ABSTRACT
Abstract
A high temperature electrolyzer assembly comprising at least one electrolyzer fuel cell including an anode and a cathode separated by an electrolyte matrix, and a power supply for applying a reverse voltage to the at least one electrolyzer fuel cell, wherein a gas feed comprising steam and one or more of CO2 and hydrocarbon fuel is fed to the anode of the at least one electrolyzer fuel cell, and wherein, when the power supply applies the reverse voltage to the at least one electrolyzer fuel cell, hydrogen-containing gas is generated by an electrolysis reaction in the anode of the at least one electrolyzer fuel cell and carbon dioxide is separated from the hydrogen-containing gas so that the at least one electrolyzer fuel cell outputs the hydrogen-containing gas and separately outputs an oxidant gas comprising carbon dioxide and oxygen.
Description
BACKGROUND OF THE INVENTION
This invention relates to production of hydrogen from fuel, such as natural gas, methane, ADG digester gas and others, and in particular, to using a fuel reformer-electrolyzer-purifier assembly for hydrogen production and capable of being integrated with a fuel cell system and other systems. This invention further relates to various applications of the fuel reformer-electrolyzer-purifier assembly and systems incorporating the same.
Hydrocarbon fuels, such as methane, propane, natural gas, coal gas, etc. are widely used in energy consumption devices as well as for production of energy. Many devices and systems utilizing hydrocarbon fuel, including fuel cells, require fuel to be reformed to produce hydrogen (H2). For example, fuel cell cars require high purity hydrogen as fuel for operation. Currently, low temperature electrolysis and steam methane reforming are used for hydrogen production from hydrocarbon fuels. In low temperature electrolysis, an electrolyzer generates hydrogen from water. This process is highly inefficient due to the high power consumption required by low temperature electrolysis.
Conventional technologies for production of hydrogen from natural gas and other fuels also suffer from lower efficiency and excess CO 2 production due to incomplete conversion of methane and CO to hydrogen and from other disadvantages. For example, conventional hydrogen production and separation systems which use a steam methane reformer (SMR) coupled to a pressure swing adsorption (PSA) device suffer from the disadvantage of not converting all of the methane to hydrogen, and thus a substantial amount of feed energy is converted to heat. This generation of heat makes it impractical for the system to use waste heat from other sources to improve efficiency and also increases CO 2 emissions. These conventional systems also suffer from efficiency losses and cost increases when scaled down from today's typical 500,000 kilograms per day systems and typically produce a significant amount of NOx in addition to the high CO2 emissions. This can make obtaining permission to install and operate these conventional systems difficult, particularly in nonindustrial areas. For renewable feeds, such systems operate even less efficiently due to the dilution of the feed with CO 2 and required compression of the feed stream.
SUMMARY OF THE INVENTION
The objective of the present invention is to provide a low cost system for producing hydrogen with low greenhouse emissions.
The present invention reforms and purifies hydrogen from natural gas and other fuels in one step. Electricity used to electrochemically purify the hydrogen at high temperature produces additional hydrogen from steam electrolysis at the same time. Moreover, waste heat is utilized to drive the endothermic reforming reaction, eliminating emissions which would otherwise be produced by burning fuel. The system of the present invention incorporates a high temperature electrochemical purification system to remove CO 2 from the reformed gas during the reforming process and to drive the conversion of methane to H 2 and CO 2 to completion, producing hydrogen from fuel in a manner which approaches the theoretical minimum of CO 2 emissions.
The single step system of the present invention simplifies operations and results in a low cost system. In addition, the system of the present invention can generate hydrogen for both central and distributed production and has other possible uses, such as enabling CO 2 capture and energy storage.
Moreover, the present invention generates hydrogen from reforming fuel, such as natural gas, and high temperature electrolysis, lowering the marginal production cost of hydrogen. As a result, the total cost of hydrogen is economically attractive.
The present invention is directed to a high temperature electrolyzer assembly comprising: at least one electrolyzer fuel cell including an anode and a cathode separated by an electrolyte matrix, and a power supply for applying a reverse voltage to the at least one electrolyzer fuel cell, wherein, when the power supply applies the reverse voltage to the at least one electrolyzer fuel cell, hydrogen-containing gas is generated by an electrolysis reaction in the anode of at least one electrolyzer fuel cell and carbon dioxide is separated from the hydrogen-containing gas so that the at least one electrolyzer fuel cell outputs the hydrogen-containing gas and separately outputs an oxidant gas comprising carbon dioxide. The hydrogen-containing gas output from the at least one electrolyzer fuel cell comprises 95% or greater hydrogen, and the oxidant gas comprises a mixture of carbon dioxide and oxygen. In certain embodiments, the high temperature electrolyzer assembly includes a plurality of electrolyzer fuel cells connected in series and formed into a fuel cell stack. In some embodiments, each electrolyzer fuel cell is a molten carbonate fuel cell. In certain embodiments, the assembly further comprises one or more reformers for reforming hydrocarbon fuel and outputting reformed or partially reformed fuel to the at least one electrolyzer fuel cell. In such cases, the at least one electrolyzer fuel cell is further adapted to react methane with water to produce hydrogen and carbon dioxide, and shift carbon monoxide with water to produce hydrogen. Particularly, the one or more reformers may comprise one or more internally reforming fuel cells including reforming catalyst, and in such embodiments, the high temperature electrolyzer assembly comprises a plurality of electrolyzer fuel cells, and the one or more reforming fuel cells and the plurality of electrolyzer fuel cells are formed into a fuel cell stack.
The high temperature electrolyzer assembly of the present invention may further include a controller for controlling the power supply to apply a predetermined amount of the reverse voltage to the at least one electrolyzer fuel cell. The predetermined amount of the reverse voltage is greater than 1.0 volt. Moreover, the high temperature electrolyzer assembly may be configured to operate in one of a hydrogen producing mode and a power producing mode, and the controller controls the power supply to apply the reverse voltage to the at least one electrolyzer fuel cell when the high temperature electrolyzer assembly operates in the hydrogen producing mode so that the at least one electrolyzer fuel cell generates the hydrogen-containing gas and controls the power supply not to apply the reverse voltage to the at least one electrolyzer fuel cell when the high temperature electrolyzer assembly operates in the power producing mode so that the at least one electrolyzer fuel cell generates power from fuel.
