Abstract:
As integrated fossil fuel power plant and a method of operating the power plant is provided. The integrated fossil fuel power plant includes a gas turbine arrangement and a carbonate fuel cell having an anode side and a cathode side. The operating method for the integrated fossil fuel power plant includes partially expanding combustion gases in the gas turbine arrangement so that the temperature of the partially expanded combustion gases is optimized for reaction in the cathode side of the carbonate fuel cell, and feeding the partially expanded combustion gases at the optimized temperature to the cathode side of the carbonate fuel cell for reaction in the cathode side of the carbonate fuel cell.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to PCT/EP2012/058990 filed May 15, 2012, which in turn claims priority to Great Britain application 1108616.2 filed May 23, 2011, both of which are hereby incorporated in their entireties. 
     TECHNICAL FIELD 
     This patent specification relates to fossil-fuelled power plants with integrated carbon capture facilities. In particular, there is disclosed a system and method of operation in which a fossil-fuelled open- or combined-cycle gas turbine plant is integrated with a molten carbonate fuel cell (MCFC) arrangement. 
     BACKGROUND 
     MCFCs are under development for natural gas and coal-based power plants. MCFCs operate at temperatures in excess of 600 deg. C., using an electrolyte comprising molten carbonate salts in a permeable, chemically inert ceramic matrix, such as beta-alumina. They are more efficient at power conversion than some other fuel cells—with capture and use of waste heat, overall fuel efficiencies of MCFCs can reach 85 percent. A further advantage is that because of their high operating temperatures, they can reform fuels such as natural gas to hydrogen internally, thereby potentially eliminating the need for an external reformer. Moreover, MCFCs are resistant to poisoning by carbon monoxide or carbon dioxide, so facilitating their integration with fossil fuel power plants. In particular, MCFCs can be used to
         increase the efficiency of fossil fuel power plants by reacting exhaust gases to generate electricity, and   reduce environmental impact by separating out carbon dioxide from the exhaust gases.       

     For example, U.S. Pat. No. 7,396,603 B2 discloses a fossil fuel power plant arranged in tandem with a MCFC, in which flue gases containing about 10% CO 2  together with 19% water and 9% oxygen are fed directly to the cathode side of the MCFC. At the same time, fuel such as natural gas is input to the anode side, where it is reformed to liberate hydrogen. Electrochemical reactions in the MCFC effectively result in a major proportion of the CO2 in the flue gas being transferred from the cathode side to the anode side. The CO 2 -enriched gas is then exhausted from the anode side for subsequent processing, including separation and sequestration of the CO 2 . 
     Published European patent application EP 0 418 864 A2 discloses a broadly similar method and apparatus. 
     The present patent specification discloses aa fossil fuel power plant having improved integration with an MCFC arrangement for CO2 separation, and a method of operating the power plant. 
     SUMMARY 
     A first aspect provides an operating method for a fossil fuel power plant comprising a gas turbine arrangement coupled to a molten carbonate fuel cell (MCFC) arrangement, the method including the steps of:
         (i) partially expanding combustion gases in the gas turbine arrangement to a predetermined temperature and pressure substantially greater than ambient pressure and temperature and compatible with reaction in a cathode side of the MCFC arrangement; and   (ii) exhausting the partially expanded combustion gases at the predetermined temperature and pressure to the cathode side of the MCFC arrangement for reaction therein.       

     Partial expansion of the combustion gases may be achieved by:
         reducing the number of turbine stages in a gas turbine relative to the number of stages that would be present if the gas turbine was intended to expand the combustion gases down to ambient pressure, and/or   expanding the combustion gases through an exhaust duct of the gas turbine whose length and profile is adapted to achieve the required partial expansion.       

     By only partially expanding the combustion gases in a gas turbine, instead of fully expanding the gases down to ambient pressure, the temperature and pressure of the combustion gases can be at least approximately matched to reaction conditions in the cathode side of an MCFC, thus eliminating the need to insert additional equipment such as heat exchangers, pumps or compressors between the gas turbine and the MCFC to adjust the temperature and pressure of the combustion gases to a suitable level for reaction in MCFC&#39;s. The operating method enables the provision of a less complex integrated fossil fuel power plant with fewer components and at the same time allows more effective operation of MCFC&#39;s. 
