Combined cycle electric power plant and heat recovery steam generator

Abstract

A combined cycle electric power plant includes gas and steam turbines and a steam generator for recovering the heat in the exhaust gases exited from the gas turbine and for using the recovered heat to produce and supply steam to the steam turbine. The steam generator includes a superheater tube and a steam drum from which heated steam is directed through the superheater to be additionally heated into superheated steam by the exhaust gas turbine gases. An afterburner serves to further heat the exhaust gas turbine gases passed to the superheater tube and a bypass conduit is disposed about the superheater tube whereby a variable steam flow determined by a bypass valve disposed in the bypass conduit may be directed about the superheater tube to be mixed with the superheated steam therefrom, whereby the temperature of the superheated steam supplied to the steam turbine may be accurately controlled. Steam temperature control means includes a first control loop responsive to the superheated steam temperature for regulating the position of the bypass valve with respect to a first setpoint, and a second control loop responsive to the superheated steam temperature for controlling the fuel supply to the afterburner with respect to a second setpoint varying in accordance with the bypass valve position. In particular, as the bypass valve position increases, the second setpoint, originally higher than, is lowered toward a value substantially equal to that of the first setpoint.