WO2012170375A1

WO2012170375A1 - Fuel cell and reciprocating gas/diesel engine hybrid system

Info

Publication number: WO2012170375A1
Application number: PCT/US2012/040830
Authority: WO
Inventors: Mingmin Wang; David James WALKER; Wenhua Li; Roy James Primus; Matthew Alexander Lehar; Bruce Philip Biederman
Original assignee: General Electric Company
Priority date: 2011-06-09
Filing date: 2012-06-05
Publication date: 2012-12-13
Also published as: EP2719008A1; JP2014519177A; CN102820480A; RU2013153197A

Abstract

A hybrid fuel cell plant includes a fuel cell and a fuel reformer that mixes a hydrocarbon fuel and steam together upstream of the fuel cell. The reformer partly or fully converts the hydrocarbon fuel and steam into a reformed fuel stream that includes hydrogen (H₂), carbon monoxide (CO) and carbon dioxide (C0₂). The fuel cell receives the reformed fuel stream at or above atmospheric pressure and also an air stream at or above atmospheric pressure that includes oxygen (0₂) and nitrogen (N₂) to generate a fuel cell hot exhaust stream that includes lean air, unoxidized CO and residual H₂. The lean air stream has an 0₂ molar fraction less than that of the fuel cell inlet air. The hot exhaust stream is generated above atmospheric pressure when the reformed fuel and air stream are received by the fuel cell above atmospheric pressure. An internal or external combustion engine directly or indirectly generates power in response to the pressurized fuel cell hot exhaust stream to increase the efficiency of the fuel cell power plant. A Rankine cycle generates power in response to waste heat extracted from at least one of the fuel cell hot exhaust stream and hot exhaust gas generated by the combustion engine to further increase the efficiency of the hybrid fuel cell plant from 50% to above 70% efficiency.

Description

FUEL CELL AND RECIPROCATING

GAS/DIESEL ENGINE HYBRID SYSTEM

BACKGROUND

[0001] This invention relates generally to fuel cell power plants, and more particularly to a fuel cell power plant integrated with a Rankine cycle such as an Organic Rankine cycle (ORC), and that employs an internal or external combustion engine downstream of a fuel cell anode, and that is driven by the unconverted fuel exiting the fuel cell to achieve higher total plant efficiencies than that achievable from conventional fuel cell power plants.

[0002] Fuel cells are electrochemical energy conversion devices that have demonstrated a potential for relatively high efficiency and low pollution in power generation. A fuel cell generally provides a direct current (DC) which may be converted to alternating current (AC) via for example, an inverter. The DC or AC voltage can be used to power motors, lights, and any number of electrical devices and systems. Fuel cells may operate in stationary, semi- stationary, or portable applications. Certain fuel cells, such as solid oxide fuel cells (SOFCs), may operate in large-scale power systems that provide electricity to satisfy industrial and municipal needs. Others may be useful for smaller portable applications such as for example, powering cars.

[0003] A fuel cell produces electricity by electrochemically combining a fuel and an oxidant across an ionic conducting layer. This ionic conducting layer, also labeled the electrolyte of the fuel cell, may be a liquid or solid. Common types of fuel cells include phosphoric acid (PAFC), molten carbonate (MCFC), proton exchange membrane (PEMFC), and solid oxide (SOFC), all generally named after their electrolytes. In practice, fuel cells are typically amassed in electrical series in an assembly of fuel cells to produce power at useful voltages or currents. Therefore, interconnect structures may be used to connect or couple adjacent fuel cells in series or parallel. [0004] In general, components of a fuel cell include the electrolyte and two electrodes. The reactions that produce electricity generally take place at the electrodes where a catalyst is typically disposed to speed the reactions. The electrodes may be constructed as channels, porous layers, and the like, to increase the surface area for the chemical reactions to occur. The electrolyte carries electrically charged matter from one electrode to the other and is otherwise substantially impermeable to both fuel and oxidant.

