Internal burning of pulverized semi-anthracite: the relation between particle structure and reactivity

Abstract Measurements have been made of structural properties of particles of pulverized semi-anthracite at various stages of combustion at temperatures in the range 1400–2200 K. Four size-graded fractions of the particles (mass-median sizes 78, 49, 22 and 6 μm) were burned at oxygen partial pressures of about 0.1 and 0.2 atm. The structural properties determined included specific surface areas, helium densities, X-ray diffraction patterns, and pore-volume distributions. The relation of these properties to the temperatures prevailing during reaction and to the degree of burn-off which took place showed that macropores (pores above 0.02 μm width) developed during reaction and that the micropore volume was reduced. The opening up of the macropores results from the combustion process, but the reduction in micropore volume is due mainly to the effect of the higher temperatures on the structure of the solid. The structural data are combined with kinetic data relating to the reaction of oxygen with the particles to determine values of the intrinsic reactivity coefficient R s (the rate of carbon consumption per unit of total internal area per unit pressure of oxygen) under various conditions. R s is independent of particle size and is given by the equation R s ( g/cm 2 s atm O 2 ) = 55 [ −40 000 (RT p ) ] where T p is the temperature of the particles and R is in cal/mol K ∗ . Calculations of the intrinsic reactivity via two routes, treating the pores as having either a unimodal or bimodal distribution of sizes respectively, gave similar values of the reactivity. Values of R s are used to predict, within a factor of two, the results of other workers' measurements of semi-anthracite combustion rates.