Abstract:
Methods and systems of operating an integrated gasification combined cycle system are provided. The method includes coupling a non-fuel fluid conduit to a fuel conduit, warming a flow of non-fuel fluid, and channeling the warmed non-fuel fluid through the fuel conduit such that heat from the warmed non-fuel fluid heats the fuel conduit to a predetermined temperature.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to integrated gasification combined cycle (IGCC) systems, and more specifically to methods and systems for facilitating reduced flaring during startup of the system. 
     At least some known IGCC systems startup on a fuel such as natural gas to provide heat to various subsystems and to warm-up components of the IGCC system such that operational temperature limits are not exceeded during startup or conversion to operation using syngas fuel. For example, a saturator generally includes an operating requirement of being pre-warmed during startup and preventing boiling of the circulation loop. For protection of internal gas turbine components, the syngas supplied to the gas turbine typically is required to be superheated to a final temperature in the range of approximately 250 degrees Celsius. Additionally, diluent nitrogen is heated with extraction air and vent to atmosphere. Currently, such temperature requirements are met by venting heated syngas to flare during the startup process. However, using the existing warm-up line between the syngas stop and control valves to channel heated syngas to flare while meeting GT syngas temperature requirements and permissive causes visible flare and higher emissions for an extended time period and vents valuable syngas fuel to flare. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one embodiment, a method of operating an integrated gasification combined cycle system includes coupling a non-fuel fluid conduit to a fuel conduit, warming a flow of non-fuel fluid, and channeling the warmed non-fuel fluid through the fuel conduit such that heat from the warmed non-fuel fluid heats the fuel conduit to a predetermined temperature. 
     In another embodiment, an integrated gasification combined cycle system includes a fuel fluid conduit comprising an inlet from a supply of fuel fluid, the conduit configured to channel a flow of fuel to a combustor and to flare, and a non-fuel fluid conduit coupled in flow communication to the fuel fluid inlet conduit inlet such that in a first mode fuel fluid is channeled through the fuel fluid conduit from the inlet to at least one of the combustor and flare and in a second mode non-fuel fluid is channeled through the fuel fluid conduit from the inlet to flare. 
     In yet another embodiment, a method of heating a fuel supply system is provided. The fuel supply system includes a fuel inlet, piping configured to channel a flow of fuel to at least one of a combustor and a flare, and a fuel supply system heat exchanger coupled in flow communication with the fuel inlet. The method includes coupling an outlet of a first flow path of a non-fuel system heat exchanger in flow communication with the fuel inlet, heating a flow of non-fuel fluid using the non-fuel system heat exchanger, channeling the heated non-fuel fluid to the fuel inlet, heating the non-fuel fluid using the fuel supply system heat exchanger, and channeling the non-fuel fluid to flare. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a portion of an exemplary integrated gasification combined-cycle (IGCC) power generation system; and 
         FIG. 2  is a schematic diagram of another portion of the IGCC system in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic diagram of a portion of an exemplary integrated gasification combined-cycle (IGCC) power generation system  50 . IGCC system  50  generally includes a main air compressor  52 , an air separation unit  54  coupled in flow communication to compressor  52 , a gasifier  56  coupled in flow communication to air separation unit  54 , a gas turbine engine  10 , coupled in flow communication to gasifier  56 , and a steam turbine  58 . In operation, compressor  52  compresses ambient air. The compressed air is channeled to air separation unit  54 . In some embodiments, in addition or alternative to compressor  52 , compressed air from gas turbine engine compressor  12  is supplied to air separation unit  54 . Air separation unit  54  uses the compressed air to generate oxygen for use by gasifier  56 . More specifically, air separation unit  54  separates the compressed air into separate flows of oxygen and a gas by-product, sometimes referred to as a “process gas.” The process gas generated by air separation unit  54  includes nitrogen and will be referred to herein as “diluent nitrogen.” The diluent nitrogen may also include other gases such as, but not limited to, oxygen and/or argon. For example, in some embodiments, the diluent nitrogen includes between about 95% and about 100% nitrogen. The oxygen flow is channeled to gasifier  56  for use in generating partially combusted gases, referred to herein as “syngas” for use by gas turbine engine  10  as fuel, as described below in more detail. In some known IGCC systems  50 , at least some of the diluent nitrogen flow, a by-product of air separation unit  54 , is vented to the atmosphere. Moreover, in some known IGCC systems  50 , some of the diluent nitrogen flow is injected into a combustion zone (not shown) within gas turbine engine combustor  14  to facilitate controlling emissions of engine  10 , and more specifically to facilitate reducing the combustion temperature and reducing nitrous oxide emissions from engine  10 . IGCC system  50  may include a compressor  60  for compressing the diluent nitrogen flow before being injected into the combustion zone. 
