WO2002069430A2

WO2002069430A2 - Internal reforming improvements for fuel cells

Info

Publication number: WO2002069430A2
Application number: PCT/US2002/005853
Authority: WO
Inventors: G.B.Kirby Meacham
Original assignee: Meacham G B Kirby
Priority date: 2001-02-23
Filing date: 2002-02-22
Publication date: 2002-09-06
Also published as: WO2002069430A9; WO2002069430A3

Abstract

Method and apparatus for internal reforming of hydrocarbon fuels in fuel cells by mixing fuel with products of reaction over the anode such that the fuel concentration is below the soot formation limit. Steam reforming takes place using the product water and heat generated at the anode by the electric power generation reaction. The reactor exhaust is a mixture that contains H2, CO, CO2, H2O and hydrocarbons, and has 40% to 60% of the original fuel content remaining. The reactor can be the first stage of a two-stage fuel cell system, with the second stage being a fuel cell operated in plug flow mode that directly utilizes fuel content of the exhaust to generate additional electric power, or the exhaust can be used for other purposes including as a reducing atmosphere or feedstock. As a further aspect of the invention allows for the utilization of solid fuels such as coal.

Description

INTERNAL REFORMING IMPROVEMENTS FOR FUEL CELLS

Technical Field

The present invention is primarily related to reforming of fuels that are consumed in fuel cells, such as methane, higher hydrocarbons, and alcohol, but it also including any liquid, solid, or gas fuels that could be employed, and particularly to process and apparatus improvements thereto. It is also related to integrated systems employing these principles, including particularly advantageous solid oxide cell and stack designs.

Background of the Invention

Fuel cells are electrochemical systems that generate electrical current by chemically reacting a fuel gas and an oxidant gas on the opposite surfaces of electrodes. Conventionally, the components of a single fuel cell include the anode, the cathode, the electrolyte, and the interconnect material. In a solid state fuel cell, such as high temperature solid oxide fuel cells (SOFCs), the electrolyte is in a solid form and insulates the cathode and anode one from the other with respect to electron flow, while permitting oxygen ions to flow from the cathode to the anode. The interconnect material is a gas barrier that electronically connects the anode of one cell with the cathode of an adjacent cell, in series, to generate a useful voltage from an assembled fuel cell stack.

The SOFC can directly utilize hydrogen and carbon monoxide as fuel gases and oxygen or air as an oxidant. These react on the active electrode surfaces of the cell to produce electrical energy, water vapor, carbon dioxide and heat.

In many important applications hydrogen and/or carbon monoxide are not readily available, and commonly available fuels including methane, higher gaseous and liquid hydrocarbons, and alcohols must be used. These fuels are lumped together under the term hydrocarbons in the following discussions and descriptions. Likewise air, oxygen or mixtures containing oxygen are lumped together under the term oxidant. Hydrocarbons may be converted into hydrogen and carbon monoxide by well-known processes including steam reforming and partial oxidation reforming. Steam reforming is an endothermic reaction that adds hydrogen and oxygen to the hydrocarbon fuel in the form of steam, and produces a mixture of hydrogen and CO and/or CO₂. Steam must be generated and sensible heat must be transferred to the reaction site. The sensible heat is often "free" since it is obtained from a fuel cell exhaust gas burner. Partial oxidation reforming is a slightly exothermic reaction that adds oxidant and optionally steam directly to the hydrocarbon fuel, and produces a mixture of hydrogen and CO and/or CO . The advantage is that sensible heat transfer is not required, but the drawback is that a portion of "expensive" fuel energy is used to drive the endothermic reaction. Background on alternative reforming methods and apparatus and a description of a compact steam reformer intended for automotive applications are described in US Patent

No. 5,938,800 to Verrill et al.. Despite the advanced design of this device, it adds significantly to the overall size and complexity of a power plant.

Internal reforming within the cell is potentially simpler and more efficient than use of a separate reformer, and helps cool the cell. Hydrocarbons have been shown to react in the presence of water vapor, CO₂ and heat at the SOFC nickel anode surface without forming soot (elemental carbon). The hydrocarbon molecules break down to form hydrogen and carbon monoxide by the classic endothermic steam reforming reaction catalyzed by the nickel anode and using water vapor, CO₂.and heat formed at the site by the power generation reaction. The hydrogen and carbon monoxide then react to generate power, and the reaction products replace the water vapor, CO₂ and heat consumed by reforming to sustain the process. The overall reaction is fuel oxidation, but water vapor, CO₂ and heat must be present in an adjunct role to break down the fuel molecules without carbon formation. The quantity of adjunct gas required is a complex issue, and depends on a number of factors including fuel type and operating temperature. Carbon formation is discussed in pages 9-23 through 9-24 of the Fuel Cell Handbook,

(Fifth Edition) by EG&G Services, Parsons Inc., and Science Applications International Corporation published by the U.S. Department of Energy, Office of Fossil Energy, National Energy Technology Laboratory, Morgantown, WN 26507-0880 (October, 2000). Generally, the higher molecular weight hydrocarbons require higher levels of adjunct species. Steam to carbon molar ratios as low as 0.25 are reported for methane pre-reforming, while ratios of over 2.5 are required for gasoline reforming.

Fuel utilization is defined as the portion of the fuel heating value consumed by the cell electrochemical reactions, and high fuel utilization is an important part of achieving high overall power generation efficiency. Prior art SOFCs typically improve fuel utilization by passing the fuel across the anode surface from an inlet region to an exit region in plug flow mode. In plug flow the fluid elements move over the anode in an orderly first-in first-out sequence with minimal mixing. This prevents fresh inlet fuel from exiting prematurely without passing over the entire anode. Power generation reactions progressively consume the fuel, and the gas composition and temperature change as it passes over the anode. Perhaps 80% of the fuel is utilized to produce electric power by the time the fuel reaches the exit region. The fuel remaining at the exit may be burned to preheat the incoming fuel and air streams, supply steam reforming heat or put to other uses. The plug flow mode, however, leads to difficulties in using unreformed hydrocarbon fuel in prior art SOFCs. Reforming is inhibited at the fuel inlet region since the fuel sweeps away water vapor and CO₂. Contact between the hydrocarbon and the anode may cause soot formation under these conditions. In addition, onset of the endothermic reforming reaction in the fuel inlet region may cause local cooling, inhibiting reforming and power generation reactions. Typical single-stage plug flow operation presents another problem. The variation in fuel concentration and Nernst potential between the anode inlet and the outlet results in thermodynamic irreversibility, since each section of the cell delivers current at an average voltage, not the optimum voltage. Passing the fuel in series through two or more cells, each operating at a different voltage, to reach the final utilization target provides higher overall efficiency. This method is described in pages 9-70 through 9-77 of the Fuel Cell Handbook, (Fifth Edition) by EG&G Services, Parsons Inc., and Science Applications International Corporation published by the U.S. Department of Energy, Office of Fossil Energy, National Energy Technology Laboratory, Morgantown, WV 26507-0880 (October, 2000). The conventional solution to soot formation and inlet chilling is to add H₂, CO,

