Patent Application
20070031718
The present application claims priority from Japanese application serial no. 2005-228999, filed on Aug. 8, 2005, the content of which is hereby incorporated by reference into this application.
The present invention relates to fuel cell power generation systems using hydrogen gas as a fuel.
Fuel cells realize energy saving, clean exhaust gas, and high energy efficiency and thereby have been received attention as possible candidates to solve environmental issues typified by air pollution caused by exhaust gases from, for examples, automobiles, and global warming caused by carbon dioxide.
Fuel cell power generation systems are energy conversion systems of feeding hydrogen gas (fuel gas) and air (oxidizing gas) to a fuel electrode (anode) and an air electrode (cathode) and of causing an electrochemical reaction so as to convert chemical energy to electrical energy. The electrochemical reaction does not yield carbon dioxide (CO2) and exhaust gas containing detrimental substances but water alone.
The hydrogen gas, however, should be improved in its storage, transportation, and cost, in order to be widely used as the fuel. The hydrogen gas has been conventionally generally prepared by subjecting a hydrocarbon to steam reforming reaction. Thus, fuel cell power generation systems for domestic stationary use include a package of a reformer and a fuel cell, and use kerosene or town gas as the fuel.
Hydrogen separation technologies are being adopted so as to improve the generation efficiency of fuel cell power generation systems using such fossil fuels as raw material fuels.
For example, hydrocarbons such as town gas and liquefied petroleum gas (LPG) are used for power generation systems for domestic stationary use. In these systems, a hydrogen separation membrane is arranged in a reforming catalyst unit so as to separate hydrogen gas to thereby increase the hydrogen concentration and improve the hydrogen generation. The hydrogen separation membrane herein also acts to remove carbon monoxide to thereby improve the power generation efficiency, because carbon monoxide adversely affects the performances of the fuel cell (see, for example, Japanese Unexamined Patent Application Publication (JP-A) No. Hei 07-57758).
Proton-exchange membrane fuel cell (PEFC or PEM) power generation systems using pure hydrogen as a fuel are mainly intended to be mounted in vehicles. In these systems, a hydrogen separation membrane is arranged at the outlet of the cell to remove impurity gas and water generated in the cell node system. Unreacted hydrogen separated from the fuel electrode (anode) exhaust gas is returned to the inlet of the fuel electrode. Thereby hydrogen gas is effectively uses and the fuel economy (power generation efficiency) is improved (see, for example, Japanese Unexamined Patent Application Publication (JP-A) No. 2005-108698).
The technique disclosed in JP-A No. Hei 07-57758, however, fails to consider to effectively use unreacted hydrogen contained in the fuel electrode exhaust gas (anode exhaust gas), in contrast to the technique disclosed in JP-A No. 2005-108698, although the former technique may improve the power generation efficiency due to improved hydrogen concentration in the fuel gas.
In contrast, the technique of removing impurities from the fuel electrode exhaust gas discharged from the cell outlet and of recirculating a hydrogen-enriched gas to the cell inlet, as disclosed in JP-A No. 2005-108698, fails to consider to improve the hydrogen concentration in and to remove impurities from the fuel gas used upstream of the cell, in contrast to the technique disclosed in JP-A No. Hei 07-57758. The technique lacks the consideration to eliminate the effects of the composition of the fuel gas, such as an unsuitable composition or a composition containing an undesirable gas for the cell, at any time on startup and during operations.
A possible solution to solve the above-mentioned problems is the combination of these conventional techniques. Specifically, the resulting fuel cell power generation system includes two hydrogen separators in the fuel gas feed system upstream of the cell and in the fuel electrode exhaust gas discharge system downstream of the cell, respectively. This technique, however, causes new problems. For example, the technique requires two hydrogen separators, and this causes an increased cost. In addition, of such separators, those using separation membranes or fine porous articles generally require the control of the gas pressure, such as pressurization, and this causes a complicated process of controlling the pressure.
An object of the present invention is to provide a fuel cell power generation system that can always remove detrimental substances and maintain a high hydrogen concentration in response to change in fuel gas composition and to contamination of impurities upstream of the fuel cell, can simultaneously recover unreacted hydrogen in the fuel cell outlet and recirculate the recovered hydrogen to the cell inlet, has a simple configuration, and can be easily operated.
A fuel cell power generation system of the present invention comprises a fuel cell being so configured as to feed a supply gas containing hydrogen gas to a fuel electrode, oxidize the supply gas, and discharge the residual gas as an exhaust gas. The supply gas comprises a mixed gas of a fuel gas containing hydrogen gas and all or part of the exhaust gas. The system further comprises a hydrogen gas separator having the function of separating hydrogen gas from the other gas, and is so configured as to feed the supply gas to the fuel electrode through the hydrogen gas separator.