Various systems utilizing the high temperature electrolyzer assembly are also described. The systems described below include, but are not limited to, a reformer-electrolyzer-purifier system that produces hydrogen-containing gas, a power production and hydrogen generation system that incorporates the high temperature electrolyzer assembly and a high temperature fuel cell system, a reforming system that generates carbon dioxide gas for capture, a system for generating electrical power including a low temperature fuel cell and the high temperature electrolyzer assembly, an energy storage system for storing excess power as hydrogen, a gas conversion system for converting one gas to another gas with lower CO2 content, a carbon dioxide capturing system for generating high purity carbon dioxide using the high temperature electrolyzer and a coal powered assembly, and a combined gasifier and hydrogen generation system. Various methods that generate hydrogen-containing gas and separate CO2 for capture are also described.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other features and aspects of the present invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings in which:
FIG. 1 shows a schematic view of the reformer-electrolyzer-purifier (REP) system including a REP assembly of the present invention;
FIG. 2 shows a more detailed view of the reformer-electrolyzer-purifier system;
FIGS. 3 A and 3 B show the reformer-electrolyzer-purifier system integrated with a DFCR fuel cell system;
FIG. 4 shows reactions occurring in the reformer-electrolyzer-purifier (REP) assembly; and
FIGS. 5 A and 5 B show the results of testing the reformer-electrolyzer-purifier system of FIGS. 2 - 3 ;
FIG. 6 shows an embodiment of a hydrogen production system that utilizes molten carbonate fuel cell based reformer-electrolyzer-purifier assembly and is followed by electrochemical hydrogen compression to produce high pressure, pure hydrogen;
FIG. 7 shows an illustrative configuration of the CO2 capturing system that combines a reformer with the REP assembly of the present invention;
FIGS. 8 A- 8 F show illustrative configurations of hydrogen generation systems, each of which includes the REP assembly that generates hydrogen for use in one or more PEM power generation systems;
FIGS. 9 A and 9 B show illustrative configurations of such energy storage systems that incorporate the REP assembly of the present invention;
FIG. 9 C shows a schematic configuration of the REP assembly and the reactions that occur therein;
FIGS. 10 A and 10 B show illustrative ADG conversion systems that use the REP assembly and a methanation assembly for converting anaerobic digester gas (ADG) to natural gas;
FIGS. 11 A- 11 C show illustrative configurations of CO2 capture systems that use the REP assembly of the present invention for electrochemically reacting flue gas output from another fuel powered device;
FIG. 12 shows an illustrative configuration of a combined gasifier and REP system that uses the REP assembly of the present invention to purify hydrogen gas produced from a gasifier and/or provide oxygen to the gasifier while producing an off gas suitable for CO2 capture.
DETAILED DESCRIPTION
The present invention is directed to a high temperature electrolyzer assembly, also referred to throughout the specification as a reformer-electrolyzer-purifier (REP) assembly, and various systems including the REP assembly. As described below, the REP assembly includes at least one electrolyzer fuel cell and may include a plurality of electrolyzer fuel cells formed in a fuel cell stack, also referred to as a REP stack. The at least one electrolyzer fuel cell is operated in reverse so as to electrolyze CO2 and water to produce hydrogen, and to purify the hydrogen by removing the CO 3 = . The CO2 may be provided by a hydrocarbon, such as methane, and removing the CO 3 = drives the reforming reaction to completion. Other reactions may occur in the at least one electrolyzer fuel cell, as described below and shown in the accompanying Figures.
The REP stack preferably comprises a molten carbonate fuel cell stack and the REP assembly includes a power supply for supplying power to the REP stack for driving the electrolysis reactions to completion. A controller may be included in the REP assembly and/or in the REP system for controlling the power supply and for controlling other operations and parts of the REP assembly and/or REP system. Control operations are described in more detail below. Although the specification describes the REP assembly, the REP stack and the REP system as including reforming, such as internal or external reforming, it is also contemplated that the REP assembly, the REP stack and/or the REP system may omit internal and/or external reforming, and may be used for electrolyzing a supply gas containing CO2 and purifying hydrogen without reforming.
FIG. 1 shows a schematic view of the reformer-electrolyzer-purifier (REP) system 100 of the present invention. As shown in FIG. 1 , fuel, such as natural gas, ADG digester gas or other suitable fuel, is pre-heated using lower level waste heat in a pre-heater 102 and thereafter supplied to the REP system 100 . The fuel may be humidified or mixed with water before or after being pre-heated. In the REP system 100 , the fuel is reformed by reacting with steam to produce hydrogen, CO, and carbon dioxide, and hydrogen is purified at high temperature (reforming temperatures) to separate it from other reaction products and drive the reforming reaction to completion. The REP system 100 outputs hydrogen and separately outputs other reaction products, including oxygen, and carbon dioxide. As shown, high level waste heat is supplied to the REP system 100 to drive the endothermic reforming reaction so that all of the fuel is converted to hydrogen, thereby reducing CO 2 emissions resulting from incomplete conversion of methane to hydrogen.
FIG. 2 shows a more detailed view of the REP system 100 which comprises a REP assembly including a REP stack 200 and a power supply 230 . The REP stack 200 comprises fuel cell components and may include one or more reforming only cells, or reforming units, 202 and one or more REP fuel cells 204 , each of which comprises an anode 204 a and a cathode 204 b separated by an electrolyte matrix. The REP fuel cells are configured the same as conventional MCFC fuel cells but are operated in reverse by applying a reverse voltage of greater than 1.0 Volt, typically in the 1.15 to 1.5 Volt range. The reforming only units 202 and REP fuel cells 204 are assembled in a stack and are connected in series so that fuel is first conveyed through the reforming only cells 202 and thereafter through the anodes 204 a of the REP fuel cells 204 . The cathodes 204 b may receive hot gas, such as air, supplied to the system and a CO2/O2 gas mixture produced in purification operation from the anode 204 a of the REP fuel cell. In one illustrative embodiment, the fuel cell stack 200 of the REP system 100 incorporates components developed for commercial molten carbonate fuel cell technology, such as MCFC/DFC® developed by FuelCell Energy, Inc. However, it is understood that other types of molten carbonate fuel cells may be used in the REP system 100 .