CO₂ and water vapor to the fuel in the inlet region. Power generation and internal reforming reactions then have the ingredients to start immediately, and the balance of the hydrocarbon fuel is internally reformed. This is effective, but increases system complexity. US Patent No. 5,082,751 to Reichner, for example, shows a system in which this is accomplished by partial fuel pre-reforming. Catalyst-filled reformers make intimate contact with the fuel cells so the fuel cell sensible heat drives the endothermic reaction. A water vapor and reformable fuel mixture is passed through the reformers before contacting the fuel cells. Recycling and mixing a portion of the fuel exhaust with the incoming fuel is an alternative to partial pre-reforming. This recycled exhaust contains water vapor, H₂, CO₂, CO, and heat, establishing the necessary conditions to start and sustain power generation and internal reforming. US Patent No.6,344,289 to Dekker et al. describes an internal reforming system in which recycled anode gas suppresses carbon formation without steam injection. It should be noted that while Dekker uses multiple cell stacks, the anode flows are in parallel, not series, and maximum fuel utilization must therefore be reached in a single plug flow stage.

U.S. Patent No. 6,162,556 to Vollmar, et al. teaches a high temperature fuel cell thermally integrated with a reformer that produces excess hydrogen beyond what the fuel cell uses. This excess hydrogen may be used for other purposes including as fuel for additional fuel cells. The fuel is vaporized, water is injected, and it is passed through the reformer into the fuel cell where it is partially utilized in an electrochemical power generation reaction. It should be noted that internal reforming is not used, and the water is added from an external source.

Summary of the Invention

The present invention goes beyond the conventional internal reforming approaches used for fuel cells, and provides a simple means of utilizing the steam, CO and heat formed within the SOFC to reform hydrocarbon fuels. The primary advantage of the present invention is that it provides an improved process for internal reforming in fuel cells and allows commonly available hydrocarbon fuels, including solid fuels, to be utilized in a fuel cell with minimal auxiliary equipment such as reformers and heat exchangers. This in turn leads to power generation systems with size, weight and cost advantages compared to prior art systems. It also leads to fuel cell based reformers that process fuel for other applications such as reducing furnace gas atmospheres, while simultaneously producing electric power. A second advantage is that the present invention is applicable to a range of high temperature fuel cell configurations and types, including SOFCs and molten carbonate fuel cells.

In the present invention a two-stage process replaces the single stage plug flow of fuel across the anode typical of prior art fuel cells. In the first stage unreformed hydrocarbon fuel is mixed with the products of reaction over the anode. This mixed fluid reactor is very different from the prior art plug flow reactor, in that the temperature and composition of the mixture of the hydrocarbon fuel and reaction products is relatively uniform between the inlet and the outlet regions. This serves to eliminate local fuel-rich areas with insufficient water vapor, CO₂ and heat, and as a consequence facilitates reforming and suppression of soot formation. In the first stage, the fuel is partially oxidized and power is generated. The heating value of the fuel in the first stage exit flow to the inlet of the second stage is only partially utilized ~ perhaps 40% to 60%. It is a partially reformed mixture of hydrogen, carbon monoxide, CO₂, water vapor and hydrocarbon fuel. The second stage is operated in plug flow mode to increase the utilization to perhaps 80%. Since the inlet gas is partially reformed, reforming is completed in the second stage without soot formation or excessive local cooling, and additional power is generated. Further, the oxidant can be routed through the second stage and then through the first stage so that sensible heat from the second stage is used to drive the endothermic reforming reaction in the first stage.

The first and second stages can be separated electrically such that each stage may be operated at a different cell voltage and/or current density to optimize performance. By using gas circulating through the first stage of the two-stage process, solid fuels such as coal can be reformed. While a variety of cell configurations may be used to carry out the present invention, the present invention includes a multiple manifold planar cell and stack design that is particularly adapted to two-stage power generation reforming. It should also be noted that the second stage fuel cell may be in principle any type of fuel cell. These may include multiple stage high temperature cells or low temperature cells employing using known processes to convert the exhaust of the first stage cells int a suitable hydrogen-containing fuel with low levels of carbon monoxide. In addition, by passing the mixture of hydrocarbon fuel and reaction products over active reforming catalyst surfaces, the catalytic reforming function of the anode surfaces can be augmented. The present invention provides means for mixing hydrocarbon fuel with reaction products in the flow passages over the anode in the first stage of the two-stage process, including a blower driven recirculating loop flow, jet pump recirculating loop flow, expansion driven reversing flow, and blower driven phase-shifted flow. The present invention also provides a means for implementing the two-stage power generation and reforming process as an integrated engine that includes fuel handling, air handling and heat recuperation in a compact package.

For the utilization of solid fuels, such as coal, anode flow is circulated through the first stage stack, a solid fuel bed, a filter and back to the first stage cell stack. This partially utilized fuel gas contains steam, CO₂ and sensible heat that contacts the solid fuel. The solid fuel reduces the steam and CO₂ to produce H₂ and CO, using the sensible heat to drive the endothermic reaction. The solid fuel reforming and fuel cell power generation reactions increase the volume of recirculating gas in the flow. This excess gas flows from the loop flow duct to the second stage. The first stage reactor exhaust of the present invention has other uses than feeding a second stage fuel cell stack. It may, for example, be used as a reducing atmosphere or feedstock for further processing, or it can be used in a variety of other energy conversion devices. Upon examination of the following detailed description the novel features of the present invention will become apparent to those of ordinary skill in the art or can be learned by practice of the present invention. It should be understood that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only. Various changes and modifications within the spirit and scope of the invention will become apparent to those of ordinary skill in the art upon examination of the following detailed description of the invention and claims that follow.

Brief Description of the Drawings The appended claims set forth those novel features which characterize the invention. However, the invention itself, as well as further objects and advantages thereof, will best be understood by reference to the following detailed description of a preferred embodiment taken in conjunction with the accompanying drawings, where like reference characters identify like elements throughout the various figures, in which: Fig. 1 is a schematic drawing showing the operating principle of the mixed fluid internal fuel reforming concept;

Fig. 2 is an illustration of the blower drive loop flow embodiment of the present invention;

Fig. 3 is an illustration of the jet pump drive loop flow embodiment of the present invention;

Fig. 4 is an illustration of the first cycle of the cyclic flow embodiment of the present invention;

Fig. 5 is an illustration of the second cycle of the cyclic flow embodiment of the present invention; Fig. 6 is an illustration of a blower drive loop flow solid fuel reforming embodiment of the present invention;

Fig. 7 is an illustration of phase shifted flow embodiment of the present invention;

Fig. 8 is a partially broken-away view of planar cell and its flow passages; Fig. 9 is an exploded view showing the individual layers of a planar cell; Fig. 10 is top and bottom views of a planar cell partially in cross-section;

Fig. 11 is a partially exploded view of a cell stack;

Fig. 12 is an exploded view of an integrated power module;

Fig. 13 is another exploded view, from a different perspective, of an integrated power module;

Fig. 14A is a plan view of an integrated power module indicating the section lines for the views shown in FIG. 15A and FIG. 15B, and FIG. 14B is an illustration of the exterior of the module; and

Fig. 15A is a section designated AA in FIG. 14A though an integrated power module illustrating the fuel flow, and Fig. 15B a section designated BB in FIG. 14A though the module illustrating the air flow.

