A study of the reaction order of the NO-carbon gasification reaction

Abstract

The heterogeneous reduction of NO with carbon can be an important process in destruction of NO in combustion systems. In this study, apparent reaction orders of NO reduction by graphite, resin char, and Wyodak coal char were investigated at different temperatures and pressures in the kinetic control regime. The results indicate that reaction orders increase from zero to unity as temperature increases. In the low-temperature regime, reaction rate is determined by a desorption process, which is a very weak function of NO pressure: in the high-temperature regime, direct NO attack, a first-order reaction, determines the overall rate. The behavior at intermediate temperatures is determined by the sum of two processes. This resolves a long-standing discrepancy between the claim that the process is desorption controlled at low temperatures and reports of apparent first order. There is evidence of very long timescale dynamics as-sociated with rearrangement of surface oxides at low temperatures, which can significantly affect the observed rates and therefore observed apparent reaction orders. Previously reported first-order kinetics with cleaned samples at low temperatures reflect complicated transient rather than steady-state behavior. The determination of correct apparent reaction order in fixed-bed reactors is complicated by the long times needed to reach true steady state and by the possibility of surface-catalyzed reaction of NO with product CO.