As also shown in FIG. 2 , the REP system <b
BACKGROUND OF THE INVENTION
This invention relates to production of hydrogen from fuel, such as natural gas, methane, ADG digester gas and others, and in particular, to using a fuel reformer-electrolyzer-purifier assembly for hydrogen production and capable of being integrated with a fuel cell system and other systems. This invention further relates to various applications of the fuel reformer-electrolyzer-purifier assembly and systems incorporating the same.
Hydrocarbon fuels, such as methane, propane, natural gas, coal gas, etc. are widely used in energy consumption devices as well as for production of energy. Many devices and systems utilizing hydrocarbon fuel, including fuel cells, require fuel to be reformed to produce hydrogen (H2). For example, fuel cell cars require high purity hydrogen as fuel for operation. Currently, low temperature electrolysis and steam methane reforming are used for hydrogen production from hydrocarbon fuels. In low temperature electrolysis, an electrolyzer generates hydrogen from water. This process is highly inefficient due to the high power consumption required by low temperature electrolysis.
Conventional technologies for production of hydrogen from natural gas and other fuels also suffer from lower efficiency and excess CO 2 production due to incomplete conversion of methane and CO to hydrogen and from other disadvantages. For example, conventional hydrogen production and separation systems which use a steam methane reformer (SMR) coupled to a pressure swing adsorption (PSA) device suffer from the disadvantage of not converting all of the methane to hydrogen, and thus a substantial amount of feed energy is converted to heat. This generation of heat makes it impractical for the system to use waste heat from other sources to improve efficiency and also increases CO 2 emissions. These conventional systems also suffer from efficiency losses and cost increases when scaled down from today's typical 500,000 kilograms per day systems and typically produce a significant amount of NOx in addition to the high CO2 emissions. This can make obtaining permission to install and operate these conventional systems difficult, particularly in nonindustrial areas. For renewable feeds, such systems operate even less efficiently due to the dilution of the feed with CO 2 and required compression of the feed stream.
SUMMARY OF THE INVENTION
The objective of the present invention is to provide a low cost system for producing hydrogen with low greenhouse emissions.
The present invention reforms and purifies hydrogen from natural gas and other fuels in one step. Electricity used to electrochemically purify the hydrogen at high temperature produces additional hydrogen from steam electrolysis at the same time. Moreover, waste heat is utilized to drive the endothermic reforming reaction, eliminating emissions which would otherwise be produced by burning fuel. The system of the present invention incorporates a high temperature electrochemical purification system to remove CO 2 from the reformed gas during the reforming process and to drive the conversion of methane to H 2 and CO 2 to completion, producing hydrogen from fuel in a manner which approaches the theoretical minimum of CO 2 emissions.
The single step system of the present invention simplifies operations and results in a low cost system. In addition, the system of the present invention can generate hydrogen for both central and distributed production and has other possible uses, such as enabling CO 2 capture and energy storage.
Moreover, the present invention generates hydrogen from reforming fuel, such as natural gas, and high temperature electrolysis, lowering the marginal production cost of hydrogen. As a result, the total cost of hydrogen is economically attractive.
The present invention is directed to a high temperature electrolyzer assembly comprising: at least one electrolyzer fuel cell including an anode and a cathode separated by an electrolyte matrix, and a power supply for applying a reverse voltage to the at least one electrolyzer fuel cell, wherein, when the power supply applies the reverse voltage to the at least one electrolyzer fuel cell, hydrogen-containing gas is generated by an electrolysis reaction in the anode of at least one electrolyzer fuel cell and carbon dioxide is separated from the hydrogen-containing gas so that the at least one electrolyzer fuel cell outputs the hydrogen-containing gas and separately outputs an oxidant gas comprising carbon dioxide. The hydrogen-containing gas output from the at least one electrolyzer fuel cell comprises 95% or greater hydrogen, and the oxidant gas comprises a mixture of carbon dioxide and oxygen. In certain embodiments, the high temperature electrolyzer assembly includes a plurality of electrolyzer fuel cells connected in series and formed into a fuel cell stack. In some embodiments, each electrolyzer fuel cell is a molten carbonate fuel cell. In certain embodiments, the assembly further comprises one or more reformers for reforming hydrocarbon fuel and outputting reformed or partially reformed fuel to the at least one electrolyzer fuel cell. In such cases, the at least one electrolyzer fuel cell is further adapted to react methane with water to produce hydrogen and carbon dioxide, and shift carbon monoxide with water to produce hydrogen. Particularly, the one or more reformers may comprise one or more internally reforming fuel cells including reforming catalyst, and in such embodiments, the high temperature electrolyzer assembly comprises a plurality of electrolyzer fuel cells, and the one or more reforming fuel cells and the plurality of electrolyzer fuel cells are formed into a fuel cell stack.
The high temperature electrolyzer assembly of the present invention may further include a controller for controlling the power supply to apply a predetermined amount of the reverse voltage to the at least one electrolyzer fuel cell. The predetermined amount of the reverse voltage is greater than 1.0 volt. Moreover, the high temperature electrolyzer assembly may be configured to operate in one of a hydrogen producing mode and a power producing mode, and the controller controls the power supply to apply the reverse voltage to the at least one electrolyzer fuel cell when the high temperature electrolyzer assembly operates in the hydrogen producing mode so that the at least one electrolyzer fuel cell generates the hydrogen-containing gas and controls the power supply not to apply the reverse voltage to the at least one electrolyzer fuel cell when the high temperature electrolyzer assembly operates in the power producing mode so that the at least one electrolyzer fuel cell generates power from fuel.
