Unraveling the NO reduction mechanisms occurring during the combustion of NH3/CH4 mixtures

Abstract

The interaction between NH3, CH4 and NO under different conditions of interest for combustion applications is analyzed, from both experimental and kinetic modeling points of view. Reduction of NO by reburn and by SNCR (selective non-catalytic reduction) strategies is evaluated, through an extense systematic study of the influence of the main variables of interest for NO reduction, by means of laboratory flow-reactor experiments at atmospheric pressure. Variables analyzed include: temperature in the 700 to 1500 K range, air stoichiometry from fuel-rich (λ = 0.31) to fuel-lean conditions (λ = 2.21), NH3/CH4 ratio in the 0.4 to 10.78 range, NH3/NO ratio in the 0.49 to 2.60 range the, and CH4/NO ratio in the 0.37 to 1.98 range, dilution level, and bath gas by using nitrogen and argon, the latter to allow the precise determination of nitrogen balances. Results are interpreted using a literature reaction mechanism, together with reaction pathway analysis tools, and the main findings are discussed. Results indicate that ammonia promotes the conversion of methane, while methane inhibits the conversion of ammonia, due to the competition for radicals of both components in the mixture. The interaction of ammonia and methane implies that the reduction of NO by NH3/CH4 mixtures is comparatively lower than the reduction obtained by NH3 and CH4 independently. Implications for practical applications of the reduction of NO by the studied mixtures are discussed.