Integrated fuel cell hybrid power plant with controlled oxidant flow for combustion of spent fuel

A hybrid power generation system for generating electrical power comprises a compressor for producing a compressed oxidant and a recuperator in flow communication with the compressor. The hybrid power generation system further comprises a fuel cell assembly comprising a plurality of fuel cells in flow communication with the recuperator to provide the compressed oxidant for the fuel cell assembly. The hybrid power generation system further comprises a tail gas burner in flow communication with the fuel cell assembly. A control system is used for controlling the amount of cathode exhaust stream introduced in the tail gas burner for stable combustion and reduction of fuel and carbon monoxide emission. The hot compressed gas from the tail gas burner is introduced to a turbine, where the hot compressed gas is expanded, thereby producing electrical power and an expanded gas.

BACKGROUND OF INVENTION

This invention relates generally to power plants, and, more specifically to hybrid power plants with integrated fuel cells, where the controlled injection of exhaust air from the fuel cell is used to burn the spent fuel from the fuel cells.

In certain hybrid power generation systems, fuel cells have been integrated with conventional gas turbines for increased power generation capacity in electrical power plants. Known fuel cells, such as, for example, solid oxide fuel cells include a plurality of solid oxide fuel cells that react a gaseous fuel, such as reformed natural gas, with air to produce electrical power and a hot gas. The gas turbine compressor supplies the air for the fuel cells, which fuel cells operate at elevated pressure and produce hot gas for expansion in the turbine. Fuel cell exhaust air is combined with fuel cell exhaust fuel and the resulting heat release is converted to work in the turbine portion of the plant. Thus, electrical power is produced by both the solid oxide fuel cell generator and the turbine.

Solid-oxide fuel cells usually do not convert all of the fuel that is fed into the inlet of the fuel cells. Composition of the outlet stream from the fuel cells primarily includes carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2), and water (H2O) along with the equilibrium species and inert species like N2. In the absence of means to burn the partly spent fuel, the heat content of these constituents is wasted, thereby reducing thermodynamic efficiency of the plant. Additionally, unburned hydrocarbons and carbon monoxide may also be undesirably emitted into the atmosphere when all the fuel components for the fuel cells are not completely converted.

Accordingly there is a need to develop a combustor or a burner that can efficiently convert the fuel components in the spent fuel stream from the fuel into useful work using the fuel cell exhaust air.

BRIEF DESCRIPTION

In one aspect, a hybrid power generation system for generating electrical power comprises a compressor for producing a compressed oxidant and a recuperator in flow communication with the compressor. The hybrid power generation system further comprises a fuel cell assembly comprising a plurality of fuel cells in flow communication with the recuperator to provide the compressed oxidant for the fuel cell assembly. The fuel cell assembly further comprises a cathode inlet for receiving the compressed oxidant, an anode inlet for receiving a fuel stream, an anode outlet in flow communication with an anode exhaust stream and a cathode outlet in flow communication with a cathode exhaust stream, wherein at least a portion of the fuel reacts with the oxidant to produce electrical power. The hybrid power generation system further comprises a tail gas burner in flow communication with the anode outlet and the cathode outlet. The tail gas burner is configured for combusting a mixture of at least a portion of the anode exhaust stream and at least a portion of the cathode exhaust stream and producing a hot compressed gas. A control system is used for controlling the amount of the cathode exhaust stream introduced in the tail gas burner for stable combustion and reduction of fuel and carbon monoxide emissions. The hot compressed gas from the tail gas burner is introduced to a turbine, where the hot compressed gas is expanded, thereby producing electrical power and an expanded gas.

In yet another aspect, a method of operating a hybrid power generation system is provided. The method comprises an initial step of supplying an oxidant flow and a fuel flow to the inlet of a fuel cell assembly comprising a plurality of fuel cells. The next step is reacting electrochemically the fuel with the oxidant to produce electricity, an oxygen depleted exhaust stream and a low heat content fuel stream. The low heat content fuel stream is introduced into a tail gas burner. The flow of the oxygen depleted oxidant flow into the tail gas burner is controlled to promote stable combustion and to produce a hot gas. The hot gas is introduced in a turbine to generate electricity.

