Patent Application
20100104903
This invention relates to membrane water gas shift reactors in a fuel processing system, and more particularly to membrane water gas shift reactors included in a fuel processing system for fuel cell power plants and the like.
There exists a need to provide hydrogen (H2) as a fuel for various end uses, particularly as a fuel in fuel cell power plants and the like. The hydrogen is typically chemically bound, as in a raw hydrocarbon and/or including alcohol, or it may be in a processed gas mixture such as syngas (H2 and CO), which, in either event, is processed by a fuel processing system to provide a hydrogen-rich fuel stream for eventual use as fuel for a fuel cell. The raw fuel is typically reformed by a process that not only provides a hydrogen-rich fuel stream, but which also results in the production of carbon monoxide. Unfortunately, the carbon monoxide is a very effective poison for low temperature fuel cells (<100° C.). The CO gets adsorbed on the noble metal catalyst in the fuel cell stack, thereby preventing the H2 from reacting. Only a very small concentration of CO is necessary to considerably reduce the number of the reaction sites available. CO concentration of <50 ppm is typically required for a proper operation of the fuel cell stack.
State of the art fuel processing systems rely on the integration of several reactors and heat exchangers (HEXs), e.g., a reformer, a water gas shift reactor train (WGS) and a preferential oxidizer train (PROX) are thermally integrated to produce reformate. This reformate can be fed into the stack after a final CO cleaning is achieved by direct injection of oxygen into reformate. The CO cleaning process (WGSs, PROXs and HEXs) adds weight, volume, complexity and cost to the fuel cell power plant.
Membrane reactors offer an inherent ability to combine reaction, product concentration, and separation in a single unit. A type of membrane reactor of particular interest is an integrated water gas shift reactor with palladium alloy based membrane for selectively removing hydrogen. Broadly speaking, a membrane reactor includes a primary chamber or region containing a catalyst for receiving a hydrogen-rich, gaseous derivative, e.g. reformate, of the raw fuel and reacting the reformate to liberate hydrogen, a secondary chamber or region for receiving nearly pure hydrogen as a permeate from the first region, and a palladium membrane separating the primary and secondary regions and providing a hydrogen-selective permeability for the exclusive transfer of hydrogen from the primary region to the secondary region.
Though membrane reactors have been discussed generally in the literature, as for example in U.S. Pat. No. 6,228,147 to Takahashi for operating a membrane reactor with a steam flow as a sweep gas, relatively little or no discussion exists in the area of the design of the membrane reactor and the integration of such a system into a fuel cell-based power plant. For instance, a recent U.S. Pat. No. 6,572,837 to Holland et al, though discussing a hydrogen-separating membrane in use in a fuel processing system for a fuel cell power plant, describes the hydrogen separation function and structure as being physically separate from the various reactor structures and functions.
An example of a fuel cell power plant that does incorporate a water gas shift reactor integrated with a hydrogen-separating membrane is illustrated and described in U.S. Pat. No. 6,423,435 to Autenrieth, et al. That system tends to work at the relatively high pressure, ie, greater than 10 bar, of 12 bar in the reformer and WGS reactor, and then relies upon a relative vacuum of 0.5 bar to remove the H2 from the hydrogen collecting space, but then further re-pressurizes that H2 to about 1.5 bar to feed the anode of the fuel cell. This system also employs some measure of heat and water management to provide limited assistance to the efficiency of the system. While the foregoing examples of fuel processing systems each discuss various ways of separating hydrogen from a reformate stream, at most only limited attention is given to the efficient integration of that process into the overall system of a power plant, particularly in a fuel cell power plant of the type having a PEM (polymer electrolyte membrane) fuel cell assembly that incorporates porous water transport plates and operates at or near, ambient pressure.
It is an objective to provide an arrangement for enhancing the efficiency with which hydrogen is removed from the reformate stream of a fuel processor in the larger context of a power plant. It is a further objective to do so through the use of a membrane water gas shift reactor as part of a fuel processing system in the context of a PEM-type fuel cell power plant. It is a still further objective to do so through the use of a water gas shift reactor as part of a fuel processing system in the context of a PEM-type fuel cell power plant having a fuel cell assembly that incorporates porous water transport plates and operates at moderate pressures near or somewhat above ambient pressure.
