The Operational Stability and Energy Efficiency of Coal-Fired Power Plant Under Ultra-Low Power Load Ratio: Feedwater Preheating System Issues

Abstract The world energy supply continues to evolve and adapt to low carbon and sustainable sources due to energy shortage and environmental issues. As a result, the share of renewable energy sources in the energy mix sharply increases in many countries. However, wind and photovoltaic power is intermittent, and high penetration of renewable power poses high peak shaving pressure on the power system. Therefore, the peak shaving techniques for coal-fired power plant (CFPP) should be developed, aiming to decrease the adjustable minimal power load ratio. When the coal-fired power plant operates under low load ratio, it significantly deviates from the design condition, which will affect the operational safety and economic performances. In this study, the drainage obstruction issues of the feedwater preheating system are analyzed and addressed. Off-design analysis models of coal-fired power plant are developed. Simulation results show that the feedwater preheating system increases the net energy efficiency by 2.90%, 3.11%, 2.49% under 100%, 30% and 20% power load ratio conditions, respectively. The pressure difference between adjacent regenerative heaters (RH) decreases with the load ratio, thus increasing the risk of the drainage obstruction phenomenon. Then, schemes, including the RH’s switch-off, connecting the drainage pipe to deaerator (DRTR) or condenser, or installing the drainage pump (DP) in the drainage pipe are proposed and evaluated. Scheme A connects RH2’s and RH3’s drainage pipes to the DRTR, which can improve the unit’s operational stability and energy efficiency greatly when the drainage obstruction occurs in high-pressure feedwater preheating system. Scheme B connects RH7s and RH8’s drainage pipes to the condenser and Scheme C installs DPs in RH7s and RH8’s drainage pipelines. Scheme B and Scheme C can both enhance the operational stability of low-pressure feedwater preheating system under low load ratio condition. Scheme C have better energy efficiency performance than scheme B, but with more retrofit expense on the pump purchases. Results of this study can provide some guidance for the safe operation of the feedwater preheating system.