Detailed Description of the Invention

The basic principle of the present invention may be implemented in a variety of ways, and is applicable in principle to any reformable fuel. Mixing, such as by mechanical mixing, of the incoming hydrocarbon fuel with the reaction products within a first set of fuel cells in the first stage cell 1, for example, as shown in Fig. 1, is the key element, since it facilitates reforming through electrochemical oxygen addition to hydrocarbon fuels. This is a desirable alternative to conventional steam reforming or POX reforming, since electric power is generated as part of the process. The anode exhaust from the first set of cells is only partially utilized, and is passed in plug flow mode through a second set of high temperature cells to achieve high fuel utilization. It should also be noted that the second stage fuel cell may be in principle any type of fuel cell. These may include multiple stage high temperature cells or low temperature cells employing using known processes to convert the exhaust of the first stage cells int a suitable hydrogen-containing fuel with low levels of carbon monoxide. Alternatively, the anode exhaust may be used for other purposes. Examples include industrial furnace atmospheres used in metal heat treating and similar applications that require a reducing gas environment. It may also be used as feedstock for other processes. The elements of the present invention are illustrated in FIG. 1 for a single first stage cell 1 and a single second stage cell 2. The first stage cell 1 is a layered structure with an anode 3, electrolyte 4 and cathode 5. Power collection conductors 6 and 7 contact the anode 3 and cathode 5 respectively. The injector 8 introduces hydrocarbon fuel 9 into the chamber 10 adjacent to the first stage anode 3, where the fuel is mechanically mixed with the reaction products (water vapor and CO₂) evolved from the anode as a result of the power generation reaction. A mechanical mixer 11 is shown to illustrate the principle, although other means are used in practice. The second stage cell 2 is a layered structure with an anode 12, electrolyte 13 and cathode 14. Power collection conductors 15 and 16 contact the anode 12 and cathode 14 respectively. Gas flows from volume 10 through transfer passage 17 to chamber 18 adjacent to the second stage anode 12, and exits through the fuel exhaust passage 19. The gas flow over the second stage anode 12 is essentially as unmixed plug flow. Oxidant is introduced into the chamber 20 adjacent to the second stage cathode 14 through the inlet passage 21. The oxidant flows across the cathode 14 and then through transfer passage 22 to chamber 23 adjacent to the first stage cathode 5, flows across the cathode 5, and exits through the oxidant exhaust passage 24.

The operating principle of the present invention is described with reference to FIG. 1. Hydrocarbon fuel 9 is added to the chamber 10 and mixed with the preexisting reaction products by the mechanical mixer. The mixing avoids the localized chilling or high local hydrocarbon fuel concentrations over the anode experienced with plug flow. Unreformed fuel reacts with the steam, CO₂ and heat to produce H₂ and CO, which produce electric power. The anode 5 will typically catalyze this reaction, although additional catalytic material may be positioned in contact with the gas mixture. The power reaction in turn replaces the steam, CO₂ and heat. The overall reaction is simply fuel oxidation, and the steam and CO₂ act as catalysts and are not consumed. Heat is a limiting factor, and heat may have to be conserved or added in some cases. The concentration of fuel relative to the reaction products is controlled by the volume of chamber 10, the rate of fuel addition, and the rate at which power generation produces products of reaction at the anode. Variation of these parameters provides the means of adjusting the fuel concentration such that soot-free fuel reforming is achieved for the specific fuel compositions. Generally, the higher molecular weight hydrocarbons require higher levels of adjunct species. Steam to carbon molar ratios as low as 0.25 are reported for methane pre-reforming, while ratios of over 2.5 are required for gasoline reforming. The actual limiting ratio depends on a number of factors, and will have to be determined experimentally for each case. The steam/carbon molar ratio is based on water and/or steam mixed with the hydrocarbon fuel. CO₂ may substitute for water vapor at a ratio of two moles of CO₂ for one mole of water vapor. Introduction of hydrocarbon fuel 9 into chamber 10 increases the quantity of gas in chamber 10 and causes excess mixed fuel and reaction products to flow out through the transfer passage 17 into the second stage cell 2. Fuel utilization is less than 50% in the primary cell stack, so the gas flows over the second stage anode 12 in plug flow mode to complete the reforming reaction and increase fuel utilization to the 70 to 80% level. The depleted fuel gas leaving the second stage cell 2 through the fuel exhaust passage 19. The sensible heat and remaining heating value may be recovered by means such as a burner and heat exchanger for uses such as preheating the inlet oxidant stream. Preheated oxidant flows into the second stage cell 2 through inlet passage 21, where it flows through chamber 20 and contacts cathode 14. The chemical potential difference between the oxidant at the cathode 14 and the fuel mixture at the anode 12 generates a voltage across the electrolyte 13 and an external current flow through power collection conductors 15 and 16. Oxygen from the oxidant passes through the electrolyte in the form of ions, and oxidizes hydrogen and CO at the anode to form water vapor and CO₂. The partially depleted oxidant flows from the second stage cell 2 through transfer passage 22, where it flows through chamber 23 and contacts cathode 5. The chemical potential difference between the oxidant at the cathode 5 and the fuel mixture at the anode 3 generates a voltage across the electrolyte 4 and an external current flow through power collection conductors 6 and 7. Sensible heat in the partially depleted oxidant stream may be transferred through the layers of the first stage cell 1 to provide additional heat to drive the endothermic reforming reaction taking place on anode 3. Depleted oxidant gas leaves the first stage cell 2 through the oxidant exhaust passage 24. The sensible heat and remaining oxidant value may be recovered by means such as a burner and heat exchanger for uses such as preheating the inlet oxidant stream. In cases in which reforming heat is the limiting factor, a controlled amount of oxidant may be added to the first stage cell 1 along with the fuel to generate heat through direct combustion. Air addition may also be used to provide additional heat during startup, part-load operation, or any other condition where additional heat is needed to sustain operation. Recirculating loop flow mixing, shown in FIG. 2 is one implementation of the basic principle of the present invention. Two cell stacks are employed. The first stage stack 30 is represented by a cross-flow bipolar stack of conventional design. It is formed of a stack of cells 31 clamped between power takeoff plates 32 and 33. Anode passages 34 pass between the cells 31 and allow flow of fuel gas from the fuel inlet manifold 35 to the fuel exhaust manifold 36 such that it flows over and contacts the anode layer of each cell. Similarly cathode passages 37, perpendicular to the anode passages 34, pass between the cells 31 and allow flow of oxidant from an oxidant inlet manifold to an oxidant exhaust manifold (oxidant manifolds not shown). Conductive separator plates 38 between the cells 31 prevent mixing of the fuel gas streams and the oxidant streams while electrically connecting the cells in series. The second stage cell stack 39 is illustrated as identical to 30, and it is understood that the same description applies. A hot blower 40 (motor not shown) and a loop flow duct 41 recirculate the fuel and reaction product mixture from the fuel exhaust manifold 36 of the first stage cell stack 30 back to its inlet fuel manifold 35. Hydrocarbon fuel 9 is added to the recirculated mixture by an injector 8 upstream of the inlet fuel manifold 35. An optional reforming catalyst bed 42 may be positioned between the fuel injection point and the manifold 35. A "T" connection 43 branches off the loop flow duct 41 and leads to the fuel inlet manifold 44 of the second stage cell stack 39. Depleted fuel gas exits though the fuel exhaust manifold 45 of the second stage cell stack 39. Additional ducts (not shown) provide oxidant flow to the cathode passages 37 of the two stacks 30 and 39. The operation of recirculating loop flow mixing is described with reference to FIG. 2. The hot blower 40 creates a recirculating loop flow through the first stage stack 30. Fuel 9 is added between the "T" connection 43 and the first stage stack fuel inlet manifold 35 so that it is carried to the first stage stack 30 rather than the second stage stack 39. The loop flow is large compared to the flow of fuel 9, with the result that the concentration of unreformed fuel is entering the stack is low relative to the reaction products. A second result is that the loop flow transfers heat from hot areas to cool areas and makes the temperature more uniform. This is the same result obtained by mixing incoming fuel with the reaction products, and has the same benefits of eliminating soot formation and facilitating reforming. The practical benefit of loop flow is that the hot blower 40 that provides the mechanical mixing energy is outside the stack and serves multiple cells. The optional reforming catalyst bed 42 provides added catalytic reforming surface area, and the incoming stream provides the reforming heat for the reaction. Addition of new fuel 9 and the resulting reforming and power generation reactions increase the quantity of recirculating gas. This excess gas flows from the loop flow duct 41 through the "T" connection 43 into the secondary stack fuel inlet 44. The gas passes once through the secondary stack 39 in slug flow mode to increase fuel utilization and power generation, and is exhausted through manifold 45. Oxidant is passed through the cathode passages 37 in the two cell stacks 30 and 39. Various oxidant flow arrangements may be used, although a preferred arrangement is to first pass the oxidant through the second stage stack 39 where it will be heated, and then through the first stage stack 30 where this heat may be utilized to drive the endothermic reforming reaction.