The present invention pertains to a fuel cell power plant system comprising at least one fuel cell stack assembly including an anode, a cathode, and a coolant channel; a fuel processing system for providing H2 to the anode and including a water gas shift membrane reactor having a reaction region and a permeate region separated by a H2 separation membrane, the reaction region being connected to receive a supply of H2-rich reformate and a supply of water for supporting a water gas shift reaction of the reformate to enhance the production of H2 and to shift CO to CO2, the produced H2 being selectively separated from the reformate stream via the membrane to form a permeate in the permeate region of the reactor, and the reformate stream issuing from the reactor as a retentate; a source of heat; and a water management system operatively connected to the fuel cell assembly coolant channel for conducting water from and to the fuel cell assembly. A stream of sweep gas is caused to flow through the permeate region of the water gas shift membrane reactor to facilitate the separation of H2 via the membrane. The water management system is additionally connected to the source of heat for converting some water to steam, and the steam may be operatively connected to the permeate region of the water gas shift membrane reactor to provide some or all of the sweep gas flowing there through. In one embodiment, steam is the only sweep gas. In another embodiment, steam is combined with an inert gas, such as nitrogen (N2) from combusted or otherwise O2-depleted air.
This integration of the power plant components and functions yields a particularly efficient arrangement, from the standpoint of overall plant efficiency, for generating H2 for use by the fuel cell of the power plant.
The fuel cell stack assembly is of the PEM type (polymer electrolyte membrane) that incorporates porous water transfer plates (WTPs) for the efficient recovery of water from the fuel cell stack assembly, which water is then available for use in the processing of fuel into H2, and for the generation of the steam for use as a sweep gas. The system also employs energy recovery devices (ERDs) that serve to recover water from hot exhaust streams while exchanging heat to incoming air. Additionally, selective use of existing thermal energy sources, such as the combustion of the retentate from the fuel processing system for steam generation, enhances the efficiency of the system.
In yet a further aspect of the invention, the sweep gas stream is preferably caused to flow through the permeate region of the water gas shift membrane reactor in a direction that is counter-current (contra) to the flow of the reformate/retentate stream to increase the efficiency for a given volume of the membrane reactor.
In a still further aspect of the invention, a connected combination of a compressor and an expander may be included respectively before and after at least the membrane reactor to efficiently provide limited pressurization of fluid flow through the membrane reactor. That pressurization is moderate, typically being in the range of 1 to 10 bars, and is preferably about 6 to 7 bars.
In yet a further aspect of the invention, the reaction region of the water gas shift membrane reactor contains catalyst loaded or arranged therein to yield an improved efficiency/cost ratio. Particularly, assuming that the sweep gas flows in a direction counter to the flow of the reformate/retentate stream, it is beneficial to use catalyst at or near the opposite ends of the reaction chamber, but to omit its use in the mid-region of the reaction chamber where it is relatively ineffective. In the event the sweep gas flows in a direction concurrent (cocurrent) with the flow of the reformate/retentate stream, it is beneficial to use a limited amount of catalyst only in the entry end or region of the reaction chamber, but to omit its use from the mid-point to the end of the reaction chamber where it is relatively ineffective.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.
Referring to
In a coolant loop 16′ that is local to the fuel cell assembly 12, water exiting from coolers 24 is circulated by pump 30 through circuit 31 to a conventional degasifier/accumulator 32 where gas is removed from the coolant and the water is then accumulated and available in liquid form for return to the fuel cell assembly coolers 24 via line 33. Water accumulated in degasifier/accumulator 32 is also available for use in the FPS 14 via line 16″.
The oxidant supplied to the cathodes 20 via line 28 preferably comprises the passage of ambient air through a gas channel of a water transfer energy recovery device (ERD) 34 of suitable known design, as for example of the type described in U.S. Pat. No. 6,274,259 to Grasso et al and incorporated herein by reference. The driver for that air flow may be, for example, a blower 35. Spent oxidant laden with moisture is exhausted from the cathodes 20 via the degasifier/accumulator 32 and thence on line 29 through the ERD 34, where it transfers heat and moisture to the incoming air.
A stream of spent H2 is exhausted from the anodes 18 and is both recycled via line 36 by blower 37 to the inlet of the anodes and is also conveyed via line 38 to a catalytic burner 40.
