A transient kientics study of the combustion reactivity of a coal char

Transient kinetics experiments have been used to characterize the combustion reactivity and the reactive surface area of char obtained from a bituminous coal. They consisted of isothermal-pulsed combustion of char samples with reiterated combustion and desorption stages. Experiments showed that the combustion rate passed through a sharp maximum at the begnning of each combustion stage, to decline afterwards as the carbon conversion degree increased. Combustion rate and reactive surface area have been determined as functions of the carbon conversion degree and of the oxygen partial pressure during the combustion stage. Direct correlation between combustion rate and reactive surface area is satisfactory at lowoxygen partial pressure; it becomes poorer when data at higher pressure are included in the comparison. A descriptive model based on a Monte Carlo simulation technique has been developed to shed light onsome features of transient and steady behaviour observed in experiments. In particular, the relevance of nonuniform reactivity of the exposed surface to the combustion behaviour and to overall char reactivity is assessed on the basis of qualitative comparison between model calculations and experimental results. Simulation results suggest that a pronounced "screening effect" of more stable surface oxides with respect to more reactive ones takes place. It should be responsible for the sharp maxima observed in the combustion rate vs time experimental relationship at the beginning of the combustion stages. The model predicts the steady combustion rate to be lower than that obtained by averaging the intrinsic reactivity of different classes of active sites on the basis of their concentrations in the unconverted solid. © 1994 Combustion Institute.