Abstract:
Method and apparatus for supplying extra/booster steam and/or fuel vapor to the fuel processor of a fuel cell system during surges in power demanded from the fuel cell. Hot water and/or liquid fuel is/are stored under pressure until needed. When needed during power surges, the pressure is rapidly reduced on the hot liquid(s) causing it/them to flash vaporize and supplement the normal, steady state supply of steam/fuel-vapor to the fuel processor.

Description:
TECHNICAL FIELD 
     This invention relates to an apparatus and method for improving the response time of fuel cell systems during increases in power demand. 
     BACKGROUND OF THE INVENTION 
     Fuel cells have been used as a power source in many applications. Some fuel cells (e.g. PEM-type or phosphoric-acid-type) use hydrogen supplied to the anode as fuel, and oxygen supplied to the cathode as oxidant. The hydrogen can be provided directly from liquefied or compressed hydrogen, or indirectly from reformed hydrogen-containing fuels such as methane, methanol, gasoline or the like. The oxygen is typically provided from air. 
     PEM fuel cells are preferred for vehicular applications (i.e. as replacement for internal combustion engine) owing to their compactness, moderate temperature operation, and high power density. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, protontransmissive, solid polymer electrolyte membrane (e.g. perfluoronated sulfonic acid) having an anode catalyst on one of its faces and a cathode catalyst on its opposite face. The MEA is sandwiched between a pair of electrically conductive current collectors that distribute the fuel cell&#39;s gaseous reactants over the surfaces of the respective anode and cathode. See U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, and assigned to General Motors Corporation. 
     For vehicular applications, it is desirable to use a liquid, hydrogen containing fuel such as methanol or gasoline as the source of hydrogen, because it is easy to store onboard, and there is an existing, nationwide infrastructure capable of supplying such fuels. However, hydrogen-containing fuels (liquid or gaseous) must be dissociated to release their hydrogen content. The dissociation reaction is accomplished within a chemical reactor known as a fuel processor. Fuel and water vaporizers located upstream of the fuel processor convert water and liquid fuel to steam and fuel vapor respectively for supply to the fuel processor. The fuel processor contains one or more catalytic reactors wherein vaporized fuel reacts with steam, and sometimes air, to yield a hydrogen-rich reformate gas that is supplied to the fuel cell. In the steam reformation of methanol, methanol vapor and steam are reacted. In the reformation of gasoline, steam, air and gasoline vapor are reacted in a fuel processor known as an autothermal reformer (ATR). The ATR contains two sections, i.e. (1) a first, partial oxidation (POX) section where the gasoline is partially reacted exothermically with air, and (2) a second steam reformer (SR) section where the effluent from the POX section is exothermically reacted with steam. The effluents from both the methanol and gasoline reformation processes contain primarily hydrogen and carbon dioxide along with some water and CO. A water-gas-shift (WGS) reactor and/or a preferential oxidation (PROX) reactor may be provided downstream of the reformer to reduce the CO content of the hydrogen stream. See U.S. Pat. No. 6,232,005 (issued May 15, 2001), U.S. Pat. No. 6,077,620 (issued Jun. 20, 2000), and U.S. Pat. No. 6,238,815 (issued May 29, 2001), each assigned to the General Motors Corporation. 
     For vehicular applications, the fuel processor must be capable of delivering hydrogen to the fuel cell over a wide range of rates depending on the power demands placed on the fuel cell. In this regard, the fuel cell has to be able to power the vehicle over a broad spectrum of operating conditions ranging from when the vehicle is standing at idle (i.e. a low power demand condition) to when the vehicle is moving at high speeds (a high power demand condition), as well as transitions (e.g. rapid acceleration) therebetween (a very high power demand condition). Heretofore, the ability of the fuel processor to quickly respond to very high power demand transitions between low and high power demand conditions (hereafter “power surges”) of the fuel cell has been hampered by the ability of the system to quickly produce enough vaporized fuel and/or steam for supply to the fuel processor. In this regard, it has not been considered practical to design a vehicular fuel cell system with vaporized fuel and steam reserves sized to accommodate power surges. Rather, the systems are typically sized to accommodate substantially steady state low and high power conditions, but not the power surges therebetween. Hence, transient response has suffered. 
