The Role of Fuel Preparation in Low Emissions Combustion

The attainment of very low pollutant emissions, in particular oxides of nitrogen (NO_x), from gas turbines is not only of considerable environmental concern but has also become an area of increasing competitiveness between the different engine manufacturers. For stationary engines, the attainment of ultra-low NO_x has become the foremost marketing issue. This paper is devoted primarily to current and emerging technologies in the development of ultra-low emissions combustors for application to aircraft and stationary engines. Short descriptions of the basic design features of conventional gas turbine combustors and the methods of fuel injection now in widespread use are followed by a review of fuel spray characteristics and recent developments in the measurement and modeling of these characteristics. The main gas turbine generated pollutants and their mechanisms of formation are described, along with related environmental risks and various issues concerning emissions regulations and recently-enacted legislation for limiting the pollutant levels emitted by both aircraft and stationary engines. The impact of these emissions regulations on combustor and engine design are discussed first in relation to conventional combustors and then in the context of variable-geometry and staged combustors. Both these concepts are founded on emissions reduction by control of flame temperature. Basic approaches to the design of “dry” low NO_x and ultra-low NO_x combustors are reviewed. At the present time lean, premix, prevaporize, combustion appears to be the only technology available for achieving ultra-low NO_x emissions from practical combustors. This concept is discussed in some detail, along with its inherent problems of autoignition, flashback, and acoustic resonance. Attention is also given to alternative methods of achieving ultra-low NO_x emissions, notably the rich-bum, quick-quench, lean-burn and catalytic combustors. These concepts are now being actively developed, despite the formidable problems they present in terms of mixing and durability. The final section reviews the various correlations which are now being used to predict the exhaust gas concentrations of the main gaseous pollutant emissions from gas turbine engines. Comprehensive numerical methods have not yet completely displaced these semi-empirical correlations but are nevertheless providing useful insight into the interactions of swirling and recirculating flows with fuel sprays, as well as guidance to the combustion engineer during the design and development stages. Throughout the paper emphasis is placed on the important and sometimes pivotal role played by the fuel preparation process in the reduction of pollutant emissions from gas turbines.