DECREASES IN THE SWELLING AND POROSITY OF BITUMINOUS COALS DURING DEVOLATILIZATION AT HIGH HEATING RATES

Concern about comparability and validity of different methods for producing coal chars for reactivity experiments has led to research on the effect of devolatilization conditions on the char physical and chemical structure. Particle diameter and porosity changes during devolatilization significantly affect char oxidation rates. In particular, physical properties of chars prepared in drop tube reactors differ greatly from chars prepared in the flat flame burner experiments. Recent data indicate that the presence of oxygen in the gas atmosphere has no effect on swelling until char oxidation has begun. The present research concentrates on the effects of heating rate, particle temperature and residence time on the swelling and porosity of a plastic coal, and compared these results with a nonplastic coal. The heating rate at which the transition from increasing swelling to decreasing swelling occurs is approximately 5 x 10(3) K/s for swelling coals. Swelling coals also reach a maximum porosity near this heating rate. At low particle heating rates swelling gradually increases versus heating rate in contrast to a decline in the swelling at high heating rates in a narrow heating rate region of 2 x 10(4) to 7 x 10(4) K/s. Nonswelling bituminous and lignite coals continue to increase in porosity beyond the heating rate of 2 x 10(4) K/s.