Various systems utilizing the high temperature electrolyzer assembly are also described. The systems described below include, but are not limited to, a reformer-electrolyzer-purifier system that produces hydrogen-containing gas, a power production and hydrogen generation system that incorporates the high temperature electrolyzer assembly and a high temperature fuel cell system, a reforming system that generates carbon dioxide gas for capture, a system for generating electrical power including a low temperature fuel cell and the high temperature electrolyzer assembly, an energy storage system for storing excess power as hydrogen, a gas conversion system for converting one gas to another gas with lower CO2 content, a carbon dioxide capturing system for generating high purity carbon dioxide using the high temperature electrolyzer and a coal powered assembly, and a combined gasifier and hydrogen generation system. Various methods that generate hydrogen-containing gas and separate CO2 for capture are also described.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other features and aspects of the present invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings in which:
FIG. 1 shows a schematic view of the reformer-electrolyzer-purifier (REP) system including a REP assembly of the present invention;
FIG. 2 shows a more detailed view of the reformer-electrolyzer-purifier system;
FIGS. 3 A and 3 B show the reformer-electrolyzer-purifier system integrated with a DFCR fuel cell system;
FIG. 4 shows reactions occurring in the reformer-electrolyzer-purifier (REP) assembly; and
FIGS. 5 A and 5 B show the results of testing the reformer-electrolyzer-purifier system of FIGS. 2 - 3 ;
FIG. 6 shows an embodiment of a hydrogen production system that utilizes molten carbonate fuel cell based reformer-electrolyzer-purifier assembly and is followed by electrochemical hydrogen compression to produce high pressure, pure hydrogen;
FIG. 7 shows an illustrative configuration of the CO2 capturing system that combines a reformer with the REP assembly of the present invention;
FIGS. 8 A- 8 F show illustrative configurations of hydrogen generation systems, each of which includes the REP assembly that generates hydrogen for use in one or more PEM power generation systems;
FIGS. 9 A and 9 B show illustrative configurations of such energy storage systems that incorporate the REP assembly of the present invention;
FIG. 9 C shows a schematic configuration of the REP assembly and the reactions that occur therein;
FIGS. 10 A and 10 B show illustrative ADG conversion systems that use the REP assembly and a methanation assembly for converting anaerobic digester gas (ADG) to natural gas;
FIGS. 11 A- 11 C show illustrative configurations of CO2 capture systems that use the REP assembly of the present invention for electrochemically reacting flue gas output from another fuel powered device;
FIG. 12 shows an illustrative configuration of a combined gasifier and REP system that uses the REP assembly of the present invention to purify hydrogen gas produced from a gasifier and/or provide oxygen to the gasifier while producing an off gas suitable for CO2 capture.
DETAILED DESCRIPTION
The present invention is directed to a high temperature electrolyzer assembly, also referred to throughout the specification as a reformer-electrolyzer-purifier (REP) assembly, and various systems including the REP assembly. As described below, the REP assembly includes at least one electrolyzer fuel cell and may include a plurality of electrolyzer fuel cells formed in a fuel cell stack, also referred to as a REP stack. The at least one electrolyzer fuel cell is operated in reverse so as to electrolyze CO2 and water to produce hydrogen, and to purify the hydrogen by removing the CO 3 = . The CO2 may be provided by a hydrocarbon, such as methane, and removing the CO 3 = drives the reforming reaction to completion. Other reactions may occur in the at least one electrolyzer fuel cell, as described below and shown in the accompanying Figures.
The REP stack preferably comprises a molten carbonate fuel cell stack and the REP assembly includes a power supply for supplying power to the REP stack for driving the electrolysis reactions to completion. A controller may be included in the REP assembly and/or in the REP system for controlling the power supply and for controlling other operations and parts of the REP assembly and/or REP system. Control operations are described in more detail below. Although the specification describes the REP assembly, the REP stack and the REP system as including reforming, such as internal or external reforming, it is also contemplated that the REP assembly, the REP stack and/or the REP system may omit internal and/or external reforming, and may be used for electrolyzing a supply gas containing CO2 and purifying hydrogen without reforming.
FIG. 1 shows a schematic view of the reformer-electrolyzer-purifier (REP) system 100 of the present invention. As shown in FIG. 1 , fuel, such as natural gas, ADG digester gas or other suitable fuel, is pre-heated using lower level waste heat in a pre-heater 102 and thereafter supplied to the REP system 100 . The fuel may be humidified or mixed with water before or after being pre-heated. In the REP system 100 , the fuel is reformed by reacting with steam to produce hydrogen, CO, and carbon dioxide, and hydrogen is purified at high temperature (reforming temperatures) to separate it from other reaction products and drive the reforming reaction to completion. The REP system 100 outputs hydrogen and separately outputs other reaction products, including oxygen, and carbon dioxide. As shown, high level waste heat is supplied to the REP system 100 to drive the endothermic reforming reaction so that all of the fuel is converted to hydrogen, thereby reducing CO 2 emissions resulting from incomplete conversion of methane to hydrogen.
FIG. 2 shows a more detailed view of the REP system 100 which comprises a REP assembly including a REP stack 200 and a power supply 230 . The REP stack 200 comprises fuel cell components and may include one or more reforming only cells, or reforming units, 202 and one or more REP fuel cells 204 , each of which comprises an anode 204 a and a cathode 204 b separated by an electrolyte matrix. The REP fuel cells are configured the same as conventional MCFC fuel cells but are operated in reverse by applying a reverse voltage of greater than 1.0 Volt, typically in the 1.15 to 1.5 Volt range. The reforming only units 202 and REP fuel cells 204 are assembled in a stack and are connected in series so that fuel is first conveyed through the reforming only cells 202 and thereafter through the anodes 204 a of the REP fuel cells 204 . The cathodes 204 b may receive hot gas, such as air, supplied to the system and a CO2/O2 gas mixture produced in purification operation from the anode 204 a of the REP fuel cell. In one illustrative embodiment, the fuel cell stack 200 of the REP system 100 incorporates components developed for commercial molten carbonate fuel cell technology, such as MCFC/DFC® developed by FuelCell Energy, Inc. However, it is understood that other types of molten carbonate fuel cells may be used in the REP system 100 .
