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

•Reduction of dilution level is recommended to increase temperature with minimum NO.•H2-lean syngas with high strain rates is compiles with syngas MILD regime criteria.•Existence of CH4 into the syngas increasing NO emission through prompt mechanism.•NNH route is main mechanism for NO emission at the moderate and low temperatures.•Reaction paths changes significantly by moving from H2-lean toward H2-rich syngas. 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%H2/90%CO, 50%H2/50%CO, and 90%H2/10%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 H2/CO ratios, oxidizer preheating temperature and O2 mole fraction. NNH mechanism by the reaction NNH + O ⇔ 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 H2/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.