Ammonia chemistry in oxy-fuel combustion of methane

Abstract

The oxidation of NH 3 during oxy-fuel combustion of methane, i.e., at high [ CO 2 ] , has been studied in a flow reactor. The experiments covered stoichiometries ranging from fuel rich to very fuel lean and temperatures from 973 to 1773 K. The results have been interpreted in terms of an updated detailed chemical kinetic model. A high CO 2 level enhanced formation of NO under reducing conditions while it inhibited NO under stoichiometric and lean conditions. The detailed chemical kinetic model captured fairly well all the experimental trends. According to the present study, the enhanced CO concentrations and alteration in the amount and partitioning of O/H radicals, rather than direct reactions between N-radicals and CO 2 , are responsible for the effect of a high CO 2 concentration on ammonia conversion. When CO 2 is present as a bulk gas, formation of NO is facilitated by the increased OH/H ratio. Besides, the high CO levels enhance HNCO formation through NH 2 + CO . However, reactions NH 2 + O to form HNO and NH 2 + H to form NH are inhibited due to the reduced concentration of O and H radicals. Instead reactions of NH 2 with species from the hydrocarbon/methylamine pool preserve reactive nitrogen as reduced species. These reactions reduce the NH 2 availability to form NO by other pathways like via HNO or NH and increase the probability of forming N 2 instead of NO.