FIG. 3 shows a variation of recirculating loop flow mixing that incorporates a fuel jet driven venturi pump. The hot blower 40 shown in FIG. 2 is omitted. Instead, the recirculating loop flow is provided by the fuel jet 9 cooperating with a venturi 50. The fuel jet momentum is transferred by mixing to the fuel and reaction product mixture in the venturi, causing it to flow from the fuel exhaust manifold 36 of the first stage cell stack 30 back to its inlet fuel manifold 35. The balance of the system configuration and operation is substantially the same as shown in FIG. 2. The jet pump offers a mechanically simple circulation pump with no moving parts. It is particularly suitable for systems operating on compressed gaseous hydrocarbon fuel. Push-pull flow mixing, shown in FIG. 4 and FIG. 5 is another implementation of the basic principle of the present invention. Two cell stacks are employed. The first stage stack 30 is represented by a cross-flow bipolar stack of conventional design. It is formed of a stack of cells 31 clamped between power takeoff plates 32 and 33. Anode passages 34 pass between the cells 31 and allow flow of fuel gas between the first inlet manifold 60 and the second fuel manifold 61 such that it flows over and contacts the anode layer of each cell. Similarly cathode passages 37, perpendicular to the anode passages 34, pass between the cells 31 and allow flow of oxidant from an oxidant inlet manifold to an oxidant exhaust manifold (oxidant manifolds not shown). Conductive separator plates 38 between the cells 31 prevent mixing of the fuel gas streams and the oxidant streams while electrically connecting the cells in series. The second stage cell stack 39 is illustrated as identical to 30, and it is understood that the same description applies. A first flow duct 62 connects the first fuel manifold 60 to the first opening of a "T" connection 43, and a second flow duct 63 connects the second fuel manifold 61 to the second opening of the "T" connection. The third opening of the "T" connection 43 branches leads to the fuel inlet manifold 44 of the second stage cell stack 39. Depleted fuel gas exits though the fuel exhaust manifold 45 of the second stage cell stack 39. A first flow control valve 64 and a second flow control valve 65 are incorporated in the first flow duct 62 and the second flow duct 63 respectively. Flow control valves 64 and 65 may be opened to allow flow through the ducts or closed to block the flow on demand by a control means (not shown). A first fuel injector 66 and a second fuel injector 67 are placed to inject fuel into volume 68 of the first flow duct 62 and volume 69 of the second duct respectively. Volume 68 is defined as the duct volume between the first flow control valve 64 and the first fuel manifold 60. Likewise, volume 69 is defined as the duct volume between the second flow control valve 65 and the second fuel manifold 61. A control means (not shown) turns the injectors 66 and 67 on and off on demand. Optional first reforming catalyst bed 70 may be positioned between the first fuel injector 66 and the first fuel manifold 60. Likewise, optional second reforming catalyst bed 71 may be positioned between the second fuel injector 67 and the first fuel manifold 61. Additional ducts (not shown) provide oxidant flow to the cathode passages 37 of the two stacks 30 and 39.

The operation of push-pull flow mixing is described with reference to FIG. 4 and FIG. 5. The basic mechanism is to mix fresh fuel and reaction products by periodically reversing the flow through the primary cell stack, using fuel expansion to drive the flow. FIG. 4 shows Cycle A, in which the flow is left to right through the first stage cell stack 30. Flow control valve 64 is closed, and flow control valve 65 is open. Fresh liquid or cool gas fuel 72 is injected into the hot fuel and reaction product mixture in volume 68, and is heated and reformed. The gas volume in volume 68 increases through both fuel heating (including vaporization in the case of liquid fuel) and the increase in the number of moles resulting from the reforming reaction. The expansion pushes the mixture of fresh fuel 72 through the first stage cell stack anode passages 34 and on through the open second flow control valve 65 and second flow duct 63. As the gas passes through the first stage cell stack 30, power is generated and reforming continues. The gas flows from second flow duct 63 through the "T" connection 43 to the second stage cell stack 31 where power generation and reforming of residual fuel proceeds further. Second stage cell stack 31 has unidirectional plug flow through its anode channels 34, and increases fuel utilization.

