Integrated power plant and system and method incorporating the same

A system and method for producing electricity is described. The system comprises a fuel cell assembly. The system may comprise a steam turbine and a generator. The fuel cell assembly may be used to provide heat to produce the steam used to power the steam turbine. The system may comprise a gasifier that is operable to produce a fuel for use in the fuel cell assembly. The system may comprise an air separation unit that is operable to supply oxygen to the gasifier and to the fuel cell assembly for reaction with the fuel. The oxygen that is not reacted in the fuel cell assembly may be recirculated through the fuel cell assembly. Spent fuel from the fuel cell assembly may be recirculated through the fuel cell assembly. A carbon dioxide removal system may be used to remove carbon dioxide from the fuel upstream of the fuel cell.

BACKGROUND

The invention relates generally to power plants producing electricity from coal or other solid or low grade liquid fuels and more specifically to an integrated gasification power plant including a fuel cell and a steam turbine power plant.

A variety of different systems have been used to generate electrical power from coal and low grade fuels. One such system is an integrated gasification gas turbine combined cycle power plant (IGCC). In this system a conventional gas turbine combined cycle power plant utilizes combustion of the gasified fuel to induce rotation of gas turbine blades coupled to a shaft that is coupled to a generator. The generator converts the rotational energy produced by the turbine into electrical energy. The fuel for combustion in the turbine may be provided by a gasifier, which produces a gaseous fuel from a solid fuel, such as coal or other solid or low grade liquid fuels (e.g. biomass, heavy oils). The exhaust waste heat can be recovered in a bottoming steam turbine system.

In addition, fuel cells have been integrated with conventional gas turbines to improve the efficiency of the power plant. Fuel cells typically cause a reaction between a fuel, such as hydrogen, and an oxidant, such as air, which produces electrical power. The fuel cells also produce hot gases that may be mixed with the fuel for combustion in the gas turbine, which improves the efficiency of the system. In addition, integration of a fuel cell into a conventional IGCC plant is a possibility.

However, all these types of power plants have several disadvantages. For example, a typical gasifier combined cycle power plant has a low thermal efficiency, approximately 40 percent. Furthermore, there may be a large amount of carbon dioxide produced by the gasifier in addition to the fuel. However, removing the carbon dioxide from the fuel before the fuel is combusted in order to reduce carbon dioxide emissions significantly reduces the performance of the system. Furthermore, the fuel cells that are used in such plants do not consume all of the fuel that is fed into the fuel cell, thereby reducing the efficiency of the system.

Therefore, it is desirable to provide a power plant with greater efficiency and with reduced emissions. The techniques described below may provide a solution to one or more of the problems described above.

BRIEF DESCRIPTION

In one aspect of the present technique, a system for producing electricity is provided. The system comprises a fuel cell and a steam turbine. The fuel cell is operable to produce electricity from a reaction between a fuel gas and an oxidant. The steam turbine is coupled to the fuel cell to receive steam heated by the fuel cell.

In another aspect, a method of producing electricity is provided. The method comprises operating a fuel cell to produce electricity, wherein the fuel cell is cooled by steam. The method further comprises using the steam from the fuel cell to drive a steam turbine coupled to an electrical generator.

In yet another aspect, a system for producing electricity is provided. The system comprises a gasifier and a fuel cell. The gasifier is operable to produce a fuel gas from a solid fuel for reaction in the fuel cell. The fuel cell is operable to receive the fuel gas from the gasifier and to produce electricity from a reaction between the fuel gas and an oxidant, wherein unreacted fuel gas from the fuel cell is recirculated through the fuel cell.

In still another aspect of the present technique, an integrated power plant is provided, which comprises an air separation unit, and a fuel cell. The air separation unit is operable to produce a supply of oxygen from air. The fuel cell is coupled to the air separation unit, wherein oxygen from the air separation unit is coupled to the fuel cell assembly to react with a fuel to produce electricity.

