Combined autothermal/steam reforming fuel processor mechanization

A fuel processor system includes first and second reactors each having an inlet that receives fuel from a fuel supply and an outlet that discharges a reformate containing hydrogen. The reactors are operable to reform the fuel to form the reformates. The second reactor is coupled in parallel with the first reactor with the reformates produced by each combining to form a reformate flow. The first reactor can be an autothermal reforming reactor and the second reactor can be a steam reforming reactor. The first and second reactors are controlled differently to provide quick startup and transient capability while providing improved overall efficiency under normal operation.

FIELD OF THE INVENTION

The present invention relates to fuel processors, and more particularly to fuel processors used to produce reformate containing hydrogen that can be used in fuel cells.

BACKGROUND OF THE INVENTION

Fuel cells are increasingly being used as a power source in a wide variety of different applications. Fuel cells have been proposed for use in electrical vehicle power plants to replace internal combustion engines. A solid-polymer-electrolyte membrane (PEM) fuel cell includes a membrane that is sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, hydrogen (H2) is supplied to the anode and air or oxygen (O2) is supplied to the cathode.

In a first half-cell reaction, dissociation of the hydrogen (H2) at the anode generates hydrogen protons (H+) and electrons (e−). The membrane is proton conductive and dielectric. As a result, the protons are transported through the membrane while the electrons flow through an electrical load that is connected across the electrodes. In a second half-cell reaction, oxygen (O2) at the cathode reacts with protons (H+) and electrons (e−) to form water (H2O).

The main function of a fuel processor in the fuel cell system is to provide a controlled hydrogen-containing stream to the fuel cell stack. The fuel cell stack converts the chemical energy in the hydrogen to electrical power to charge capacitors or batteries or to directly power a device such as an electric motor. In hybrid applications, a storage medium such as capacitors or batteries removes some of the problems that are associated with transient demand. For non-hybrid applications, and to a lesser extent in hybrid applications, the fuel processors need to provide a dynamic flow rate of hydrogen-containing gas to the fuel cell stack. When a device is directly powered by the fuel cell, the amount of hydrogen that is required is determined by the demand for power output from the fuel cell. For example in automotive applications, the driver demands power by depressing the accelerator pedal. Acceleration requires the electric motor to turn faster, which requires more current. When the accelerator is depressed, the fuel processor increases the hydrogen that is provided to the fuel cell. The current output by the fuel cell increases and the electric motor accelerates the vehicle.

The fuel processor produces a reformate stream that is composed primarily of hydrogen, carbon dioxide, nitrogen, water, methane and trace amounts of carbon monoxide. During operation, the fuel processor provides the flow rate of hydrogen that is required to meet the current demand for power. As can be appreciated, the demand for power can vary significantly. For example, a vehicle moving in rush hour traffic may repeatedly require sudden acceleration followed by deceleration or braking. Thus, the delivery of hydrogen to the fuel cell stack must vary accordingly. Fuel processors may also require careful metering of air and fuel to maintain precise oxygen to carbon ratio control.

Additionally, a typical fuel processor may use an autothermal reforming reactor as a primary reactor to initiate the production of the hydrogen-containing reformate stream. Autothermal reforming reactors introduce reactants (fuel, oxidants, steam, etc.) into the front of the reactor and allow the associated reactions to occur to completion as the reactants flow through the reactor. The fuel can come in a variety of forms, such as methanol, gasoline, ethanol, etc. The oxidant is typically provided in the form of oxygen (O2) or air (O2mixed with N2). The steam is typically superheated steam which supplies heat and water to the reactor. An autothermal reforming reactor is capable of converting the fuel into a nitrogen/steam diluted reformate stream containing hydrogen and carbon oxides that result from the combined partial oxidation reaction and steam reforming reaction, the extent of each being dependant on the operating conditions (e.g., availability of an oxidant and/or steam and temperature of the reactor). A steam reformer may also serve as the primary reactor which eliminates the nitrogen diluent that is present when partial oxidation is also included as in autothermal reforming. These two different reactions differ in their efficiencies, the operating conditions that increase and/or maximize the efficiencies, and their ability to quickly adjust to transient changes in the demand for the hydrogen-containing reformate stream. For example, the steam reforming reaction is typically more efficient at producing the hydrogen-containing reformate stream than the partial oxidation reaction. Additionally, the steam reforming reaction is more efficient at higher pressures (5-7 bars). The partial oxidation reaction is able to respond more quickly to transient changes in the demand for the hydrogen-containing reformate stream than the steam reforming reaction. Transient response requires rapid response and control in fuel, air and steam delivery, but the rates of response may vary. Furthermore, all the reactors downstream of the primary reactor must also be able to respond rapidly as well.

