Determination of reactivity parameters of model carbons, cokes and flame-chars

Reactivity profiles are defined and measured with thermogravimetry for a dense metallurgical Longyear coke, a polymer-derived porous active carbon, Carboxen 1000, and three flame-chars, Illinois #6, Pittsburgh #8 and New Mexico Blue #1. For each sample it is found that the reactivity profile can be separated into a chemical kinetics expression and a structural profile describing reactivity evolution during conversion. Determinations of the reactivity profiles have been performed for reaction rates varying up to a factor of 100 and for burn-off from 20 up to 80%. The apparent reaction order and activation energy in oxygen are estimated for each sample over a 100 K temperature range (cases vary from 573 to 885 K) and a maximum oxygen partial pressure range of 0.01-1 bar. For the different fuel samples reaction orders range fromn =0.65 to 0.78, and global activation energies are found in the narrow range of 130-133 kJ/mol. For Carboxen n=0.91 and E=146 kJ/mol. The reactivity differences between the coal chars are proposed mainly due to variations in the physical structure. Over the pressure-temperature domain examined reactivity varies considerably, but the structural profile is approximately invariant, i.e. each sample exhibits consistently the same structural evolution for a broad span of kinetic conditions. The structural profile is different for each carbonaceous material. In the 20-80% burn-off range, Carboxen undergoes a four-fold increase in reactivity, while for coal-derived chars the reactivity decreases by a factor ranging from 2 to 5. These results emphasize the importance of a sound understanding of the structural evolution of a given sample in evaluating and using kinetic parameters. Global kinetic parameters are obtained by isolating kinetics from structural effects. A flexible reactivity measure is used to rank fuels with very different structural profiles, at varying levels of burnout, temperature and oxygen partial pressure.
Keyword: TGA,Reactivity,Coal