     Step (i) typically comprises partially expanding the combustion gases to a predetermined optimum pressure, greater than ambient pressure, at which the temperature of the partially expanded combustion gases is optimised for reaction in the cathode side of an MCFC. The temperature of the exhaust gases at the predetermined optimum pressure may be equal, or substantially equal, to the optimal cathode inlet temperature of the cathode side. Partial expansion of the combustion gases to a pressure greater than ambient pressure enables efficient operation of MCFC&#39;s under pressurised conditions. 
     The method may include recirculating a proportion of the cathode exhaust gases from an outlet of the cathode side of an MCFC to an inlet of the cathode side for reaction therein. Such partial recirculation is intended to improve the efficiency of the integrated fossil fuel power plant. The method may include mixing the recirculated cathode exhaust gases with the partially expanded temperature-optimised combustion gases provided by step (i) and feeding the mixture to the cathode side for reaction therein. This may enable the temperature of the partially expanded combustion gases provided by step (i) to be further adjusted for reaction in the cathode side. 
     The method may further include expanding through a gas expander the cathode exhaust gases that are not recirculated to the cathode inlet. This is possible because the combustion gases are only partially expanded, and not fully expanded to atmospheric pressure, prior to being fed to the cathode side of the carbonate fuel cell. Further useful work can, thus, be extracted from the non-recirculated proportion of the cathode exhaust gases by expanding them in a gas expander. 
     In enable the desired electrochemical reaction in the MCFC arrangement, the method includes feeding fuel gas to the anode side of the MCFC arrangement. 
     The method may further include recirculating a proportion of the anode exhaust gases from an outlet to an inlet of the anode side for reaction therein. Again, this partial recirculation is intended to improve the efficiency of the integrated fossil fuel power plant. The method may include mixing the recirculated anode exhaust gases with the fuel gas and feeding the mixture to the anode side of the MCFC arrangement. 
     The method may also include expanding through a gas expander the anode exhaust gases that have not been recirculated to the anode inlet. Again, this allows further useful work to be extracted from the non-recirculated proportion of the anode exhaust gases, thus further improving the efficiency of the integrated fossil fuel power plant. 
     A second aspect provides a fossil fuel power plant comprising a gas turbine arrangement and a molten carbonate fuel cell (MCFC) arrangement, the gas turbine arrangement in use being coupled to the MCFC arrangement to exhaust its combustion gases to a cathode side thereof, the gas turbine arrangement being configured to partially expand combustion gases produced by combustion of a carbon-containing fossil fuel to a predetermined pressure and temperature above ambient and compatible with reaction in the cathode side of the MCFC; arrangement. 
     To enable the desired electrochemical reaction to proceed in the MCFC arrangement, the anode side thereof is coupled to a source of fuel gas when in use. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The sole drawing is a schematic representation of an integrated fossil fuel power plant in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will now be described by way of example only and with reference to the accompanying drawing. 
     The integrated fossil fuel power plant  10  includes a fuel processor  12  operable to desulphurise fossil fuel such as natural gas (NG). The power plant further includes a steam reformer  14 , a gas turbine  15 , a molten carbonate fuel cell (MCFC)  16  and small gas turbines  18 ,  20 , otherwise known as gas expanders. The fuel cell  16  has an anode side  16   a , a cathode side  16   c  and an electrolyte  16   b.    
     The integrated fossil fuel power plant  10  is operated on natural gas (NG) by initially desulphurising it in the fuel processor  12 . Of course, if the fuel fed to the power plant  10  were sufficiently pure, the fuel processor  12  would not be required. A first proportion of the desulphurised natural gas is then fed to the external natural gas steam reformer  14 . As known, this creates fuel gas (FG), which is fed to the anode side  16   a  of the MCFC  16 . A second proportion of the desulphurised natural gas is fed to a combustion section  15   a  of the gas turbine engine  15 , which, as known, also comprises a compressor section  15   b , and a turbine section  15   c . The compressor operates to pressurize ambient air and pass it to the combustor  15   a  where it is mixed and burnt with the natural gas to produce combustion gases (CG), which are expanded through the turbine  15   c . Shaft power produced by the turbine is used to drive an electrical generator (not shown) to generate electricity. The expanded combustion gases from the gas turbine  15  are fed to the cathode side  16   c  of the MCFC  16 , where they are reacted. 