While the fuel cell stack can consume as much hydrogen as it needs based on the electrical load applied to the fuel cell stack, mismatching the hydrogen flow and the electrical load is problematic. An under-fueled stack may cause some of the fuel cells to temporarily have reverse polarity, which may damage the fuel cell stack. An over-fueled stack will not damage the fuel cell stack but will increase the H2exhausted. If the exhausted hydrogen is fed to a combustor, for example, increased combustion temperature may damage the combustor or cause NOx emissions to increase if additional air control is not used.

Therefore, what is needed is a fuel processor that can provide a required flow rate of hydrogen-containing reformate and respond quickly to transient changes in the demand for the hydrogen-containing reformate. Additionally, it is advantageous to provide these capabilities in an efficient manner.

SUMMARY OF THE INVENTION

A fuel processor according to the principles of the present invention is capable of providing a required flow rate of hydrogen-containing reformate. The fuel processor is also capable of responding quickly to transient changes in the demand for the hydrogen-containing reformate. Furthermore, the fuel processor incorporates two primary reactors to meet the demand for converting the fuel into the carbon oxide/hydrogen-containing reformate. The two primary reactors enable the fuel processor to meet the transient changes in the demand for the hydrogen-containing reformate while more efficiently producing the hydrogen-containing reformate during nontransient operation.

A fuel processing system according to the principles of the present invention includes a fuel supply and first and second primary reactors. The first reactor has an inlet that receives fuel from the fuel supply and an outlet that discharges a first reformate containing hydrogen and carbon oxides. The first reactor is operable to reform the fuel to form the first reformate. The second reactor also has an inlet that receives fuel from the fuel supply and an outlet that discharges a second reformate containing hydrogen and carbon oxides. The second reactor is operable to reform the fuel to form the second reformate. The second reactor is coupled in parallel with the first reactor with the first and second reformates combining to form a reformate flow which continues through additional process units to convert carbon monoxide to carbon dioxide.

In another aspect of the present invention, the fuel processor is part of a fuel cell system. The fuel cell system includes a fuel supply and an oxidant supply. There are first and second primary reactors that each have an inlet that receives fuel from the fuel supply and an outlet that discharges respective first and second reformates containing hydrogen and carbon oxides. The first and second primary reactors are operable to reform the fuel to form the respective first and second reformates. The second reactor is coupled in parallel with the first reactor with the first and second reformates combining to form a reformate flow which subsequently is fed to additional reactors such as water gas shift, preferential CO oxidizers, etc. prior to being fed to the fuel cell stack. A fuel cell stack receives an oxidant flow from the oxidant supply and the reformate flow and uses these to produce electricity.

The present invention also discloses a method of operating a fuel processing system to produce a reformate flow containing hydrogen at a predetermined rate. The method includes the steps of: (a) determining a target H2production rate; (b) producing a first reformate flow containing hydrogen at a first rate in a first primary reactor receiving fuel from a fuel supply; (c) producing a second reformate containing hydrogen at a second rate in a second primary reactor receiving fuel from the fuel supply, the second reactor operating in parallel with the first reactor; (d) combining the first and second reformate flows to form a reformate flow containing hydrogen and carbon oxides; and (e) adjusting at least one of the first and second rates so that the reformate flow is produced at the target H2production rate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1schematically illustrates a cross-section of a fuel cell assembly10that includes a membrane electrode assembly (MEA)12. Preferably, the MEA12is a proton exchange membrane (PEM). The MEA12includes a membrane14, a cathode16, and an anode18. The membrane14is sandwiched between the cathode16and the anode18.

