Factors Influencing the Time-Resolved Evolution of NO, HCN, and N₂O during Char Oxidation at Fluidized Bed Conditions

There is increasing evidence that HCN is an important intermediate to the production of N₂O during the oxidation of char nitrogen at fluidized bed combustion conditions. The experimental evidence shows that N₂O is formed primarily during oxidation so that the fundamental question is on which fuel N intermediate is formed during char consumption. This paper models first the conversion of char nitrogen assumed to be released as HCN, simulating the homogeneous nitrogen chemistry in the boundary layer of a single char particle. This pathway, which is based on 180 reversible gas-phase reactions and 33 chemical species, assumes that HCN is released at a rate proportional to carbon consumption. The oxidation of HCN occurs in the boundary layer of the particle. An alternative pathway assumes that during oxidation, NO is formed in addition to a cyano species by heterogeneous processes and then both NO and the cyano species react in the boundary layer. The temporal evolution of NO, N₂O, and cyano compounds from both models have been compared against the observations obtained at fluidized bed conditions. The results suggest that solely HCN oxidation on the boundary layer does not predict the most important trends observed for the production of NO and N₂O for char oxidation at fluidized bed combustion conditions. On the contrary, the simultaneous heterogeneous production of NO and a cyano-like species can predict to a certain extent the experimental observations but will require a more detailed understanding of the surface chemistry which controls the temporal evolution of NO and cyano species.