As also shown in FIG. 2 , the REP system 100 may include one or more pre-heaters which utilize waste heat from the cells 204 of the REP system and/or produced by other devices external to the REP system and/or integrated with the REP system. The pre-heater 102 uses waste heat from the fuel cells 204 and reforming only cells 202 to pre-heat fuel, which may be mixed with water or humidified, prior to supplying the fuel to the reforming only cells 202 . Other pre-heater(s) 104 may be used for pre-heating gas supplied to the system using waste heat from other devices such as a high temperature fuel cell being used to produce power. Moreover, as shown in FIG. 2 , an oxidizer 106 may be provided for increasing the heat to the REP system using supplemental fuel by oxidizing the supplemental fuel with air and generating hot oxidant gas which is then supplied to the REP fuel cell cathodes 204 b.
In the present invention, the REP fuel cell stack 200 is operated in purification mode, or a hydrogen producing mode, as a purifying reforming electrolyzer and during such operation, removes almost all of the carbon from the system as CO 2 and produces nearly pure hydrogen from the reformed methane. In addition, the REP fuel cell stack 200 also efficiently produces additional hydrogen by dissociation of steam (electrolysis) at the same time. Thus, when natural gas is supplied to the REP system, about 80% of the hydrogen output is produced from the natural gas reformation and the other 20% of the hydrogen is provided by the electrolysis reaction. This reformer-electrolyzer-purifier (REP) system 100 produces hydrogen efficiently and with minimal CO 2 emissions.
As seen in FIG. 2 , fuel, such as natural gas and/or renewable fuel, plus water are fed into the system 200 . This fuel feed is heated in the pre-heater 102 and then routed to the reforming cells 202 and the REP fuel cells 204 where the almost all of the gas is reformed to hydrogen and CO. Heat for this endothermic reforming reaction is provided by external waste heat 104 , which is provided from other waste heat generating devices. In certain embodiments, supplemental or extra fuel is used as a backup or to raise the level of the waste heat, particularly when interruptible renewable waste heat such as wind power or solar heat is used as the source of waste heat. For example, in FIG. 2 , an oxidizer 106 is provided in the system which receives supplemental fuel and air and oxidizes the supplemental fuel to produce heated gas for use in the cathode. In this way, the oxidizing reaction raises the level of waste heat that is used in the REP cells.
In the illustrative embodiment shown in FIG. 2 , first the fuel gas is partially reformed in the reforming only cells (RU's) 202 . The reaction occurring between water and methane in the RU's (reformer) is shown in FIG. 4 . As shown in FIGS. 2 and 4 , the partially reformed gas from the RU's 202 is then fed to the anode side 204 a of an MCFC fuel cell 204 operating in purification mode as an electrolyzer (REP cells) (hydrogen producing mode). In the fuel cells 204 , water is dissociated to hydrogen and oxygen, the oxygen combines with the carbon dioxide in the reformed gas to produce CO 3 = , and the CO 3 = is removed electrochemically across the molten carbonate membrane. These reactions in the anode side 204 a of the fuel cell 204 are shown in FIG. 4 . This operation in the fuel cell 204 removes almost all of the carbon in the system and forces the equilibrium reforming and shift reactions to essentially complete conversion of the CH4 and CO to hydrogen. Thus, as shown in FIGS. 2 and 4 , the exiting hydrogen-containing gas stream is almost pure hydrogen (greater than 98%) with a small amount of CO 2 and CH4. This small amount of CO 2 and CH4 can easily be removed as the hydrogen is pressurized for systems requiring high purity hydrogen. However, many systems are able to use the low purity hydrogen directly, without the need for removing the small amount of impurities.
As shown in FIG. 2 , the operation of the REP fuel cell 204 as an electrolyzer may be controlled by a controller. The controller 250 is programmed to control the supply or flow rate of reactant gases to the REP fuel cell 204 . The controller 250 also controls the voltage and current applied to the fuel cell, which is supplied from the power supply (e.g., DC power supply) 230 so that the ion transfer is in the reverse direction of the normal fuel cell operation. The reactions that occur in the fuel cells of the REP system 100 are shown in FIG. 4 . When a gas containing CO2 and oxygen is used as the cathode side gas, the controller 250 may further control the switching of the operation modes of the fuel cell 204 between operation as an electrolyzer and normal power production operation. This operation is described in more detail below.
Moreover, although the reforming cells 202 in FIG. 2 are shown as part of the REP fuel cell stack, so that the stack is an indirect internally reforming stack, in other embodiments, an external reformer may be used instead or in addition to the internal reforming cells for reforming the fuel.
In certain illustrative embodiments, the components used in the REP system 100 of FIG. 2 are the same or similar to the commercially available components of DFC® fuel cells developed by FuelCell Energy, Inc. By using commercially available components for the REP system, this invention can be rapidly commercialized with competitive costs, which results in further cost savings.
FIGS. 3 A and 3 B show an assembly that integrates the REP system 100 of FIGS. 1 and 2 with a high temperature fuel cell system, such as a standard DFC® fuel cell system. In the assembly shown in FIG. 3 A , the high temperature fuel cell system 300 is a power producing fuel cell, which can provide the waste heat, controls, feed gas treating, water treating, power, and auxiliary support equipment to the REP system 100 , thus minimizing the REP system capital cost. As shown in FIG. 3 A , water and fuel are supplied to the high temperature fuel cell system 300 , which also receives returning cathode exhaust from the REP system 100 . Part of the purified and humidified fuel used by the standard fuel system is sent to a REP unit 100 A (REP assembly) of the REP system 100 . Hot cathode exhaust comprising unspent oxidant gas is also output from the high temperature fuel cell system 300 and is then supplied to the cathode side of the REP system 100 to supply heat to the REP unit 100 A and a dilute the CO2 and oxygen produced by the REP unit 100 A (which lowers the voltage and power requirements of the REP). Cathode exhaust output from the REP system 100 is recycled back to the high temperature fuel cell system 300 for use as oxidant in the cathode side. This recycle is enriched with CO2 and oxygen which slightly improves the performance of the standard DFC fuel cell system. As described above with respect to FIG. 2 , humidified fuel supplied to the REP system is first pre-heated in a preheater 102 , then conveyed to the reforming cells 202 and thereafter provided to the anode side 204 a of the REP unit 100 A, which comprises a fuel cell assembly operating as an electrolyzer. The anode side 204 a of the REP unit 101 A outputs hydrogen with a small amount of CO 2 and CH4. Hydrogen produced by the REP system 100 may be further purified to remove the CO 2 and CH4 so that high purity hydrogen can be provided to devices that operate and require high purity hydrogen, such as fuel cell cars. Such applications are described in more detail below.