     SUMMARY OF THE INVENTION 
     The present invention is a fuel cell system that includes fuel and/or steam buffer(s) that supplement(s) the system&#39;s primary fuel and/or water vaporizer(s) by rapidly responding to surges in the power demanded from the fuel cell. The buffer(s) supply(s) needed steam/fuel-vapor during power surges, and allow(s) time for the primary vaporizer(s) to catch-up, or increase its/their output sufficiently to meet the fuel vapor and/or steam requirements of the fuel processor after the power surge has ceased. 
     One aspect of the invention, apparatus-wise, involves a fuel cell system comprising (1) a hydrogen-consuming fuel cell, (2) a fuel processor that produces the hydrogen from a hydrogen-containing fuel and steam, and (3) a primary steam vaporizer that produces the steam from water and supplies a first quantity of the steam to the fuel processor at a first pressure. The invention is an improvement to the foregoing comprising a steam buffer that communicates with the fuel processor in parallel with the primary steam vaporizer for supplying a second, extra/booster quantity of steam to the fuel processor during surges in the electrical power demanded from the fuel cell. The steam buffer comprises a vessel containing a pool of water held under a second pressure greater than the first pressure and at an elevated temperature greater than the boiling point of water at the first pressure. A pump supplies water to the vessel at the second pressure, and a heat exchanger heats the water to the elevated temperature. An inlet valve communicates the pump with the vessel, and an outlet valve communicates the vessel with the fuel processor. A controller controls closing of the inlet valve and opening of the outlet valve during surges in the power demanded from the fuel cell to rapidly reduce the pressure in the vessel and thereby cause the heated water in the pool to flash vaporize and provide the second quantity of steam needed by the fuel processor to keep up with the rising power demands on the fuel cell. 
     A similar buffer may also be provided for generating extra hydrogen-containing fuel vapor from liquid hydrogen-containing fuel (e.g. gasoline). In this embodiment, the system&#39;s primary fuel vaporizer converts liquid fuel into fuel vapor and supplies a first quantity of the fuel vapor to the fuel processor at a first pressure. A fuel buffer communicates with the fuel processor fur supplying a second extra/booster quantity of fuel vapor to the fuel processor during surges in the electrical power demanded from the fuel cell The fuel buffer comprises a vessel containing a pool of liquid hydrogen-containing fuel held under a second pressure greater than the bubble point of the liquid hydrogen-containing fuel at the first pressure, where the bubble point is the temperature at which the lowest boiling constituent of a mixture of liquid hydrogen-containing fuels (e.g. gasoline is typically a mixture of low-boiling alkanes) begins to vaporize. A pump supplies the liquid hydrogen-containing fuel to the vessel at the second pressure, and a heat exchanger heats the liquid hydrogen-containing fuel to the elevated temperature. An inlet valve communicates the pump with the vessel, and an outlet valve communicates the vessel with the fuel processor. A controller controls opening and closing of the outlet and inlet valves during power surges to rapidly reduce the pressure in the vessel so as to cause the liquid hydrogen-containing fuel in the pool to flash vaporize and provide the extra/booster quantity of fuel vapor. 
     The invention further contemplates a method of operating a hydrogen-fueled fuel cell system having a fuel processor that produces the hydrogen from a hydrogen-containing fuel vapor and steam. The method comprises the steps of vaporizing water to provide a first quantity of steam to the fuel processor at a first pressure when operating the fuel cell at a first power level, and supplying a second extra/booster quantity of steam to the fuel processor in parallel with the first quantity to supplement the first quantity during surges in the electrical power demand to a higher power level. The second, extra/booster quantity of steam is provided from a pool of water maintained at a second pressure greater than the first pressure and at a temperature greater than the boiling point of water at the first pressure. When a power surge occurs, the pressure on the pool is rapidly reduced from the second pressure to the first pressure to flash vaporize the water in the pool and provide it to the fuel processor. 
     The invention is also applicable to a method for supplying fuel vapor to the fuel processor. This embodiment comprises the steps of vaporizing liquid fuel to provide a first quantity of fuel vapor to the fuel processor at a first pressure when operating the fuel cell at a first power level, and supplying a second, extra/booster quantity of fuel vapor to the fuel processor in parallel with the first quantity to supplement the first quantity during surges in the electrical power demand to a higher power level. The invention provides a pool of the liquid, hydrogen-containing fuel at a second pressure greater than the first pressure and at a temperature greater than the bubble point of the liquid fuel at the first pressure. When the power demand surges, the pressure on the pool is rapidly reduced from the second pressure to the first pressure to flash vaporize the fuel in the pool and provide it to the fuel processor. 