FIG. 5 shows Cycle B, in which the flow is right to left through the first stage cell stack 30, and is the mirror image of FIG. 4. Injection through first injector 66 is stopped and the expansion flow is allowed to drop to a reduced level. Second flow control valve 64 is closed, and first flow control valve 65 is opened. Fresh liquid or cool gas fuel 73 is injected into the hot fuel and reaction product mixture in volume 69, and is heated and reformed. The gas volume in volume 69 increases through both fuel heating (including vaporization in the case of liquid fuel) and the increase in the number of moles resulting from the reforming reaction. The expansion pushes the mixture of fresh fuel 73 and reaction products through the first stage cell stack anode passages 34 and on through the open second flow control valve 65 and second flow duct 63. As the gas passes through the first stage cell stack 30, power is generated and reforming continues. The gas flows from second flow duct 63 through the "T" connection 43 to the second stage cell stack 31 as described for FIG. 4.

Optional reforming catalyst beds 70 and 71 may be useful for both fuel processing and thermal management. In FIG. 4 the second catalyst bed 71 is heated by the first stage stack 30 exhaust, and in the process cools the gas entering the second stage stack 31. The first catalyst bed 70 is hot from the previous cycle, and this sensible heat and the catalytic surface help pre-reform the fuel entering the first stage stack 30. During the reversed flow shown in FIG. 5, the first catalyst bed 70 is heated by the first stage stack 30 exhaust, and in the process cools the gas entering the second stage stack 31. The second catalyst bed 71 is hot from the previous cycle, and this sensible heat and the catalytic surface again help pre-reform the fuel entering the first stage stack 30. The effect of the push-pull flow mixing is generally the same as loop flow mixing. The hydrocarbon fuel is reformed, local areas of high fuel concentration are avoided and temperature is made more uniform. The practical advantage is that the expansion from heating, including vaporization in the case of liquid fuel, is utilized to provide the mixing energy. This is "free" energy, since heat must be supplied to heat and vaporize the fuel in any case.

Recirculating loop flow mixing may be extended to include direct solid fuel utilization. This is illustrated in FIG. 6 in which solid fuel replaces liquid or gas fuel injection. As in recirculating loop flow mixing, two cell stacks are employed. The first stage stack 30 is represented by a cross-flow bipolar stack of conventional design. It is formed of a stack of cells 31 clamped between power takeoff plates 32 and 33. Anode passages 34 pass between the cells 31 and allow flow of fuel gas from the fuel inlet manifold 35 to the fuel exhaust manifold 36 such that it flows over and contacts the anode layer of each cell. Similarly cathode passages 37, perpendicular to the anode passages 34, pass between the cells 31 and allow flow of oxidant from an oxidant inlet manifold to an oxidant exhaust manifold (oxidant manifolds not shown). Conductive separator plates 38 between the cells 31 prevent mixing of the fuel gas streams and the oxidant streams while electrically connecting the cells in series. The second stage cell stack 39 is illustrated as identical to 30, and it is understood that the same description applies. A hot blower 40 (motor not shown) and a first flow duct 80 connects the first fuel manifold 36 to the first opening of a "T" connection 43, and a second flow duct 81 connects the second opening of the "T" connection to the inlet plenum 82 of the solid fuel bed 83 such that the gas passes through and contacts the solid fuel. A third flow duct 84 routes the fuel gas through a fuel cleanup system shown schematically as filter 85, and a fourth flow duct 86 carries the fuel gas to the fuel inlet manifold 35 of the first stage cell stack 30. An optional reforming catalyst bed 42 may be positioned between the fuel injection point and the manifold 35. The third opening of the "T" connection 43 branches leads to the fuel inlet manifold 44 of the second stage cell stack 39. Depleted fuel gas exits though the fuel exhaust manifold 45 of the second stage cell stack 39.

Additional ducts (not shown) provide oxidant flow to the cathode passages 37 of the two stacks 30 and 39.

The operation of recirculating loop flow solid fuel reforming is described with reference to FIG. 6. The hot blower 40 creates a recirculating loop flow through the first stage stack 30, the solid fuel bed 83, the filter 85 and back to the inlet of the first stage cell stack 30. This partially utilized fuel gas contains steam, CO₂ and sensible heat that contacts the solid fuel in bed 83. The solid fuel reduces the steam and CO to produce H₂ and CO, using the sensible heat to drive the endothermic reaction. The solid fuel reforming and fuel cell power generation reactions increase the volume of recirculating gas in the flow. This excess gas flows from the loop flow duct 41 through the "T" connection 43 into the secondary stack fuel inlet 44. The gas passes once through the secondary stack 39 in slug flow mode to increase fuel utilization and power generation, and is exhausted through manifold 45. Oxidant is passed through the cathode passages 37 in the two cell stacks 30 and 39. The optional reforming catalyst bed 42 provides added catalytic reforming surface area, and the incoming stream provides the reforming heat for the reaction.

The overall effect of the solid fuel reaction is similar to injection of liquid or gas fuel. The solid fuel is oxidized by oxygen passing though the primary call stack electrolyte in the form of ions, in the process generating electric power. The main difference is that at least part of the endothermic fuel reforming reaction is carried out in the solid fuel bed 81 rather than the first stage 30 as is the case with hydrocarbon fuels. The likely result is higher stack operating temperature. Coal, coke, charcoal, solid waste and biomass are obvious candidates for the process, but in theory any combustible material containing carbon or carbon and hydrogen can be used. Technology developed for gasification of coal and other solid fuels, in particular solids and ash handling and gas cleanup, is applicable to this process. The fact that H₂ and CO, are the desired reformate, not pipeline quality methane, simplifies the process. The direct thermal integration of the gasification process with the high temperature fuel cell further simplifies the system since heat exchangers are not required.

FIG. 7 illustrates a phase shifted fuel flow embodiment of the present invention. Two cells 90 are stacked in a conventional bipolar arrangement. Oxidant flows straight through the stack from the inlet region A to the exhaust region C through cathode passages 91. Anode passages 92 are divided into two segments. The first anode segments 93 extend from region A to region B, and the second anode segments 94 extend from region B to region C. Openings 95 separate segments 93 and 94 in region B. Loop flow duct 96 connects the outlet of the anode passages 92 in exhaust region C back to the inlet of these same passages in inlet region A, and contains a hot blower 40. A fuel injector 97 with nozzles 98 extends into the openings 95 such that the nozzles deliver fuel into the second anode passage segments 94. Exhaust flow openings 99 are provided by a gap between the fuel injector 97 and the anode openings 95.

