Method and apparatus for removal of contaminants from a hydrogen processor feed stream, as in a fuel cell power plant

Method and apparatus are provided for removing contaminants from a hydrogen processor feed stream, as in a fuel cell power plant (110). Inlet oxidant (38), typically air, required by a catalytic hydrogen processor (34) in a fuel processor (14) for a fuel cell stack assembly (12) in the power plant (110), may contain contaminants such as SO2 and the like. A cleansing arrangement, which includes an accumulator/degasifier (142, 46) acting as a scrubber, and possibly also a water transfer device (118), receives the inlet oxidant and provides the desired cleansing of contaminants. Water in the water transfer device and in the accumulator/degasifier serves to dissolve the water-soluble contaminants and cleanse them from the oxidant stream. The cleansed oxidant stream (138′) is then delivered to the hydrogen processor and to the fuel cell assembly, with minimal inclusion of detrimental contaminants such as sulfur.

TECHNICAL FIELD

This invention relates to hydrogen processors, and particularly to hydrogen processing and processors as used in fuel processing systems for fuel cell power plants for providing a hydrogen-rich stream to a fuel cell stack assembly. More particularly, the invention relates to an arrangement for removing contaminants, especially sulfur, from the process flow stream.

BACKGROUND ART

Catalytic systems exist for processing a fuel stream to derive hydrogen, typically for use as a fuel or reactant. One example of such a fuel processing system resides in the fuel processors associated with fuel cell power plants. Fuel cell power plants are becoming well known for a variety of applications requiring a relatively clean and reliable source of electrical power. The power plant typically includes one or more fuel cell stack assemblies (CSA) having anode and cathode regions separated by an electrolyte. The electrolyte may take various forms, with phosphoric acid and solid polymer proton exchange membrane (PEM) being two of the more common, and gives rise to the characterization and operation of the fuel cell.

The generation of electrical power results from an electrochemical reaction occurring between fuel (a reducing agent) supplied at the anode and an oxidant (oxidizing agent) supplied at the cathode. The oxidant is typically air and the reducing agent is hydrogen in a pure, or more typically near-pure, form. The hydrogen may be obtained from various sources, with various forms of hydrocarbon feedstock being the most common. The hydrocarbon feedstock is catalytically processed by a fuel processing system (FPS) also associated with the power plant. The FPS converts the hydrocarbon feedstock to a hydrogen-rich fuel stream via reformation and shift reactions, and may also include selective oxidation to reduce CO levels.

The PEM-type fuel cell is enjoying increased interest, in part because of certain stabilities afforded by the solid membrane electrolyte and in part because of its ability to operate at relatively lower temperatures and pressures and higher current densities. However, the PEM fuel cell also has certain challenging requirements, one of which is the proper management of the water coolant in the system. The coolant water exists as a byproduct of the electrochemical reaction and is used for thermal management in the fuel cell, as a source of water and/or steam for the fuel processing system, and also to maintain appropriate moisture levels at the anode electrode adjacent the PEM membrane.

FIG. 1illustrates a fuel cell power plant10generally in accordance with the prior art. The power plant10comprises a fuel cell stack assembly (CSA)12, a fuel processing system (FPS)14, a water management/treatment system16, and may also include a water transfer device (WTD)18. The CSA12is a PEM-type fuel cell, having an anode region20, a cathode region22, an electrolyte region24between the anode region20and the cathode region22, and a coolant region and flow path26. The electrolyte is PEM.

A hydrogen-rich fuel stream28is supplied to the anode20of CSA12. That fuel stream28is derived from a source of hydrocarbon feedstock30, by means of the FPS14. The hydrocarbon feedstock may typically be methane, natural gas, gasoline, LPG, naphtha, or the like, and the FPS14converts the feedstock, as by reformation, to various component quantities and compounds, including H2, CO, CO2, NH3, etc. As there is normally some sulfur content in the hydrocarbon feedstock30which may be deleterious to the catalysts (typically a noble metal such as platinum) in various portions of the FPS14and particularly the catalyst of anode20of the CSA12, provision is made for removing, or at least reducing the level of, that contaminant from the fuel stream. That removal is typically accomplished with a desulfurizer, or hydrodesulfurizer (HDS)32, normally connected between the source of hydrocarbon feedstock30and an initial or secondary portion of the FPS14. The desulfurized feedstock is delivered to the FPS14via conduit33.