     Preferably, the water or fuel, as appropriate, will be maintained at, or near, its boiling or bubble point at the second pressure to more quickly produce the most extra/booster steam/fuel vapor possible when the pressure in the vessel is reduced. A preferred feature of the invention involves replenishing the pool by providing a two phase mixture of water/steam or liquid-fuel/fuel-vapor, as appropriate, to the vessel between power surges when the system is operating under substantially steady state conditions at either high or low power levels. 
     The invention will be better understood when considered in the light of the following detailed description of a preferred embodiment thereof which is given hereafter in conjunction with the drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically depicts a fuel cell system in accordance with one embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The FIGURE depicts a fuel cell system comprising a fuel cell  2  and a fuel processor  4  for supplying hydrogen to the fuel cell  2 . The fuel processor  4  catalytically reacts hydrogen-containing fuel  6  with steam  12 , and possibly air  22 , to form the hydrogen. When the hydrogen-containing fuel  6  is a liquid, it is first vaporized in a primary vaporizer  8  before being supplied to the fuel processor  4  via the pipe  10 . If the hydrogen-containing fuel is a gas (e.g. methane), the primary fuel vaporizer  8  is eliminated and the gaseous fuel fed directly to the fuel processor  4 . The primary fuel vaporizer  8  is sized to accommodate the fuel needs of the fuel processor  4  when the fuel processor is operating under substantially steady state low and high power conditions (i.e. between power surges). However, the time response of such a primary vaporizer  8  is relatively slow during a power demand surge such as may occur during the rapid acceleration of a fuel-cell powered vehicle. Similarly, steam  12  is also provided to the fuel processor  4  from a vaporizer  14  which vaporizes water  16 . Like the fuel vaporizer  8 , the water vaporizer  14  is sized to accommodate the low and high power, steady state condition, but its response will be slow during transient surges in power between the low and high power conditions. Air  22  may also be provided to the fuel processor  4  depending on the nature of the fuel  6 . In this regard, if the fuel  6  were methanol, no air is needed. But if the fuel  6  were gasoline, it would typically be processed in an ATR-type fuel processor  4  that requires air for the POX reaction, as described above. 
     According to the present invention, any additional fuel and/or steam required by the fuel processor  4  during surges over and above that available from the primary vaporizers  8  and  14  is/are provided by a fuel buffer system  18  and/or a steam buffer system  20 . 
     The steam buffer  20  includes a pressure vessel  24  containing a pool of water  26 , a heat exchanger  28  for heating the water  26 , a pump  32  fur pumping water under pressure into vessel  24 , an inlet valve  30  communicating the pump  32  with the heat exchanger  28  and vessel  24 , and an outlet valve  34  communicating the vessel  24  with the fuel processor  4  via pipe  36  to provide booster steam to the fuel processor  4  in parallel with the steam  12  from the primary water vaporizer  14 . The heat exchanger  28  may either be a discrete heat exchanger, as shown, or integrated with the vessel  24  (not shown). The inlet and outlet valves  30  and  34  are motor actuated in response to control signals  31  and  55  emanating from a controller  38 , as will be discussed in more detail hereinafter. A liquid level sensor  40 , associated with the vessel  24 , senses the level of the liquid in the pool  26  and reports it to the controller  38  via signal  41 . Any convenient type of liquid level sensor or gauge (e.g. a float switch, or electrical resistance type) may be used. A thermocouple  42 , and pressure sensor  44 , at the top of the vessel  24  are used to measure the temperature and pressure of the fluid thereat where it exits the vessel  24  and report those measurements to the controller  38  via signals  43  and  45  respectively. Additional thermocouples (not shown) may be provided at different levels within the vessel  24 , above the surface of the pool  26 , to indicate the temperatures thereat. A thermocouple  46  provided in the lower portion of the vessel  24  indicates the temperature of the water in the pool  26  and reports it to the controller  38  via signal  35 . 
     A more compact, alternative structure has the primary steam vaporizer  14  integrated into the heat exchanger  28  such that all of the steam generated passes through the pool  26 . 
