Non-dissociative and dissociative adsorption of oxygen on carbon: A theoretical comparison with prediction of reaction order

Abstract

For oxygen reacting with carbon during combustion, normalized reaction rates (NRs,N) predicted from the nondissociative form of the Langmuir adsorption isotherm (NLAI) equation are shown to correlate almost linearly with normalized rates (NRs,D) predicted from the dissociative LAI equation (DLAI). The curve obtained was a slight sigmoid with a high linearity showing that a single-valued “universal” multiplier could be used to correct the NLAI predictions to the DLAI values. This conclusion was supported by application to experimental data selected arbitrarily from the literature. The pattern of oxidation reaction “order” with temperature was also examined and was shown to predict a rise of (effective) order with rise in temperature, from zero to unity in a sigmoid form, with the variation only marginally dependent on the LAI equation form. The common assumption of quasi equilibrium between adsorption and desorption with changing temperature was also examined by calculating the equilibration time constant. The values obtained were very sensitive to the rate constants used and to the temperature, but with ranges from 10 ms to one μs. The values are nevertheless consistent with establishing near-equilibration between adsorption and desorption at heating rates in standard flames up to 105°C/s. In high-intensity flames, however, at heating rates of 106°C/s, the reaction may well be out of equilibrium for some chars, and this could affect values of rate constants and reaction order measured in such flames.