Oxygen composition modulation effects on flame propagation and NOx formation in methane/air premixed flames

The effect of O-2 composition modulation on the dynamics and structure of lean CH4/air premixed flames was computationally and experimentally investigated, in light of its relevance to oxygen-enhanced combustion (OEC). OEC can result in conditions of stable combustion with reduced NOx emissions. Experiments were conducted in the stagnation flow configuration. Laminar flame speeds, as well as temperature and NOx concentration profiles, were determined. The laminar flame speeds were measured by using laser Doppler velocimetry. Temperatures were measured by using thermocouples, and the NOx profiles were measured by using a chemiluminescence analyzer. The experiments were modeled in detail by using the CRI 3.0 mechanism, and satisfactory agreement was found between the experimental and simulated results, for both laminar fame speeds and nitrogen chemistry. It was found that addition of extra O-2 in the oxidizer noticeably extends the lean flammability limit. Thus, stable combustion can be achieved at leaner renditions, which are also characterized by reduced fuel consumption, lower flame temperatures, and reduced NOx emissions. The reduction of NOx is caused by the synergistic effect of temperature and Np concentration reductions as more O-2 is added. Furthermore, studies conducted by extensively modulating the O-2 composition in the oxidizer, and by keeping the ratio of N-2 to CH4 supplies constant, revealed non-monotonic effects on both the laminar flame speed and the maximum NOx concentration. The nonmonotonic effect on the laminar flame speed was found to be a result of the competing effects of O-2 concentration and temperature variations on the main branching H + O-2 --> OH + O reaction, which critically affects propagation. The non-monotonic effect on the maximum NOx concentration was explained from first principles by assessing the relative importance of the reactions that are largely responsible for NO production and destruction.