A cathode diffusion medium20is layered adjacent to the cathode16opposite the membrane14. An anode diffusion medium24is layered adjacent to the anode18opposite the membrane14. The fuel cell assembly10further includes a cathode flow channel26and anode flow channel28. The cathode flow channel26receives and directs air or oxygen (O2) from a source to the cathode diffusion medium20. The anode flow channel28receives and directs hydrogen (H2) from a source to the anode diffusion medium24.

In the fuel cell assembly10, the membrane14is a cation permeable, proton conductive membrane having H+ions as the mobile ion. The fuel gas is hydrogen (H2) and the oxidant is oxygen or air (O2). The overall cell reaction is the oxidation of hydrogen to water and the respective reactions at the anode18and the cathode16are as follows:
H2=>2H++2e−
0.5O2+2H++2e−=>H2O
Since hydrogen is used as the fuel gas, the product of the overall cell reaction is water. Typically, the water that is produced is rejected at the cathode16, which is a porous electrode including an electrocatalyst layer on the oxygen side. The water may be collected in a water collector (not shown) as it is formed and carried away from the MEA12of the fuel cell assembly10in any conventional manner.

The cell reaction produces a proton exchange in a direction from the anode diffusion medium24towards the cathode diffusion medium20. In this manner, the fuel cell assembly10produces electricity. An electrical load30is electrically connected across a first plate32and a second plate34of the fuel cell assembly10to receive the electricity. The plates32and/or34are bipolar plates if a fuel cell is located adjacent to the respective plate32or34or end plates if a fuel cell is not adjacent thereto.

Referring now toFIG. 2, a block diagram of a fuel cell system40is illustrated. The fuel cell system40includes a fuel cell stack42that includes multiple fuel cell assemblies10. A fuel processor44provides a reformate stream46to the anode flow channel28. The fuel processor44includes a pair of primary reactors48and50. The first primary reactor48is an autothermal reforming (ATR) reactor and the second primary reactor50is a steam reforming (SR) reactor. As used herein, the term “primary reactor” refers to reactors wherein the hydrocarbon fuel conversion (breakdown) takes place to provide a reformate stream. The ATR reactor48and the SR reactor50are coupled in parallel, as will be described in more detail below. The fuel processor44receives fuel from a fuel supply52, water from a water supply54, and air from an air supply56that may come from a variable speed compressor58. Fuel processor44uses the fuel, water and air to produce reformate stream46.

The ATR reactor48receives a fuel stream60from the fuel supply52, a water stream62from the water supply54and an air stream64from the air supply56. The fuel stream60, water stream62, and air stream64are all controlled by metering devices (not shown) so that the quantities of fuel, water and air are controlled and monitored. The ATR reactor48uses fuel stream60, water stream62and air stream64to produce a first reformate stream66that contains hydrogen (H2) as well as carbon monoxide (CO) and carbon dioxide (CO2). As is known in the art, the ATR reactor48can produce the first reformate stream66with a combination of a partial oxidation reaction and a steam reforming reaction. The ATR reactor48operates at the same pressure as the fuel cell system40. Preferably, the fuel cell system40and the ATR reactor48are operated at a pressure in the range of about 1.5-3.0 bars. The first reformate stream66exits the ATR reactor48and flows through a combustor air preheater68which extracts thermal energy from the first reformate stream66and uses it to heat an oxidant stream70.

The SR reactor50receives a fuel stream72from fuel supply52and a water stream74from water supply54. The fuel stream72and water stream74are controlled by metering devices (not shown) that allow for the fuel stream72and water stream74to be provided to the SR reactor50in controlled quantities. The SR reactor50uses the fuel stream72and water stream74to produce a second reformate stream76in a steam reforming reaction. The second reformate stream76contains H2, CO and CO2. To improve efficiency, the SR reactor50is operated at an elevated pressure relative to the fuel cell system pressure40. Preferably, the SR reactor50is operated in the range of about 5.0-7.0 bars. The fuel stream72passes through a fuel vaporizer78prior to entering the SR reactor50. The fuel vaporizer78vaporizes the fuel stream72so that it is in gaseous form when entering the SR reactor50. The water stream74passes through a water vaporizer80prior to entering the SR reactor50. The water vaporizer80heats up and vaporizes the water stream74so that it is in the form of steam when entering the SR reactor50. Preferably, the water stream74is in the form of super heated steam when entering the SR reactor50.

