Comparison between Regenerative Feed Water Heating and Regenerative Fuel Drying in Biomass Power Plant

Abstract

Regeneration is a widely implemented method in a thermal power plant for increasing the power plant efficiency. In a typical regenerative method, extracted steam from a steam turbine is mixed with cooling water in an open feed water heater to increase feed water temperature before feed water is heated, and becomes superheated steam. An alternative method of regeneration is possible for a biomass power plant that consumes the fuel with high moisture content. In this method, extracted steam is used to decrease the fuel moisture content before the fuel is combusted. The main objective of this paper is to compare both methods of regeneration. The reference power plant consists of a biomass boiler, a steam turbine, a condenser, and an open feed water heater. The boiler consumes 5.2 kg/s of fuel with a moisture content of 52 %, and generates superheated steam with a pressure of 4.5 MPa. Steam is extracted from the steam turbine at a pressure of 1.2 MPa, and supplied to the open feed water heater. The condenser pressure is 10 kPa. Turbine isentropic efficiency is 85 %, and pump isentropic efficiency is 70 %. The power output of the reference power plant is 10.0 MW. Two methods of increasing the power output are compared. In the first method, another open feed water heater is added to the reference power plant. The pressure of extracted steam supplied to this open feed water heater is 0.2 MPa. In the second method, a steam dryer is added to the reference power plant. The pressure of extracted steam supplied to the steam dryer is also 0.2 MPa. Simulation results indicate the first method increases the power output up to 3 %, whereas the second method increases the power output up to 7 %.