In still another aspect, a method of operating a power plant to produce electricity is provided. The method comprises operating an air separation system to produce oxygen from air. The method further comprises operating a fuel cell assembly to produce electricity by reacting a fuel with the oxygen from the air separation system.

In still another aspect of the present technique, an electrical power generating system is provided. The system comprises a gasifier and a fuel cell. The gasifier is operable to produce a fuel gas from a solid fuel. The fuel cell is operable to receive the fuel gas from the gasifier and to produce electricity from a reaction between the fuel gas and an oxidant. The system further comprises a sulphur and carbon dioxide removal system from, for removing sulphur and carbon dioxide the fuel gas prior to entering the fuel cell.

DETAILED DESCRIPTION

Referring now toFIG. 1, an exemplary integrated power plant10comprising a fuel cell12and a steam turbine system14is illustrated. In this embodiment, the fuel cell12and the steam turbine system14are each operable to supply electricity16to an electrical grid17. The illustrated power plant10also comprises a gasifier18. The gasifier18is supplied with coal20and steam22for reaction therein. In addition, in the illustrated embodiment, the gasifier18receives oxygen24from an air separation unit (ASU)26that separates the oxygen24from nitrogen and other gaseous components of air. The reaction in the gasifier18involves a multi-stage combustion of coal20in the presence of steam22and oxygen24to produce a fuel28comprising hydrogen and carbon monoxide for the fuel cell12.

The use of substantially pure oxygen24in the gasifier18is more desirable than the use of air. Atmospheric air has a relatively high concentration of nitrogen, which is largely un-reacted in the gasifier18. Thus, the fuel28supplied by the gasifier18and oxygen24supplied to the fuel cell12contain an amount of nitrogen, which, over a period of time, accumulates over various system components, thereby lowering system performance. In such systems, it is necessary to have means to extract (bleed) the nitrogen from the spent fuel50and spent oxygen56(bleed not shown)

In the illustrated embodiment, the fuel28produced by the gasifier18is directed along a flow path toward a heat exchanger30. The fuel28flows through one flow path through the heat exchanger30while liquid water32exhausted from the steam turbine system14flows through another flow path through the heat exchanger30. Inside the heat exchanger30heat is transferred from the fuel28to the liquid water32without mixing the fuel28and the water32. As a result, the liquid water32exits the heat exchanger30as steam34.

As described earlier, the fuel28produced by the gasifier18is primarily hydrogen and carbon monoxide. In addition, the fuel28may also comprise sulphur and particulate impurities, such as mercury (Hg). Preferably, these impurities are removed from the fuel28before the fuel28is utilized in the fuel cell12. In the illustrated embodiment, the fuel28is directed through a particle separation unit (PSU)36that is operable to remove these particulate impurities from the fuel28. The PSU36may have a cyclone separator, a high temperature ceramic filter, or another device for particulate removal. The fuel free of particulate impurities38may be directed toward another heat exchanger40, which is similar in operation to the previously discussed heat exchanger30. Thus heat exchanger40includes isolated flow paths for fuel38and liquid water32exhausted from the steam turbine system14.

In the illustrated embodiment, the fuel38flowing from the heat exchanger40is directed toward a sulphur and carbon dioxide removal unit42for sequestration of sulphur and carbon dioxide from the fuel38. Various techniques of de-sulphurizing may be used for the separation of sulphur from the fuel38. In one exemplary technique, the fuel38can be made to flow through a vessel containing a bed of sulphur absorbent. Likewise, various techniques may be used for the separation of carbon dioxide from the fuel38, including but not limited to pressure swing adsorption, chemical absorption and membrane separation. A certain amount of water44may be removed from the fuel38in the process, and may be advantageously directed toward the inlet of a water pump68. The water pump68is used to pump condensate from the steam turbine system14to the aforementioned heat exchangers30and40. After the removal of sulphur and carbon dioxide, the fuel free of sulphur and carbon dioxide48largely comprises hydrogen and carbon monoxide, with lesser amounts of water and carbon dioxide, and is hereinafter referred to as fresh fuel48. The fresh fuel48is directed into the fuel cell12for reaction therein.