[0005] Typically, the fuel cell converts hydrogen (fuel) and oxygen (oxidant) into water to produce electricity. If air is employed as the oxidant, the nitrogen in the air is substantially inert and typically passes through the fuel cell. Hydrogen fuel may be provided via local reforming (e.g., on-site steam reforming) of carbon-based feedstocks, such as reforming of the more readily available natural gas and other hydrocarbon fuels and feedstocks. Examples of hydrocarbon fuels include natural gas, methane, ethane, propane, methanol, syngas, and other hydrocarbons. The reforming of hydrocarbon fuel to produce hydrogen to feed the electrochemical reaction may be incorporated with the operation of the fuel cell. Moreover, such reforming may occur internal and/or external to the fuel cell. For reforming of hydrocarbons performed external to the fuel cell, the associated external reformer may be positioned remote from or adjacent to the fuel cell.

[0006] Fuel cell systems that can reform hydrocarbon internal and/or adjacent to the fuel cell may offer advantages, such as simplicity in design and operation. For example, the steam reforming reaction of hydrocarbons is typically endothermic, and therefore, internal reforming within the fuel cell or external reforming in an adjacent reformer may utilize the heat generated by the typically exothermic electrochemical reactions of the fuel cell. Furthermore, catalysts active in the electrochemical reaction of hydrogen and oxygen within the fuel cell to produce electricity may also facilitate internal reforming of hydrocarbon fuels. In SOFCs, for example, if nickel catalyst is disposed at an electrode (e.g., anode) to sustain the electrochemical reaction, the active nickel catalyst may also reform hydrocarbon fuel into hydrogen (H₂) and carbon monoxide (CO). Moreover, both hydrogen and CO may be produced when reforming hydrocarbon feedstock. Thus, fuel cells, such as SOFCs, that can utilize CO as fuel (in addition to hydrogen) are generally more attractive candidates for utilizing reformed hydrocarbon and for internal and/or adjacent reforming of hydrocarbon fuel.

[0007] The capability of a fuel cell to convert hydrocarbon fuel into electrical energy is limited by loss mechanisms within the cell that produce heat and by partial utilization of the fuel. Reforming of the hydrocarbon primary fuel occurs upstream of the fuel cell in a conventional combined cycle fuel cell system. Tail gas from the fuel cell including unoxidized fuel and products of combustion are then sent to tailgas burners, the heat from which can be incorporated in a combined cycle system, sometimes into the fuel reformer. Present day examples of fuel cells routinely achieve about 50% conversion efficiency.

[0008] In view of the foregoing, there is a need to provide a technique that further increases the efficiency of a fuel cell plant through increased fuel cell efficiency.

BRIEF DESCRIPTION

[0009] An exemplary embodiment of the present invention comprises a hybrid fuel cell plant comprising: a fuel cell; a fuel reformer configured to mix a hydrocarbon fuel and steam together upstream of the fuel cell, and to partly or fully convert the hydrocarbon fuel and steam into a reformed fuel stream comprising hydrogen (H₂), carbon monoxide (CO) and carbon dioxide (C0₂), wherein the fuel cell receives the reformed fuel stream at or above atmospheric working pressure and further receives an air stream at or above atmospheric working pressure, the air stream comprising oxygen (0₂) and nitrogen (N₂), wherein the fuel cell generates a hot exhaust stream comprising lean air, unoxidized CO and residual H₂ in response to the reformed fuel stream and the air stream, wherein the lean air comprises an 0₂ molar fraction less than that of the fuel cell inlet air; an internal or external combustion engine configured to generate power in response to the hot exhaust stream generated via the fuel cell; and a Rankine cycle driven via heat recovered from at least one of the hot exhaust stream generated via the fuel cell and hot exhaust gas generated via the combustion engine.

