NONISOTHERMAL ANALYSIS OF THE KINETICS OF THE COMBUSTION OF COKED SAND

The kinetic parameters for the combustion of carbonaceous residue or coke deposited on the sand during the fluidized bed pyrolysis of tar sands have been determined. Nonisothermal thermo-gravimetric experiments at a fixed heating rate were performed at various oxygen partial pressures on coked sands produced at three different retorting temperatures. Direct Arrhenius plot analysis which invoked a first-order assumption with respect to the unreacted fraction of coke yielded an oxygen partial pressure dependence of 0.5 and activation energies in the range of 127-148 kJ/mol. A second-order assumption with respect to the residual carbon gave an oxygen partial pressure dependence of 0.75 and activation energies of 175-217 kJ/mol. The first-order model was found to be applicable for lower oxygen partial pressures (< 5 kPa), and the second-order model was appropriate for high oxygen partial pressures (> 5 kPa). In the overall range of oxygen partial pressures, the second-order model seems to provide a better fit to the data. These observations were consistent with the chemisorption/surface reaction mechanism proposed for the combustion of small carbonaceous particles. The Freeman and Carrol analysis indicated a reaction order between 1 and 2 with respect to the uncovered coke fraction, with high oxygen partial pressures resulting in reaction orders close to 2 and lower oxygen partial pressures yielding orders nearer to 1.