Combined gas-steam turbine power plant and method of operating the same

Claim 1

IN A COMBINED GAS TURBINE-STREAM TURBINE ELECTRICAL GENERATING PLANT INCLUDING A STEAM GENERATING AND HEATING UNIT FOR SUPPLYING STEAM TO SAID STEAM TURBINE AND WHICH INCLUDES WALLS FORMING A FURNACE AND A CONNECTED CONVECTION GAS PASS, FUEL BURNING MEANS FOR SUPPLYING FRESHLY GENERATED HEATING GASES TO THE FURNACE, STEAM GENERATING TUBES LINING THE WALLS OF THE FURNACE, STEAM HEATING TUBES DISPOSED IN SAID CONVECTION GAS PASS AND CONNECTED FOR FLOW OF FLUID FROM THE STEAM GENERATING TUBES AND TO SAID STEAM TURBINE, AND AN ECONOMIZER DOWNSTREAM GAS-WISE OF SAID STEAM HEATING TUBES AND SUPPLYING FEEDWATER TO SAID STEAM GENERATING TUBES, THE METHOD FOR IMPROVING THE THERMAL EFFICIENCY OF SAID UNIT DURING LOW LOAD OPERATION THEREOF COMPRISING THE STEPS OF OPERATING THE GAS TURBINE AT SUBSTANTIALLY FULL LOAD FOR GENERATING THE BASE ELECTRICAL OUTPUT OF SAID PLANT, AND VARYING THE LOAD OF SAID STEAM GENERATING UNIT TO COMPENSATE FOR ANY SWING IN THE ELECTRICAL OUTPUT OF SAID PLANT ABOVE SAID BASE OUTPUT DURING THE COMBINED GASSTEAM CYCLE OPERATION THEREOF, RECOVERING THE HOT EXHAUST GASES FROM THE GAS TURBINE TO INCREASE THE CYCLE EFFICIENCY OF THE STEAM GENERATING UNIT DURING SUCH LOAD SWINGS BY DIVIDING THE RECOVERED EXHAUST GASES, COMBINING AT THE INLET END OF THE ECONOMIZER A DIVIDER PORTION OF THE EXHAUST GASES TOGETHER WITH THE FRESHLY GENERATED HEATING GASES OF THE UNIT AND DIRECTING SAID COMBINED GASES OVER ALL THE ECONOMIZER HEATING SURFACE TO HEAT THE FEEDWATER BY RECOVERING THE HEAT FROM SAID COMBINED GASES BY INDIRECT HEAT EXCHANGE, DELIVERING ANOTHER DIVIDED PORTION OF SAID EXHAUST GASES DIRECTLY TO THE FUEL BURNING MEANS FOR UTILIZATION AS A PREHEATED COMBUSTION SUPPORTING MEDIUM, INTRODUCING STILL ANOTHER DIVIDED PORTION OF THE EXHAUST GASES DIRECTLY INTO THE FURNACE TO REDUCE THE AMOUNT OF RADIANT HEAT TRANSMISSION OF THE PRODUCTS OF COMBUSTION TO THE STEAM GENERATING TUBES LINING THE WALLS OF THE FURNACE AND TO INCREASE THE GAS MASS FLOW THROUGH THE UNIT, INCREASING THE RATE OF FLOW OF GAS TURBINE EXHAUST TO SAID FURNACE AS THE LOAD OF THE STEAM GENERATING UNIT DECREASES, AND DECREASING THE RATE OF SUPPLY OF FRESHLY GENERATED HEATING GASES TO THE FURNACE AS THE LOAD OF THE STEAM GENERATING UNIT DECREASES.