Dominant chemistry and physical factors affecting no formation and control in oxy-fuel burning

NO formation in oxygen-enriched counterflow methane diffusion flames has been computationally studied with detailed chemistry and transport properties. Particular interests are the identification of the dominant chemistry in the production of NO and the investigation of the sensitivity of NO production due to effects of air infiltration, fuel contamination, flame radiation, and aerodynamic straining when applying oxy-fuel combustion as a control technique for NO emissions. It is shown that oxygen displacement significantly modifies the diffusion flame structure from the familiar one obtained with air as the oxidizer, with thermal NO gradually replacing Fenimore NO as the dominant production pathway with increasing oxygen enrichment. Based on GRI-Mech 2.11, the net production of NO through the Fenimore mechanism is shown to be negative in oxygen-enriched combustion such that simulation of NO formation in oxy-fuel combustion by considering thermal mechanism alone will yield an overprediction of NO emissions. Important reactions leading to NO formation and destruction are also identified and compared under various levels of oxygen enrichment. It is further demonstrated that NO emission is very sensitive to the extent of air leakage and that it is advantageous to apply oxy-fuel control with even more than 0.6% nitrogen contamination of methane, provided that there is no severe air infiltration in the oxidizer stream. For the nonadiabatic oxygen-enriched combustion with radiative heat loss, the net production through Fenimore mechanism is negative over a wide range of strain rate, while NO emissions decrease with increasing strain rate. The study suggests that because NO formation can be substantially reduced with increasing strain rate and because oxy-fuel combustion is more resistant to stretch-induced extinction, the strategy to minimize effects of air infiltration and fuel contamination is to operate the burner at high turbulence intensities and, thereby, correspondingly high local strain rates.