Operational analysis of natural gas combined cycle CHP plants: energy performance and pollutant emissions

The natural gas-fired combined cycle (NGCC) plants are among the best technologies for power production, especially when operating in combined heat and power (CHP) generation feeding a district heating (DH) network. Even if usually designed to operate with very high utilization factors, thus satisfying mainly the base load, nowadays these plants are often used also as backup power. This is due mainly to the necessity to compensate the nonprogrammable renewable energy sources (RES) production, and it can be done, thanks to the good flexibility of these plants. However, in off-design conditions, the energy performance and the pollutant emissions may not be as good as the expected nominal ones. In this paper, the real operation of three NGCC units has been analysed in detail by considering mean hourly data over several years. A gas turbine efficiency curve at partial loads has been obtained, showing a decrease of conversion efficiency at lower unit loads. The CO emissions during the start-up and shut-down procedures of the plant reached values that are some orders of magnitude higher than in normal operation. This criticality should not be forgotten when using these units for frequent on-off operations.