Having described the relatively conventional structure and operation of the PEM fuel cell assembly 12, attention is turned to a description of the FPS 14 and its integration with the fuel cell assembly 12 and the coolant system 16. A supply of carbon-based fuel 42, as for instance gasoline, natural gas or other similar hydrocarbons, is delivered by fuel pump 43 via heat exchangers 44 and 45 where it receives heat, and line 46, to a hydrodesulfurizer (HDS) 47 where sulfur is removed from the fuel. The HDS 47 may be optional if desulfurized fuel is used. The desulfurized fuel is then delivered via line 48 to an inlet region of a reformer 50, which may be an autothermal reformer (ATR), a catalytic partial oxidizer (CPOX), a catalytic steam reformer (CSR), or similar reactor for the reformation of the fuel stock. For the reformation process or reaction, the reformer 50 additionally typically requires sources of oxidant (air) and water or steam.
A feedwater pump 51 in the water line 16″ delivers the liquid water to various feeders to points of use in the FPS 14 as will be described. Water from line 16″ is delivered to the inlet region of the reformer 50 via feeder line 16a″ after receiving some heat passing through the cold side of an anode precooler 52 in line 16″.
Air for the reformer 50 is initially heated and humidified by passage thru the ERD 34, and is then delivered via lines 54 and 54′ by a driver, such as the gas compressor 53, through a heat exchanger 56 where it receives heat indirectly from the exhaust of the catalytic burner 40, and then to an inlet region of the reformer via line 57. The compressor 53 is preferably paired with and directly or indirectly driven by, a gas expander 55, the functions of which will be described in greater detail hereinafter. One or more water vapor injectors (sprays) 58 receive water from line 16″ via feeder line 16b″, and are positioned and operative to introduce water vapor to the air stream in line 57 prior to and/or after passage through the heat exchanger 56. To the extent the catalytic burner 40 may not be available as a source of heat, as during system start-up, a separate, limited-capacity start burner 59 is selectively and operatively connected (shown in broken line) to the supply of fuel 42 and the inlet air supplied by the ERD 34, such that the air may be warmed and supplied to line 57 for delivery to the reformer 50. Similarly, a limited-capacity electric heater may serve as the hot-side thermal source for the heat exchanger 45 during start up, or as needed.
The reformer 50 operates to react the air, water and carbon-based fuel in a well known manner to produce a stream of reformate containing a mixture of H2, CO, CO2, H2O (and N2). That reformate, after perhaps receiving a charge of water vapor in a vaporizer section 49, issues from the reformer 50 via line 60 and passes through the hot side of a heat exchanger 61 and to the inlet region of a water gas shift (WGS) membrane reactor 62. The WGS membrane reactor 62 will be described in greater detail, but suffice it to say at this point that the WGS reaction on the reformate creates a gaseous mixture rich in H2 and in which much of the CO has been desirably converted to CO2, then most of the H2 is separated from the mixture via a separation membrane 64, and the remaining constituents of the reformate stream issue from the reactor 62 as an H2-depleted retentate stream on line 66.
The retentate stream on line 66 continues to contain a small amount of H2, and is supplied via expander 55 and line 66′, as one fuel source for the catalytic burner 40. Another fuel source for that burner 40 is provided by unspent H2 in the anode exhaust stream of line 38. Oxidant (air) for the combustion reaction in burner 40 is supplied to the burner via blower 68. A heated stream of air and combustion products exhausted from burner 40 is extended on line 69 through the heat exchanger 56, to provide heat to the air being heated therein for supply on line 57 to the reformer 50. After exiting the heat exchanger 56, the burner exhaust stream 69 extends through the hot side of heat exchanger 44 and thence through the hot side 70A of a steam generating heat exchanger 70A&B and is connected to the cathode exhaust line 29 prior to being exhausted through the hot/warm side of ERD 34. A supplemental supply of heated water may be supplied as a spray to the stream 69 by vapor injector 71 connected between heat exchanger hot side 70A and the ERD 34, and receiving water from line 16″ via feeder line 16c″. This serves to cool the exhaust, if needed, and to add moisture to the stream that is recycled via the ERD 34.
System performance is enhanced by providing a portion of the H2-containing reformate from the reformer 50, via line 60′ and through an ejector 72, to the HDS 47 for use in the desulfurization process. The ejector is driven by a pressurized stream of water on feeder line 16d″ connected to line 16 prior to precooler 52.
Attention is now turned to the WGS membrane reactor 62, which forms a principal component of the invention. Referring to
To separate the H2 from the reaction products in the reaction region 74 of the WGS membrane reactor 62, a membrane 64 of H2-selective, permeable material forms an H2-permeable boundary of the reaction region 74. The H2-selective material is typically palladium or the like. Gaseous H2 that diffuses through the H2-permeable membrane 64 accumulates as a permeate in a permeate region 76, for transport to and use in, at least the anodes 18 of the fuel cell assembly 12.
