Experimental and Numerical Study of the Effect of High Steam Concentration on the Oxidation of Methane and Ammonia during Oxy-Steam Combustion

The effect of high H₂O concentration during oxy-steam combustion on the oxidation of methane and ammonia was investigated both experimentally and numerically. Comparison experiments between O₂/N₂ and O₂/H₂O atmosphere were performed in a flow reactor at atmospheric pressure covering fuel-rich to fuel-lean equivalence ratios and temperatures from 973 to 1773 K. Experimental results showed that the presence of high H₂O concentration dramatically suppressed CO formation at temperatures above 1300 K. High H₂O concentrations inhibited NO formation under stoichiometric and fuel-lean conditions but enhanced NO formation under fuel-rich conditions. The chemical kinetic mechanism, which was hierarchically structured and updated, satisfactorily reproduced the main characteristics of CO and NO formation. High H₂O concentrations significantly alter the structure of radical pool and subsequently the formation of CO and NO. Ultralow CO concentrations above 1300 K are attributed to the enhancement of CO + OH ⇄ CO₂ + H by high OH radical concentrations. NO suppressions under stoichiometric and fuel-lean conditions are caused by strong suppression of NH₂ + O ⇄ H + HNO in the pathway NH₂ → HNO → NO. This suppression is due to the lack of O radicals. By contrast, NO enhancement under fuel-rich conditions is caused by the significant enhancement of NH₂ + OH ⇄ NH + H₂O in the pathway NH₂ → NH → HNO → NO. This enhancement is due to the fairly high OH concentration in the O₂/H₂O atmosphere