Reduction of nitric oxide on the char surface at pulverized combustion conditions

This study presents an experimental evaluation of the rate of nitric oxide reduction on the char surface. It addresses the claim that the rate for the destruction of nitric oxide on the char surface has been underpredicted due to char deactivation in the process of char formation. Experiments conducted with chars produced in situ, char previously produced at pulverized combustion conditions, and char produced with an activated carbon showed the existence of three phenomena during the reduction of nitric oxide: (1) the homogeneous reaction of the volatiles that evolved after the injection of the solid into the reaction with nitric oxide; (2) the accumulation of nitrogen on the char surface, probably through the formation of C(N) complexes, and (3) the heterogeneous reaction of nitric oxide with char. The nitric oxide reaction with char was found to be dominant at pulverized combustion conditions (T > 1500 K) with a rate within I order of magnitude of that predicted by an expression recommended in previous studies. At fluidized-bed conditions (T < 1300 K), the second phenomenon may be important and traditional rate expressions may underpredict the nitric oxide conversion to N-2 when used at combustion conditions when the nitric oxide-char reactions begin immediately after char formation and before a pseudo-steady state is reached. For the solid used in this study, the increase in nitric oxide reduction due to formation of C(N) sites was a factor of 2-3.