The FPS14may take a variety of forms, but typically includes at least a hydrogen generator and a shift reactor for segregating the hydrogen from the hydrocarbon feedstock and for shifting resulting CO to CO2. The FPS may also provide subsequent selective oxidation (SOX) to further reduce CO concentrations. While the hydrogen generator may take various forms depending on feedstocks, system dynamics, and/or cost considerations, one common general configuration is that of a partial oxidizer (POX), a catalytic partial oxidizer (CPOX), or an autothermal reformer (ATR), in which the typically desulfurized hydrocarbon feedstock is directly burned or combusted with a supply of oxidant (air) and, except with a POX, water and/or steam in a reformation reaction, which reaction components jointly and severally represent feed streams to the hydrogen generator.

In the illustrated example ofFIG. 1, the initial hydrogen generator in the FPS14is a CPOX34, which receives desulfurized hydrocarbon feedstock30via HDS32. An oxidant source apparatus, such as blower36, supplies the CPOX34with an oxidant38, such as inlet air, passed through the WTD18. The blower36, or preferably a separate blower37, also supplies inlet air as oxidant to the cathode22of the CSA12. The WTD18serves to transfer water, otherwise exiting the system, into the incoming air and thereby enhances water balance and energy efficiency of the plant10. The WTD18may be of the general type described in U.S. Pat. No. 6,048,383 to Breault, et al and assigned to the assignee of the present invention, and alternatively referred to as a mass transfer device or, where the transfer of thermal energy is the principal application, an energy recovery device (ERD). The WTD18typically includes a mass transfer medium39, such as one or more plates, membranes, or the like, for permitting mass transfer between the exiting and entering flow streams, while also maintaining their distinct flow paths. The CPOX34is also provided with a supply of water and/or steam40, which may be taken from the coolant flow path26and/or the water treatment portion16of the system. The CPOX acts in a known manner to catalytically reform, or at least partly reform, the hydrocarbon feedstock30in the presence of oxidant38and water and/or steam40. Reference may be made to U.S. Pat. No. 6,299,994 for a better understanding of the relevant reformation and shift reaction formulas, as well as a general functioning of the CPOX34, with the recognition, however, that the steam and fuel feedstock therein are premixed and reacted in a pre-reforming zone and the resulting effluent is reacted with air in the presence of a catalyst in the POX to provide the reformed effluent stream of H2, CO, CO2and H2O.

Although only the CPOX34of the FPS14ofFIG. 1is depicted as an identified block in the FPS, it will be understood that the FPS additionally includes a shift reactor and typically also a selective oxidizer, collectively represented by block41, with those components functioning in a well-known manner. As noted above, the CPOX34and the remaining components of the FPS14, as well as the catalyst of the anode20, include catalysts that may be sensitive to various contaminants such as CO and sulfur, and components such as the HDS32, the shift reactor and the selective oxidizer serve to reduce those contaminants.

The water management/treatment system16is provided to maintain coolant water in appropriate amount (supply) and temperature, and also to prevent or minimize problems that might occur as a result of contaminants in the coolant water. The process of reforming hydrocarbon feedstock to produce a hydrogen-rich fuel stream has the normal consequence of introducing various gases, such as NH3and CO2, into the fuel stream. Those gases tend to dissolve into the water created in the fuel cell and thus enter the coolant. Those dissolved gases in the coolant represent contaminants in that they may cause the conductivity of the water to increase and support destructive shunt current corrosion. Accordingly, the water management/treatment system16is provided with an accumulator/degasifier42for interaction between an oxidant, typically cathode exhaust air44exhausted from the cathode region22, and coolant water46collected in accumulator42from the coolant region26, to facilitate removal of dissolved gases from the coolant.

