The kinetics of combustion of petroleum coke particles at 1000 to 1800 K: The reaction order

Abstract

As part of a program on the combustion of residual materials from coal liquefaction processes, a study has been made of the combustion kinetics of sized fractions of petroleum coke particles (median sizes 85 and 88 μm). Experiments were carried out at 101.3 kPa total pressure and at oxygen partial pressures between 5.1 and 30.4 kPa. Two parameters were determined: the ratio χ which is the ratio of the actual rate of combustion to the maximum possible (i.e. mass transfer limited rate) and the rate coefficient Rc, Rc is the rate of carbon oxidation per unit external surface of the particle per (unit pressure of oxygen)n and n is the apparent order of reaction. χ was found to vary inversely with oxygen pressure, a situation that can be shown analytically to occur for fractional values of n. Rc was found, using a statistical analysis, to be least dependent on oxygen pressure for a value of n close to 0.5. The conditions of the present experiment were close to those used in an earlier study of the combustion of sized fractions of petroleum coke (median sizes 18, 67 and 77 μm), when it was shown by experiment, and by calculation, that the rate was controlled by pore diffusion as well as by chemical reaction, i.e. the reaction was occurring under Zone II conditions. In the present work, particle size and density were each found to decrease with increasing burn-off in the manner characteristic of the Zone II regime. From these considerations, on the basis of the relation between the apparent (n) and true (m) order of reaction in Zone II (n=(m+1)/2) it follows that the true order is close to zero. When the results of the present and earlier work on petroleum coke are combined, the following relation between Rc and particle temperature (Tp) for combustion under the combined rate control by pore diffusion and chemical reaction is obtained. R c = A e x p [ - E a / R ¯ T p ) ] where A = 7.0 ± 0.6 kgC / [m2s(kPa O2)0.5] and Ea = 82,400 ± J/mol.