Studies on Coal Reactivity: Kinetics of Lignite Pyrolysis in Nitrogen at808°C

Abstract

Lignites pyrolyzed in nitrogen have provided direct evidence for a two-component model of coal constitution. The pyrolysis was studied in an electrically heated 5.1 cm i.d. laminar-flow furnace using four closely sized cuts (58, 64, 86 and 180 microns). Pyrolysis was under isothermal conditions, at 808 °C, for periods up to 1 sec, following a heating period of 95 msec, at 8000 °C/sec rate of heating. Pyrolysis times in excess of 200 msec were obtained by recycling, experiments having shown that this did not affect the kinetics. Variations of weight loss with time were found to be in agreement with equations based on a two component model in which two independent parallel reactions are assumed to occur. Notably, weight loss declined approximately exponentially with time, with a sharp break in the lines obtained for pyrolysis times in excess of 200 msec. Slopes of the lines yielded values of the relevant kinetic constants, with Component “1” decomposition dependent on particle size over the range studied, but the slower-reaction of Component “II” being independent of size. The ratio of Component I to Component II is estimated at about 10 to 1. In other results, the proportionality between weight loss and ASTM volatile loss of the char, as first reported by Badzioch and Hawksley was reconfirmed, with Q factors for the coals studied of 1.8 and 2