Flame characteristics and NO emission behaviors of methane/ammonia counterflow premixed flames with opposed N2 or air stream

• Published in: Fuel (Guildford) 2024-08, Vol.370, p.131851, Article 131851
• Main Authors: Lee, Huido, Moon Lee, Jong, Park, Jeong, Ho Chung, Suk, Sang Yoo, Chun
• Format: Article
• Language: English
• Subjects: Ammonia; Counterflow flame; Extinction; Heat loss ratio; Local equilibrium temperature; NO emission
• ISSN: 0016-2361; 1873-7153
• DOI: 10.1016/j.fuel.2024.131851

•A systematic study on flame characteristics and NOx emissions in NH3/CH4/air premixed counterflow flames with opposed air stream has not yet been reported.•The flame characteristics in counterflow premixed flames with opposed air stream mimic those for a small distance between the primary and secondary combustion zones in two-stage combustor.•Burnt gas temperature derived by an integral method can be applied to describe downstream interactions between rich premixed flame and diffusion flame.•Extinction mechanism was successfully explained by introducing local equilibrium temperature concept. Behaviors of methane/ammonia counterflow premixed flames and NO emission are numerically and experimentally investigated with opposed N2 or air stream by varying strain rate (ag) and equivalence ratio (ϕ). The premixed stream is 80% methane and 20% ammonia by volume mixed with air considering the utilization of ammonia by blending hydrocarbon fuels. The opposed stream is either N2 or air considering the potential application in gas turbines having single- or two-stage combustion. The simulations are conducted by increasing ag up to flame extinction for a specified ϕ and an experiment is conducted to evaluate various kinetic mechanisms. The extinction strain rate increases monotonically with ϕ with opposed air stream (AS), while increases and then decreases with opposed nitrogen stream (NS). The extinction mechanisms are explained by adopting the concepts of the local equilibrium temperature and heat loss ratio. For lean and rich mixtures with NS and lean mixtures with AS, the extinction occurs due to comparable effects of incomplete reaction and heat loss from the flame to the downstream burnt region. While for rich mixtures with AS, the extinction is attributed mainly to incomplete reaction. The contribution of thermal NO as compared with prompt NO in NO production is weak for all cases with NS and AS. The emission index (EINO) for lean and rich mixtures with NS and for lean mixtures with AS decreases monotonically with the increase of strain rate for all ϕ. Increasing equivalence ratio at a specified ag decreases NO production for lean and rich mixtures with NS and lean mixtures with AS. While for rich mixtures with AS, the EINO increases monotonically with ag. From the reaction pathway analysis, for lean mixtures, the effect of increasing ag on the reduction of NO production is attributed by weakening NH-related reaction paths, while NH2-related reaction paths chan