Characteristics and mechanistic analysis of CO formation in MILD regime with simultaneously diluted and preheated oxidant and fuel

Abstract

The innovative combustion modality of moderate or intense low-oxygen dilution (MILD) combustion has high combustion efficiency and low pollutant emissions. In this paper, the characteristics and mechanisms of CO formation in the MILD combustion regime were studied using opposed diffusion flame (OPPDIF) with simultaneously diluted and preheated oxidant and fuel. The GRI 3.0 mechanism was adopted to represent the chemical reactions. The effects of the entrainment ratio (Kv), the equivalence ratio (ϕ), and the inlet temperature (Tin) on CO formation were studied under three calculation conditions: (1) the range of Kv from 1 to 8 at Tin=1300K, (2) the range of ϕ from 0.6 to 1.6 at Tin=1300K, and (3) the range of Tin from 1000 to 1300K at Kv=3 and ϕ=1. The results show that in the MILD combustion regime, increasing entrainment ratio and equivalence ratio can reduce the emission of CO. However, their effects become weak when entrainment ratio exceeds 6. From the threshold states to MILD combustion regime, the EICO sharply decreases with increasing entrainment ratio and equivalence. Increasing inlet temperature can gently enhance the emission of CO in MILD combustion regime.The main differences of the mechanisms of CO formation between in MILD combustion regime and in threshold state are two: (1) in the threshold states, C2H2 makes contribution comparable with CH3 to the CO formation in the threshold state; due to oxidative reforming process, the path from CH3 to C2H2 (CH3→C2H6→C2H5→C2H4→C2H3→C2H2→CO) is greatly weakened in MILD combustion regime, causing that the contribution of CH3 is up to be comparable with HCCO and the contribution of C2H2 is down to the lowest; (2) in threshold states, the ROP of CO of the elementary reaction, OH+CO⇔H+CO2, changes from positive value to negative value due to the increase in [OH]/[H]; the direct reaction of this elementary reaction predominate in MILD combustion regime, because its direct reaction prefers the lower temperature compared to its reverse-direction reaction. According to the sensitivity analysis of CO formation, the CO formation is mainly controlled by the direct reaction of the elementary reaction, OH+CO⇔H+CO2 , in the MILD combustion regime. The importance of the role of this elementary reaction increases with increasing the dilution and the equivalence ratio and decreases with increasing the inlet temperature.