[0010] According to another embodiment, a method of generating power via a hybrid fuel cell plant comprises: reforming a hydrocarbon based fuel together with a stream of steam upstream of a fuel cell via an external reformer and generating a substantially pure hydrogen fuel stream at or above atmospheric working pressure therefrom; generating a stream of air at or above atmospheric working pressure; generating a hot exhaust stream comprising carbon monoxide and residual hydrogen, and further comprising a lean air stream via a fuel cell in response to the reformed fuel and stream of air, such that the lean air stream has an 0₂ molar fraction less than that of the fuel cell inlet air, wherein the hot exhaust stream is generated above atmospheric pressure when the reformed fuel and stream of air enter the fuel cell above atmospheric pressure; driving a combustion engine in response to the fuel cell hot exhaust stream to generate power; and generating power via a Rankine cycle in response to waste heat recovered from both the fuel cell hot exhaust stream and hot exhaust generated via the combustion engine to provide a fuel cell plant having an efficiency greater than 60%.

DRAWINGS

[0011] The foregoing and other features, aspects and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

[0012] Figure 1 is a simplified diagram illustrating a hybrid fuel cell power plant that employs a fuel cell running on pressurized reformed fuel to generate a hot exhaust stream including lean air that is employed to feed a downstream combustion engine according to one embodiment in which additional air feeds the combustion engine if the lean air generated via the fuel cell is deficient; and

[0013] Figure 2 is a simplified diagram illustrating a hybrid fuel cell power plant that employs a fuel cell running on pressurized reformed fuel to generate a hot exhaust stream that is employed to feed a downstream combustion engine and that also employs an Organic Rankine cycle (ORC) driven via exhaust waste gases generated by both the fuel cell and/or the combustion engine to generate additional power according to one embodiment.

[0014] While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. DETAILED DESCRIPTION

[0015] The embodiments described herein with reference to the Figures advantageously provide increased fuel cell plant efficiencies greater than 60% and even greater than 70% in particular embodiments that employ combustion engine principles also described herein. Because any size combustion engine may be employed downstream of a fuel cell, including the smallest gas engines of lOOkW output power, the fuel cell-combustion engine complex embodiments described herein can advantageously be easily deployed to provide distributed energy.

[0016] Other embodiments of the present invention are also contemplated, as noted in the discussion. The principles described herein can just as easily be applied for example, to comparable fuel-cell technologies that are not strictly solid-oxide fuel cells or molten carbonate fuel cells. A vast variety of waste heat recovery cycles and methods for integrating those cycles are also possible using the principles described herein.

[0017] Figure 1 is a simplified diagram illustrating a hybrid fuel cell power plant

10 that employs a fuel cell 12 that may comprise for example, without limitation, a solid- oxide fuel cell (SOFC) or molten carbonate fuel cell (MCFC) running on pressurized reformed fuel 17 to generate a hot exhaust stream 14 that ultimately feeds a combustion engine 16 according to one embodiment. According to particular embodiments, the hot exhaust stream 14 may be employed to directly feed the combustion engine 16, or may be first cooled down to remove excess water before feeding the combustion engine 16. The combustion engine 16 may comprise for example, without limitation, a reciprocating 4- stroke, reciprocating 2-stroke, opposed piston 2-stroke or gas turbine.

[0018] A pressurized hydrocarbon fuel 11 such as CH₄ is combined with pressurized steam 13 in a reformer 18 that functions to partly or fully convert the pressurized hydrocarbon fuel and steam into a pressurized reformed fuel stream 17 comprising hydrogen (H₂), carbon monoxide (CO) and carbon dioxide (C0₂) upstream of the fuel cell 12. The fuel cell 12 receives the pressurized reformed fuel stream 17 comprising H₂, CO and C0₂ and further receives a pressurized air stream 15 comprising oxygen (0₂) and nitrogen (N₂) to generate a hot exhaust stream 14 comprising lean air, unoxidized CO and residual H₂ in response to the pressurized reformed fuel stream 17 and the pressurized air stream 15, while generating electricity.

