System and method for operating a power plant

Abstract

A system including a fuel-supply system including, an auxiliary-fuel-gas compressor configured to compress a fuel for use by a gas-turbine system, an expander configured to generate power by expanding an oxidant from the gas-turbine system, and a motor/generator configured to function in a motor mode and in a generator mode, wherein the motor/generator drives fuel compression with the auxiliary fuel-gas compressor in the motor mode, and the motor/generator generates power in the generator mode as the expander uses oxidant from the gas-turbine system to drive the motor/generator

1 . A system, comprising:
 an auxiliary-fuel-gas compressor configured to compress a fuel for use by a gas-turbine system;   an expander configured to generate power by expanding an oxidant from the gas-turbine system; and   a motor-generator configured to function in a motor mode and in a generator mode, wherein the motor-generator drives fuel compression with the auxiliary fuel-gas compressor in the motor mode, and the motor-generator generates power in the generator mode as the expander uses oxidant from the gas-turbine system to drive the motor-generator.   
     
     
         2 . The system of  claim 1 , comprising a fuel-gas compressor-expander having both the auxiliary-fuel gas compressor and the expander as a single dual-purpose unit. 
     
     
         3 . The system of  claim 1 , comprising a controller configured to change the motor-generator between the motor mode and the generator mode. 
     
     
         4 . The system of  claim 3 , comprising a compressor clutch configured to engage and disengage the auxiliary-fuel-gas compressor from the motor-generator, wherein the controller controls operation of the auxiliary-fuel-gas compressor by engaging and disengaging the compressor clutch. 
     
     
         5 . The system of  claim 4 , comprising an expander clutch configured to engage and disengage the expander from the motor-generator, wherein the controller controls operation of the expander by engaging and disengaging the expander clutch. 
     
     
         6 . The system of  claim 3 , wherein the controller controls the release of oxidant from the gas-turbine system to the expander. 
     
     
         7 . The system of  claim 3 , comprising a primary-fuel-gas compressor. 
     
     
         8 . The system of  claim 7 , wherein the controller enables operation of the auxiliary-fuel gas compressor during a startup mode of the gas-turbine system. 
     
     
         9 . The system of  claim 8 , wherein the controller enables a transition from the auxiliary-fuel-gas compressor to the primary-fuel-gas compressor during a steady-state mode of the gas-turbine system. 
     
     
         10 . The system of  claim 9 , wherein the transition from auxiliary-fuel-gas compressor comprises a disengagement of the auxiliary-fuel-gas compressor. 
     
     
         11 . The system of  claim 3 , wherein the controller is configured to transition the motor-generator from the motor mode to generator mode once a steady-state mode of the gas-turbine system has been reached after startup. 
     
     
         12 . The system of  claim 11 , wherein the expander is configured to drive the motor-generator in the generator mode once the steady-state mode of the gas-turbine system has been reached after startup. 
     
     
         13 . The system of  claim 11 , wherein the controller is configured to flare or recycle fuel gas until a pressure of the fuel gas reaches a threshold pressure during a transition from the startup to the steady-state mode. 
     
     
         14 . The system of  claim 1 , wherein the system comprises the gas-turbine system. 
     
     
         15 . A method, comprising:
 initiating a startup mode of a gas-turbine system;   starting a motor-generator in a motor mode for the startup mode;   compressing fuel for the startup mode using an auxiliary-fuel-gas compressor driven by the motor-generator in the motor mode;   transitioning the gas-turbine system into a steady-state mode;   driving a primary-fuel-gas compressor with the gas-turbine system to compress fuel for use in the gas-turbine system during the steady-state mode;   disengaging the auxiliary-fuel-gas compressor; and   transitioning the motor-generator from the motor mode to a generator mode after disengaging the auxiliary-fuel-gas compressor.   
     
     
         16 . The method of  claim 15 , comprising:
 extracting oxidant from an oxidant compressor of the gas-turbine system; and   driving an expander with oxidant from the compressor.   
     
     
         17 . The method of  claim 16 , comprising:
 driving the motor-generator in the generator mode with power from the expander; and   outputting power from the motor-generator in the generator mode.   
     
     
         18 . Tangible, non-transitory, and computer-readable medium having instructions stored thereon, wherein the instructions are configured to cause a processor to:
 start up a gas-turbine system in a startup mode;   utilize a motor-generator in a motor mode during the startup mode;   compress fuel gas for the startup mode using an auxiliary-fuel-gas compressor driven by the motor-generator in the motor mode;   transition the gas-turbine system into a steady-state mode from the startup mode;   during the steady-state mode, compress fuel gas for use in the gas-turbine system using a primary-fuel-gas compressor driven using the gas-turbine system;   during the steady-state mode, disengage the auxiliary-fuel-gas compressor; and   transition the motor-generator from the motor mode to a generator mode after disengaging the auxiliary-fuel-gas compressor to generate auxiliary power for the gas-turbine system.   
     
     
         19 . The tangible, non-transitory, and computer-readable medium of  claim 18 , wherein disengaging the auxiliary-fuel-gas compressor comprises disengaging a clutch that drivingly couples auxiliary-fuel-gas compressor to the motor-generator when engaged. 
     
     
         20 . The tangible, non-transitory, and computer-readable medium of  claim 18 , wherein disengaging the auxiliary-fuel-gas compressor comprises:
 opening a first valve to flow fuel gas to the primary-fuel-gas compressor; and   closing a second valve to block flow of fuel gas to the auxiliary-fuel-gas compressor.