Reactivity of pyrolysis chars related to precursor coal chemistry

Changes in the air reactivity of sets of chars obtained from pyrolysis of five coals were determined. The effect of coal rank and pre-oxidation was assessed. The reactivities of the chars were determined by isothermal thermogravimetric methods performed at 500 degrees C, temperature at which the reaction of chars with oxygen is kinetically controlled. The values of maximum reactivity, R-max, the reactivity at various conversions, R-X, the semi-reaction time, t(0.5), and the reaction time, t(0.9), were evaluated. Chars from high-rank coals showed lower reactivities and longer reaction times. Char reactivity increased with an increase in the pre-oxidation; the higher the O/C ratio of precursor coal, the higher the reactivity of the correspondent char. The presence of oxygen-containing groups in precursor coal is likely a key factor in char reactivity, related to the carbon active sites formed after decomposition of C(O) complexes during coal pyrolysis. Pre-oxidation caused an increase in the rate of heat flow during the reaction of chars. However, differential thermal analyses indicated that the calorific values of the chars are similar, regardless of rank and oxidation of precursor coals. Thus, the main factor responsible for the loss of calorific value of coals as a consequence of oxidation is the decrease in the energy produced from the burning of the volatile matter and not from the residual char. (C) 2000 Elsevier Science B.V. All rights reserved.