[0019] The hot exhaust gas 14 generally comprises substantial amounts of water as a steam phase. Therefore, it is generally better to remove the excess water prior to feeding the hot exhaust gas 14 into the combustion engine 16. A condenser or flash condenser 22 is employed to cool and remove some or all of this water that enters the condenser 22 above atmospheric pressure. A Rankine cycle such as described herein may be employed to recover the waste heat of the fuel cell 12 as well as the combustion engine 16.

[0020] Hybrid fuel cell power plant 10 further comprises a compressor 20 that functions to compress the condensed exhaust gas stream prior to entering the combustion engine 16 that is disposed downstream of the fuel cell 12. In general, the anode of the fuel cell (SOFC) 12 may utilize about 80% of the pressurized reformed fuel 17 that may include reaction of carbon monoxide. The unconverted H₂ and CO fuel exits the anode of the fuel cell 12 in a pressurized exhaust stream 14. Adding an internal or external combustion engine 16 downstream of the fuel cell 12 such as depicted in Figure 1 allows the unconverted portion of the reformed fuel 17 to be ultimately consumed, producing additional power and significantly increasing the overall conversion efficiency of the fuel cell plant 10.

[0021] Additional fuel 19 may be added to the fuel cell anode exhaust stream 14 to enrich the stream of fuel entering the combustion engine 16 and optimize the engine performance. Air for the downstream combustion may be drawn from the atmosphere, or optionally from the exhaust of the fuel cell cathode. The fuel cell cathode provides a stream with lower 0₂ concentration than atmospheric air, thus mitigating NOX emissions when used in place of air drawn from the atmosphere in the downstream combustion.

[0022] The pressure at which combustion occurs in both the anode and the downstream engine 16 may be atmospheric, or elevated above atmospheric. If the pressure is higher than atmospheric, the higher pressure is maintained throughout the fuel cell anode and cathode. According to one embodiment, atmospheric air is compressed by a turbocharger or compressor-turbine system 40 driven by the exhaust flow from the combustion engine 16 such as depicted in Figure 2. This pressurized air can be used for example, without limitation, to preheat fuel and air and/or to generate power to drive compressors. Oxygen ions transfer from the pressurized air stream to combine with fuel, also at elevated pressure, within the anode, producing a pressurized anode exhaust stream 14 that ultimately feeds into the combustion engine 16 downstream as well as producing electricity. According to one embodiment, a condenser 22 removes excess water from the exhaust stream 14 as stated herein. A compressor 20 disposed downstream of the condenser 22 further elevates the exhaust stream working pressure prior to its use by the combustion engine 16.

[0023] According to one embodiment depicted in Figure 2, a Rankine cycle 32, such as for example, without limitation, a DReSCO-type C0₂ Rankine cycle, recovers heat simultaneously from the anode exhaust stream 14 and the ultimate exhaust stream exiting the downstream combustion engine 16. Figure 2 is a simplified diagram illustrating a hybrid fuel cell power plant 30 that employs a fuel cell 12 running on pressurized reformed fuel 17 to generate a hot exhaust stream 14 that ultimately feeds a combustion engine 16 and that also employs an Organic Rankine cycle (ORC) 32 driven via waste exhaust gases generated by both the fuel cell 12 and/or the combustion engine 16 to generate additional power according to one embodiment. The Rankine cycle 32 cools both streams and produces the additional power. The Rankine cycle 32 may also employ a condenser to remove water from the fuel cell exhaust gas prior to use by the combustion engine 16.

[0024] The foregoing embodiments described with reference to Figures 1 and 2 operate to increase the fuel cell plant efficiency above the current practical limit of 50% by improving the utilization of fuel. Simulations have shown the overall hybrid system efficiency of some embodiments, excluding effects that can be attributed to a Rankine cycle, can be increased up to 65%, including the heat consumed by reformer 18. The overall system efficiency can theoretically be higher than 70%.