The present invention further provides a method of operating a fuel cell, comprising the steps of:
mixing a fuel gas conveyed from a fuel feed unit with an exhaust gas discharged from a fuel electrode of a fuel cell;
pressurizing the resulting mixed gas and feeding the pressurized mixed gas to a unit for separating hydrogen gas so as to separate hydrogen gas; and
feeding the separated hydrogen gas to the fuel electrode of the fuel cell.
In addition and advantageously, the present invention provides a method for operating a fuel cell power generation system. The fuel cell power generation system comprises a fuel feed unit, a hydrogen gas separator, and a fuel cell comprising a fuel electrode, wherein an exhaust gas from the fuel cell has an impurity gas concentration higher than that of a fuel gas fed from the fuel gas feed unit. The method comprises the steps of:
mixing the exhaust gas with the fuel gas so as to allow the resulting mixed gas to have an impurity gas concentration lower than that of the exhaust gas;
conveying the mixed gas to the hydrogen gas separator to thereby separate hydrogen gas; and
feeding the separated hydrogen gas to the fuel electrode of the fuel cell.
According to a power generation system of the present invention, impurity gases detrimental to the cell, such as impurity gas contained in the fuel gas upstream from the fuel cell and/or impurity gas contained in the recirculated gas, can be eliminated effectively, and a high-concentration hydrogen gas can be fed to the cell at anytime including startup of operation and during operations. Additionally, an operation having a high utilization rate of the fuel can be continuously carried out. The power generation system can have a high generation efficiency and can be simplified in the configuration.
A fuel cell power generation system according to an embodiment of the present invention is a solid polymer electrolyte fuel cell comprising a multilayer structure of single cells, each of single cells comprises a fuel electrode (anode) for oxidizing hydrogen gas; an air electrode (cathode) for reducing oxygen gas; and a solid polymer electrolyte membrane arranged between the fuel electrode and the air electrode.
The fuel cell system according to this embodiment has a fuel cell comprising a fuel inlet and an exhaust outlet. The fuel inlet serves to feed a gas containing hydrogen gas as a feed gas to the fuel electrode. The exhaust outlet serves to discharge the supply gas passed through the fuel electrode as an exhaust gas.
The supply gas is a mixed gas of the exhaust gas and a fuel gas comprising a hydrogen-enriched gas supplied from, for example, a hydrogen cylinder or a reformer.
The mixed gas comprises hydrogen gas and impurity gas other than hydrogen gas, and is thereby passed through a hydrogen gas separator having the function of separating hydrogen gas from another gas, and is fed to the fuel electrode as the supply gas.
The hydrogen gas separator comprises a hydrogen separation membrane comprising a ceramic having micropores on the order of nanometers, and is so configured as to pass a gas through the hydrogen separation membrane to thereby separate hydrogen gas from impurity gas. Accordingly, if the supply gas to be treated contains the impurity gas in a high concentration, the hydrogen gas separator does not effectively separate hydrogen gas from the impurity gas. Hydrogen gas in the fuel gas is consumed after passing through the fuel electrode, and the resulting exhaust gas has a relatively high concentration of the impurity gas and a relatively low concentration of the hydrogen gas. Therefore, the exhaust gas is mixed with the fuel gas having a high hydrogen gas concentration and having a low impurity gas concentration, and the resulting mixed gas is passed through the hydrogen gas separator according to the present invention. This effectively realizes efficient separation of hydrogen gas from the impurity gas.
The pressure of the mixed gas is set depending on the pressure of the exhaust gas and the pressure of the fuel gas. In this connection, a pressure sensor is preferably arranged upstream from the hydrogen gas separator. The pressure sensor detects the pressure of the mixed gas passing through the hydrogen gas separator, and the pressure of the mixed gas is then adjusted within a suitable range for the hydrogen gas separator so as to efficiently separate hydrogen gas from another gas. By satisfying this, hydrogen can be easily and efficiently separated from the impurity gas, and a feed gas enriched in hydrogen gas can be fed to the fuel electrode.
The present invention will be illustrated in further detail with reference to several specific embodiments of the fuel cell power generation systems according to the present invention and with reference to the attached drawings.
