain amount of H2. This increases the H2 concentration in the anode exhaust gas downstream of the H2 separator 12, resulting in a decrease in the carbon capture rate. Avoiding this issue, an even higher carbon capture rate reduces the CO2 concentration in the circuit, improving the H2 separation efficiency. This reduces the amount of H2 in the residual gas downstream of the H2 separator, thus improving the carbon capture efficiency and consequently reducing the required power consumption. Therefore, up to a certain limit, increasing the carbon capture efficiency improves the overall system efficiency.

Claims

1. A method for operating a fuel cell system (1), wherein the fuel cell system (1) A fuel supply unit (2) that supplies fuel, A reforming apparatus (7) for catalytically reforming the fuel into a synthesis gas containing hydrogen H2, A fuel cell (10) includes an anode (10A), a cathode (10B), and a separation device membrane (10C) between them, wherein the anode inlet (9A) of the anode (10) is conduited by a synthesis gas conduit (8) to the reformed gas outlet (7B) of the reformer (7) which uses the H2 for power production, for receiving the H2 from the reformer (7) via the synthesis gas conduit (8), A reformer heater (6) for heating the reformer (7) for catalytic reforming the fuel into the synthesis gas, The H2 separator (12) has an upstream inlet connected to the anode outlet (9B) of the anode (10A) by the anode exhaust conduit (11) in order to receive anode exhaust gas from the anode (10A) by the electrochemical H2 separator (12), wherein the H2 separator (12) is configured to separate H2 from the anode exhaust gas and guide the separated H2 gas to an H2 conduit (13) which is fluidly connected to the anode inlet (9A) in order to recirculate it to the anode (10A); The method described above is The reforming device (8) receives the fuel from the fuel supply unit (2), reforms the fuel into synthesis gas with a catalyst, and supplies the synthesis gas to the anode (10A); The fuel cell (10) produces electricity, and the electricity production generates anode exhaust gas, the exhaust gas containing hydrogen gas (H2), water vapor (H2O), carbon dioxide (CO2), and carbon monoxide (CO); The H2 separation device (12) receives the anode exhaust gas, and the H2 separation device (12) separates the first fraction of H2 into the H2 conduit (13); The method is characterized by including limiting the first fraction to less than 90% of the total amount of H2 in the anode exhaust gas.

2. The method according to claim 1, wherein the method includes limiting the first fraction of H2 to 84% or less of the total amount of H2 in the anode exhaust gas.

3. The method according to claim 1 or 2, wherein the H2 separation device (12) includes at least one electrochemical cell (12a) for the H2 separation, the electrochemical cell (12A) receives power for the H2 separation, and the method includes adjusting the voltage of the electrochemical cell (12A) to a predetermined maximum voltage level or less.

4. The method according to claim 3, wherein the method includes pre-determining the maximum voltage level in the range of 50 mV to 100 mV.

5. The method according to any one of the prior claims, wherein the method includes, during the power generation operation of the fuel cell system (1), changing the voltage of the electrochemical cell (12A) according to the generation rate of the anode exhaust gas, and adjusting the voltage so as to separate 90% or less, optionally 84% or less, of the H2 in the anode exhaust gas.

6. The method according to any of the prior claims, wherein the reformer heater (6) includes a burner for heating the reformer by burning H2 from a second fraction of H2, the second fraction of H2 corresponds to the H2 remaining in the anode exhaust gas after the removal of the first fraction of H2 by the H2 separator, and the method includes adjusting the efficiency of the reformer heater (6) by changing the second fraction of H2 to adjust the separation efficiency of the H2 separator.

7. The method according to any one of the prior claims, wherein the method comprises repeatedly adjusting the first fraction during the power production operation of the fuel cell system (1), measuring the increase or decrease in output power of the fuel cell system (1) caused by the adjustment of the first fraction, and repeatedly increasing and decreasing the first fraction to maximize the output power of the fuel cell system (1).

8. The method according to any of the prior claims, wherein the method comprises comparing a change in power consumption by the H2 separator (12) with a change in power output by the fuel cell system (1) caused by the change in power consumption by the H2 separator (12), and increasing the power consumption by the H2 separator (12) if the immediate preceding increase in the power consumption by the H2 separator (12) results in an increase in power output by the fuel cell system (1), or if the immediate preceding decrease in the power consumption by the H2 separator (12) results in a decrease in power output by the fuel cell system (1).