The second reformate stream76exits the SR reactor50and passes through the backside of fuel vaporizer78and the water vaporizer80wherein thermal energy is extracted from the second reformate stream76to help vaporize the fuel stream72and the water stream74. The second reformate stream76then passes through a pressure let down valve82that lowers the pressure of the second reformate stream76to the fuel cell system pressure. The first and second reformate streams66and76then combine together to form reformate stream46.

Further processing of the reformate stream46to reduce the CO content prior to being fed to the fuel cell stack then takes place. The reformate stream46passes through a low temperature shift inlet cooler84wherein thermal energy is extracted from the reformate flow46and is used to heat air stream64prior to entering the ATR reactor48. In this fuel processor design, the reformate stream46exits the cooler84and passes through a water adsorber86wherein water is removed from or added to the reformate stream46depending upon the operating condition (e.g., temperature) of the fuel processor44. The water adsorber86contains a desiccant (e.g., silica, zeolite). The desiccant retains water either through physisorption or chemisorption. The water adsorber86may be a temperature swing device that, depending upon temperature, will either adsorb water from the reformate stream46or release water already held to the reformate stream46. For a discussion of water adsorbers see U.S. patent application Ser. No. 09/853,398 entitled “Rapid Startup of Fuel Processor Using Water Adsorption,” which is herein incorporated by reference in its entirety.

In this fuel processor design the reformate stream46then flows through a CO adsorber88. The CO adsorber88has a similar structure to the water adsorber86. The CO adsorber86contains a metal oxide or metal salt, such as copper, silver, or tin salt or oxide impregenated or exchanged on activated carbon, alumina or zeolites, and mixtures thereof. The CO adsorber88is a temperature swing device that, depending upon temperature will either adsorb CO from the reformate stream46or release CO already held to the reformate stream46. For a discussion of CO adsorbers see U.S. patent application Ser. No. 09/780,184 entitled “Carbon Monoxide Adsorption for Carbon Monoxide Clean-Up In A Fuel Cell System;” U.S. Pat. No. 4,917,711 issued to Xie et al.; U.S. Pat. No. 4,696,682 issued to Hirai et al.; U.S. Pat. No. 4,587,114 issued to Hirai et al.; and U.S. Pat. No. 5,529,763 issued to Peng et al., each of the disclosures of which is incorporated herein by reference in its entirety.

The reformate stream46then flows through a catalytic oxidizer90. The catalytic oxidizer90also receives air stream92from air supply56. A metering device (not shown) precisely controls the amount of air stream92that enters the catalytic oxidizer90. The catalytic oxidizer90consumes a portion of the H2and CO contained within the reformate stream46to generate heat. The amount of H2consumed from the reformate stream46is controlled by controlling the amount of air stream92that enters the catalytic oxidizer90. Since the heat generated is typically only used for start-up or when the reactor94is below operating temperature, the addition of air and operation of the catalyst oxidizer90is only applied for these requirements.

The heat generated in the catalytic oxidizer90is used to heat a low temperature water gas shift (WGS) reactor94. The heat can be transferred to the WGS reactor94via radiation and/or reformate stream46which flows from the catalytic oxidizer90and through the WGS reactor94. The WGS reactor94reduces the amount of CO in the reformate stream46according to the reaction CO+H2OCO2+H2and in the process generates heat. The heat generated by the WGS reactor94is used to heat an anode effluent96that is exhausted by the fuel cell stack42.

The reformate stream46then passes through a preferential oxidation reactor inlet cooler98that extracts thermal energy from reformate stream46. A coolant stream100also passes through the backside of the inlet cooler98. The inlet cooler98transfers the thermal energy extracted from the reformate stream46to the coolant stream100. The coolant stream100passes through the fuel cell stack42and removes heat therefrom prior to passing through the inlet cooler98. The coolant stream100then passes through a radiator101that expels heat from the coolant stream100to the environment.