DETAILED DESCRIPTION

FIG. 1schematically illustrates an exemplary integrated gas turbine and fuel cell hybrid power plant10including a fuel cell portion31and a turbine portion13for producing electricity in tandem with one another. The turbine portion13typically includes a compressor12, a turbine14, a rotor16by which turbine14drives compressor12, an electrical generator18, and a recuperator20. The fuel cell portion31typically includes a fuel pump30, a de-sulfurizer32, a fuel cell assembly34, a fuel reformer36for fuel cell assembly34and a tail gas burner38(hereinafter TGB). As explained in some detail below, while the basic components of plant10are well known, efficiency improvements in relation to known plants are obtained through strategic interconnection of plant components with re-circulation flow paths to enhance performance and efficiency of the system. The efficiency of the hybrid power plant is enhanced by converting the fuel components from the spent fuel stream from the fuel cells into useful work. As will be seen below, plant efficiency is also improved by recycling oxidant and fuel streams exhausted from the fuel cell portion11to extract useful work from oxidant and fuel streams in the fuel cell and turbine portions of the system, and utilizing heat generated in the turbine portion for the benefit of the fuel cell portion. In the various embodiments of the hybrid power generation systems described herein, the oxidant is ambient air. It is understood that any other oxidant stream comprising the required amount of oxygen for the reaction in the fuel cell may be used for the same purpose.

In operation, an exemplary compressor12is a multi-stage compressor that includes rows of stationary vanes and rotating blades. Compressor12inducts air and produces a compressed air stream40at an outlet11of compressor12. The compressed air stream40is directed towards recuperator20, which recuperator20is a known type of heat exchanger including isolated flow paths. Compressed air stream40enters recuperator20in a first recuperator flow path41, and a turbine exhaust stream42is passed into recuperator20in a second recuperator flow path43, whereby heat from the turbine exhaust42is transferred to the compressed air stream40from the compressor outlet11without mixing of the compressed air stream40and the turbine exhaust stream42. Compressed air stream40is heated within recuperator20by the turbine exhaust stream42, and a heated compressed air stream44exits recuperator20and flows to a cathode inlet46of fuel cell assembly34to provide an oxidant thereto. By heating the compressed air stream40with turbine exhaust42, the costs of conventional heaters or regenerative heat exchangers to raise a temperature of the fuel cell oxidant are avoided, and turbine exhaust stream42is cooled before being discharged into the atmosphere. In some embodiments, the exit stream112from the recuperator20is further cooled in a desulferizer32.

Fuel cell assembly34comprises a plurality of the fuel cells (not shown). Fuel cells are energy conversion devices that produce electricity by electrochemically combining a fuel and an oxidant, such as air across an ion conduction layer. More particularly, each fuel cell includes an anode, an electrolyte, and a cathode (not shown) arranged for example in a tubular or planer configuration. In a hydrogen fuel cell, hydrogen is used as fuel and the hydrogen and oxygen from an oxidant stream react to produce water and electricity. It is understood that although in various embodiments disclosed herein, the arrangement of a plurality of fuel cell is called a fuel cell assembly. It may alternatively be called a fuel cell bundle, in the case of the tubular arrangement of the fuel cells. The term fuel cell assembly, as used herein may refer to either a fuel cell stack or a fuel cell bundle. In an exemplary embodiment, fuel cell assembly34comprises a plurality of solid oxide fuel cell (SOFC) units with an oxygen-ion conducting solid electrolyte, such as yttria stabilized zirconia (YSZ), ceria-doped zirconia, or lanthanum strontium gallium manganate. In alternative embodiments, fuel cell assembly34may include, for example, proton exchange membrane (PEM) electrolytes, molten carbonate electrolytes or other known electrolyte materials suitable for use. In the various embodiments of the hybrid power generation system described herein, the fuel cell is selected from the group consisting of solid oxide fuel cells, proton exchange membrane fuel cells, molten carbonate fuel cells, phosphoric acid fuel cells, alkaline fuel cells, direct methanol fuel cells, regenerative fuel cells, zinc air fuel cells, and protonic ceramic fuel cells.

