Loss of gasification reactivity toward O2 and CO2 upon heat treatment of carbons

The effect of heat treatment on the reactivity of different carbons toward oxygen and carbon dioxide was studied for heat-treatment temperatures in the range 500-2000°C. Results were fitted to a kinetic model of annealing based on an nth order power law reaction kinetics. The sensitivity of the model to the choice of the ultimate char reactivity upon prolonged heat treatment was assessed. The thermodeactivation model was used to fit other experimental data of annealing available in the literature. As far as the loss of oxyreactivity is concerned, different thermodeactivation patterns (reflected by a change of annealing kinetic parameters) are observed at heat-treatment temperatures below and above about 1200°C, depending on the carbon. Activation energy of thermodeactivation toward oxygen is larger under severe heat-treatment conditions than under moderate ones for the coals; it is relatively insensitive to heat-treatment temperature for the petroleum coke. On the contrary a single set of annealing kinetic parameters fairly well represents the loss of carbon reactivity toward carbon dioxide throughout the temperature range investigated and for all the carbons. It is speculated that changes of the turbostratic carbon structure and modifications of the ash constituents, affecting the heterogeneous reaction of carbon along different pathways for the two oxidants, may both be relevant to the observed behavior.