Waste Heat Recovery From Fly Ash of 210 MW Coal Fired Power Plant Using Organic Rankine Cycle

Abstract

Combustion of coal in thermal power plants generates ash as a residue, which depends on the quality of coal, specific to its ash content and calorific value. In a typical Indian scenario, a standard 210 MW thermal plant produces ∼57 T/hr total ash, which has 80:20 fly and bottom ash share, considering coal with 40% ash content. This study aims to harness the waste heat of fly ash collected at the bottom of the electrostatic precipitator (ESP) by coupling organic Rankine cycle (ORC) with 210 MW subcritical coal-fired thermal power plant works on R134a. Thermodynamic properties of R134a are taken from the PYroMAT library (python 3.6) to develop a computer-based program that estimates the variability of key parameters with respect to log mean temperature difference (LMTD). The main plant's efficiency was 28.714%, with main steam pressure, reheat pressure, and temperature being about 134.35 bar, 24.02 bar, and 540 °C, respectively, and combustion of coal is about 141.5 T/hr. The study shows additional generation from fly ash waste heat is about 30.5 kW with an increase in net power output (0.0145%) and net energy efficiency (0.0146%). The optimum value of LMTD for the evaporator, condenser, and recuperator is 40, 7, and 16 K, which yields the optimum energy efficiency and developed cost-effective design. The proposed system is economically analyzed, considering 25 years of equipment life and 14% of loan interest. The study shows that the payback period and the generation cost of electricity of ORC is about 6.22 years and INR 3.14 per kWh, respectively.