Off-design performance of integrated waste-to-energy, combined cycle plants

This paper focuses on the off-design operation of plants where a waste-to-energy (WTE) system fed with municipal solid waste (MSW) is integrated with a natural gas-fired combined cycle (CC). Integration is accomplished by sharing the steam cycle: saturated steam generated in a MSW 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 CC and the WTE plant. Most likely, the WTE section and the natural gas-fired CC section are subject to different operation and maintenance schedules, so that the integrated plant operates in conditions different from those giving full power output. In this paper we discuss and give performance estimates for the two situations that delimit the range of operating conditions: (a) WTE plant at full power and gas turbine down; (b) WTE plant down and gas turbine at full power. This is done for two integrated plants having the same WTE section, i.e. grate combustors with an overall MSW combustion power of 180 MWLHV, coupled with Combined Cycles based on two different heavy-duty gas turbines: a medium-size, 70 MW class turbine and a large-size, 250 MW class turbine. For each situation we discuss the control strategy and the actions that can help to achieve safe and reliable off-design operation. Heat and mass balances and performances at off-design conditions are estimated by accounting for the constraints imposed by the available heat transfer areas in boilers, heaters and condenser, as well as the characteristic curve of the steam turbine. When the gas turbine is down the net electric efficiency of the WTE section is very close to the one of the stand-alone WTE plant; instead, when the WTE section is down, the efficiency of the CC is much below the one of a stand alone CC. These performances appear most congenial to what is likely to be the operational strategy of these plants, i.e. paramount priority to waste treatment and CC dispatched according to the requirements of the national grid.