The operation of the fuel cell power generation system illustrated in
The air, from which dust has been removed typically by an air filter (not shown), is compressed by an air feeder 7 and is conveyed to a humidifier 8 via an air-piping 10. The air humidified by the humidifier 8 is fed to an air electrode (cathode) inlet of the fuel cell 2. The air electrode inlet of the fuel cell 2 is provided with an air-pressure regulator 20, and the pressure of the air electrode is regulated by the degree of opening of the pressure regulator 20 and by the driving force of the air feeder 7. The air-pressure regulator 20 discharges the exhaust air to outside of the system via an air exhaust pipe 11.
The fuel gas containing hydrogen gas fed from the hydrogen reservoir 1 is regulated to a predetermined operation pressure by an on-off valve 21 and a pressure regulator 22, is controlled to a predetermined flow rate by a flow-rate controller 30, and is fed to the mixer 3 via a fuel feed piping 15.
The exhaust gas discharged from the fuel electrode outlet of the fuel cell 2 is conveyed to a water separator 6 via an exhaust gas circulation piping 13. Entrained water is separated from the exhaust gas, and is discharged to the outside of the system via a drain exhaust tube 18. Incidentally the entrained water in the exhaust gas is derived from humidified air and water produced as a result of a cell reaction, and it undergoes condensation in the inside pathway of the fuel cell 2. The exhaust gas discharged from the water separator 6 is fed to the mixer 3 via the exhaust gas circulation piping 13. The exhaust gas is mixed with the fuel gas in the mixer 3. The mixed gas is conveyed to the hydrogen gas separator 4 by the action of the circulation blower 5 arranged in a mixed gas piping 14. The hydrogen gas separator 4 comprises, for example, a porous membrane having micropores capable of allowing hydrogen to pass through. The mixed gas is separated into hydrogen gas and a secondary gas other than hydrogen gas. The system comprises a pressure sensor 31 arranged upstream from the hydrogen gas separator 4 for optimizing the hydrogen separation performance. Depending on the sensed pressure, the pressure regulator 22 and the driving force of the circulation blower are controlled so that the mixed gas is in the optimum pressure for the hydrogen gas separator 4. Unnecessary gas components such as inert gas and gas components detrimental to the fuel cell have been removed from the mixed gas as a result of separation, and the separated hydrogen gas has a high hydrogen concentration of substantially 100%. The hydrogen gas is fed to the fuel cell 2, in which hydrogen is consumed as a result of power generation in the fuel cell 2, and is discharged as an exhaust gas from the fuel electrode outlet of the fuel cell 2. The exhaust gas contains unreacted hydrogen gas, water, inert gas, and other impurity gases, as described above.
The secondary gas containing inert gas and other unnecessary or detrimental components, which is separated in the hydrogen gas separator, is discharged to outside the system continuously or intermittently by an on-off valve 23 via an exhaust pipe 12, according to the flow rate of the gas passing through the hydrogen gas separator 4 and the pressure of the gas sensed by the pressure sensor 31.
A residual gas in a purge piping 16 is discharged to outside by opening and closing a purge valve 25 to control the circulation flow rate and pressure of the mixed gas piping on operation startup and on operation stop. The discharged residual gas is diluted with the air and discharged from the system (not shown).
The system further comprises a bypass piping 17 and a bypass valve 24. The bypass piping 17 is used for bypassing the hydrogen gas separator 4 and serves to feed another gas than hydrogen gas, such as nitrogen gas, to the fuel cell by opening the bypass valve 24. This configuration is intended, for example, to feed another gas than hydrogen gas, such as nitrogen gas, to the fuel electrode during a stop of operation.
The system according to the first embodiment can prevent the cell from deteriorating, because the impurity gas and detrimental components contained in the exhaust gas from the fuel electrode outlet and in the fuel gas in the hydrogen reservoir 1, respectively, can be removed from upon startup of operation. Furthermore, as the fuel gas at the inlet of the fuel cell has a high hydrogen gas concentration, the hydrogen gas partial pressure in the fuel electrode is increased, and thereby the cell voltage is improved. In addition, unreacted hydrogen gas in the exhaust gas from the fuel electrode outlet is always circulated, and fresh hydrogen gas has only to be fed to the cell in an amount corresponding to the hydrogen consumed as a result of the cell reaction. Thus, substantially 100% of the fuel (hydrogen gas) can be utilized, and the generation efficiency is improved.
The system can have a simplified configuration, because only one hydrogen gas separator 4 is used to separate hydrogen from two different gases, i.e., the exhaust gas and the fuel gas. In addition, the system is capable of increasing the gas pressure so as to achieve the hydrogen gas separation and of circulating the exhaust gas by only one circulation blower 5, because the fuel gas and the exhaust gas are mixed, and the resulting mixed gas is conveyed to the blower 5 which serves to perform the above exhaust gas circulation and pressurization for the hydrogen gas separation. This also contributes to the simplification of the system.