The operation of phase shifted fuel flow reforming is described with reference to FIG. 7. Preheated oxidant enters the cathode passages 91 in region A, and flows to region C and exhausts. During this transit the oxidant supplies oxygen to the cathode surfaces for the fuel cell reaction and exchanges heat with the cell structures. The hot blower 40 circulates a mixture of fuel and fuel reaction products containing steam, CO and sensible heat through the anode passages 92 and the loop flow duct 96. Fuel is added through the nozzles 98. This fuel is progressively reformed and consumed in the anode passages, first in anode segments 99 and then in anode segments 98. The flow splits and excess depleted gas exits through exhaust flow openings 99, and the rest mixes with the fresh fuel added by the nozzles 98 within second anode passage segments 94. The depleted gas is hot and high in water vapor and CO₂, while still containing significant quantities of hydrogen and CO. As a consequence, the reforming and fuel cell power generation reactions start immediately. Additional reforming heat is transferred to the second anode passage segments 94 from the parallel cathode passages 91. The fuel gas is then depleted to exhaust conditions in the first anode passage segments 93. The overall result is that water vapor, CO₂ and heat are supplied to reform hydrocarbon fuel within the cell without local fiiel concentrations that could cause soot formation. The use of two stacks provides flexibility in system management. In all of the implementations of the present invention the primary cell stack and secondary cell stack will in general have different voltages, and at minimum should be connected in series to allow each stack to establish its own voltage at a common current value. This also provides the highest overall output voltage, which is important for efficient power conditioning in small systems. Optimally, each stack will supply a separate electrical load such that the voltage and current may be independently optimized. The principal physical requirement for the first stage cell stack is that the fuel and air exhaust streams are separate, not combined, so that the loop flow recirculation reforming process may be implemented. The second stage stack may use a common exhaust.

A novel hollow cathode multiple manifold planar cell and stack design is a further aspect of the present invention, and is particularly applicable to two stage internal reforming systems in which the fuel and air streams must be kept separate after passing through the cells. It also scales easily for different system sizes and is relatively straightforward to manufacture and seal. The basic multiple manifold idea is covered by

US Patent No. 5,268,241 to Meacham, owned by Michael A. Cobb & Company, and incorporated herein by reference.

FIG. 8 is partial section through the midplane of a planar hollow cathode cell 100 showing the flow fields and directions. Four "flavors" of holes penetrate the cells: oxidant in 101, oxidant out 102, fuel in 103 and fuel out 104. When the cells are stacked in series, the aligned holes form internal manifolds that serve all the cells in the stack. Passages in mating parts connect to each manifold hole. The hollow cathode flow passages 105 connect the oxidant in 101 and oxidant out 102 holes within the hollow cathode 106. Oxidant flowing through the passages diffuses through the porous cathode material to the active power generation sites at the electrolyte. The holes and flow passages are arranged to provide a reasonably uniform distribution of the oxidant flow over the cell area. The anode flow passages are formed between the cells by a set of grooves 109 on the anode surface contacting the flat surface of the next cell in the bipolar cell stack. The anode passages 108 lead from the fuel inlet holes 102 to the fuel out holes 103, and result in fuel flow across the anode. Fuel gas flowing through the passages diffuses through the porous anode layer to the active power generation sites at the electrolyte. The holes and flow field passages are arranged to provide a reasonably uniform distribution of the fuel flow over the cell area. The overall result is the oxidant flow through the cathode passages 105 and the fuel flowing through the anode grooves 109 are in cross flow on each side of the cell electrolyte. The hollow cathode construction confines the oxidant within the cell structure. Face seals between the cells around the oxidant holes 101 and 102 and the cell perimeter 112 prevent the fuel from contacting the oxidant or leaking out of the edge of the stack. FIG. 9 and FIG. 10 show the construction of a single small hollow cathode planar cell according to the present invention. The hollow cathode 106 foπns the structural base of the cell. It is formed of green porous cathode material and fired to reach it final properties. The internal hollow cathode flow passages 105 are formed by a means such as molding around a fugitive core that is burned out during firing. The other layers are applied as coatings and fired as required. The electrolyte coating 107 is applied to the grooved anode side of the hollow cathode 106. The coating continues around the cathode edges 113 and into the fuel passage holes 103 and 104. The lanthanum chromite interconnect coating 114 is applied to the flat back surface 115 of the hollow cathode 106. It joins the electrolyte coating 107 to seal the cathode surface and prevent fuel contact. An anode coating 110 is applied to the anode side over the electrolyte coating 107 except in the face seal areas 111 around the oxidant passage holes and the cell perimeter 112. The anode coating does not continue around the edges 113 or through the fuel passage holes 103 and 104. A conductive interface layer 116 similar to the anode layer is applied to the flat back surface 115 over the interconnect layer 114 except in the seal areas around the oxidant passage holes. The interface layer does not continue through the fuel passage holes 103 and 104 so that it is electrically isolated from the anode coating 110 of the cell. Its purpose is to contact the anode coated ridges of the next cell and distribute current over the interconnect coating 114. The cells 100 are assembled as shown in FIG. 11 to form an internal manifold stack 120. The anode coated ridges between the grooves 109 on one cell contact the conductive interface layer 116 on the next cell to make electrical contact and form anode flow passages 108. Gaskets 121 surrounds the oxidant passage holes and extends around the cell perimeter. The aligned oxidant in 101, oxidant out 102, fuel in 103 and fuel out 104 holes in the stack form respectively the oxidant in 122, oxidant out 123, fuel in 124 and fuel out 125 internal manifold passages. The internal manifolds provide gas access to each cell in the stack from ducts (not shown) connected to the stack ends, and fluid is confined to the air manifold passages and the hollow cathode interiors, and the fuel is confined to the flow passages between the cells. The overall result is a cell stack in which oxidant is hermetically sealed within the hollow cathode passages 105 and oxidant mamfold passages 122 and 123. Similarly, fuel is sealed within the anode passages 108 and fuel mamfold passages 124 and 125.

The hollow cathode multiple manifold planar cell and stack design of the present invention has important advantages. It is cathode supported, and the conductivity balance of cathode supported cells is favorable since the cathode is thick and the anode is thin. Fuel cells require open reactant gas contact areas on the anode and cathode surfaces. Current generated in these open areas must flow laterally through the electrodes to collection points. The cathode support structure is a half-millimeter or more thick, and therefore has sufficient cross-sectional area to carry lateral current efficiently despite the relatively low electronic conductivity of cathode material. The high electronic conductivity coated anode carries lateral current efficiently with a layer thickness on the order of 50 microns. The combination of thick cathode and thin anode results in low overall resistive losses. Anode support, typical of many prior art SOFC designs, reverses this situation, and resistive losses in the cathode become a problem. A tightly integrated system that combines two stage mixed internal reforming with thermal management and fluid handling subsystems into a compact air-breathing engine, is a further implementation of the present invention. A number of engine configurations are possible, and the design described is only an example and is not limiting. The example illustrates in FIG. 12 through FIG. 15. has several particular features. Hollow cathode multiple manifold planar cell and stack design planar cells are used that combine high power density with simple sealing and flexibility in gas manifold arrangement, and are a key element in achieving tight integration. The counterflow thermal confinement stack enclosure is a multishell structure that surrounds the cell stack and heats the inlet air while cooling the exhaust and maintaining the outside skin at near-ambient temperature. A single extended shaft motor drives an air blower and anode gas recirculation blower. It also delivers and atomizes the liquid fuel. The engine subsystems are combined into a compact assembly with simple interconnections and short flow paths. Like any engine, supporting subsystems are required to form a complete power plant. These include a fuel supply, power conditioner, automatic controls, starting system, user interface and system enclosure. This aspect will focus on the engine, and largely exclude the supporting subsystems. FIG. 12 and FIG. 13 are exploded views that illustrate the principal elements of the integrated engine 130, and FIG. 14 shows an overall view of the assembled unit. The first stage cell stack 131 and the second stage cell stack 132 are assembled such that they are connected by the electrically conductive afterburner housing 133 and cover 134. An electrical insulator 135 and conductive power takeoff 136 contact the cathode end of the second stage stack 132, and the electrically conductive fuel blower housing 137 contacts the anode end 138 of the first stage stack 131. The air plenum and heater 139 is a multilayer structure that includes the exhaust pipe 140 and encloses the cell stacks. The blower motor 141 has an extended hollow shaft 142 that rotationally drives and supports the fuel recirculation blower impeller 143 and air blower impeller 144. The air blower housing 146 and the blower base 147 contain and support the blowers and motor. The integrated engine operation is best illustrated by FIG. 15 A that illustrates the fuel flows and by FIG. 15B that illustrates the air flows and exhaust flows.

