Effect of carbon monoxide on the rate of oxidation of charcoal, graphite and coke in carbon dioxide

Abstract

The rate of oxidation of coconut charcoal, electrode graphite and metallurgical coke granules in CO 2-CO and CO 2-CO-He gas mixtures at various pressures was measured within the range of 700°–1400°C. The rate measurements were made under conditions such that almost complete internal burning prevailed, at temperatures below 1000° or 1200°C depending on the type of carbon. Carbon monoxide is found to have a pronounced retarding effect on the rate of oxidation in carbon dioxide. This retarding effect is attributed to the strong chemisorption of carbon monoxide on the pore surface of carbon. The experimental results (for the case of almost complete pore diffusion) can be interpreted reasonably well in terms of a reaction mechanism involving two consecutive rate-controlling reactions in series: 1. (1) dissociation of CO 2 on the surface of carbon and 2. (2) formation of CO on the surface of carbon. The reactions in series are analogous to resistances in series; the rate is controlled by the reaction step which exerts most of the resistance to the overall reaction. At low CO contents, resistance of (2) ⪢ resistance of (1) and at CO contents above about 10 per cent, resistance of (1) ⪢ resistance of (2). The apparent heat of activation for rate (1) is 60 kcal/g-atom C and for rate (2) is 69 kcal/g-atom C. Carbon monoxide has a two-fold poisoning effect: 1. (a) covering of the surface site due to strong adsorption and 2. (b) increasing the activity coefficient of the activated complex for the dissociation of CO 2; hence CO changes the rate-controlling mechanism from (2) to (1). The temperature dependence of the rate parameters is obtained from the rate data for these two mechanisms. In addition, adsorption isotherms are derived indirectly from the rate data for adsorption of CO on the surface of carbons.