Chemical kinetics of Lean − Rich − Lean fuel-air staged NH3/H2-air combustion for emission control

Abstract

Ammonia has garnered considerable attention among researchers as a carbon–neutral fuel option. Addressing the challenge of its inherently slow combustion kinetics, significant research efforts have been directed toward mitigating NOx emissions, which represents a critical hurdle in advancing the commercial viability of ammonia as a sustainable fuel. The rich-lean combustion approach has been advocated and demonstrated as the primary means to keep NO_X within acceptable emission limits. However, in the current study, a distributed fuel and air injection strategy is proposed and investigated as a superior alternative compared to the traditional rich-lean method for NO_X reduction for NH₃/H₂-air mixtures with higher ammonia fuel fraction ($X_{{NH}_3}$ = 50 − 90 %). The proposed lean-rich-lean strategy with multi-staging of fuel and air supply is achieved by injecting NH₃-air mixtures into H₂-air combustion products followed by a downstream supply of preheated dilution air. Such an injection strategy manages the surge of NH_i and O/H radicals while avoiding local temperature peaks by implementing a two-stage ignition pattern of NH₃/H₂ blends combustion in series. NO_X reductions of approximately 48 % and 7 % for$X_{{NH}_3}$ = 50 % and 70 %, respectively, is achieved with the lean-rich-lean combustion strategy compared to the rich-lean strategy over a wide range of global equivalence ratios$({\varphi }_G$ = 0.3 to 0.7). The lean-rich-lean combustion strategy for atmospheric NH₃/H₂-air swirl flames, when integrated with advanced SCR technology, is anticipated to demonstrate a viable approach to achieving zero carbon and low NO_X emissions in industrial applications.