| Combustion of hydrocarbon fuels is a major source of two common pollutants, soot particulates and NO, that are of health and environmental concern. Formation and control of soot and NO in partially premixed methane flames were studied. Diluted methane-oxygen flames were stabilized between opposing axisymmetric flows of fuel and oxidizer. The distribution of soot, NO, OH, major stable species, temperature and flow-field along the burner axis was measured. Computations, using a version of the opposed flow flame code, OPPDIF, modified to include soot and gas radiation, were carried out to supplement experimental measurements. With partial premixing the energy release takes place in a non-premixed reaction zone and a premixed reaction zone. An analytical model was developed to understand the effect of progressive partial premixing on the location of the premixed and non-premixed flamesheets with respect to the stagnation plane. The model showed good agreement with previously reported experimental data of flame positions in methane-air counter-flow flames of Tsuji and Yamaoka (1978). The effects of fuel/oxidizer (small amount of oxidizer/fuel added to the fuel/oxidizer) side partial premixing on flame structure and soot formation are reported. Various flame configurations obtained by progressive fuel side partial premixing were investigated for energy release, flame radiation, soot loading and NO emission. A novel flame configuration is reported in which soot is formed between spatially separated premixed and non-premixed reaction zones on the fuel side of the stagnation plane. This flame configuration has the potential for providing enhanced soot radiation with simultaneous reduction of soot emission. Although the energy release and the flame radiation both increased with progressive fuel side partial premixing, gas radiation per unit energy release decreased. Implications of these observations for NO formation in turbulent partially premixed flames are discussed. Peak NO concentration increased and the NO distribution profile became broader with progressive fuel side partial premixing. Computations showed that the spatial separation between the two peaks in the NO production rate distribution increased and the importance of thermal NO pathway to NO formation increased with progressive fuel side partial premixing. Emission Index of NO (gm NO/gm fuel) first increased and then decreased. |
