Thermodynamic Analysis of Semi-Closed Gas Turbine Combined Cycles With High Temperature Diluted Air Combustion

Abstract

In the last years many research studies have been focused on the features of MILD (Moderate and Intensive Low oxygen Diluted) or Flameless combustion, that is a stable form of combustion characterized by low flame temperature and, consequently, low Nox emissions. Early studies showed that flameless conditions can be obtained using high temperature air diluted with a large amount of exhaust gas. MILD combustion is presently applied in industrial furnaces where ceramic regenerators provide to raise the temperature of the entering diluted air, the main advantages being high efficiency and low emissions. Attractive features of MILD combustion (low NOx emissions, stable combustion) addressed in the last years towards investigations about new combustors suitable for applications in gas turbines. Although there is an intense activity aiming at better understanding the features of this form of combustion, there is a limited research effort to understand which could be the power cycles that could be better suitable to the application of MILD combustion. MILD combustion allows for increasing the temperature of the entering reactants beyond the self-ignition temperature thus decreasing combustion exergy losses. High temperature of the reactants can be obtained through recuperative heat exchangers. Recirculation, on the other hand, is the origin of new losses that may reduce partially the advantages produced in the combustion process. The oxidizer and flue gas can be mixed at different points influencing the final cycle efficiency. The paper presents a thermodynamic analysis of semi-closed Joule-Brayton cycles with high temperature diluted air and flue gas recirculation at intermediate pressure. This arrangement shows some favorable characteristics: reduction of the combustion exergy losses due to the increase of the temperature of the oxidizer, limited dimensions of the recuperative heat exchanger, efficient part load operation, favorable conditions for CO2 separation. The effects of the main cycle parameters on the plant efficiency are presented in order to outline the best trade-off that can be reached between the advantages given by high temperature of the reactants and the penalties caused by the recirculation of the flue gas. The combination of the semi-closed cycle with a bottoming steam plant is then examined, assuming state-of-the-art technologies. As applications, two plant configurations are considered. The first one, suitable for small plants using low calorific fuels, is characterized by lower combustor outlet temperature and simple air cooling technology for the turbine blades. The second one, suitable for large power plants, is characterized by higher turbine outlet temperature and steam cooling of the turbine blades. Advantages and disadvantages in comparison modern conventional CCGT power plant fueling natural gas are discussed.