Optimization of steam parameters in double-pressure heat recovery steam generators

Double-pressure heat recovery steam generators are installed in medium-size combined cycle power plants. For a fixed total heating surface area of heat recovery steam generator, the net power output of the power plant depends on steam pressures and temperatures. This paper presents a method of determining the optimal set of steam parameters in double-pressure heat recovery steam generators without reheating and with reheating. The mathematical models of both heat recovery steam generators are presented, and the optimization procedure is described. For given exhaust gas temperature and total heating surface area, the optimal steam parameters of both heat recovery steam generators that maximize net power output can be found subjected to the constraint that the steam quality at turbine outlet is not less than 88 %. Simulation results show that reheating increases optimal net power output by 2.4% and exhaust gas temperature by about 80 degrees C.