Coolant water46from accumulator/degasifier42is circulated to the inlet of coolant flow path26in the CSA12, through the CSA12where it acquires gases from the fuel reformation process, and from the CSA12for return to the water management/treatment system16. The coolant exiting from the coolant flow path26of the CSA12is typically directed through a separator48that removes entrained gas bubbles from coolant, then through a radiator50for thermal control of the coolant, and is then returned to the accumulator/degasifier42. Removal of certain dissolved minerals from the coolant water46is provided by a demineralizer52(DMN) connected in a coolant circuit from the accumulator/degasifier42that by-passes the CSA12and separator48and leads to the radiator50. A circulation pump54connected between the outlets of the separator48and the DMN52and the inlet to radiator50serves to provide the requisite circulation in those liquid circuits. The cathode exhaust air44is admitted to the accumulator/degasifier42and caused to pass in gas-absorbing contact with the coolant water46to cause the gases dissolved in the coolant water to diffuse into the cathode exhaust air44. That “gas-laden” air, which also contains significant water and some thermal energy, then leaves the accumulator/degasifier42via conduit56and is conveyed out of the plant via passage through the WTD18where water and thermal energy are transferred to the inlet air38. Further, the exhaust from anode20is represented by conduit58and may be conveyed directly out of the system, or more typically is utilized for unburned hydrogen content to fuel a burner system (not shown), but in either event may be optionally directed through the WTD18(represented by broken line inFIG. 1) with the cathode exhaust56, in order to recover water and/or thermal energy content it may possess.

An additional understanding of a water management/treatment system may be derived from reference to U.S. Pat. No. 6,207,308 to Grasso, et al and assigned to the assignee of the present invention. That patent describes a water management/treatment system that is similar in many respects to that ofFIG. 1, but with the additional provision that it first passes its inlet air, ultimately destined for the cathode, through the degasifying apparatus to cleanse dissolved gases from the water coolant flowing in that degasifying apparatus. Moreover, the inlet air and the water containing the dissolved gases are caused to flow in a counter-current manner in the degasifying apparatus to maximize the release of gas from the coolant water.

While the prior art has addressed concerns with contaminants such as dissolved gases in the coolant water and sulfur in the hydrocarbon feedstock, as discussed with respect to the description of the fuel cell power plant10ofFIG. 1, an additional concern remains that the level of the sulfur in the fuel stream delivered to the CSA12may be excessive as the result of other sources of sulfur. Specifically, the sulfur level target output from the HDS32in the fuel stream is 25 ppb/by vol. This fuel then feeds the CPOX34(reformer). Assuming no additional sulfur from another source, the 25 ppb becomes about 5 ppb/by vol. coming out of the CPOX34. The drop in concentration is due to the dilution effect of air and steam that are added in the CPOX process. It has now been recognized that air has about 5 ppb sulfur on average, which contributes to the sulfur coming out of the CPOX. This is particularly true in large urban areas such as the Northeastern United States, where the sulfur level may be 5–10, to as much as 30, ppb/by vol. As a result, the level can be more like 10 ppb or greater, with additional sulfur possibly also coming from the water that is used for the steam. Thus it can be seen that the sulfur in the air can easily double the sulfur load affecting the water gas shift reactor (WGS) catalyst, any possible SOX catalyst, and possibly the catalysts of the fuel cell itself.

Accordingly, it is an object of the invention to provide method and apparatus for improved removal of contaminants from catalytic fuel processors for making hydrogen, and particularly for use in a fuel cell power plant.

It is a further object of the invention to improve the removal of sulfur as a potential contaminant to one or more of the catalysts in the fuel processor and/or cell stack assembly of a fuel cell power plant.

It is yet a further object of the invention to provide a relatively efficient and cost-effective arrangement for the improved removal of contaminants, such as sulfur, in a fuel cell power plant.

It is an even further object of the invention to provide an improved arrangement for reducing the level of sulfur present in the fuel and/or oxidant flow streams of a fuel processor and/or cell stack assembly for a PEM fuel cell power plant.