[0025] These embodiments further increase the efficiency of fuel cell plant above that which is achievable using conventional gas turbines, thus creating a substantial justification for choosing fuel cell systems over conventional technology. Because any size of combustion engine may be employed downstream of the fuel cell 12, including the smallest gas engines of lOOkW output, the fuel cell-combustion engine complex 10, 30 can be easily deployed for a distributed energy generation as stated herein. Previous fuel cell power plants have not recognized the possibility of using a high pressure fuel cell integrated with a combustion engine and a Rankine cycle and that simultaneously generates and uses lean air generated via the fuel cell cathode to decrease combustion engine NO_x emissions.

[0026] In summary explanation, hybrid fuel cell plant embodiments 10, 30 have been described herein. Each of these embodiments comprise a fuel cell 12 and a fuel reformer 18 configured to mix a pressurized hydrocarbon fuel 11 and pressurized steam 13 at elevated pressure together upstream of the fuel cell 12, and to partly or fully convert the hydrocarbon fuel 11 and steam 13 into a reformed fuel stream 17 comprising hydrogen (H₂), carbon monoxide (CO) and carbon dioxide (C0₂). The fuel cell 12 is configured to receive the reformed fuel stream 17 comprising H₂, CO and C0₂ and further configured to receive a pressurized air stream 15 at elevated pressure comprising oxygen (0₂) and nitrogen (N₂) and to generate a hot exhaust stream 14 comprising lean air, unoxidized CO and residual H₂ in response to the pressurized reformed fuel stream 17 and the pressurized air stream 15. An internal or external combustion engine 16 that may be a reciprocating gas engine generates power in response to the hot exhaust stream 14 either directly or subsequent to removal of water from the exhaust stream 14 and/or compression of the exhaust stream 14.

[0027] While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

Claims

CLAIMS:

1. A hybrid fuel cell plant comprising: a fuel cell comprising an anode and a cathode; a fuel reformer configured to mix a hydrocarbon fuel and steam together upstream of the fuel cell, and to partly or fully convert the hydrocarbon fuel and steam into a reformed fuel stream comprising hydrogen (H₂), carbon monoxide (CO) and carbon dioxide (C0₂), wherein the fuel cell receives the reformed fuel stream at or above atmospheric working pressure and further receives an air stream at or above atmospheric working pressure, the air stream comprising oxygen (0₂) and nitrogen (N₂), wherein the fuel cell generates a hot exhaust stream comprising lean air, unoxidized CO and residual H₂ in response to the reformed fuel stream and the air stream, wherein the lean air comprises an 0₂ molar fraction less than that of the fuel cell inlet air; an internal or external combustion engine configured to generate power in response to the pressurized hot exhaust stream generated via the fuel cell; and a Rankine cycle driven via heat recovered from both the hot exhaust stream generated via the fuel cell and hot exhaust gas generated via the combustion engine.

2. The hybrid fuel cell plant according to claim 1, wherein the fuel cell comprises a solid-oxide fuel cell.

3. The hybrid fuel cell plant according to claim 1, wherein the fuel cell comprises a molten carbonate fuel cell.

4. The hybrid fuel cell plant according to claim 1, wherein the fuel cell comprises both a solid-oxide fuel cell and a molten carbonate fuel cell.

5. The hybrid fuel cell plant according to claim 1, wherein the Rankine cycle comprises a DReSCO-type C02 Rankine cycle.

6. The hybrid fuel cell plant according to claim 1, wherein the Rankine cycle comprises an Organic Rankine cycle.

7. The hybrid fuel cell plant according to claim 1, further comprising a turbocharger or compressor-turbine system driven via hot exhaust gas generated via the combustion engine to generate compressed air.

8. The hybrid fuel cell plant according to claim 7, further comprising a recuperator configured to extract heat from at least one of the fuel cell hot exhaust stream and the hot exhaust gas generated via the combustion engine to further heat the pressurized air entering the fuel cell cathode.

9. The hybrid fuel cell plant according to claim 1, further comprising a compressor configured to further compress the pressurized exhaust stream generated via the fuel cell prior to use by the combustion engine.

