DEVELOPMENT OF COLORLESS DISTRIBUTED COMBUSTION FOR GAS TURBINE APPLICATION

Abstract

Colorless Distributed Combustion (CDC) is investigated for gas turbine engine application due to its benefit for ultra-low pollutant emission, improved pattern factor, low noise emission, stable combustion and low pressure drop, alleviation of combustion instabilities and increased life of turbine blades with less air cooling requirements. The CDC is characterized by discrete and direct injection of fuel and air at high velocity and the reaction zone is stabilized due to controlled aerodynamics inside the combustor and wider (radially) shear layer mixing. Mixing between the injected air and product gases to form hot and diluted oxidant is required followed by rapid mixing with the fuel. This results in distributed reaction zone instead of a concentrated flame front as observed in conventional diffusion flames and hence, to avoid hot spot regions and provide reduced NOx and CO emissions. The focus of this dissertation is to develop and demonstrate CDC for application to stationary gas turbine combustors which generally operate at thermal intensity of 15MW/m3-atm. However, higher thermal intensity is desirable to reduce hardware costs due to smaller weight and volume of the combustors. Design of high thermal intensity CDC combustor requires careful control of critical parameters, such as, gas recirculation, fuel/oxidizer mixing and residence time characteristics via careful selection of different air and fuel injection configurations to achieve desirable combustion characteristics. This dissertation examines sequential development of low emission colorless distributed combustor operating from thermal intensity of 5MW/m3-atm up to 198MW/m3-atm. Initially, various fuel and air injection configurations were investigated at a low thermal intensity of 5MW/m3-atm. Further investigations were performed for a simpler combustor having single air and fuel injection ports for medium thermal intensity range of 28-57MW/m3-atm. Among the flow configurations investigated, reverse cross-flow configuration was found to give more favorable results possibly due to higher residence time because of reverse flow geometry and faster mixing with the fuel injection in cross-flow. This configuration was investigated in detail by further reducing the combustor volume to give ultra-high thermal intensity of up to 198MW/m3-atm. At thermal intensity of 53MW/m3-atm NO emissions were 4ppm in non-premixed mode and 1ppm in premixed mode and CO emissions were 30ppm in both the modes. The pressure loss was less than 5% and heat loss was less than 15%. The pressure fluctuations were less than 0.025% suggesting very stable combustion. At ultra-high thermal intensity of 170MW/m3-atm NO emissions were 8ppm and 3ppm in non-premixed and premixed modes respectively and CO emissions were about 100ppm in both the modes. Dilution of fuel with nitrogen, carbon dioxide and air resulted in significant reduction in NO emission in non-premixed mode from 8ppm to about 2ppm. Methane was used as fuel for all these investigations. Liquid fuel (ethanol) was also tested and very low NO emission of about 6ppm was obtained in direct injection mode and 2ppm in premixed prevaporized mode. CO emission of about 200ppm was observed in both the modes.