AN EXPERIMENTAL AND MODELING STUDY OF HEATING RATE AND PARTICLE-SIZE EFFECTS IN BITUMINOUS COAL PYROLYSIS

Heating rate (10-20 000 K/s) and particle size (63-75 mum at 1073 K; 106-125 mum at 1073 and 1273 K) effects on tar yields and total weight loss from pyrolysis of Pittsburgh No. 8 bituminous coal in an electrical screen heater were measured under 1 atm of helium. Tar molecular weight distributions and gravity column elution behavior were also determined. With increasing heating rate, weight loss increased for both particle sizes. For the 106-125-mum particles, tar yield plateaued or maximized at about 1000-2000 K/s. Tar yields at 1073 K were similar for both particle sizes at 10 K/s, but larger for the 63-75 mum particles at all other heating rates. Tar yields and intraparticle liquids inventories were mathematically modeled in terms of the kinetics of liquids generation by coal pyrolysis, depletion by intraparticle secondary reactions, diffusion through molten coal, and release to ambient by diffusion of vaporized liquid (tar) through a circumambient gaseous boundary layer. Experimental and predicted effects of heating rate, particle size, and temperature on tar yields were in reasonable agreement when the model allowed for liquids depletion by two parallel secondary reactions, e.g., cracking and repolymerization.