9. The method according to any of the prior claims, wherein the method comprises separating H2O from the remaining anode exhaust gas after separating H2 from a first fraction of the anode exhaust gas by the H2 separation device, and introducing at least a portion of the residual dry anode exhaust gas after moisture separation to the reforming device (7) or the reforming device heater (6), wherein the residual dry exhaust gas contains H2 from a second fraction, and the reforming device heater (6) includes a burner that burns the H2 derived from the second fraction of H2 to heat the reforming device.

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

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

12. A fuel cell system (1) for the method according to any of the prior claims, A fuel supply unit (2) that supplies fuel, A reforming apparatus (7) for catalytically reforming the fuel into a synthesis gas containing hydrogen H2, A fuel cell (10) includes an anode (10A), a cathode (10B), and a separation device membrane (10C) between them, wherein the anode inlet (9A) of the anode (10) is conduited to the reformed gas outlet (7B) of the reformer (7) by a synthesis gas conduit (8) to receive the H2 from the reformer via the synthesis gas conduit (8) and to use the H2 for power production. A reformer heater (6) for heating the reformer (7) for catalytic reforming the fuel into the synthesis gas, The system includes an electrochemical H2 separator (12) having an upstream inlet connected by the anode exhaust conduit (11) to the anode outlet (9B) of the anode (10A) in order to receive anode exhaust gas from the anode (10A) via the anode exhaust conduit (11), wherein the H2 separator is configured to separate H2 from the anode exhaust gas into an H2 conduit (13) fluidly connected to the anode inlet (9A) in order to recirculate the separated H2 gas back to the anode (10A); A water separator (15, 16) and an H2O separator (12) have the H2 separator (12) receiving the residual anode exhaust gas after the removal of the first fraction of H2, and the H2O separator (12) having the H2O separator (15, 16) connected downstream of the H2 separator (12) via a conduit to remove H2O from the residual anode exhaust gas; The CO2 separation device (17, 18, 19) has an upstream side of the CO2 separation device (17, 18, 19) connected downstream of the H2O separation device (15, 16) for receiving dry residual anode exhaust gas and separating CO2 therefrom; In this case, downstream of the CO2 separation devices (17, 18, 19), a conduit (20) is provided for supplying at least a portion of the second fraction of H2 from the anode exhaust gas to the reforming device (7) or the reforming device heater (6), after the first fraction of H2, H2O, and CO2 have been removed by the respective separation devices (12, 15-16, 17, 18, 19); The system is characterized in that it is configured to limit the first fraction to 90% or less of the total H2 content in the anode exhaust gas.

13. The system according to claim 12, wherein the system (1) includes a computer system (21) configured to control the operation of the fuel cell system (1), including the control of the power supplied to the H2 separator (12), and to measure and control the output power from the fuel cell system (1), wherein the computer system (21) is programmed to repeatedly adjust the first fraction during the operation of the fuel cell system (1), measure the increase or decrease in output power from the fuel cell system (1) resulting from the adjustment, and repeatedly increase and decrease the first fraction in order to maximize the output power.

14. The system according to claim 13, wherein the computer system (21) is programmed to compare the change in power consumption by the H2 separator (12) with the change in power production by the fuel cell system (1) caused by the change in power consumption by the H2 separator (12), and is configured solely to increase the power consumption by the H2 separator (12) when an immediate increase in the power consumption by the H2 separator (12) results in an increase in power production by the fuel cell system (1), or when an immediate decrease in the power consumption by the H2 separator (12) results in a decrease in power production by the fuel cell system (1).

15. The system according to any one of claims 12 to 14, wherein the system (1) includes a computer system (21) configured to control the operation of the fuel cell system (1), including the control of the power supplied to the H2 separator (12), the computer system (21) is programmed to change the voltage of the electrochemical cell (12A) during the power production operation of the fuel cell system (1) in accordance with the generation rate of the anode exhaust gas, and to adjust the voltage to separate 90% or less, optionally 84% or less, of the H2 in the anode exhaust gas.

16. The system according to any one of claims 12 to 15, wherein the fuel (2) comprises methanol to be mixed with water, the system (1) comprises an evaporator (4) that receives and evaporates the mixture of the fuel and water for reforming in the reformer (7), and the fuel cell (10) is an HT-PEM fuel cell (2) configured to operate in the range of 120°C to 200°C.

17. Use of the method according to any one of claims 1 to 11 or the system according to any one of claims 12 to 16 for power generation in an electric vessel.

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