Introduction and Perspectives

Publisher Summary Studies of lean combustion are among the oldest in the combustion literature because its extreme represents the lean limit of inflammability, which has been a well-recognized hazard marker from the inception of combustion science. Lean combustion was considered only with regard to explosion hazards until the late 1950s, when lean flames were introduced as useful diagnostic tools for identifying detailed reaction behavior. This chapter presents the foundations of lean combustion including the roles of turbulence, flame front instabilities, and flame speed in controlling the robustness of the reaction. Lean combustion is employed in nearly all combustion technology sectors, including gas turbines, boilers, furnaces, and internal combustion (IC) engines. This wide range of applications attempts to take advantage of the fact that combustion processes operating under fuel lean conditions can have very low emissions and very high efficiency. Pollutant emissions are reduced because flame temperatures are typically low, reducing thermal nitric oxide formation. In addition, for hydrocarbon combustion, when leaning is accomplished with excess air, complete burnout of fuel generally results, reducing hydrocarbon and carbon monoxide (CO) emissions. Unfortunately, achieving these improvements and meeting the demands of practical combustion systems is complicated by low reaction rates, extinction, instabilities, mild heat release, and sensitivity to mixing.