THE ROLE OF FUEL PREPARATION IN LOW-EMISSION COMBUSTION

The attainment of very low pollutant emissions, in particular oxides o f nitrogen (NOx), from gas turbines is not only of considerable enviro nmental concern but has also become an area of increasing competitiven ess between the different engine manufacturers. For stationary engines , the attainment of ultralow NOx has become the foremost marketing iss ue. This paper is devoted primarily to current and emerging technologi es in the development of ultralow emissions combustors for application to aircraft and stationary engines. Short descriptions of the basic d esign features of conventional gas turbine combustors and the methods of fuel injection now in widespread use are followed by a review of fu el spray characteristics and recent developments in the measurement an d modeling of these characteristics. The main gas-turbine-generated po llutants and their mechanisms of formation are described along with re lated environmental risks and various issues concerning emissions regu lations and recently enacted legislation for limiting the pollutant le vels emitted by both aircraft and stationary engines. The impacts of t hese emissions regulations on combustor and engine design are discusse d first in relation to conventional combustors and then in the context of variable-geometry and staged combustors. Both these concepts are f ounded on emissions reduction by control of flame temperature. Basic a pproaches to the design of ''dry'' low-NOx and ultralow-NOx combustors are reviewed. At the present time lean premix, prevaporize combustion appears to be the only technology available for achieving ultralow NO x emissions from practical combustors. This concept is discussed in so me detail, along with its inherent problems of autoignition, flashback , and acoustic resonance. Attention is also given to alternative metho ds of achieving ultralow NO, emissions, notably the rich-bum, quick-qu ench, lean-bum, and catalytic combustors. These concepts are now being actively developed, despite the formidable problems they present in t erms of mining and durability. The final section reviews the various c orrelations now being used to predict the exhaust gas concentrations o f the main gaseous pollutant emissions from gas turbine engines. Compr ehensive numerical methods have not yet completely displaced these sem i-empirical correlations but are nevertheless providing useful insight into the interactions of swirling and recirculating flows with fuel s prays, as well as guidance to the combustion engineer during the desig n and development stages. Throughout the paper emphasis is placed on t he important and sometimes pivotal role played by the fuel preparation process in the reduction of pollutant emissions from gas turbines.