     Gasifier  56  converts a mixture of fuel, the oxygen supplied by air separation unit  54 , steam, and/or limestone into an output of syngas for use by gas turbine engine  10  as fuel. Although gasifier  56  may use any fuel, in some known IGCC systems  50 , gasifier  56  uses coal, petroleum coke, residual oil, oil emulsions, tar sands, and/or other similar fuels. In some known IGCC systems  50 , the syngas generated by gasifier  56  includes carbon dioxide. The syngas generated by gasifier  56  may be cleaned in a clean-up device  62  before being channeled to gas turbine engine combustor  14  for combustion thereof or may be channeled to other systems for further processing, for example, to a Fischer-Tropsch synthesis reaction system for conversion to liquid hydrocarbons. Carbon dioxide may be separated from the syngas during clean-up and, in some known IGCC systems  50 , vented to the atmosphere, sequestered for storage, or processed to industrial use gases. Gas turbine engine  10  develops power by expanding the combustion gases from combustor  14  in a turbine  15 . The power output from gas turbine engine  10  drives a generator  64  that supplies electrical power to a power grid (not shown). Exhaust gas from gas turbine engine  10  is supplied to a heat recovery steam generator  66  that generates steam for driving steam turbine  58 . Power generated by steam turbine  58  drives an electrical generator  68  that provides electrical power to the power grid. In some known IGCC systems  50 , steam from heat recovery steam generator  66  is supplied to gasifier  56  for moderating the syngas. 
       FIG. 2  is a schematic diagram of another portion  200  of IGCC system  50  in accordance with an embodiment of the present invention. In the exemplary embodiment, portion  200  is a two unit portion of IGCC system  50  wherein two independent gas turbine generators are supplied by components of portion  200  that are shared between the two gas turbine generators. A flow of syngas  202  from for example, a gasification system or a gasification portion of IGCC system  50  is channeled to a syngas saturator  204  through a saturator inlet  205  where it is contacted with heated water that is circulated between a low temperature gas cooling system (not shown) and saturator  206  using a pump  214 . The heated water circulating through saturator is kept from boiling by a pressurized nitrogen blanket. Alternatively, the flow of syngas  202  may be bypassed around saturator  206  through a bypass line  216 . The flow of syngas  202  may be flared wet (after having passed through saturator  206 ) or dry (after having passed through bypass line  216 ) through flare line  218 . If not flared, the flow of syngas  202  is channeled to a performance heater  220  where heat from a flow of water circulating through the tube side of performance heater  220  and a tube bundle in HRSG  206 ,  207  is transferred to the water. A circulating pump  222  provides the motive force for the water in performance heater  220  circuit. The flow of syngas  202  is then channeled to a respective fuel skid  224 ,  226  for each gas turbine engine  210 ,  212 . The flow of syngas is then channeled to a combustor  228 ,  230  of a respective gas turbine engine  210 ,  212  where it is combusted to generate high pressure and high temperature gases to drive a turbine  232 ,  234  of gas turbine engines  210 ,  212  respectively. The exhausted gases are channeled through HRSG  206 ,  207  where remaining heat is extracted through a series of tube bundles to generate steam for a steam turbine (not shown in  FIG. 2 ) and to heat water for various streams used in IGCC system  50 . Turbines  232 ,  234  drive a shaft  236 ,  238  that in turn drives a compressor  240 ,  242  and a generator  244 ,  246 . 
