Physical and chemical effects of steam dilution on premixed NH3/O2/H2O flames

Abstract

NH₃/O_2/H₂O combustion combines oxygen-rich enhanced combustion with steam to control flame temperature and NO_x emissions, and it is a promising ammonia combustion technology for future clean and efficient power generation. For investigating the regulation mechanisms of steam dilution in NH₃/O₂/H₂O flames, this study employed premixed free flame and counterflow flame models to investigate the impact of steam dilution on laminar flame characteristics, including flame speed (S_L), temperature (T_f), NO emissions (E_NO), and extinction strain rate (κ_ext). The effects of steam dilution are decoupled into physical (dilution effect, thermal effect, transport effect, radiation effect) and chemical effects (direct reaction effect and third-body effect) to investigate the competitive and synergistic mechanisms between these effects on the kinetic scale. Results indicate that the NH₃ primary decomposition and the H/O radical pools determine NH₃/O₂/H₂O flame speed. H/O radical formation and NH₃ staged dehydrogenation dominate the heat release and consumption, respectively. NO is mainly produced from HNO and NH radicals, and the reduction reactions of NH_i(i=0–2) and NNH-N₂O mechanism dominate NO consumption in NH₃/O₂/H₂O flame. The dilution effect strongly inhibits S_L, T_f, E_NO, and κ_ext of NH₃/O₂/H₂O flames, as it reduces fuel concentration and leads to low active radicals. The thermal and radiation effects increase the reaction heat loss to decrease S_L, T_f, and κ_ext, while the transport effect helps the rapid reactant diffusion with contrast results. The thermal/radiation/transport effects extend the heat release and OH/HNO accumulation region to increase E_NO at fuel-lean conditions. For chemical effects, the direct reaction and third-body effect synergistically suppress T_f and promote E_NO, exhibiting competition on S_L. Under most conditions, the direct reaction effect dominates the chemical effects, which are reversed at a high dilution ratio (Z_H2O). For the extinction strain rate, the direct reaction effect and three-body effect transition from competitive to synergistic with increased Z_H2O. To balance the NH₃/O₂/H₂O combustion efficiency, combustor thermal resistance, emissions, and flame stability, future strategies may involve adopting steam staged injection and RQL (Rich-Quench-Lean) combustion organization, as well as combining with plasma-assisted/porous media combustion technology to enhance combustion efficiency, cleanliness, and flame stability.