An aspect of the invention is the reliance upon an energy-efficient, intermediate-pressure regime for operation of the fuel cell power plant 10. In that regard, it may be advantageous to provide some pressurization in the FPS 14 to increase the pressure differential across the membrane 64, yet also advantageous to limit the size and capacity of equipment necessary to provide and contain the resulting pressures. To that end, and in conjunction with the PEM fuel cell assembly 12 that typically operates near ambient pressure, the air on line 57 delivered to the reformer 50 is pressurized in the range of 1-10 bar, and is preferably about 6 bar. This is accomplished by the compressor 53. Some of the retentate exiting the WGS membrane reactor 62 on line 66 is fed to the expander 55, where its pressure is reduced and then used to partially fuel the catalytic burner 40. This expansion of the retentate at the expander 55 also serves to recover energy which may then be used to power a motor/generator 67 connected thereto, and/or to drive the compressor 53 which is placed on the same shaft 65 as the expander, thus resulting in efficient energy usage. Whereas the operating pressure in much of the FPS 14 is preferably at an intermediate pressure of about 6 bar, it will be understood that if operation at or near the ambient condition of 1 bar is alternatively preferred, the need for the pressure controlling equipment described above may be avoided.
At this juncture it is important to consider an important aspect of the invention, that being the use of a sweep gas flowing through permeate region 76 as not only facilitating transport of the H2 to the anodes 18, but importantly also, as facilitating the shifting of the WGS reaction towards the products within the reaction region 74. This is accomplished by continued removal of the H2 from the permeate region 76 so as to enable high H2 partial pressure differentials across the membrane portion 64, which facilitates flow of H2 across the membrane portion 64 to decrease the H2 present in the reaction region 74, which in turn acts to shift the WGS reaction equilibrium in a direction favorable to the production of H2.
Although the sweep gas might be inert gas, nitrogen, the H2-lean, moisture-laden exhaust from the anodes 18, or other suitable fluids, including phase-change materials, the sweep gas, designated 78 in
In accordance with yet another aspect of the preferred embodiment of the invention, the sweep gas 78 in
Still further, it has been found that the catalyst 75, to the extent it exists in the middle of the WGS reaction region 74, here designated intermediate portion 74B, relative to longitudinal flow of reformate therethrough, is not utilized effectively in a counter flow configuration because the H2 and CO fractions are too close to equilibrium to have appreciable affect on the rate of the WGS reaction. However, towards the entry and exit end(s) or portions 74A and 74C of the reaction region 74, as the hydrogen partial pressure decreases, the shift reaction is promoted towards the product side, such that having the catalyst at the end of the reactor enables complete conversion of CO. Accordingly, in the operational configuration where the sweep gas flows countercurrent to the reformate flow, the tubes of membrane material 64 that collectively define the WGS reaction region 74 are filed or otherwise loaded with catalyst 75 at or near the entry portion 74A and the exit portion 74C of the region/tubes, and the intermediate portion remains relatively vacant for use only for hydrogen separation. This loading is reflected in
Referring to
Additionally in
Referring to
A second catalytic burner 292 receives H2 and water from the anode exhaust via line 238, and an oxidant-depleted supply of N2-rich air via line 229 from the exhaust of fuel cell cathode 20. The burner 292 and the supply of N2-rich air are regulated carefully to provide a gaseous exhaust stream that is rich in N2 and substantially devoid of O2, and is connected via line 290 to a vapor injector 280. Also connected to the vapor injector 280 is the steam supply line 16f″, such that a measure of steam (H2O) may be mixed with the N2 to provide the resulting sweep gas 278. This particular arrangement for using N2 as a significant portion of the sweep gas has the advantages, relative to steam alone, that the vaporizer and heat exchanger elements may be down-sized, and the use of steam is decreased. While this arrangement may have the disadvantage of slightly lowering the partial pressure of hydrogen supplied to the fuel cell stack assembly 12 (due to the presence of gaseous nitrogen in the fuel feed stream), it is a modest penalty.
Although the invention has been described and illustrated with respect to the exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made without departing from the spirit and scope of the invention.
The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract No. DE-FC26-05NT42453 awarded by the Department of Energy.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2005/047012 | 12/23/2005 | WO | 00 | 6/20/2008 |
