Combined Cycles for High Performance, Low Cost, Low Environmental Impact Waste-to-Energy Systems

Abstract

This paper assesses the integration between natural gas-fired combined cycles and grate combustors for municipal solid waste (MSW). Saturated steam generated in the grate combustor is exported to the heat recovery steam generator (HRSG) of the combined cycle, where it is superheated and then fed to a steam turbine serving both the combined cycle and the Waste-to-Energy (WTE) plant. Using a single steam turbine reduces costs and increases efficiency; in addition, superheating steam with the clean combustion products discharged by the gas turbine avoids all penalties (and extra-costs) caused by the corrosive gases generated in the grate combustor, which follow a path and are discharged from a stack completely separated from those of the CC. The optimal CC/WTE plant match is achieved when evaporation is carried out almost exclusively in the grate combustor, with the HRSG bearing the load for superheat (and reheat) and part of feedwater heating. Performance estimates for a combined cycle centered around a medium-size, heavy-duty gas turbine show that WTE/CC integration increases the efficiency of energy recovery from waste by 50% and more, with MSW disposal costs lower by 30–40%. Higher energy conversion efficiencies imply lower environmental impact, notably greater reductions of greenhouse gas emissions.