It is a still further object of the invention to provide a water transfer device (WTD) with enhanced capabilities for the reduction of contaminants.

DISCLOSURE OF INVENTION

In accordance with the invention, there is provided an improved contaminant removal method and system for a catalytic fuel processor for making hydrogen. The contaminant removal system finds particular use in a fuel cell power plant having a fuel cell stack assembly (CSA) and a fuel processing system (FPS), though is not exclusive thereto. The FPS includes a hydrogen generator, such as a CPOX or the like, and a shift reactor for converting a hydrocarbon fuel stream to provide a hydrogen-rich stream for use in the CSA. The FPS further includes a supply of oxidant for connection to the hydrogen generator for combustion therein with the fuel stream to convert some of the hydrogen. In anticipation that the supply of oxidant contains a contaminant at least including sulfur, deleterious to one or both of the FPS and the CSA, the contaminant removal system of the invention comprises a cleansing arrangement connected intermediate the supply of oxidant and the hydrogen generator for reducing at least the sulfur contaminant in the supply of oxidant prior to admission of the oxidant to the hydrogen generator.

In a general arrangement according to the invention, the oxidant containing the sulfur contaminant, typically including SO2, is caused to pass in intimate mass transfer association with water to permit dissolution of the sulfur/sulfur compound into the water. That intimate mass transfer association is achieved by maximizing the area and time of the oxidant/coolant interface. The oxidant thus freed of that sulfur is then passed to the hydrogen generator for combustion with the fuel stream.

In accordance with one preferred embodiment, the degasifying/accumulator apparatus typically associated with the water management/treatment system of present power plant systems is configured, or reconfigured, to act as a scrubber, and receive the contaminant/sulfur-containing oxidant and pass it in intimate contact with water circulating in the system to remove soluble solids and gases and entrained solids, particularly the soluble sulfur/sulfur compounds, from the oxidant. In addition to sulfur compounds, other contaminants such as NaCl (sea salt) may be scrubbed from the oxidant. The cleansed oxidant is then passed to the hydrogen generator, and also to the cathode of the CSA. The dissolved sulfur and other contaminants may be removed from the water by means of a demineralizer typically forming part of the water treatment system.

In accordance with another embodiment of the invention, a water transfer device (WTD) typically present in existing power plants for managing the water content of inlet oxidant through mass transfer interrelation with exhaust reactant gas streams, is configured to use water in that exhaust gas stream and/or from other sources to enhance dissolution of sulfur/sulfur compounds contained in the inlet oxidant.

In a still further embodiment, the sulfur-cleansing capabilities of both the degasifying apparatus and the enhanced WTD are combined to optimize the removal of sulfur from the oxidant supplied to the hydrogen generator and to the cathode of the CSA.

In yet a further embodiment of the invention, the cleansing of the oxidant by its basic intimate mass transfer association with coolant, typically water, is further enhanced/supplemented by one or a combination of the following techniques and arrangements: a) the addition of water to the oxidant at or near the entry to the WTD, as by spray, and/or the inclusion of a condensation zone in, or near, the WTD to facilitate formation of sulfate aerosols in the oxidant stream for subsequent removal in the degasifying/accumulating apparatus; b) the inclusion of scavenger aids, such as iron oxide and/or calcium oxide, present either on surfaces in the WTD or in the accumulating apparatus for forming salts (soluble or insoluble) of sulfite and sulfate to thereby limit back diffusion of SO2into the gas phase; c) the inclusion of aqueous-based catalyst agents, such as iron oxide and/or hydrogen peroxide, typically in the coolant loop, to increase the oxidation rate of SO2in the accumulating apparatus to form highly soluble sulfate ions (i.e., sulfuric acid); and d) the inclusion of gas-phase catalyst agents, such as ozone, typically in a portion of the system through which the inlet oxidant passes to increase the oxidation rate of SO2in the accumulating apparatus to form sulfite and then to form sulfate aerosols.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference is made toFIG. 2for an understanding of the fuel processor (hydrogen generator) contaminant removal system of the present invention, particularly in the exemplary context of a fuel cell power plant and relative to the prior art ofFIG. 1. Reference numbers identical to those ofFIG. 1are used inFIG. 2for those components that are the same, or substantially the same, in the two configurations. However, where there is some functional, compositional, or structural difference occasioned by the invention, but the components ofFIG. 2nevertheless remain analogous to components inFIG. 1, they have been given the same reference number, but preceded by a “1”. The following description will emphasize the novel character, structure, and/or function of the contaminant removal system of the invention, and will attempt to minimize repetition of description that is duplicative of that provided with respect toFIG. 1. Moreover, while the description is focused on the removal of sulfur, including sulfur compounds, from the inlet air, it is intended to similarly include and apply to other water-soluble solids and gases, as well as entrained solids, that may be removed as contaminants by the method and apparatus of the invention.