The reformate stream46next passes through a preferential oxidation reaction (PROX) reactor102. The PROX reactor10also receives an air stream104from air supply56that is added to the reformate stream46. The air stream104is controlled by a metering device (not shown) so that precise quantities of air stream104are added to the PROX reactor102. The air stream104and reformate stream46react within the PROX reactor102to remove CO from the reformate stream46in a preferential oxidation reaction. For a discussion of PROX reactors and their control see U.S. Pat. No. 5,637,415 entitled “Controlled CO Preferential Oxidation,” which is herein incorporated by reference in its entirety. Oxidant stream70passes through the PROX reactor102where thermal energy is extracted from reformate stream46and added to oxidant stream70prior to oxidant stream70passing through the combustor air preheater68. Air stream106from air supply56is combined with a cathode effluent108exhausted from the fuel cell stack42to form the oxidant stream70.

The reformate stream46then enters the anode flow channels28of the fuel cell stack42. Air stream110from air supply56passes through the cathode flow channels26of the fuel cell stack42. The air stream110and the reformate stream46are reacted within the fuel cell stack42to produce electricity, cathode effluent108and anode effluent96. The anode effluent96contains unreacted H2that is used in the fuel processor44. Anode effluent96exits the fuel cell stack42and passes through the WGS reactor94wherein thermal energy is transferred from WGS reactor94and/or reformate stream46to anode effluent96. The cathode effluent108contains unused oxidant that is also used in the fuel processor44. Cathode effluent108, as was stated above, combines with air stream106to form oxidant stream70. Oxidant stream70after passing through the combustor air preheater68combines with the anode effluent96to form effluent stream112.

Effluent stream112flows through a combustor114. Fuel stream116from fuel supply52flows into combustor114. The fuel stream116is controlled by a metering device (not shown) that accurately meters the amount of fuel stream116that enters combustor114. Combustor114combusts effluent stream112and/or the fuel stream116to produce thermal energy that is used to heat a catalyst bed (not shown) within the SR reactor50. The combustor114can catalytically combust and/or thermally combust effluent stream112and fuel stream116. The combustor114produces a hot exhaust stream118that passes through the backside of fuel vaporizer78and the water vaporizer80prior to being exhausted to the environment. The hot exhaust stream118is used to help vaporize the fuel stream72flowing through fuel vaporizer78and water stream74flowing through water vaporizer80.

In addition to the above-described components of the fuel processor44, various water collectors can be incorporated into the fuel processor44and/or fuel cell system40to provide a source of water to supplement and/or be the water supply54as will be apparent to those skilled in the art. For example, a water collector (not shown) can be positioned between the PROX reactor102and the fuel cell stack42such that the reformate stream46flows through the water collector prior to entering the fuel cell stack42. Water can then be collected from the reformate stream46. Additionally, a water collector (not shown) can be positioned downstream of the fuel cell stack42such that the cathode effluent passes through the water collector prior to forming oxidant stream70. Water can then be collected from the cathode effluent108. Furthermore, a water collector can also be positioned in the exhaust stream118downstream of the gas and water vaporizer78and80. Water can then be collected from the exhaust stream118.

The operation of the fuel processor44will vary depending upon the operating state of the fuel processor44. For example, the fuel processor44will operate differently during a cold startup then during a normal run mode. Additionally, fuel processor44will operate differently during transient changes in a demand for H2within reformate stream46. Furthermore, transient operation of the fuel processor44will also vary depending upon whether there is an upswing or downswing transient response in the need for H2in reformate stream46. The mechanization of the fuel processor44allows for partial oxidation or autothermal reforming within the ATR reactor48during cold startup, with transition to pure high pressure steam reforming in the SR reactor50during normal run mode, if desired, and the potential for transient supplementation by the ATR reactor48if the SR reactor50pressure is inadequate due to response time, duration or steam reforming thermal balance. This aspect of the present invention will be more fully appreciated with a description of the operating process.

