Effects of preheating temperature and dilution level of oxidizer, fuel composition and strain rate on NO emission characteristics in the syngas moderate or intense low oxygen dilution (MILD) combustion

摘要

The aims of present study are to investigate the effective parameters in syngas MILD combustion, including preheating temperature and dilution level of oxidizer, syngas composition and flow structure on NO emission characteristics. For this purpose, counterflow syngas flame simulations are employed at the oxidizer preheating temperatures 1100-2100 K, oxygen mole fractions 2-12%, three syngas mixtures (10%H-2/90%CO, 50%H-2/50% CO, and 90%H-2/10%CO) and the strain rate ranges 25-4000 s(-1). Moreover, the effect of methane in the syngas composition as a diluent is studied on NO emission index. For the independence of the results from other parameters, simulations are performed at the equivalence ratio 1 and for the formation of reaction zone on the middle plane of fuel and oxidizer nozzles. The results show that NO emission is minimum at lower H-2/CO ratios, oxidizer preheating temperature and O-2 mole fraction. NNH mechanism by the reaction NNH + O double left right arrow NH + NO plays the most important role in the NO emission in low temperature conditions while the thermal mechanism is responsible for NO formation in high temperature flames. The use of pure H-2/CO mixture as fuel indicates that prompt mechanism has the minimum contribution to the NO release from MILD syngas combustion. However, the small values of methane in the syngas composition changes the reaction pathways and makes the prompt mechanism as dominant route in the NO emission at low and moderate temperatures.