FIG. 3 B shows a photograph of a 30 cell DFC® stack and is similar to a possible arrangement of a 30 cell REP system 100 of FIG. 3 A . The REP system 100 of FIG. 3 B includes a fuel cell stack 204 , positioned on a base and various connections and ports for supplying inlet gases to the stack and conveying exhaust gases out of the stack. As shown in FIG. 3 B , the REP system 100 also includes a plurality of manifolds 206 a - c for directing the respective inlet and outlet gases, including a fuel turn manifold 206 a for directing reformed fuel to the anode side of the REP fuel cell, a fuel out manifold 206 b for receiving anode exhaust (purified hydrogen), and a cathode out manifold 206 c for outputting cathode exhaust. An exemplary fuel cell module which can be adapted for use in the REP system 100 of FIG. 3 A is shown and described in U.S. Pat. Nos. 7,323,270 and 7,070,874, assigned to the same assignee herein and incorporated herein by reference.
Although in the illustrative embodiment of FIGS. 3 A and 3 B , the reforming cells 202 are shown as part of the fuel cell stack, so that the stack is an indirect internally reforming stack, in other embodiments, an external reformer may be used instead or in addition to the internal reforming cells for reforming the fuel.
As discussed above, the REP system of present invention utilizes a MCFC fuel cell operating as a high temperature electrolyzer to convert water, methane, and/or carbon monoxide in the reformed gas supplied from the reforming cells to hydrogen by removing the CO2 from the gas. In order to operate the fuel cell of the REP system as the electrolyzer, a voltage is applied to the fuel cell so that the CO 3 = ions, generated from CO2 and H2O, flow is in the reverse direction of the normally occurring flow direction in fuel cells. The voltage applied to the fuel cell operating as an electrolyzer is supplied from a power supply, which may be a battery, another fuel cell or fuel cell assembly operating in a power production mode (or even fuel cells in the REP stack operating in the power production mode), or any other power storage or power supply device. The reactions in the fuel cell of the REP system require CO 2 and water on the anode side and generate a mixture of CO 2 and oxygen on the cathode side, as the CO 3 = ion is pumped across the electrolyte membrane or matrix. The oxygen needed to create CO 3 = is generated by the dissociation of water on the anode side. In the present illustrative embodiment, this reaction is produced by applying a reverse voltage of about 1.2V to the MCFC cell, and in the system shown in FIG. 3 A , power generated by the fuel cell system 300 , or portion thereof, may be used for applying the reverse voltage to the REP unit 100 A. The reactions occurring in the anode side and in the cathode side of the fuel cell in the REP system, as well as the application of DC power to the anode side to drive the electrolysis reaction are shown in FIG. 4 .
As discussed above, the operation of the REP system and in particular, of the fuel cell in the REP system is controlled by the controller 250 or the like. The controller 250 controls the power supply and the application of the voltage required for the electrolysis reactions in the fuel cell, as well as the flow rates of the inlet gases to the REP system. The voltage required is a function of the following Nernst equation:
Nernst
â¢
Voltage
â¢
Equation
ïº
E
=
E
T
â¦
+
RT
2
â¢
F
â¢
ln
â¢
Ï
H
2
â¢
Ï
O
2
1
/
2
â¢
Ï
CO
2
(
c
)
Ï
H
2
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CO
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(
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)
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4
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By configuring and controlling the REP system to dilute the cathode CO 2 and oxygen concentration with another gas such as air, a lower voltage and more efficient operation is realized. In the anode, at the high temperature of about 1100° F., methane is reformed by reacting with water to produce hydrogen and CO. The CO is then reacted with water to produce hydrogen and CO 2 . Although these reactions are reversible, when the CO 2 is pumped out of the system, these reactions are driven towards complete or near complete conversion. The pumping out of the CO 2 from the system may also be controlled by the controller 250 .
Theoretically pure hydrogen can be produced from the anode, but complete CO 2 removal is not possible due to the vapor pressure of CO 2 from the molten carbonate membrane and the CO 2 on the cathode side of the cell. Testing has shown that the CO 2 can be reduced to around 1% on a dry basis which can be easily removed from the hydrogen using downstream purification systems if necessary. This level of CO 2 is sufficient to convert essentially all the methane to hydrogen. Moreover, if a downstream purification step is used, the hydrogen and CO 2 ejected from a downstream purification step can easily be recycled to the REP system so that 100% conversion to hydrogen can be realized. In some embodiments, the REP system can be integrated with reactor off gases, such as the off gas from a Fischer-Tropes reactor, to facilitate recycling of hydrogen off gas from the system. Moreover, the REP system can be integrated with low temperature fuel cell systems, with power generating systems operating on coal, with a gasifier, and other systems. Specific examples of systems that use the REP system of the present invention are described below.
In the present invention, the reforming of natural gas to hydrogen is driven to completion by removal of almost all carbon from the gas being reformed. This carbon removal, in the form of CO 3 = , is done at high temperature so that the reforming reaction continues to completion. The power used to remove the CO 2 by the fuel cell of the REP system provides a double benefit to the system in that it generates additional hydrogen while purifying the hydrogen from the reforming reaction. The hydrogen generated from the electrolysis reaction in the fuel cell is highly efficient due to the high temperature and the fact that the reaction is based on steam electrolysis rather than water. It is expected that the electrolysis power requirements will be roughly 55% of the power used in low-temperature electrolysis systems per kilogram of hydrogen from electrolysis. Since up to 80% of the total hydrogen is from reforming, the power needs are roughly 11% based on total hydrogen produced.