During a cold startup of the fuel processor44, the temperatures of the various components will not be at their normal operating temperatures and will have limited functionality. For example, the SR reactor50, when below operating temperature, may not be able to produce the second reformate stream76. Additionally, water may also not be available during the cold startup depending upon the design and configuration of the fuel processor44and/or the fuel cell system40. During a cold startup, the ATR reactor48is provided with fuel stream60and air stream64and fuel conversion is initiated as a partial oxidation reactor. The fuel stream60and air stream64are metered so that an oxygen to carbon ratio within the ATR reactor48is equal to or greater than 1.0. The ATR reactor48then produces hot H2-containing first reformate stream66via the partial oxidation reaction. The startup partial oxidation can be thermal or electrically supplemented catalytic partial oxidation. First reformate stream66then passes through the combustion air preheater68to preheat oxidant stream70prior to combining with the anode effluent96and entering the combustor114. The SR reactor50is not operated during the cold startup due to the SR reactor50being below operating temperature and incapable of producing a H2-containing second reformate stream76. Because the SR reactor50is not producing second reformate stream76, the first reformate stream66is the reformate stream46. Reformate stream46then passes through the low temperature shift inlet cooler84to extract heat from reformate stream46and heat air stream64prior to entering ATR reactor48.

Reformate stream46then passes through water adsorber86which, due to being below its normal operating temperature, adsorbs water from the first reformate stream46. The water adsorber86will continue to adsorb water from the reformate stream46until the WGS reactor94and the water adsorber86are up to operating temperature. The use of water adsorber86allows for a cold dry reformate46to pass into downstream reactors without having condensation problems on the catalysts within those downstream reactors. Reformate stream46then passes through the CO adsorber88which, due to being below its normal operating temperature, adsorbs CO from the first reformate stream66. The CO adsorber88continues to extract CO from the reformate stream46until the WGS reactor94and the CO adsorber88are at their normal operational temperature. The use of the CO adsorber88provides a means to supply H2to the fuel cell stack42before the low temperature WGS reactor94is at its normal operating temperature.

The reformate stream46then passes through the catalytic oxidizer90which, during the cold startup, is used to heat WGS reactor94. Specifically, air stream92is added to the catalytic oxidizer90in a controlled amount so that a portion of the reformate stream46is combusted and generates thermal energy that heats the WGS reactor94. The catalytic oxidizer90is operated until the WGS reactor94reaches its start-up temperature. The use of the catalytic oxidizer90provides a means to oxidize a portion of the reformate stream46to supply heat to the downstream WGS reactor94to bring it to start-up/normal operation as soon as possible.

The reformate stream46then passes through the WGS reactor94which is inactive until it reaches its start-up/normal operational temperature. Reformate stream46then passes through the inlet cooler98. Air stream104is added to the reformate stream46, prior to entering the PROX reactor102, in a controlled quantity so that the air stream104oxidizes a portion of the reformate stream46in the PROX reactor102. The oxidizing of a portion of reformate stream46generates thermal energy that heats the PROX reactor102up to normal operating temperature.

The reformate stream46then enters the fuel cell stack42along with air stream110wherein electricity is produced along with anode and cathode effluents96and108. The anode effluent96passes through the WGS reactor94wherein, depending upon the temperature of the WGS reactor94, thermal energy is added to or removed from the WGS reactor94and transferred from/added to the anode effluent96. Concurrently, cathode effluent108is added to air flow106to form oxidant flow70that passes through the PROX reactor102wherein, depending upon the temperature of the PROX reactor102and of the oxidant stream70, heat transfer will also occur between the oxidant stream70and the PROX reactor102. The oxidant stream70then passes through the combustion air preheater68wherein thermal energy from the first reformate stream66is transferred to the oxidant stream70. The oxidant stream70and anode effluent96then combine to form effluent stream112that is supplied to the combustor114.

Concurrently to producing first reformate stream66in the ATR reactor48via partial oxidation reactions, combustor114is supplied with fuel stream116and, when available, effluent stream112which are combusted, either thermally or catalytically, to heat the SR reactor50. Additionally, the hot exhaust stream118exiting the combustor114passes through fuel vaporizer78and water vaporizer80to assist in vaporizing the respective fuel and water streams72and74entering SR reactor50.