10. The hybrid fuel cell plant according to claim 1, further comprising a condenser configured to cool the pressurized exhaust stream generated via the fuel cell and further to remove water from the pressurized exhaust stream generated via the fuel cell prior to use by the combustion engine.

11. The hybrid fuel cell plant according to claim 1, wherein the combustion engine comprises a reciprocating gas engine.

12. The hybrid fuel cell plant according to claim 1, wherein the combustion engine comprises a gas turbine.

13. The hybrid fuel cell plant according to claim 1, wherein the reformer, fuel cell, combustion engine and Rankine cycle are together configured to provide a fuel cell plant that operates at greater than about 60% efficiency.

14. The hybrid fuel cell plant according to claim 1, wherein the reformer, fuel cell, combustion engine and Rankine cycle are together configured to provide a fuel cell plant that operates at greater than about 65% efficiency.

15. The hybrid fuel cell plant according to claim 1, wherein the reformer, fuel cell, combustion engine and Rankine cycle are together configured to provide a fuel cell plant that operates at greater than about 70% efficiency.

16 The hybrid fuel cell plant according to claim 1, wherein the reformer, fuel cell, combustion engine and Rankine cycle are together configured to provide a fuel cell plant that operates between about 50% and about 75% efficiency.

17. A method of generating power via a hybrid fuel cell plant, the method comprising: reforming a hydrocarbon based fuel together with a stream of steam upstream of a fuel cell via an external reformer and generating a substantially pure hydrogen fuel stream at or above atmospheric working pressure therefrom; generating a stream of air at or above atmospheric working pressure; generating a hot exhaust stream comprising carbon monoxide and residual hydrogen, and further comprising a lean air stream via a fuel cell in response to the reformed fuel and stream of air, such that the lean air stream has an 0₂ molar fraction less than that of the fuel cell inlet air, wherein the hot exhaust stream is generated above atmospheric pressure when the reformed fuel and stream of air enter the fuel cell above atmospheric pressure; driving a combustion engine in response to the fuel cell hot exhaust stream to generate power; and generating power via a Rankine cycle in response to waste heat recovered from at least one of the fuel cell hot exhaust stream and hot exhaust generated via the combustion engine to provide a fuel cell plant having an efficiency between 50% and about 75%.

18. The method of generating power via a hybrid fuel cell plant according to claim 17, wherein driving a combustion engine comprises driving a reciprocating gas engine.

19. The method of generating power via a hybrid fuel cell plant according to claim 17, further comprising compressing air above atmospheric pressure via a turbocharger or compressor-turbine system driven via hot exhaust gas generated via the combustion engine.

20. The method of generating power via a hybrid fuel cell plant according to claim 19, further comprising extracting heat from at least one of the fuel cell hot exhaust stream and the hot exhaust gas generated via the combustion engine via a recuperator to further heat the pressurized air generated via the turbocharger or compressor-turbine system.

21. The method of generating power via a hybrid fuel cell plant according to claim 17, further comprising condensing the hot exhaust stream generated via the fuel cell to cool and remove water from the hot exhaust stream generated via the fuel cell prior to use by the combustion engine.

22. The method of generating power via a hybrid fuel cell plant according to claim 21, further comprising compressing the condensed hot exhaust stream generated via the fuel cell prior to use by the combustion engine.

23. The method of generating power via a hybrid fuel cell plant according to claim 17, wherein generating a hot exhaust stream comprises generating a hot exhaust stream via a solid-oxide fuel cell or molten carbonate fuel cell.

24. The method of generating power via a hybrid fuel cell plant according to claim 17, wherein generating power via a Rankine cycle in response to waste heat recovered from at least one of the fuel cell hot exhaust stream and hot exhaust generated via the combustion engine comprises generating power via an Organic Rankine cycle.

25. The method of generating power via a hybrid fuel cell plant according to claim 17, wherein generating power via a Rankine cycle in response to waste heat recovered from at least one of the fuel cell hot exhaust stream and hot exhaust generated via the combustion engine comprises generating power via a DReSCO-type C0₂ Rankine cycle.