High-purity hydrogen gas as the fuel gas from the hydrogen reservoir 1E is mixed with the exhaust gas, and the mixed gas is conveyed to the hydrogen gas separator 4. Therefore, even if a gas detrimental or inhibitory to the cell performance, such as nitrogen, water, or hydrocarbon, finds its way into the exhaust gas, the detrimental (inhibitory) gas contained in the exhaust gas is diluted with high-purity hydrogen gas into a lower concentration. The hydrogen gas after separation thereby has a concentration of such detrimental gas much lower than that in a conventional system having a hydrogen gas separator at the outlet of a cell. Thus, the cell is prevented from deteriorating in performance and can have a longer lifetime.
Furthermore, even if the ratio of another gas than hydrogen gas in the fuel gas from the hydrogen reservoir 1 increases or even if a detrimental gas to the fuel cell finds its way into the fuel gas, such other gas and detrimental gas can be eliminated in the hydrogen gas separator 4 before fed to the fuel cell 2. Thus, hydrogen gas containing less impurities can be fed to the fuel cell at any time including startup of operation, and the fuel cell can have high reliability.
The secondary gas other than hydrogen gas separated in the hydrogen gas separator 4 contains unreacted hydrocarbons that have not been converted into hydrogen by the action of the reformer 40. The secondary gas, from which water is removed in a water separator 29, is conveyed to the burner unit of the reformer 40 via a secondary gas return piping 57, is burnt by the action of the air fed via a combustion air feed pipe 54, and the heat of combustion 55 is used as part of heat source for the reforming reaction in the reforming reaction unit 41. The system may further comprise a combustion raw fuel feed piping 56 so as to use part of the raw fuel for combustion when the heat is insufficient. A residual gas is discharged from a purge piping 16 while a purge valve 25 is opened and closed so as to control the circulation flow rate and pressure of the mixed gas piping on operation startup and on operation stop. The discharged residual gas is diluted with the air and discharged from the system (not shown) or is introduced into the burner unit 44 and is discharged as burnt gas from the system, as illustrated in
The system according to the second embodiment can effectively use the heat of combustion of the residual hydrocarbons and carbon monoxide contained in the secondary gas and contributes to the improvement in generation efficiency.
The system according to the third embodiment (
The hydrogen gas passed through the hydrogen separation membrane at the outlet of the hydrogen gas separator contains carbon monoxide, because the reformed gas contains carbon monoxide in a high concentration, while the concentration of carbon monoxide in the hydrogen gas varies depending on the separation performance of the hydrogen gas separator 4. Carbon monoxide should be preferably removed as far as possible, because it adversely affects the fuel cell as discussed above. The system according to the third embodiment comprises the heat exchanger 45 arranged downstream from the hydrogen gas separator 4 and the CO-selective oxidizer 43 arranged downstream from the heat exchanger 45. The system is so configured as to control the temperature of the CO-selective oxidizer 43 by flowing a cooling medium 101 and to oxidize carbon monoxide into carbon dioxide by the action of the air fed from the air feed pipe 53 to the CO-selective oxidizer 43. The secondary gas separated from hydrogen gas in the hydrogen gas separator 4, from which water is removed in a water separator 29, is conveyed to the burner unit 44 of the reformer 40 via a second gas return piping 57, is burnt by the action of the air fed through a combustion air feed pipe 54, and the heat of combustion 55 is used as part of heat source for the reforming reaction in the reforming reaction unit 41, as in the system according to the second embodiment (
The system according to the third embodiment eliminates the need of a CO-shift converter necessary in conventional equivalents and eliminates the need of operation control for elevating and maintaining the temperature of the shift converter catalyst. This results in shorter time for system startup and reduced energy consumption upon startup. Thus, the generation efficiency and the reliability of the system in operation control are improved.
The system according to the fourth embodiment (
In comparison with the system according to the third embodiment (
According to the sixth embodiment, the separation membrane 106 is arranged diagonally, and the guide plate 107 is arranged so as to constitute the passage 108. This configuration increases the contact time and contact area between the separation membrane 106 and the mixed gas 103 and contributes to efficient separation of hydrogen, because the residual gas 105 can be smoothly discharged by the action of the guide plate 107.
The present invention can be applied to various fuels for use in domestic cogeneration fuel cells, and vehicle-mounted and other movable fuel cell power generation systems. Some embodiments according to the present invention can be applied to gas separators.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-228999 | Aug 2005 | JP | national |