The fuel cell power plant110includes a CSA12which receives, at anode20, hydrogen-rich fuel128with reduced sulfur levels. The hydrogen-rich fuel is derived from a source of hydrocarbon feedstock30, which is preliminarily desulfurized by HDS32and is then processed by FPS14. The initial hydrogen generation stage of the FPS14comprises CPOX34, which receives the fuel via conduit33, and further receives desulfurized inlet oxidant138′ in accordance with the invention and water/steam140for a reformation reaction through direct combustion of the fuel and oxidant in the presence of a high-temperature platinum catalyst. The reformed, or at least partly reformed, fuel stream from the CPOX34is then further processed in a known manner in the FPS14to yield the hydrogen-rich fuel128.

The desulfurized inlet oxidant138′ supplied to the CPOX34is obtained in the following manner. The inlet oxidant38, typically inlet air, is subjected to one or more cleansing processes for removing (or at least significantly reducing the level of) sulfur from the air stream. Of main importance and impact is the removal of water-soluble SO2from the inlet oxidant stream38, principally by means of an accumulating/degasifying apparatus142acting as a scrubbing apparatus. The accumulating/degasifying apparatus142is part of a water management/treatment system116. The water management/treatment system116comprises a circuit that maintains coolant water in appropriate amount (supply), temperature, and quality in the fuel cell power plant110. Alternatively, or typically additionally, SO2may be removed from the inlet oxidant38by means of an enhanced WTD118.

The accumulating/degasifying apparatus142is functionally similar to the accumulator/degasifier42of theFIG. 1embodiment, and perhaps even more similar to the degasifying apparatus described in the aforementioned U.S. Pat. No. 6,207,308. Notably, however, while those prior art arrangements had the objective of removing dissolved gases from the coolant exiting the coolant channel26of the CSA12, the present arrangement additionally cleanses, or scrubs, soluble SO2, and other soluble solids and gases and entrained solids, from the inlet oxidant38through intimate counter-current contact of the inlet oxidant stream with the coolant admitted to the degasifying apparatus142from the radiator50. The inlet oxidant is supplied to the degasifying apparatus142via conduit138after passing through the WTD118. The structure of the degasifying apparatus142and the points of entry thereto of the inlet oxidant in conduit138and the coolant from radiator50, as well as the point of egress of the subsequently cleansed air, are such as to promote the intimate contact of the two fluids to dissolve the SO2from the air stream. Typically, the inlet oxidant and the coolant water in the water management/treatment system116are caused to pass each other in counter-current manner and over such interface surface area and duration as to maximize the intimate mass transfer association of the two fluids to cleanse the inlet oxidant. The degasifying apparatus may include appropriate baffles, flow direction changers, and or agglomerating screens to effect the cleansing action as well as subsequent demisting of the inlet air exiting therefrom. The cleansed inlet oxidant is then removed from the degasifying apparatus142typically, though not necessarily, near its upper end, via conduit(s)138′ connected through blowers36and37for delivery of the cleansed air to the CPOX34and to the cathode22of CSA12, respectively. The cleansed oxidant will contain little or substantially no SO2, with a majority having been removed by the cleansing action. In this way, the initial contaminant content of the inlet oxidant does not adversely impact the objective of obtaining less than about 5 ppb/by vol. sulfur content in the fuel stream issuing from the CPOX34.

