Numerical study of NH3/CH4 MILD combustion with conjugate heat transfer model in a down-fired lab-scale furnace

Abstract

Ammonia (NH3) is a promising carbon-free fuel but suffers from low flame stability and high NO emission. Moderate or intense low-oxygen dilution (MILD) combustion has been indicated to be a feasible technology for resolving these drawbacks of NH3 during its combustion. However, the effect of NH3 substitution on combustion and NO emission characteristics as well as heat transfer behaviors under MILD combustion is not well-understood. This paper reports a numerical study of NH3/CH4 combustion in a laboratory-scale furnace by using conjugate heat transfer (CHT) model, which eliminates the commonly-used fixed-temperature or fixed-heat flux assumptions for furnace walls. Main emphasis is laid on the comparison of NO emission, temperature distribution, flame structure and furnace heat transfer behaviors between traditional swirling combustion and MILD combustion under various NH3 blending ratio (XNH3) conditions. The results show that NO emission reaches to the maximum at XNH3 of 50% regardless of combustion mode., while MILD combustion generates a lower NO emission and conversion ratio than swirling combustion in the whole range of XNH3 (0% - 100%). Numerical simulation without using CHT model produces negative heat transfer in certain furnace regions, indicating the necessity of adopting CHT model on simulating heat transfer process in combustion furnaces. With the CHT model, the heat flux on the furnace wall is found to follow the same trend between the two different combustion modes, but the heat utilization efficiency of MILD combustion is slightly lower mainly due to the increased exhausted gas temperature, which is resulted from the postponed reaction zone. With the replacement of CH4 by NH3, the heat utilization efficiency gradually decreases under both two combustion modes as a result of lower peak temperature and less soot radiation.