Air inlet46and air outlet48of the fuel cell assembly34are referred to herein as a cathode inlet and a cathode outlet, respectively, as they provide oxidant airflow for the cathodes of fuel cell assembly34. Similarly inlet for fuel and outlet for fuel are referred to herein as an anode inlet50and an anode exhaust52, respectively, as they provide fuel flow for the anodes of fuel cell assembly34.

The heated compressed air stream44from the recuperator20enters the fuel cell assembly34through cathode inlet46and flows through the fuel cell units in the fuel cell assembly34. At least a portion of the fuel reacts electrochemically with the oxidant air flowing through the fuel cell assembly to produce electricity. Spent air54is exhausted from fuel assembly34through cathode outlet48and is designated as cathode exhaust stream54.

The cathode exhaust stream54flows to a reformer36, in which reformer gaseous hydrocarbons, for example, natural gas may be reformed in the presence of steam and a nickel catalyst into hydrogen and carbon monoxide. In different embodiments, fuel reformation may be accomplished in an external fuel reformer36or in a reformer integral with fuel cell assembly34.

Gaseous fuel, which in different embodiments may be, for example, natural gas, methane, propane, n-heptane, diesel, kerosene, gasoline, or a coal derived fuel gas, is driven by fuel pump30through a de-sulferizer32, which de-sulferizer in an exemplary embodiment includes a vessel containing a bed of sulfur sorbent through which fuel flows. In some embodiments, the fuel is an aviation fuel comprising fillers. Heat from the turbine exhaust42is transferred to the de-sulferizer32to warm fuel therein before being exhausted from the plant10. Complexity and expense of an external heater for the de-sulferizer32is therefore avoided, and the turbine exhaust is cooled before being discharged from the plant.

De-sulferized fuel56flows from the de-sulferizer32to the reformer36so that fuel may be reformed in the reformer36prior to entering the fuel cells of fuel cell assembly34. In another embodiment, steam (not shown) is introduced into the incoming fuel stream56to facilitate the reforming process. Once treated therein, the reformed fuel58flows from the reformer36to anode inlet50and into the fuel cells of assembly34. Once expanded in the fuel cells, spent fuel60(also designated as anode exhaust stream) is exhausted from fuel cell assembly34through anode exhaust52. The fuel cells usually do not convert all the fuel that is fed into the inlet of the fuel cells. Typically the anode exhaust stream60comprises carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2), unutilized fuel and water. At least a portion of the anode exhaust stream60is fed to a tail gas burner38for combustion therein. A portion of the spent (i.e., oxygen depleted) air62from fuel cell assembly34is also fed into the tail gas burner38, and a mixture of spent air64and spent fuel60is combusted in tail gas burner38. Combustion exhaust68from the TGB38is fed to the working fluid paths in the gas turbine14to provide added heat and pressure for expansion of gas in the turbine14.

By controlling the injection of spent air62and exhausted fuel60in tail gas burner38, it can be ensured that the fuel/air mixture is lean and within flammability limits. Thus, all of the fuel components remaining in the spent fuel stream60are combusted in the tail gas burner38, thereby fully utilizing the fuel in the system and preventing discharge of unburnt fuel and carbon monoxide in exhaust114from the plant10.