The fuel path is as follows. The recirculation blower 143 drives a loop flow of partially utilized fuel gas through the first stage stack 131. Gas is pumped from the fuel out internal manifold passage 125 at the center of the stack, through the recirculation blower impeller, and into the four fuel internal manifolds 124 at the corners of the stack. Liquid fuel is introduced through the hollow motor shaft 142 and sprays into the recycled fuel gas through an outlet hole 162 in the center of the recirculation blower 143. Here it is atomized and thoroughly mixed with the gas flowing through the blower and pumped through the primary stack anode passages 108 back to the central passage 125. The fuel flow through the hollow shaft also cools the shaft and recirculation blower and preheats the fuel. The addition of fuel to the recirculated fuel gas flow and the reforming and power generation reactions in first stage stack increases the gas volume. This excess volume spills over through the passage 148 in the afterburner housing 133 and cover 134 into the central fuel inlet manifold 149 of the secondary stack 132. The gas then passes in plug flow mode though the anode passages 108 and exhausts into the afterburner volume 150.

The air path is as follows. The air blower 144 draws in atmospheric air below the motor 141 and delivers it through the air blower housing 146 to the air plenum 151. The air plenum surrounds the air distribution can 152 and forms a cool outer surface. A gap 153 at the top of the distribution can and holes 154 in the sides of the can distribute cool air over the exhaust plenum 155 where it is heated. The heated air then enters the two oxidant inlet manifolds 122 in the second stage stack 132, flows through the hollow cathode passages 105, and exits through the two oxidant outlet manifolds 123. The air is heated as it passes through the second stage stack. It then flows through passages (not shown) in the afterburner housing 133 and cover 134 into the two oxidant inlet manifolds (not shown) of the first stage stack 131. The gas passes in plug flow mode though the cathode passages 105 and exhausts into the afterburner volume 150. Typically the air gains heat as it passes through the second stage stack 132 that is transferred to the first stage stack 131 to drive the endothermic reforming reaction.

The exhaust flow path is as follows. The depleted fuel and air mix and burn in the afterburner volume 150. Baffles are included to increase mixing, and catalytic material may be added to promote complete combustion. The exhaust gas leaves the afterburner through ports 157 in the side, and is directed to the bottom of the stack assembly by the exhaust baffle 158. The exhaust then flows up through the exhaust plenum 159 and exits through the exhaust pipe 140. In the process it is cooled while heating the inlet air in counterflow. A conventional high temperature thermal insulation blanket (not shown) is desirable and may be employed on the exposed lower portion 160 of the distributor shell lo prevent heat loss from the inlet air. The electrically hot current collector 161 is at the bottom of the stack, and is isolated by the electrical insulator 135. The top of the first stage stack 131 is grounded to the blower housing structures. The metal afterburner assembly 133 and 134 connects the primary and secondary cell stacks. Further, an axial,, load is applied to the insulator 135 by a spring in the housing (not shown) to clamp the . cell stacks 131 and 132 together with the current collector 136, afterburner assembly 133 and 134, and the fuel blower housing 137. The purpose of the clamping is to maintain sealing 2nd electrical contact between these elements. A secondary system (not shown) is required for starting. SOFCs have an operating temperature in the range of 800°C to 1000°C, and must be heated to start the reforming and power generation reactions. A battery spins up the motor 141 to start the fluid flows, and air is bled into the fuel inlet area of the first stage stack 131. Electrical ignition sources are activated in the fuel inlet and the afterburner volume.150 to start partial oxidation combustion in the fuel inlet and to assure complete fuel combustion in the afterburner. The inlet air is heated by the hot exhaust flov/ and the fuel gas is heated by the partial combustion. In addition, afterburner heat is transferred to the cell stacks on each side. When the stack is heated to operating temperature by these combined processes, the air flow into the fuel inlet and the ignition sources are turned off, and the engine operation is self-sustaining.

It is important to adjust parameters including air flow and fuel flow during startup and operation to accommodate changes in electrical load, maintain efficient power generation, and protect the system from temperature extremes. While these parameters may be adjusted manually, a microprocessor based "engine control computer" is desirable. Such a system automatically controls the sequence of operations to start up and shut down the system, and control operating variables such as fuel flow and blower motor speed to follow load changes and keep the process within its operating envelope. Further, it monitors variables such as temperatures and stack voltage as inputs to control algorithms and to trigger alarm and emergency shutdown sequences. The principle of counterflow thermal confinement is part of the present invention. In its most basic form three air cooled metal shells provide both thermal insulation and gas stream heat exchanger functions. The outer shell serves as an inlet air plenum that supplies cooling air to the inner shells, and in the process forms a cool outer skin for the package. Cool inlet air flows through calibrated openings in intermediate distributor shell, and flows over the hot outer surface of the inner exhaust plenum and then enters the fuel cell. In this process the air is heated and the exhaust gas is cooled. The fuel cell stack is inside the exhaust plenum, so it is maintained at high temperature. The spent air and fuel from the stack are mixed to afterburn the remaining fuel, and then flow into the exhaust plenum. The exhaust gas flows out through the exhaust pipe after it has transferred a significant part of its heat to the incoming air. The advantage of the arrangement is that the shells do triple duty as a recuperative heat exchanger, stack thermal insulation, and mechanical stack enclosure. This results in a particularly light and compact assembly. It also provides effective heat management, since all the heat release, including fuel and air afterburning, takes place inside the exhaust plenum. This is particularly important for maintaining operating temperatures in small systems with large surface to volume ratios.

The foregoing embodiments of the present invention have been presented for the purposes of illustration and description. These descriptions and embodiments are not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above disclosure. The embodiments were chosen and described in order to best explain the principle of the invention and its practical applications to thereby enable others skilled in the art to best utilize the invention in its various embodiments and with various modifications as are suited to the particular use contemplated. In particular, while a specific cell and stack design is a part of the present invention, many aspects of the invention may be implemented with a variety of high temperature fuel cells. It is intended that the invention be defined by the following claims.