     During startup and prior to a transition to operation using syngas, fuel skids  224 ,  226  receive a flow of natural gas  248  from a source of natural gas. The flow of natural gas  248  is channeled to fuel skids  224 ,  226  to be controllably delivered to combustors  228 ,  230 . Prior to transitioning to syngas fuel supplying gas turbines  210 ,  212  certain temperature limits and other permissions are required to be met. Included in these permissives are a requirement of the Saturator being pre-warmed during startup and preventing boiling of the circulation loop, a GT Syngas temperature of approximately 250 degrees Celsius, and to heat the diluent N 2  with extraction air and vent to atmosphere. 
     Various embodiments of the present invention use a non-fuel fluid such as diluent nitrogen extracted from an air separation unit (not shown) and heated using exhaust heat from gas turbines  210 ,  212  to facilitate meeting the above described permissives and reducing visible flare, and lowering emissions, lowering O&amp;M fuel cost, and realizing high plant revenue by transferring from natural gas to syngas operation by gas turbines  210 ,  212 , earlier in the warm-up process. 
     In the exemplary embodiment, a flow of diluent nitrogen  250  is channeled from for example, the air separation unit to a diluent nitrogen heater  252 ,  254 . Diluent nitrogen heater  252 ,  254  heats the flow of diluent nitrogen  250  using heat from a flow of extraction air  256 ,  258  from turbines  210 ,  212 , respectively. Rather than being supplied to combustors  228 ,  230  or released to atmosphere, the flow of diluent nitrogen  250  is channeled to saturator inlet  205  through a portion of piping  256  that couples the diluent nitrogen  250  circuit to the syngas  202  circuit to permit warm-up of the syngas piping using heated diluent nitrogen. The heated flow of diluent nitrogen is channeled through saturator  206  while picking up additional heat from a Low Temperature Gas Cooling section (not shown) from water circulated by pump  214 . The flow of diluent nitrogen  250  is channeled to performance heater  220  where additional heat is received from HRSG  206 ,  207 . The flow of diluent nitrogen  250  is then channeled to fuel skids  224 ,  226  where through a valve manifold (not shown), the flow of diluent nitrogen  250  is sent to flare. Maintaining the system piping warm (approximately 220 degrees Celsius or more) using the flow of diluent nitrogen  250  rather than by flaring syngas to warm up the piping and system components permits reducing visible flare, and lowering emissions, lowering O&amp;M fuel cost, and realizing high plant revenue by transferring from natural gas to syngas operation by gas turbines  210 ,  212 , earlier in the warm-up process. 
     During single unit operation, a crossover line  260  between fuel skid  224  and  226  is used during turn-down when one gas turbine is secure to maintain line  260  warm or for start-up of the second gas turbine engine when the first gas turbine engine is operating. 
     Exemplary embodiments of IGCC systems and methods of minimizing emissions are described above in detail. The IGCC system components illustrated are not limited to the specific embodiments described herein, but rather, components of each system may be utilized independently and separately from other components described herein. For example, the IGCC system components described above may also be used in combination with different IGCC system components. 
     The above-described IGCC systems and methods are cost-effective and highly reliable. The method permits maintaining the system piping warm using the flow of diluent nitrogen rather than by flaring syngas to warm up the piping and system components which permits reducing visible flare, and lowering emissions, lowering O&amp;M fuel cost, and realizing high plant revenue by transferring from natural gas to syngas operation by the gas turbines, earlier in the warm-up process. Accordingly, the systems and methods described herein facilitate the operation of IGCC systems in a cost-effective and reliable manner. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.