In addition to the cleansing of sulfur from the inlet air provided by the degasifying apparatus142, additional benefits in this and other regards are obtained with a WTD118, optionally having enhanced capabilities relative to those of the WTD18described inFIG. 1. Referring further toFIG. 2and additionally toFIG. 3, the WTD118typically includes, as mentioned above, a mass transfer medium139, such as one or more plates, membranes, or the like, for permitting mass transfer of at least water between the exiting and entering flow streams, while also maintaining their distinct flow paths. Although the mass transfer medium139has been depicted, for simplicity, as a single broken-line surface, it in fact is comprised of a number of plates or the like, which present a large surface area to the flowing gas streams. Indeed, the mass transfer medium139may be a number of such plates arranged to define a number of generally-parallel adjacent flow channels of incoming oxidant and exhausting gases respectively to maximize the interface area of the medium between the counter-flowing gas streams. The cathode exhaust air is here designated144and is depicted as being connected from cathode22to the WTD118without passing through the degasifying apparatus142. The CSA12produces humid reactant exhausts of the oxidant, as cathode exhaust air, and the reducing agent, as anode exhaust. Migration of water in the humid exhausting gas, through the plates to the relatively drier, contaminant-containing inlet oxidant, serves to dissolve some of the water-soluble sulfur contaminants (i.e., SO2) to form sulfate aerosols. Those sulfate aerosols are removed from the inlet oxidant in the degasifying/accumulating apparatus142.

In addition to the sulfur-cleansing of the inlet air by its basic intimate mass transfer association with water in the degasifying/accumulating apparatus142and further the WTD18/118as described above, additional enhancement of that cleansing capability may optionally be obtained, if required, via one or a combination of, the following described arrangements.

Referring to the WTD118ofFIG. 2and particularlyFIG. 3, one or more water injection devices, depicted as spray nozzles70, typically at or near the inlet end of the WTD118for the inlet oxidant38, are disposed to inject a spray of water into the inlet oxidant stream38of the WTD118, such that the excess water helps to dissolve SO2and form sulfate aerosols. Alternatively, or additionally, such a sulfate aerosol may be derived by condensing moisture contained in the inlet oxidant38, or more particularly the inlet oxidant in the conduit138after exiting the WTD118, as by the optional condensing coils72depicted in broken-line form inFIGS. 2 and 3.

Further optional arrangements for enhancing the cleansing of sulfur from the inlet oxidant include one or more of a) the inclusion of scavenger aids, such as iron oxide and/or calcium oxide, present either on surfaces in the WTD118or in the accumulating apparatus142for forming salts (soluble or insoluble) of sulfite and sulfate to thereby limit back diffusion of SO2into the gas phase; b) the inclusion of aqueous-based catalyst agents, such as iron oxide and/or hydrogen peroxide, typically in the coolant loop, to increase the oxidation rate of SO2in the accumulating apparatus to form highly soluble sulfate ions (i.e., sulfuric acid); and c) the inclusion of gas-phase catalyst agents, such as ozone, typically in a portion of the system through which the inlet oxidant38,138passes, to increase the oxidation rate of SO2in the accumulating apparatus to form sulfite and then to form sulfate aerosols.

Because the demineralizer52serves to cleanse the dissolved sulfur, amongst other things, from the circulating coolant stream, that coolant stream at the outlet side153of the DMN52may be used as a source of cleansed water for use in the FPS14, as, for example, for the water or steam140at the inlet to the CPOX34.

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. For example, while the fuel processor to which the air cleansed of sulfur and other contaminants is supplied is associated with the production of hydrogen for a fuel cell power plant in the illustrated embodiment, it will be understood that the invention is also applicable to other similar catalytic fuel processors that provide hydrogen for other end-use applications. Moreover, although the accumulating/degasifying apparatus has been depicted in a structurally-integrated form in the preferred arrangement, it will be understood that multiple, closely-associated, separate components may be used to provide the similar function and results.