Thus, during a cold startup all of the reformate stream46is initially provided by the ATR reactor48in the form of first reformate stream66. As the various components of the fuel processor44reach their operational temperatures, the SR reactor50begins to operate in a parallel fashion with the ATR reactor48to produce second reformate stream76. Additionally, as the ATR reactor48increases in temperature and steam becomes available, the ATR reactor48will shift from a partial oxidation reaction to a combination partial oxidation and steam reforming reactions to produce first reformate stream66. In the transition from the startup mode to the normal run mode, the water and CO adsorbers86and88function to desorb water vapor and CO from the reformate stream46. Under normal run mode operation, the adsorption processes would not perform any function as mechanized and the now operational WGS reactor94and PROX reactor102operate to remove CO from reformate stream46, as will be described below.

When the fuel processor44is at its normal operational temperature, both the ATR reactor48and the SR reactor50are operated in a parallel fashion wherein both reactors48and50produce H2-containing reformates that form the reformate stream46. Specifically, the ATR reactor48will operate at the system pressure (in the range of about 1.5-3.0 bars) and will receive fuel and air streams60and64, as previously discussed, along with receiving water stream62which may already be in the form of steam or will be heated within the ATR48to form steam so that the ATR reactor48produces first reformate stream66through a combination of partial oxidation reaction and steam reforming reaction.

Concurrently, the SR reactor50will receive a vaporized fuel stream72and the vaporized water stream74in the form of steam which together undergo a steam reforming reaction within the SR reactor50to produce hot H2-containing second reformate stream76. The SR reactor50is operated at an elevated pressure (in the range of about 5.0-7.0 bars) relative to the system pressure. The second reformate stream76passes through the fuel and water vaporizers78and80wherein thermal energy is extracted from the second reformate stream76and used to assist in vaporizing fuel stream72and water stream74. The second reformate stream76then passes through the pressure let down valve82to drop to the system pressure and combine with first reformate stream66and form the reformate stream46.

The reformate stream46then passes through the low temperature shift inlet cooler84wherein thermal energy is extracted from reformate stream46and added to air stream64supplied to ATR reactor48. The reformate stream46then passes through the water adsorber86and CO adsorber88. The water adsorber86and CO adsorber88, being at their operational temperature no longer adsorb water or CO from the reformate stream46. Depending upon the equilibrium points and temperature, the water adsorber86and/or the CO adsorber88may desorb water and/or CO which would be added to reformate stream46.

The reformate stream46then passes through catalytic oxidizer90which no longer receives air stream92and is inactive. The reformate stream46then passes through WGS reactor94wherein CO is converted via the water gas shift reaction to CO2. Reformate stream46then passes through inlet cooler98and PROX reactor102wherein the reformate stream46is further conditioned to convert remaining CO to CO2and provide stack grade quality reformate to the fuel cell stack42. The reformate stream46then flows into fuel cell stack42out of which electricity and anode and cathode effluents96and108flow. The anode and cathode effluents96and108then follow the same process as was discussed above. The amount of air supplied to the oxidant stream70via air stream106will vary depending upon the needs of the combustor114for oxidants in addition to that contained within cathode effluent108.

During transient changes in the demand for H2by the fuel cell stack42, the operation of the fuel processor44will change. The transient change can be an upswing transient wherein additional H2is demanded by the fuel cell stack42or a downswing transient wherein less H2is required by the fuel cell stack42. The upswing and downswing transients are met by the fuel processor44in different ways.

ATR reactor48, and particularly the partial oxidation reaction, is able to more quickly increase H2production than SR reactor50. Therefore, during an upswing transient ATR reactor48is operated to quickly provide the additional H2required by the fuel cell stack42while relying on indirect heat transfer to gradually ramp up the SR reactor50to produce the required additional H2. The rate at which the SR reactor50can produce the additional required H2will vary depending upon the operational pressure of the SR reactor50and the heat transfer limitations associated with the design of the fuel processor44.