     In the operation of the steam buffer  20 , the vessel  24  is filled with water  26  that has been heated to a temperature, and pressurized to a pressure determined by the operating conditions of the fuel processor  4 . For example, if the primary vaporizer  14  supplies steam to the fuel processor  4  at pressure P 1 , the pressure P 2  in the vessel  24  will be maintained significantly higher than the pressure P 1 , and the temperature of the water in the pool  26  in the vessel  24  will be maintained above the boiling point of water at pressure P 1 . Preferably, the temperature of the water in pool  26  will be at or near the boiling point of water at pressure P 2  for optimum steam buffering. When the fuel cell is operating at substantially steady state conditions (i.e. between power surges), the inlet valve  30  is mostly open (e.g. 95%), the outlet valve  34  is mostly closed (e.g. 95%), and the pump  32  pumps the water into the vessel  24  at pressure P 2 . When the fuel cell is operating at substantially steady state and the level of the water in the pool  26  in the vessel  24  is below a prescribed level, fresh water is added to the pool  26  until the prescribed level is reached. To this end, the pump  32  pumps more water through the heat exchanger  28  than the heat exchanger  28  can completely vaporize so that a two-phase water-steam mixture exits the heat exchanger  28 . When the fuel cell is operating at substantially steady state, heated water collects in the pool  26  while the vapor collects above the pool  26  and bleeds slowly past the valve  34  into the fuel processor  4  along with the steam from the primary steam vaporizer  14 . When the water in the pool  26  reaches the prescribed level, the flow rate of water to the heat exchanger  28  is slowed to the point where only steam exits the heat exchanger  28 . The steam exiting the heat exchanger bubbles up through the pool  26  to heat the water therein. Hence, both water and heat are added back into the pool  26  and vessel  24  to compensate for the water and heat lost during a power surge and corresponding buffering event. The pressure P 2  will be below its prescribed level immediately after a power surge, but will build back up again between surges. 
     The controller  38  monitors (1) the output (i.e. current and voltage) from the fuel cell  2  via signal  37 , (2) the temperature and pressure at the top of the vessel  24  via signals  43  and  45  from sensors  42  and  44 , (3) the level of the water in pool  26  via signal  41  from level sensor  40 , and (4) the temperature of the pool  26  via signal  35  from thermocouple  46 . When the output from the fuel cell  2  indicates a surge in the power demanded from the fuel cell  2 , and hence an increased need for hydrogen from the fuel processor  4 , the inlet valve  30  will be closed (e.g. 95%) and the outlet valve  34  opened (e.g. 95%), as needed, to cause the pressure P 2  in the vessel  24  to quickly drop to a level closer to P 1 , the pressure of the steam entering the fuel processor  4 . When the pressure P 2  in the vessel  24  rapidly drops to P 1 , or near P 1 , the water  26  which is at a temperature above its boiling point at pressure P 1 , will rapidly flash-vaporize in the vessel  24  and flow to the fuel processor  4  via the outlet line  36 . The latent heat required to vaporize the steam will come from a sensible temperature drop in the remaining liquid and in the construction materials of the vessel  24 . The controller  38  triggers the buffer  20  to produce extra/booster steam when it senses an upward spike in the current, and concurrent drop in the terminal voltage, of the fuel cell. At the same time, the controller  38  signals ( 39 ) the primary water vaporizer  14  to increase its output in order for it to be able to accommodate the higher power level required by the fuel cell after the surge to that level has ceased. 
     The speed of the pump  32 , and hence the flow rate of the water therefrom, is controlled via signal  33  from controller  38  to insure that only steam exits the vessel  24  and enters the fuel processor. Alternatively, the operating temperature of the heat exchanger  28  may be increased to insure that only steam exits the vessel  24 . To this end, the thermocouple  42  and pressure sensor  44  at the upper end of the vessel  24  measure the temperature and pressure of the fluid thereat and report the measurements to the controller  38  via signals  43  and  45 . If the temperature measured by thermocouple  42  exceeds the boiling point of water at the pressure measured by sensor  44  at that location, then it can be inferred that the fluid thereat is all steam (i.e. no liquid water present), and no flow rate or heat exchanger temperature adjustments are needed. On the other hand, if the temperature at the upper end of the vessel  24  (measured by the thermocouple  42 ) is at the boiling point of water at the pressure measured by the pressure sensor  44 , it can be inferred that the fluid at the upper end of the vessel  24  is a two-phase mixture of water and steam, and adjustments are needed to achieve steam only. The thermocouple  46  measures the temperature of the water in the pool  26  and reports that measurement to the controller  38  via signal  35 . The temperature of the water will be at a prescribed level above the boiling point of water at pressure P 1 . If the temperature of the water falls below this prescribed temperature, the pump  32  is slowed to produce hotter steam in the heat exchanger  28  which, in turn, heats the water in the pool  26  as it bubbles therethrough. Alternatively, the operating temperature of the heat exchanger  28  may be increased to provide the heat needed to raise the temperature of the water in pool  26 . 