The other important element in the present invention is the use of waste heat to drive the endothermic reforming reaction. Although one source of waste heat may be a high temperature fuel cell providing power, such as in the integrated assembly of FIG. 3 , many other sources of waste heat can be used. Some of the waste heat used is relatively low temperature (approximately 250° F.) waste heat, which is used to convert the feed water into steam and to pre-heat the gases for the reforming reaction. The reforming reaction, however, requires a higher level of heat, such as is available from a high temperature fuel cell, a gas turbine, solar heat, nuclear, gasification, electrical heat or other sources.
Moreover, for systems requiring very high purity hydrogen, the low purity off gas produced can easily be recycled to the REP system to maintain a very high overall efficiency and low CO 2 emissions.
The REP system of the present invention was tested to determine its efficiency in terms of power consumption and purity of hydrogen produced and to compare the efficiency of the REP system to conventional electrolyzers. FIG. 5 A shows a graph of test data analyzing estimated voltage required by the fuel cell of the REP system compared to conventional electrolyzers. As shown in FIG. 5 A , when the fuel cell of the REP system is operated as an electrolyzer in a CO 2 pump mode (purification mode), the voltage needed to be applied to each cell is between 1000 and 1300 mV/cell with a voltage between 1150 and 1300 mV/cell needed to produce high purity hydrogen. In contrast, conventional low temperature electrolyzer voltage range is between 1600 and 2000 mV. Thus, this test shows that high temperature electrolysis in REP system of the present invention is much more efficient than conventional low temperature electrolyzers.
FIG. 5 B demonstrates the relationship between hydrogen purity obtained in the REP system and cell voltage applied to the fuel cells of the REP system. As shown in FIG. 5 B , the purity of hydrogen increases up to about 98-99% as more voltage is applied, and the amount of CO and CO2 in the gas output by the REP system decreases as the cell voltage increases. The purification of the reformed gas by the electrolysis reaction in the fuel cell multiplies the benefits of the power consumed by both producing hydrogen and purifying the reformed gas.
The present invention provides substantial improvements in hydrogen production. Because the REP system is fully scalable, it can be sized to provide the exact amount of hydrogen needed at a given site, eliminating the need for hydrogen transportation. Transportation costs can easily double or triple the cost of hydrogen at some sites and greatly increase CO 2 emissions due to emissions from trucks or other transportation means. Hydrogen storage is also expensive. A single high temperature stack, such as a DFC® stack, of the size currently used for power generation can produce over 1,500 kg per day of hydrogen when operated as part of the REP system. A large scale fuel-cell system typically incorporates multiple fuel cell stacks, so that, for example, a REP system using 8 fuel-cell stacks would thus produce over 12,000 kg per day of hydrogen. Thus, large, industrial scale hydrogen can be generated with the REP system of the present invention.
On the other end of the scale, the REP system will maintain efficiency even as it is scaled down. For example, a home refueling system would scale the REP system down to the 1 to 2 kg of hydrogen per day production level needed for typical fuel-cell vehicles. Such a system could potentially solve the hydrogen infrastructure problem which is a concern for these types of vehicles. As described in more detail below, an electrochemical hydrogen compression (EHC) system which compresses and purifies the H2 in one step may also be used. By combining the REP assembly and the EHC systems, the high pressure, high purity hydrogen needed by the vehicles can be easily and cost-effectively generated at this small scale.
The REP system produces a 33% oxygen/67% CO 2 stream in the cathode. As described in more detail below, this gas could potentially be used as the oxidant in a gasifier or even in a standard boiler to produce a high purity CO 2 stream for capture. Even without CO 2 capture, the use of this gas as the oxidant in place of air would eliminate NOx formation. In some cases, this stream can be diluted with air or cathode exhaust gas so that the composition of the gas on the cathode side is similar to the composition used in commercial DFC® power generation cells developed by FuelCell Energy, Inc. This dilution helps maintain the heat balance in the system and reduces the voltage requirement on the cell. Nevertheless, the system of the present invention makes CO2 capture practical. Examples of systems incorporating the REP and providing CO2 capture are described in more detail below.
As discussed above, the REP system also incorporates a high temperature electrolyzer which is much more efficient than current low temperature technology, using only approximately 55% of the conventional power. This electrolyzer could be run without any fuel when integrated with a high temperature fuel cell system, such as a DFC® fuel-cell, to efficiently store excess electrical power as hydrogen.
FIG. 6 shows another embodiment of a hydrogen production system 400 that utilizes the REP system followed by an electrochemical hydrogen compression (EHC) to produce high pressure high purity hydrogen in order to produce a high pressure high purity H2. As shown in FIG. 6 , the system 400 includes a desulfurizer 15 for desulfurizing fuel supplied to the system, a pre-heater/humidifier 16 for pre-heating desulfurized fuel, and humidifying desulfurized fuel with water, a further pre-heater 19 , a preconverter or reformer 25 for reforming humidified fuel, a fuel cell REP stack 28 operating as a high temperature electrolyzer, a methanator 26 and an electrochemical hydrogen compression (EHC) system 27 . In FIG. 6 , fuel is desulfurized in the desulfurizer 15 , mixed with water or humidified in the humidifier 16 , pre-heated using one or more pre-heaters 16 , 19 and fed to the preconverter (reformer) 25 to convert methane and water in the fuel to H 2 and CO 2 via the following reaction:
Heat from an external source (not shown) is added into the preconverter 25 . The reformed fuel comprising hydrogen and CO2 is then conveyed to an anode side of the REP fuel cell stack 28 operating as a high temperature electrolyzer (CO2 pump). In the REP fuel cell stack 28 , CO2 in the fuel is removed by electrolyzing additional water to produce more H2 via the following reaction:
The removal of the CO2 from the hydrogen-containing gas generated in the REP stack drives the CH4 conversion to near completion and a 95-99% H2 stream is generated from the fuel cell stack 28 . The resulting hydrogen-containing gas stream output from the fuel cell stack 28 is cooled slightly in the heat exchanger 19 , which also pre-heats humidified fuel, and then conveyed to the methanator 26 where the gas is methanated. In the methanator 26 , all traces of CO are removed from the gas by converting it to CH4 so that a 98% H2/2% CH4 stream with 0% CO2 and CO is produced.