In operation, the anode exhaust steam60from the fuel cell assembly34is a low heat content stream as most of the reformed fuel components fed into the anode inlet50are utilized in the electrochemical reaction in the fuel cell assembly34. The main feature while burning low heat content fuel is to control the injection of air into the TGB38so that the mixture of the fuel and oxygen depleted air is within the flammability limit. The heat content of the anode exhaust stream60, which is fed into the TGB38is equal to or less than 250 British Thermal unit (herein after BTU) per cubic feet of the gas stream at standard conditions. The standard conditions herein are defined as a condition where the temperature is at 0 Deg C. and the pressure is at 1 atmosphere. In one embodiment, the heat content of the anode exhaust stream fed into the TGB38is less than or equal to 100 BTU per cubic feet of the gas stream at standard conditions. Since a portion of the spent air stream62is used to burn the available fuel components in the anode exhaust stream60, the oxygen content of the spent air62is less than 21% by volume. Part of the oxygen content in the cathode inlet stream44is used in the electrochemical reaction in the fuel cell assembly34. As mentioned in the preceding sections, any oxidant stream comprising the required amount of oxygen may be used in a fuel cell. In accordance with the present technique, in some embodiments, the oxygen content in the spent oxidant stream or cathode exhaust stream62is less than or equal to 25% by volume. It is very important to control the injection of the amount of this oxygen depleted spent air64into the TGB38for burning the low heat content anode exhaust stream60. The presence of nitrogen in the oxygen depleted spent air64makes the mixture leaner once the spent air64is mixed with the anode exhaust stream60in the TGB38and combustion of the fuel content in the anode exhaust stream60becomes difficult to achieve.

FIG. 4shows a diagrammatical view of an exemplary embodiment of the TGB38. The TGB38is designed to handle fuel with very low heat content. The TGB38is divided into two zones, a primary zone86and a secondary zone88. The primary zone86may also be described as the flame stabilization zone. A portion of the oxygen-depleted spent air is injected in both these zones. A portion of the spent air64is injected into the primary zone86via a control valve92. The oxygen-depleted spent air enters the primary zone86through the air inlet94. The reason for controlling the air injection in the primary zone86is to achieve the stability of the flame90.

Another portion of the spent air64is injected in a controlled fashion into the secondary zone88via a control valve96and the oxygen-depleted spent air enters the secondary zone88through inlet98. The secondary zone88may also be described as the auxiliary burning zone. The secondary zone88is connected to the outlet of the TGB38through a transition piece100. In the secondary zone88, the controlled air injection is done to achieve maximum combustion of the fuel content in the anode exhaust stream60and to avoid carbon monoxide or unburnt fuel emission from the hybrid power plant10. The anode exhaust stream60is typically injected into the TGB38through a nozzle (not shown). In some embodiments, the TGB38is configured to inject the anode exhaust stream60into the TGB38through more than one nozzles. Increasing the number of nozzles to inject the incoming fuel in the TGB38may be one of the ways to handle an increased fuel flow rate.

The degree and the type of controls needed for the primary zone86and secondary zone88in the TGB38are different and may be achieved through automatic control schemes.FIG. 5shows an exemplary control scheme of the oxygen depleted spent air injection into the TGB38. The air injection may be controlled by a passive or an active control or a combination of both.FIG. 5shows an exemplary scheme for a passive control scheme for the spent air injection into the TGB38. In this control scheme, the spent air injection to the primary zone86is done through a primary control algorithm G(U) and the spent air injection to the secondary zone88is achieved through a secondary control algorithm H(V), wherein both (U) and (V) are functions of several process parameters related to the operation and performance of the fuel cell assembly34. These algorithms, G(U) and H(V), determine the mass of air to be injected into each of the zones to achieve flame stability and reduced emissions.

Since the control algorithms work to achieve different end results in two zones, the functions (U) and (V) may be different although they may depend on the same set of process parameters. The input parameters for the control algorithms are provided by a set of sensors102. The parameters can also be obtained from other sensors (not shown) available in the fuel cell assembly34. These parameters include but are not limited to fuel utilization in the fuel cell assembly34, pressure and temperature of the fuel cell assembly34, mass flow of fuel, and power generated in the fuel cell assembly. The algorithms G(U) and H(V) could also be dependent on the parameters, such as, oxygen content in the oxygen depleted cathode exhaust stream and residence time of the fuel in the primary and the secondary zone. The parameters are fed into the actuators104for primary zone86and106for secondary zone88. Accordingly the opening of the control valves92and96determines the mass of air to be injected into the primary and secondary zones. The mass of air injected into the primary zone86may be higher or lower than the corresponding stoichiometric amount of oxygen needed for burning the fuel content of the anode exhaust stream60. The mass of air injected into the primary zone86may be less than about 20% of the total cathode exhaust stream54. Accordingly the mass of air injected into the secondary zone may vary from about 5% to about 100% of the cathode exhaust stream54.