Claims

What we claim is:

1. A reactor comprising a high temperature fuel cell having an anode, means of introducing fuel to the anode of said fuel cell, means for mixing said fuel with anode reaction products, and a means for forming an exhaust stream of fuel and reaction product mixture.

2. The reactor of Claim 1 wherein the concentration of fuel in the mixture of fuel and reaction products is lower than the soot formation limit.

3. The reactor of Claim 1 wherein said means for mixing is a recirculating loop flow.

4. The reactor of Claim 3 wherein said recirculating loop flow is established by a duct and pumping means external to said fuel cell.

5. The reactor of Claim 4 wherein said pumping means is a blower.

6. The reactor of Claim 4 wherein said pumping means is a jet pump.

7. The reactor of Claim 4 wherein said pumping means is a cyclic gas volume expansion of added fuel which results in cyclically reversing loop flow through said duct.

8. The reactor of Claim 7 wherein said cyclic gas volume expansion of added fuel resulting in said cyclically reversing loop flow is controlled by valves and fuel injectors operating in a timed relationship.

9. A solid fuel reactor comprising a high temperature fuel cell having an anode and means for circulating the anode reaction products so that said anode reaction products alternately contact said solid fuel and said anode.

10. The solid fuel reactor of Claim 9 wherein said means for circulating the anode reaction products is a recirculating loop flow.

11. The solid fuel reactor of Claim 10 wherein said recirculating loop flow is established by a duct and pumping means external to said fuel cell.

12. The solid fuel reactor of Claim 11 wherein said pumping means is a blower.

13. The solid fuel reactor of Claim 11 wherein said pumping means is a jet pump.

14. The solid fuel reactor of Claim 1 wherein reforming catalyst is placed in contact with the fuel and product gas mixture.

15. A fuel cell power generation comprised of the reactor of Claim 1 and a second stage fuel cell stack wherein said fuel and reaction product mixture exhaust stream comprises the fuel gas stream to said second stage fuel cell stack.

16. The fuel cell power generation system of Claim 15 wherein said second stage fuel cell is operated in plug flow mode to increase fuel utilization.

17. The fuel cell power generation system of Claim 15 wherein oxidant is first passed over the cathode of said second stage fuel cell and then passed over the cathode of said fuel cell in said reactor.

18. The fuel cell power generation system of Claim 15 wherein said fuel cell in said reactor and said second stage fuel cell are electrically connected in series.

19. The fuel cell power generation system of Claim 14 wherein said fuel cell in said reactor and said second stage fuel cell each operate at a different optimum voltage and electric current.

20. A fuel cell including a anode passage and an anode gas recirculation loop in which fuel is added to the anode gas stream at an intermediate point in the anode passage such that the fuel is mixed with anode reaction products, and an oxidant flows in a cathode passage parallel to the anode passage.

21. The fuel cell of Claim 20 wherein excess anode reaction products exit at said intermediate point.

22. The fuel cell of Claim 20 wherein the oxidant stream transfers sensible heat to said anode stream in the region downstream of said intermediate point.

23. A multiple manifold hollow cathode supported solid oxide planar bipolar fuel cell stack comprised of multiple cells in which: oxidant flows between oxidant inlet and oxidant outlet holes through oxidant passages within the hollow cathodes; fuel flows between fuel inlet and fuel outlet holes through fuel passages between adjacent cells; one side of each cell is coated with electrolyte and anode layers; the opposite sides are coated with a lanthanum chromite interconnect layers and a conductive current distribution layers; and current flows from one cell to the next through a multiplicity of contact areas between the anode of one cell and the conductive current distribution layer of the next cell.

24. The fuel cell stack of Claim 23 wherein said electrolyte layer and said lanthanum chromite interconnect layer of said cells join at the cell outer perimeter and at the inner perimeter of each of the fuel inlet and fuel outlet holes, such that oxidant gas is confined within the hollow cathodes.

25. The fuel cell stack of Claim 23 wherein seals surround said oxidant holes and contact adjacent cells such that oxidant gas is confined within said holes and prevented from flowing between said cells.

26. The fuel cell stack of Claim 23 wherein reducing or inert gas surrounds the cell stack.

27. The fuel cell of Claim 26 wherein said reducing or inert gas is fuel, depleted fuel or mixed fuel and oxidant exhaust.

28. An integrated fuel cell engine that comprises the fuel cell power generation system of Claim 15 together with means for interconnecting said reactor and said second stage fuel cell stack, common thermal enclosure means, heat recovery means, fluid pumping means, fuel introduction means, startup means and control means.

29. The integrated fuel cell engine of Claim 28 wherein said reactor and second fuel cell stage stack are closely spaced and interconnected in said common thermal enclosure such that fluid flow paths are short and heat losses are minimized.

30. The integrated fuel cell engine of Claim 28 wherein said common thermal enclosure means and heat recovery means include an afterburner adjacent to said reactor and second stage stack within said common thermal enclosure means.

31. The integrated fuel cell engine of Claim 30 wherein said common thermal enclosure means and heat recovery means comprise heat conducting structures arranged such that fluids flowing into the enclosure are heated by the structure and fluids flowing out are cooled by the structure, with the result that the structure serves as a counterflow heat exchanger.

32. The integrated fuel cell engine of Claim 31 wherein at least a portion of said heat conducting structures are multilayer metal shells.

32. The integrated fuel cell engine of Claim 28 wherein said fluid pumping means include an anode gas recirculation blower and an air supply blower within and part of said common thermal enclosure means and heat recovery means.

33. The integrated fuel cell engine of Claim 28 wherein said fuel introduction means include a fuel passage through the center of the anode gas recirculation blower shaft, with the result that the fuel is preheated and the blower and shaft are cooled.

34. The integrated fuel cell engine of Claim 28 wherein said startup means includes introducing air with the fuel and igniting the mixture such that the system is raised to operating temperature.

35. The integrated fuel cell engine of Claim 28 wherein said control means includes adjusting air flow, fuel flow, and air addition to the fuel to accommodate changes in operating conditions and electrical load.

36. A method of processing hydrocarbon fuel comprising adding oxygen from the anode of a fuel cell in the form of CO and H₂O, reforming the hydrocarbons by reaction them with said CO₂ and H₂O and sensible heat from the power generation reaction to produce H and CO, reacting said H₂ and CO at the anode to produce power, and mechanically mixing the fuel and the reaction products.

37. The hydrocarbon fuel processing method of Claim 36 wherein the fuel concentration is below the level for soot formation.

38. The hydrocarbon fuel processing method of Claim 36 wherein the steam/carbon molar ratio is between 0.25 and 3.

39. The reactor of claim 1 wherein the concentration of fuel in the mixture of fuel and reaction products is measured by the steam to carbon molar ratio and the ratio is maintained at between 0.25 and 3.0.