     The fuel buffer  18  is structured and operated similar to the steam buffer  20 , and comprises a pressure vessel  48  containing a pool of liquid hydrogen-containing fuel  47 , a liquid level sensor  50 , a heat exchanger  52 , an inlet valve  54 , a pump  56 , an outlet valve  58 , a thermocouple  60  for the liquid, a thermocouple  62  at the top of the vessel  48 , and a pressure sensor  64  also at the top of the vessel  48  adjacent the vessel&#39;s exit  51 . The fuel buffer  18  is controlled by the same controller  38  as the steam buffer  20 . A more compact, unshown, alternative structure has the primary fuel vaporizer  8  integrated into the heat exchanger  52  such that all of the fuel vapor generated passes through the pool  47 . 
     The fuel buffer  18  functions the same as the steam buffer  20  except that the temperature of the fuel pool  48  is maintained above the bubble point of the fuel at pressure P 3 , the pressure at which the fuel vapor is admitted to the fuel processor  4 . More specifically, in the operation of the fuel buffer  18 , the vessel  48  is filled with liquid fuel  47  that has been heated to a temperature, and pressurized to a pressure determined by the operating conditions of the fuel processor  4 . For example, if the primary vaporizer  8  supplies fuel vapor to the fuel processor  4  at pressure P 3 , the pressure P 4  in the vessel  48  will be maintained significantly higher than the pressure P 3 , and the temperature of the fuel in the pool  47  in the vessel  48  will be maintained above the bubble point of the fuel water at pressure P 3 . Preferably, the temperature of the fuel in pool  47  will be at or near the bubble point of the fuel at pressure P 4  for optimum fuel buffering. When the fuel cell is operating at substantially steady state conditions (i.e. between power surges), the inlet valve  54  is mostly open (e.g. 95%), the outlet valve  58  mostly closed (e.g. 95%), and the pump  56  pumps the fuel into the vessel  48  at pressure P 4 . When the fuel cell is operating at substantially steady state, and the level of the liquid fuel in the pool  47  in the vessel  48  is below a prescribed level fresh liquid fuel is added to the pool  47  until the prescribed level is reached. To this end, the pump  56  pumps more liquid fuel into the heat exchanger  52  than the heat exchanger  52  can completely vaporize so that a two phase liquid-fuel/fuel-vapor mixture exits the heat exchanger  52 . When the fuel cell is operated at substantially steady state, heated liquid fuel collects in the pool  47  while the fuel vapor collects above the pool  47  and bleeds slowly past the outlet valve  58  into the fuel processor  4  along with the fuel vapor from the primary fuel vaporizer  8 . When the liquid fuel in the pool  47  reaches the prescribed level, the flow rate of liquid fuel to the heat exchanger  52  is slowed to the point where only fuel vapor exits the heat exchanger  52 . The fuel vapor exiting the heat exchanger  52  bubbles up through the pool  47  to heat the liquid fuel therein. Hence, both liquid fuel and heat are added back into pool  47  and vessel  48  to compensate for the liquid fuel and the heat lost during a power surge. The pressure P 4  will be below its prescribed level immediately after a power surge, but will build back up again between surges. 
