The role of in situ reforming in plasma enhanced ultra lean premixed methane/air flames

This paper describes a mechanism for the stabilization of ultra lean premixed methane/air flames by pulsed nonequilibrium plasma enhancement. It is shown that the pulsed discharge plasma produces a cool (∼500–600 K) stream of relatively stable intermediate species including hydrogen (H2) and carbon monoxide (CO), which play a central role in enhancing flame stability. This stream is readily visualized by ultraviolet emission from electronically excited hydroxyl (OH) radicals. The rotational and vibrational temperature of this “preflame” are determined from its emission spectrum. Qualitative imaging of the overall flame structure is obtained by planar laser-induced fluorescence measurements of OH. Preflame nitric oxide (NO) concentrations are determined by gas sampling chromatography. A simple numerical model of this plasma enhanced premixed flame is proposed that includes the generation of the preflame through plasma activation, and predicts the formation of a dual flame structure that arises when the preflame serves to pilot the combustion of the surrounding non-activated premixed flow. The calculation represents the plasma through its ability to produce an initial radical yield, which serves as a boundary condition for conventional flame simulations. The simulations also capture the presence of the preflame and the dual flame structure, and predict preflame levels of NO comparable to those measured. A subsequent pseudo-sensitivity analysis of the preflame shows that flame stability is most sensitive to the concentrations of H2 and CO in the preflame. As a consequence of the role of H2 and CO in enhancing the flame stability, the blowout limit extensions of methane/air and hydrogen/air mixtures in the absence/presence of a discharge are investigated experimentally. For methane/air mixtures, the blowout limit of the current burner is extended by ∼10% in the presence of a discharge while comparable studies carried out in lean hydrogen/air flames fail to extend this limit.