The control of the spent air injection into the TGB38can also be achieved by active control scheme (not shown), wherein the input parameters are measured downstream of the TGB38and a feedback loop is provided, which feedback loop controls the opening and closing of the valves92and96. The performance and the life of the TGB38are further enhanced by a cooling arrangement using a portion of the cold compressed air stream40available in the hybrid power plant10. A portion of the compressed air stream40may be diverted (as shown inFIG. 5) to the TGB38for cooling purpose. In operation, several hot spots may exist in the TGB38, for example, the nozzles (not shown) through which the spent fuel stream60is introduced into the TGB. The cold compressed air may be utilized to cool these hot spots, increasing the life of the burner and simultaneously increasing the thermal efficiency of the entire hybrid plant10, as the heat taken out from the parts of the TGB38will be utilized to further heat up the air stream to the cathode inlet46. A control valve108controls the mass of the cold compressed air110, diverted for the cooling of the hot spots in the TGB38.

Coming back toFIG. 1, the hot exhaust68from tail gas burner38is mixed with the portion of remaining spent air66and the mixed stream70is fed into the working fluid paths of gas turbine14. Thermodynamic expansion of the exhaust70produces work to drive the turbine14, which, in turn, generates electricity in generator18. Electricity from generator18and fuel cell assembly34are converted to an appropriate form and to a distribution power supply network, illustrated as grid72inFIG. 1.

FIG. 2is a schematic diagram of another exemplary embodiment of an integrated fuel cell hybrid power plant200sharing the basic components of power plant10(shown inFIG. 1), in which like features are designated with like reference characters.

In the gas turbine portion of plant200, compressor12supplies compressed air to a recuperator20, and compressed air within recuperator20is heated by turbine exhaust42as described above to produce a heated air stream supply44to cathode inlet46. In the fuel cell assembly34, the air is reacted with a fuel to generate electricity as described above.

Cathode exhaust stream54exhausted from the cathode outlet48of the fuel cell assembly34is passed to reformer a36. Gaseous fuel, which in different embodiments may be natural gas, methane, propane, n-heptane, diesel, kerosene, gasoline, or a coal derived fuel gas, is driven by fuel pump30through the de-sulferizer32as discussed in the preceding sections. In some embodiments, the fuel is an aviation fuel comprising fillers. De-sulferized fuel56flows from the de-sulferizer32to the reformer36so that fuel may be reformed therein prior to entering the fuel cells of the fuel cell assembly34. Once treated therein, reformed fuel58flows from the reformer36to the anode inlet50and into the fuel cells of assembly34. Once expanded in the fuel cells, spent fuel60(also designated as anode exhaust stream) is exhausted from the fuel cell assembly34through anode exhaust52. The anode exhaust stream60is sent to a separation unit74, where the carbon dioxide in the anode exhaust stream60is separated. The separation of carbon dioxide may be achieved using chemical absorbents like calcium oxide. Techniques like pressure swing adsorption (PSA) and membrane separation suitable for high temperature application may also be used for carbon dioxide separation from the anode exhaust stream60. Once the separation of carbon dioxide is achieved, a carbon dioxide rich stream78is generated that may exported to be industrially used elsewhere.

After separation of carbon dioxide, the anode exhaust stream60is fed to a tail gas burner38for combustion therein. A portion of the spent (i.e., oxygen depleted) air64from fuel cell assembly34is also fed into tail gas burner38, and a mixture of spent air64and exhausted fuel76is combusted in tail gas burner38. The combustion of the low heat content anode exhaust stream60is achieved by controlling the injection of oxygen-depleted spent air64as described in the preceding sections.

