Reaction rates for the oxidation of highly sulphurised petroleum cokes : the influence of thermogravimetric conditions and some coke properties

International audienceThe reaction with air of a large number (22) of high-sulphur petroleum cokes was studied by temperature-ramped thermogravimetric analysis. The kinetic parameters for each coke were established, based on BET surface areas. The oxidation rates (kg C m 22 s 21 atm 21) were found to vary with sample mass. This was a result of limitations on oxygen transfer, despite the small masses and low heating rates used. Limitations were present both externally (from the crucible mouth to the bed surface) and internally (from the sample surface to the bed interior). A method to take these effects into account was adopted, based on an analysis of the relevant diffusion rates. Application of this method reconciled the rate data for four different sample masses, except at high temperatures. The formation of a partially fused ash crust is believed to be the reason for this latter effect. The activation energies of the cokes varied between 195 and 280 kJ mol 21 , and the absolute rates varied by a factor of 10. They were between 1000 and 10,000 times higher than the average reactivity of carbon as reported in the literature. The elevated apparent rates are believed to have two causes, one in the combustion process and the other in the interpretation of the results. The first cause is the strong catalytic effect of the inorganic components, although the ash contents ranged only from 0.3 to 1.5%. The most active metal is vanadium, which is present in significant concentrations. The effectiveness of V 2 O 5 as a gasifying catalyst is believed to be due to its low melting point. Increasing sulphur content in the cokes produces no perceptible change in the combustion rates. The second cause for poor combustion correlation is the inadequacy of BET surface area for expressing combustion rates