Numerical study of HCN and NH3 reduction in a two-stage entrained flow gasifier by implementing MILD combustion

Moderate or intense low-oxygen dilution (MILD) combustion is a novel technology owing to the ability to reduce NOx emissions from combustion significantly. The development of this technology and its combination with gasification are of increasing interest for syngas impurities control. Therefore, the present study aims to numerically assess the feasibility of implementing MILD combustion to a two-stage entrained flow gasifier for reducing nitrogen-containing impurities (mainly HCN, NH3, and NO) in the produced syngas. Specifically, a conventional combustion gasifier (CCG) is modified to a MILD combustion gasifier (MCG) by replacing the original burners with a conceptually designed MILD burner. Detailed comparisons have been made between CCG and MCG in terms of combustion characteristics and evolutions of the nitrogen-containing impurities. The results show that the present conceptually designed MILD burner can achieve more uniform temperature and species distributions in comparison with the conventional burner. Different from the preferable neutralization reactions with NO in the CCG combustor, HCN and NH3 tend to be oxidized to form NO in the MCG combustor, resulting in higher NO content at the MCG combustor outlet. As a result, concentrations of HCN and NH3 in syngas at the MCG gasifier outlet can be reduced by 22% and 9% respectively compared to those at the CCG gasifier outlet. Furthermore, results from the equilibrium-reaction-network model show that the final NO emission from MCG is 14% less than that from CCG.