Combustion exhaust68along with the rest of the spent air66is fed to the working fluid paths in gas turbine14to provide added heat and pressure for expansion of gas in turbine14. The thermodynamic expansion of the exhaust70produces work to drive the turbine14, which, in turn, generates electricity in generator18. Electricity from generator18and fuel cell assembly34are converted to an appropriate form and to a distribution power supply network, illustrated as grid72.

FIG. 3is a schematic diagram of yet another exemplary embodiment of an integrated fuel cell hybrid power plant300sharing the basic components of power plant10(shown inFIG. 1), in which like features are designated with like reference characters.

In accordance with the present technique as illustrated inFIG. 3, spent air54is partly diverted into a negative pressure re-circulation flow path24in flow communication with blower22. Blower22forces air therefrom in a positive pressure re-circulation flow path26to provide a re-circulated air stream which is fed back to compressed and heated air stream44from recuperator20. The re-circulated air stream in re-circulation flow path26is therefore mixed with fresh air stream44at a flow path junction28. Mixing of re-circulated spent air24exhausted from the fuel cell assembly34with fresh air44through re-circulation flow path26is advantageous in several respects.

For example, re-circulation of hot exhaust air26from fuel cell assembly34and mixing it with fresh air from compressor air44raises an air temperature at cathode inlet46by a direct mass and heat transfer process. A need for diffusive heat transfer provided by a heat exchanger in conventional systems is therefore eliminated. Coupled with turbine exhaust flow42in recuperator20to heat compressed air40, a considerably lower cost and less complex heat exchanger, such as recuperator20, may be employed.

Additionally, re-circulated air24from fuel cell assembly exhaust54via flow path26increases an air mass flow rate to fuel cell assembly34at cathode inlet46and facilitates a substantially constant total system air flow rate for increased system performance. The increased air mass flow to the fuel cell assembly34at cathode inlet46produces greater temperature uniformity within the fuel cell assembly34and further enhances performance of fuel cell assembly34. As such, higher fuel flow rates are possible for a given constant range of assembly temperatures. Higher fuel flow rates at substantially constant total system air flow reduces the amount of total excess air, and thereby raises the firing temperature of turbine14enhancing overall system performance.

Still further, with sufficient amounts of re-circulated air26mixing with fresh air supply44through flow path26, a limit of a stoichiometric operation of the fuel cell assembly may be approached relative to the incoming fresh air.

Even further, re-circulated airflow path26effectively reduces cathode concentration of oxygen (O2) in fuel cell assembly34, which is known to be a key degradation mechanism in hot fuel cells. It is therefore believed that re-circulated airflow path26provides enhanced performance and longer life of the hot fuel cell assembly.

As shown inFIG. 3, a portion of the spent fuel60is diverted into a re-circulation fuel stream flow path82that mixes with fresh de-sulferized fuel56. Re-circulation of hot exhausted fuel via re-circulation flow path82further avoids external fuel heaters and re-introduces unspent fuel into fuel cell assembly34, thereby increasing fuel efficiency in the system. Re-circulation of exhausted fuel could be accomplished, for example, with a blower, an ejector pump84, or the like as those in the art will appreciate. In a further or alternative embodiment, steam (not shown) may be introduced to the fuel stream56prior to reforming to facilitate reforming.

For at least the reasons set forth above, power plants10,200and300provide better overall plant performance in relation to known systems while providing thermal efficiency and improved temperature control of the fuel cell assembly through re-circulation flow paths and while avoiding complexity and costs of conventional heat exchangers to maintain the fuel cell assembly at desired temperatures. The controlled injection of the oxygen depleted spent air64in the tail gas burner38enhances the performance of the hybrid power plants in all the embodiments described above. In hybrid power plant300, re-circulation of fuel cell assembly cathode exhaust also facilitates inlet air temperature control to the fuel cell assembly, which, in turn, provides for more precise control of temperature rise and uniformity within the fuel cell assembly. Re-circulation of fuel cell assembly cathode exhaust provides increased turbine section inlet temperature to provide more work in the turbine, provides for increased performance retention via reduced cathode side oxidation, permits fuel cell assembly operation at stoichiometric conditions, and simplifies exhaust after-treatment before discharging plant exhaust to the atmosphere.