After the methanation process, the resulting converted hydrogen-containing stream (98% H2/2% CH4) is conveyed to the electrochemical hydrogen compression (EHC) system 27 , which is used to compress the H2 from near atmospheric pressure to 2000+psig. At the same time, the EHC system 27 purifies the H2 to 99.9+% needed for certain uses, such as in a fuel cell vehicle. The left over gas from the EHC system 27 comprising methane, H2 and H2O is cooled in a heat exchanger 8 and then recycled back to the supply feed using a blower 32 . In this way, 100% of the CH4 is converted to H2 and 100% of the H2 generated is eventually exported as a final product H2 having purity of >99.9% and compressed at >2000 psig pressure.
A material balance for the system shown in FIG. 6 is shown below:
Stream No.
11
3
15
16
17
10
29
19
23
4
Feed
Re-
Wet NG
RU
RU
CO2/
MCFC
Meth
EHC
H2
236 Name
NG
cycle
to RU
in
Out
O2
Raw H2
Out
In
Product
Molar flow
100.00
148.27
739.33
739.33
864.30
158.93
841.44
839.33
666.14
517.87
lbmol/hr
Mass flow
1,604.3
578.0
11,028.9
11,028.9
11,028.9
6,286.6
4,742.3
4,742.3
1,622.0
1,043.9
lb/hr
Components
lb-mole/hr
mole %
lb-mole/hr
mole %
lb-mole/hr
mole %
lb-mole/hr
mole %
lb-mole/hr
mole %
lb-mole/hr
Hydrogen
0.00
0.00
129.47
87.32
129.47
17.51
129.47
17.51
361.64
41.64
0.00
Methane
100.00
100.00
11.00
7.42
111.00
15.01
111.00
CLAIMS
Claims ( 21 )
1 .- 133 . (canceled)
134 . A method for capturing carbon dioxide from a reformed gas comprising:
supplying a reformed gas to CO 2 pump; outputting, from the CO 2 pump, a first exhaust stream comprising carbon dioxide and oxygen and a second exhaust stream comprising hydrogen; and transporting the carbon dioxide and oxygen back to a reformer to convert reformer fuel comprising methane and hydrogen to reformer flue gas comprising carbon dioxide and water.
135 . The method of claim 134 , further comprising sequestering substantially all of the carbon dioxide from the reformer flue gas.
136 . The method of claim 134 , further comprising:
cooling the second exhaust stream; transporting the cooled second exhaust stream comprising hydrogen to a methanator to generate a third exhaust stream; and transporting the third exhaust stream from the methanator to an electrochemical hydrogen compressor.
137 . The method of claim 134 , further comprising:
cooling the second exhaust stream; transporting the cooled second exhaust stream comprising hydrogen to a methanator to generate a third exhaust stream; transporting the third exhaust stream from the methanator to an electrochemical hydrogen compressor; separating hydrogen from residual methane in the electrochemical hydrogen compressor to produce a purified hydrogen stream; and increasing a pressure of the purified hydrogen stream.
138 . The method of claim 134 , further comprising:
transporting the second exhaust stream comprising hydrogen to a methanator to generate a third exhaust stream; transporting the third exhaust stream from the methanator to an electrochemical hydrogen compressor; and separating hydrogen from residual methane in the electrochemical hydrogen compressor to produce a purified hydrogen stream comprising greater than 98% H 2 .
139 . The method of claim 134 , further comprising:
transporting the second exhaust stream to a methanator to convert residual carbon monoxide and carbon dioxide in the second exhaust stream into methane.
140 . The method of claim 134 , further comprising:
transporting the second exhaust stream to a methanator to generate a third exhaust stream; and cooling the third exhaust stream.
141 . The method of claim 134 , further comprising:
transporting the second exhaust stream to a methanator to generate a third exhaust stream; and transporting the third exhaust stream from the methanator to an electrochemical hydrogen compressor.
142 . The method of claim 141 , further comprising:
outputting a purified hydrogen stream from a cathode of the electrochemical hydrogen compressor.
143 . The method of claim 141 , further comprising:
outputting methane from an anode of the electrochemical hydrogen compressor.
144 . The method of claim 134 , further comprising:
receiving, by the reformer, natural gas and water; and outputting, by the reformer, the reformed gas comprising hydrogen and carbon monoxide.
145 . The method of claim 134 , further comprising:
outputting, by the reformer, the reformed gas comprising hydrogen and carbon monoxide to an anode of the CO 2 pump.
146 . The method of claim 134 , wherein the second exhaust stream comprises residual carbon monoxide and carbon dioxide.
147 . The method of claim 134 , wherein the first exhaust stream comprising a mixture having a ratio of carbon dioxide to oxygen of about 2:1.
148 . The method of claim 134 , further comprising:
transporting the second exhaust stream to a methanator to generate a third exhaust stream; transporting the third exhaust stream from the methanator to an electrochemical hydrogen compressor; and receiving, by an anode gas oxidizer, methane and hydrogen from a cathode of an electrochemical hydrogen compressor.
149 . The method of claim 148 , further comprising:
transporting the second exhaust stream to a pressure swing adsorption-based polishing system.
150 . A system, comprising:
a reformer configured to output reformed gas; and a CO 2 pump configured to:
receive the reformed gas from the reformer;
output a first exhaust stream comprising carbon dioxide and oxygen to the reformer to convert reformer fuel comprising methane and hydrogen to reformer flue gas comprising carbon dioxide and water; and
output a second exhaust stream comprising hydrogen.
151 . The system of claim 150 , further comprising:
a methanator configured to convert residual carbon monoxide and carbon dioxide in the second exhaust stream into methane.
152 . The system of claim 150 , further comprising:
a methanator configured to receive the second exhaust stream and output a third exhaust stream comprising methane and hydrogen; and an electrochemical hydrogen compressor configured to receive the third exhaust stream, output a purified hydrogen stream from a cathode of the electrochemical hydrogen compressor, and output a methane from an anode of the electrochemical hydrogen compressor.
153 . The system of claim 150 , wherein the first exhaust stream comprising a mixture having a ratio of carbon dioxide to oxygen of about 2:1.
US19/418,115
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Reformer-electrolyzer-purifier (rep) assembly for hydrogen production, systems incorporating same and method of producing hydrogen
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