     The controller  38  monitors (1) the output (i.e. current and voltage) from the fuel cell  2  via signal  37 , (2) the temperature and pressure at the top of the vessel  48  via signals  63  and  65  from sensors  62  and  64 , (3) the level of the water in pool  47  via signal  61  from level sensor  50  and (4) the temperature of the pool  47  via signal  49 . When the output from the fuel cell indicates a surge in the power demanded from the fuel cell  2 , and hence an increased need for hydrogen from the fuel processor  4 , the inlet valve  54  will be mostly closed (e.g. 95%) in response to signal  66  from controller  38 , and the outlet valve  58  mostly opened (e.g. 95%) in response to signal  68  from controller  38  as needed to cause the pressure P 4  in the vessel  48  to quickly drop to a level closer to P 3 , the pressure of the fuel vapor entering the fuel processor  4  from the primary vaporizer  8 . When the pressure P 4  in the vessel  48  rapidly drops to P 3 , the liquid fuel  47  which is at a temperature above its bubble point at pressure P 3 , will rapidly flash-vaporize in the vessel  48  and flow to the fuel processor  4  via the outlet line  59  in parallel with the fuel  10  supplied by the primary fuel vaporizer  8 . The latent heat required to vaporize the liquid fuel will come from a sensible temperature drop in the remaining liquid and in the construction materials of the vessel  48 . The controller  38  triggers the fuel buffer  18  to produce extra/booster fuel vapor when it senses an upward spike in the current and concurrent drop in the terminal voltage of the fuel cell. At the same time, the controller  38  signals ( 53 ) the primary fuel vaporizer  8  to increase its output in order for it to be able to keep up with the extra steam production from steam buffer  20 , and to accommodate the higher power level required by the fuel cell after the surge to that level has ceased. 
     The speed of the pump  56 , and hence the flow rate of the fuel therethrough is controlled via signal  67  so as to insure that only fuel vapor exits the vessel  48  and enters the fuel processor  4 . Alternatively, the operating temperature of the heat exchanger  52  may be increased to insure that only fuel vapor exits the vessel  48 . To this end, the thermocouple  62  and pressure sensor  64  at the upper end of the vessel  48  measure the temperature and pressure of the fluid thereat, and report the measurements to the controller  38  via signals  63  and  65 . If the temperature measured by thermocouple  62  exceeds the dew point of the fuel at the pressure measured by sensor  64  at that location, then it can be inferred that the fluid thereat is all vapor (i.e. no liquid fuel present), where the “dew point” is the temperature where the fuel vapor begins to condense (i.e. the highest boiling constituents of the fuel vapor precipitate). Alternatively the “dew point” could be viewed as the temperature at which the last bit of liquid fuel vaporizes. Regardless of how viewed, above the dew point no flow rate or heat exchanger temperature adjustments are needed. On the other hand, if the temperature at the upper end of the vessel  48  (measured by the thermocouple  62 ) is between the bubble point and the dew point of the fuel at the pressure measured by the pressure sensor  64 , it can be inferred that the fluid at the upper end of the vessel  48  is a two phase mixture of liquid fuel and fuel vapor and pump speed and/or heat exchanger temperature adjustments are needed to achieve fuel vapor out only. 
     Between power surges, the water in pool  26 , and/or the liquid fuel in pool  47 , will be replenished by adjusting the flow rate to, and/or heat applied to, the heat exchangers  28  and/or  52  so that the effluents from each are a two-phase mixture of water/steam from heat exchanger  28 , and/or liquid-fuel/fuel-vapor from heat exchanger  52 , as appropriate. The liquid level sensors  40  and  50  will signal (i.e.  41  and  61 ) when the liquid level is low and when it has reached its prescribed upper limit. When the prescribed upper limit is reached the sensor will trigger flow rate and/or heat exchanger adjustments that will produce only steam from the heat exchanger  28  and/or fuel vapor from heat exchanger  52 . The liquid level sensors  40  and  50  also signal the controller  38  when the liquid levels in the vessels  24  and  48  become depleted. This intelligence is particularly important for operation of the steam buffer  20  to insure that the steam to carbon (i.e. s/c) ratio of the reactants entering the fuel processor  4  does not fall below a prescribed level, which could occur if there were insufficient steam to react with extra fuel being supplied to the fuel processor during power surges. The tolerable s/c ratio for any given system will vary as a function of the temperature, pressure and oxygen-to-carbon ratio in the fuel processor. If the s/c ratio is too low, soot can form and foul the fuel processor Hence, when the water in pool  26  is depleted, the controller  38  will cut back the flow rate of fuel to the fuel processor to keep the s/c ratio above a soot-forming level. Preferably, the amount of steam produced will be determined, and the flow rate of the fuel (and air when applicable) modulated, in direct proportion to the steam rate in order to maintain a prescribed s/c ratio through the full range of operation of the fuel processor. 
