Deep part load operation of combined cycles and combined heat and power plants

Abstract

The world energy market has become very volatile due to the increase of renewable sources of power (solar and wind energy) and it asks more flexibility to fossil power stations. Gas turbines are of great importance for the generation of power both in combined cycles and in combined heat and power installations. If in the past gas turbines operated on a continuous basis (base load), nowadays there is the necessity for gas turbines to operate at part load during periods of excess of renewables. Up to now, many combined cycles and combined heat and power installations show little flexibility and their contribution to grid stability is limited. Vendors use several traditional techniques to part-load gas turbines: decreasing firing temperature, decreasing inlet airflow (inlet guide vane closure), recirculating the compressor airflow or a combination of these techniques. The cycle efficiency drops significantly at part load when using these traditional control techniques. Furthermore, traditional gas turbine control techniques struggle to keep emission levels low (NOx and CO). An option to overcome these problems is the use of Flue Gas Recirculation (FGR) for which the gas turbine flue gases are recirculated to the gas turbine inlet. When recirculating the flue gases, the inlet temperature of the gas turbinewill rise resulting in a lower mass flowthrough the installation and lower power production. Flue Gas Recirculation has been studied before in the literature to increase the CO2 content and decrease the NOx content in flue gases but not to allow part load operation. The uniqueness of this research is to enable part load operation of combined heat and power plants and combined cycles with FGR. In particular, there is a lack of studies on the impact of FGR on the thermodynamic cycle and the gas turbines components (e.g. compressor and turbine). In order to investigate the part load controls, a model of a Brayton cycle has been developed and validated with THERMOFLEX. The results show that FGR is more efficient than the other part load controls for both the combined heat and power plants and combined cycles. In particular, the use of FGR can theoretically increase the cycle efficiency of the combined heat and power plants at part load up to 9% with respect to traditional control techniques.