The role of H2O in NO formation and reduction during oxy-steam combustion of bituminous coal char

To improve the CO2 capture efficiency and reduce energy penalties in conventional oxy-fuel combustion systems, one possibility is to eliminate the use of recycled flue gas, by using steam and O2. This is known as oxy-steam combustion. However, the role of H2O in NO formation and reduction under oxy-steam combustion is not yet clear. Here, NO emissions during oxy-steam and air combustion of coal char were compared using both experiments and theoretical calculations, with an emphasis of the role of H2O and its resulting radicals in NO formation and reduction reactions. High concentration of H2O promotes both OH radicals and hydroxyl groups formation during oxy-steam combustion, which in turn effects NO formation and reduction reactions and eventually NO emissions. Specifically, NO formation under air combustion is largely determined by HCN conversion to NCO and then NO, whereas under oxy-steam combustion, high concentrations of OH radicals cause HCN to form NH2 and eventually NO as the dominant route. However, the conversion of HCN to NH2 and then NO is faster, and hence these OH radicals accelerate HCN oxidation to NO under oxy-steam combustion. Given that OH radicals and hydroxyl groups have little influence on NO reduction to N2, there is more NO production in oxy-steam combustion that air combustion.