     The controller  38  may either be a controller that is dedicated strictly to the steam/fuel buffering of the present invention, or, preferably, will be part of a central controller that is used to control the many aspects of the entire fuel cell system. Such a central controller  38  contains the necessary hardware and software for receiving inputs, converting inputs to other values correlated to inputs, summing inputs, generating internal signals based on those inputs, conditioning (i.e. integrating/differentiating) the internal signals to provide smooth output signals, and whatever other functions might be needed to control the fuel cell system. The controller  38  may take the form of a conventional general purpose digital, computer-based controller programmed to periodically carry out the described process at predetermined intervals (e.g. every 10 milliseconds). The controller  38  includes such well known elements as (1) a central processing unit (CPU) with appropriate arithmetic and logic circuitry for carrying out arithmetic, logic, and control functions, (2) read-only memory (ROM), (3) read-write random access memory (RAM), (4) electronically programmable read only memory (EPROM), and (5) input and output circuitry which interfaces with the fuel cell and the several sensors, valves and pumps of the steam/fuel vapor buffers, inter alia. The ROM contains the instructions read and executed by the CPU to implement the several processes carried out by the controller  38  including the steam/fuel buffering technique of the present invention. The EPROM contains appropriate look-up tables, and any needed calibration constants, for converting and comparing appropriate inputs/outputs. A specific program for carrying out the invention may be accomplished by standard skill in the art using conventional information processing languages. 
     The controller  38  contains a first lookup table that correlates the steam and fuel vapor requirements for the fuel processor at various power surge rates and durations, and the inlet/outlet valve settings to achieve those requirements. The values for the first lookup table are determined empirically in the laboratory through a series of experiments wherein a fuel cell system identical to the fuel cell system to be operated according to the present invention is operated under various steady state and power surge conditions, the steam and fuel vapor requirements for those conditions determined and the inlet and outlet valve settings corresponding to those requirements determined and tabulated. The controller  38  contains a second lookup table that correlates the amount of fuel (and air where applicable) that needs to be supplied to the fuel processor for a given amount of steam being provided to the fuel processor to keep the s/c ratio above a soot-forming ratio. The controller  38  contains a third lookup table that correlates (1) the various combinations of temperature and pressure at the exits of the vessels  24  and  48  that yield only steam and/or fuel vapor to (2) the speed of the pumps  32  and  56  (also determined empirically) needed to achieve only steam and fuel vapor thereat for various heat exchanger operating temperatures. 
     EXAMPLE 
     Using conventional Steam Tables, a steam buffer  20  is designed to provide steam to a fuel processor  4  at a pressure (P 1 ) of 2.8 bar. Pump  32  pumps water from atmospheric pressure (1 bar) to 4 bar pressure. At low power (e.g. less than 20% full power) steady state conditions, the vessel will have an internal pressure (P 2 ) of  3 . 9  bar, allowing for a  0 . 1  bar pressure drop (ΔP) across the mostly-open inlet valve  30  and heat exchanger  28 , and a 1.1 bar pressure drop across the mostly-closed outlet valve  34 . At steady state conditions, the vessel  24  contains liquid water at its boiling point of 144° C. at pressure P 2 . When a power surge occurs, there is an increased demand for steam at 2.8 bar (P 1 ). Inlet valve  30  is moved to a more closed position while the outlet valve  34  is moved to a more open position such that the pressure P 2  in the vessel  24  quickly drops from 3.9 bar to 2.9 bar (allowing 0.1 bar ΔP for valve  34 ). This 1 bar drop in pressure will result in the flash generation of 20.5 grams of steam for each liter of liquid water held at its 144° C. boiling point. Higher amounts of steam per liter of water can be obtained by going to higher pressures. For example, flashing a liter of water from 5.9 bar at its boiling point of 159° C. down to 2.9 bar results in the generation of 51.5 grams of steam, and flashing from 9.9 bar to 2.9 bar with 180° C. water results in the generation of 94.6 grams of steam. 
     While the invention has been described primarily in terms of certain specific embodiments thereof, it is not intended to be limited thereto, but rather only to the extent set forth hereafter in the claims which follow.