Therefore, during an upswing transient, fuel stream62and air stream64flowing into ATR reactor48are increased so that additional partial oxidation reaction can occur and the additional H2demanded by the fuel stack42can be met. The SR reactor50can be operated so that it continues to produce the same amount of H2within second reformate stream76as it does during the normal operational mode or, alternatively, can be operated to increase the amount of H2provided via the second reformate stream76as the SR reactor50is provided with the additional heat required to produce the additional H2. If the transient upswing is temporary, the fuel processor44can be operated so that the entire transient upswing is provided by the ATR reactor48. If the transient upswing is a step change in the H2requirement of the fuel cell stack42, the ATR reactor48can be operated to quickly provide the needed additional H2while the output of the SR reactor50is also increased. As the output of the SR reactor50increases, the output of the ATR reactor48can be decreased so that a balance between the amount of H2provided by first reformate stream66and second reformate stream76are a desired ratio. Because SR reactor50is more efficient, balancing the amount of H2provided by the first and second reformate streams66and76can yield the highest overall fuel processor efficiency.

During a downswing transient, the second reformate stream76and/or SR reactor50can be restricted so that less H2is provided to reformate stream46via the second reformate stream76to quickly decrease the amount of H2provided to fuel cell stack42. The output and operational conditions of ATR reactor48and SR reactor50are then adjusted so that less H2is produced overall via the first and second reformate stream66and76and each reactor produces a desired proportion of the H2in the reformate stream46.

The above described invention combines multiple processes into one fuel processor mechanization which is capable of producing a hydrogen-reformate quickly by use of a thermal or electric catalyst for start-up. The ATR reactor48operates in a partial oxidation mode during startup and upswing transients and is capable of producing a hydrogen reformate before the complete reactor volume reaches its operation equilibrium temperatures. The parallel SR reactor50requires a longer period of time for the associated catalyst within the SR reactor50to reach the required activation temperature.

As stated above, under startup and transient upswings, ATR reactor50is used to supply the H2and/or additional H2respectively, that is required by the fuel cell stack42. Under normal operation, the SR reactor50provides a majority of the total H2required by the fuel cell stack42with the ATR reactor48in an idle or reduced operating mode. For example, during normal operation run mode, the SR reactor50can be operated to supply approximately 70% of the H2required by the fuel cell stack42while the ATR reactor48is operated to provide approximately 30% of the H2required by the fuel cell stack42. This mechanization supports transients with the ATR reactor48and quasi steady state operation using the SR reactor50. An example of transient operation is illustrated inFIG. 3in which the H2generation of ATR reactor48is shown by curve122, the H2generation of SR reactor50is shown by curve124, and the total H2demand of the fuel cell stack42is shown by curve126. The transient operation example is as follows. If the fuel processor44is at a total H2generation of 10 kW (ATR reactor48at 3 kW and the SR reactor50at 7 kW), and would like to increase the power to 100 kW then ATR reactor48would be initially commanded to produce 93 kW. The SR reactor50would also perform a transient change but at a slower rate. As the H2generation of SR reactor50increases, the output of ATR reactor48is reduced until reaching a desired system balance set point. The end result would be the ATR reactor48at 30 kW and the SR reactor50at 70 kW. While this transient example is graphically represented inFIG. 3, it should be understood that the changes in H2production are shown as being linear for exemplary purposes only. The actual changes in H2production do not need to be linear or at the rates shown to be within the scope of the present invention.

This mechanization of having an ATR reactor48and a SR reactor50in parallel takes advantage of positive aspects of each by combining their attributes in a modular parallel reacting system. The ATR reactor48allows for quick partial oxidation startups and quick transients by not relying on heat transfer (to support steam reforming and vaporization) to perform transients. The ATR reactor48during normal operation or startup partial oxidation is less efficient than the SR reactor50. The SR reactor50provides an efficient hydrocarbon conversion process but may be limited in transient performance. The SR reactor50performs load follow (transient changes) at a slower rate than the ATR reactor48and does not include a quick start mechanization. Transient performance improvements are possible in the SR reactor50when operated at elevated pressures by allowing a pressure decay to perform transients. Therefore, depending upon the desired operational conditions of the fuel cell system40and/or the fuel processor44, the ATR reactor48and SR reactor50are operated in a parallel fashion with each providing various amounts of H2. The use of these modular or parallel reactors can provide for quick startup and transient capability through the ATR reactor48and improved overall efficiency through the use of SR reactor50under normal operation. During the transient conditions, the ATR reactor48and SR reactor50can be increased and/or decreased to achieve the desired operational H2output while providing the highest overall fuel processor efficiency.