The fuel cell12may comprise a plurality of fuel cells that are coupled together to form the fuel cell12. A fuel cell12is an energy conversion device that produces electricity by electrochemically combining the fuel48with the oxygen24across an ion conduction layer. More particularly, the fuel cell12comprises an anode, a cathode, and an electrolyte (not shown). In the fuel cell12, the hydrogen and oxygen react to produce water and electricity. Electrochemical conversion of carbon monoxide to carbon dioxide is another mechanism that also produces electricity. In an exemplary embodiment, the fuel cell assembly12comprises 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, the fuel cell12may comprise, for example, molten carbonate electrolytes or other known electrolyte materials suitable for use in the fuel cell12.

The products of the reaction in the fuel cell12are primarily water and carbon dioxide. However, there is an amount of unreacted hydrogen and carbon monoxide that is also present. The unreacted hydrogen and carbon monoxide is referred to hereinafter as spent fuel50and is directed away from the fuel cell12by a fuel pump52.

In the illustrated embodiment, the spent fuel50is recirculated back through the fuel cell12. The spent fuel50is mixed with the fresh fuel38from the gasifier18at the inlet to the sulphur and carbon dioxide removal unit42. The heat generated by the operation of the fuel cell12raises the temperature of the spent fuel50. In this embodiment, a heat exchanger54is provided to transfer heat from the spent fuel50to the liquid water32from the steam turbine system14. Recycling the un-reacted fuel from the fuel cell12enables the power plant10to have a greater efficiency. The efficiency is also improved by the recovery of the heat generated by the fuel cell12.

Similarly, the oxygen56that is not consumed in the fuel cell12is recirculated back through the fuel cell12. A pressure-increasing device, such as blower58, is used to raise the pressure of the exhaust oxygen, which is fed back into the fuel cell12along with the fresh oxygen24from the ASU26. Mixing of re-circulated oxygen56exhausted from the fuel cell12with fresh oxygen24is advantageous in several respects. Namely, it improves the overall efficiency of the power plant10. In addition, recirculating hot exhaust oxygen56from the fuel cell12and mixing it with fresh oxygen24from the ASU26raises the temperature of the oxygen24prior to entering the fuel cell12, which improves the efficiency of the reaction within the fuel cell12.

As opposed to conventional air-cooled fuel cells, the illustrated fuel cell12utilizes steam34from the various heat exchangers to cool the fuel cell12. The steam34flows along an isolated flow path through the fuel cell12, wherein heat from the fuel flow12is transferred to the steam34. As a result, the steam34, the steam flowing from the fuel cell12is raised to a higher temperature. The heated steam34is directed toward the steam turbine system14.

The steam turbine system14uses the steam from the fuel cell12to produce electricity. The steam turbine system14comprises a steam turbine60, a generator62, and a condenser64. The steam34from the fuel cell12is used to cause turbine blades within the steam turbine60to rotate a shaft66. The shaft66is coupled to the generator62. The mechanical rotational energy produced by the steam turbine60is converted into electrical energy by the generator62. The electricity16produced by the generator62and the fuel cell12is coupled to a distribution power supply network, represented generally as grid17.

The steam34exhausted from the steam turbine60is condensed into a liquid condensate32in the condenser64. The water pump68is used to pump the condensate32to the heat exchangers30,40, and54, wherein, as described above, the condensate32is heated to steam for delivery to the fuel cell12.

The techniques described above improve power plant efficiency by recycling oxidant and fuel streams exhausted from the fuel cell portion to extract as much work as possible from the oxidant and fuel streams. In addition, heat generated by the fuel cell is used to provide steam for a steam turbine, further improving the efficiency of the system. Although the illustrated embodiment shows a number of features, many of the features have individual benefits and are not dependent upon the other features.