The Role of Fuel Preparation in Low Emissions Combustion

Abstract

The attainment of very low pollutant emissions, in particular oxides of nitrogen (NOx), 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 NOx 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 NOx and ultra-low NOx combustors are reviewed. At the present time lean, premix, prevaporize, combustion appears to be the only technology available for achieving ultra-low NOx 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 NOx 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.

Experimental investigation of performance of high-shear atomizer with discrete radial-jet fuel nozzle: mean and dynamic characteristics

“…Spatial distribution of spray mass/volume flux in the azimuthal plane is of equal importance to drop size in low-emission combustion from an emission point of view in any gas turbine combustor. Failure to achieve a symmetrical volume flux distribution can result in a local region of mixture inhomogeneity (Lefebvre, 1995). The fuel spray's volume flux distribution controls the engine's temperature distribution, which helps to avoid hot spots inside the combustor liner.…”

“…The high-pressure requirement of the fuel nozzle makes the system bulky and costly. The main drawback of a duplex nozzle is the potential plugging of small orifices and passages by containments present in the fuel (Lefebvre, 1995). The duplex nozzle also has an operating limit since the hollow-cone sheet collapses at high operating pressures of the combustor.…”

Application of Advanced Experimental Techniques in the Development of a Cooled Turbine Nozzle

“…The main objective of a turbine nozzle cooling design is to provide long component life (durability) at high turbine inlet temperatures using minimum cooling air. Minimizing of cooling air saves an even more critical role as the engine emission characteristics am becoming a primary concern for industrial engines and the cooling air saved from turbine componan coaling can be used to improve combustor emission (Lefebvre, 1995). In order to have an effective cooling design, it is necessary to accurately predict the metal temperatures.…”

Effect on Particulate and Gas Emissions by Combusting Biodiesel Blend Fuels Made from Different Plant Oil Feedstocks in a Liquid Fuel Burner

“…For the axial swirler, the swirl number is related to the swirl angle, θ, inner r i and outer radius r 0 as given by [47] where the swirl number was proportional to tan θ [48]:…”