     In accordance with the present disclosure, the combustion gases (CG) are partially expanded in the gas turbine engine  15  to a predetermined optimum pressure greater than ambient pressure, the expansion ratio of the turbine  15   c  being selected so that the partially expanded combustion gases (CG) exhausted from the gas turbine  15  are at the predetermined optimum pressure. The temperature of the combustion gases varies with their pressure, according to the amount of expansion undergone in the turbine, and the predetermined optimum pressure is determined based on the optimal operating temperature of the cathode side  16   c  of the MCFC  16 . More particularly, the temperature of the partially expanded combustion gases (CG) at the predetermined optimum pressure is equal or substantially equal to the optimal cathode inlet temperature of the cathode side  16   c  of the MCFC  16 . 
     The electrochemical reaction that takes place inside the MCFC  16  causes carbon dioxide (CO 2 ) contained within the partially expanded combustion gases to be transferred from the cathode side  16   c  of the MCFC  16  through the electrolyte  16   b  to the anode side  16   a . The electrochemical reaction produces electricity and the transfer of carbon dioxide to the anode side  16   a  produces anode exhaust gases (AEG) that are rich in carbon dioxide. 
     The temperature of the partially expanded combustion gases increases in the cathode side  16   c  of the MCFC  16  as a result of the electrochemical reaction. It can, therefore, be advantageous to recirculate a proportion of the cathode exhaust gases (CEG) back to the inlet of the cathode side  16   c  via a recirculation line  22 , as this may enable the temperature of the partially expanded combustion gases to be further optimised for reaction in the cathode side  16   c . In the illustrated embodiment, the recirculated cathode exhaust gases are mixed with the partially expanded combustion gases (CG) from the gas turbine  15  before the resulting mixture is then fed to the cathode side  16   c  of the MCFC  16 . 
     The cathode exhaust gases that are not recirculated to the inlet of the cathode  16   c  are fed via a feed line  24  to the gas expander  18  for further expansion to generate electricity. This further expansion is possible due to the fact that the combustion gases (CG) are not fully expanded to ambient pressure in the gas turbine  15 . However, in the illustrated embodiment, some of the non-recirculated cathode exhaust gases are bled off upstream of the gas expander. Firstly, some of the cathode exhaust gases (CEG) are used to raise steam in a specific heat recovery steam generator  25 , before being fed back into line  24 . The steam thus raised from water input  23  is fed to the external reformer  14  on feed line  27  for use in the natural gas steam reforming process. Secondly, some of the cathode exhaust gases may be bled off via a feed line  26  and passed through heat exchange passages of the reformer  14  to raise the temperature of the reactants before the cathode exhaust gases are returned to the gas expander via a feed line  28 . The efficiency of the power plant  10  may be further improved by extracting further useful work from the hot gas stream exhausted from the gas expander  18 , for example by passing the exhaust gases through a heat recovery steam generator  29  to produce process steam. Hence, heat requirements for the natural gas steam reforming process, and for steam generation, are both satisfied by cooling down the cathodic outlet gas. Note that a variation of the illustrated layout could comprise combining units  25  and  29  in a single heat recovery steam generator. 
     The power plant  10  also includes a recirculation line  30  for recirculating a proportion of the anode exhaust gases (AEG) from the anode side  16   a  of the MCFC  16  to the inlet of the anode side  16   a . The recirculated anode exhaust gases are mixed with the fuel gas (FG) before the mixture is fed to the anode side  16   a  of the MCFC  16 . The anode exhaust gases that are not recirculated are fed, via a feed line  32 , to the gas expander  20  where they are expanded to generate electricity. 
     After expansion in the gas expander  20 , water is removed from the anode exhaust gases in a condenser  33 . The expanded anode exhaust gases are rich in carbon dioxide and hence downstream of condenser  33  can be either passed direct to CO 2  compression and subsequent sequestration, or the concentration of CO2 can be further increased by utilising known means such as oxy-combustion, in which the anode gases are burnt with pure oxygen, or CO2 chemical capture processes in which the gases are treated with CO2 solvents such as amines. The use of the MCFC  16  in the power plant  10  thus advantageously facilitates carbon capture and storage whilst at the same time maximising the operating efficiency of the power plant  10 . 
     It should be understood that various modifications could be made to the embodiments described above within the scope of the appended claims. For example, the carbonate fuel cell  16  could be an internally reforming MCFC, in which case the external steam reformer  14  could be omitted. Recirculation of the exhaust gases from the anode side  16   a  and/or the cathode side  16   c  of the MCFC  16  could also be omitted.