The Role of Fuel Preparation in Low-Emission 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 ultralow NOx has become the foremost marketing issue. This paper is devoted primarily to current and emerging technologies in the development of ultralow 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 impacts 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 ultralow-NOx combustors are reviewed. At the present time lean, premix, prevaporize combustion appears to be the only technology available for achieving ultralow 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 ultralow NOx emissions, notably the rich-burn, 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 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.

Stability Investigation of Combustion Chambers with LES

“…In order to get a high density of heat release flux i.e. power density and simultaneously low NO x emission highly turbulent lean premixed or partially premixed flames are mostly used (Lefebvre, 1995). Significant property of these flames is that any disturbances in the equivalence ratio through turbulence or in the air/fuel mixture supply produce a very fast change in the heat release.…”

“…At the Karlsruhe Institute of Technology the longterm project Collaborative Research Centre (CRC) 606: "Non-stationary Combustion: Transport Phenomena, Chemical Reactions, Technical Systems" was founded to investigate the basics of combustion and for the implementations relevant processes coupled to combustion (Bockhorn et al, 2003;SFB 606, 2002). Modern combustion concepts comprise lean premixed (LP) combustion, which allows for the reduction of the pollutant emissions, in particular oxides of nitrogen (NO x ) (Lefebvre, 1995). Lean premixed combustors are, however, prone to combustion instabilities with both low and high frequencies.…”

Flame Color as a Lean Blowout Predictor

“…A popular approach is to lower combustion temperature by operating the combustor in a lean premixed or partially premixed mode [1]. However, a combustion process fed by a lean mixture is inherently susceptible to lean blowout (LBO), particularly due to perturbations in a flame under such conditions [2,3].…”

Investigation of performances of innovative aeronautic injection systems using advanced laser diagnostics

“…It is well known that atomization of liquid sprays (shear-coaxial injectors) occurs via di¨erent physical processes compared to breakup of liquid sheets (¦lm injectors), which often results in di¨erent droplet size, velocity, and density [1,2]. Therefore, optimizing the atomization of liquid fuel into ¦ne droplets and their subsequent evaporation are of great importance for fuel spatial distribution within the combustor [3,4].…”