PRESSURE AND TEMPERATURE EFFECTS IN BITUMINOUS COAL PYROLYSIS - EXPERIMENTAL-OBSERVATIONS AND A TRANSIENT LUMPED-PARAMETER MODEL

Total weight loss and yields of tar, light gases and pyridine-extractables of cooled chars from pyrolysis of 63-75 mum particles of Pittsburgh No. 8 bituminous coal, as affected by total pressure (0.1-1 M Pa He) and final temperature (753-1233 K) were measured at a nominal heating rate of 1000 K s-1, using an electrical screen heater reactor. Molecular weight distributions of tars and changes in coal particle morphology during pyrolysis were determined. Above 973 K, with an increase in pressure from 0.1 to 1 M Pa, tar yield declined by 6-7wt% (daf coal basis); increases in the yields of char (approximately 5 wt%) and light gases (mainly C1-C3 hydrocarbons) (approximately 1 wt%) accounted for most or all of this tar loss. Tar MW declined slightly with increasing temperature from -850 to 973K at 0.1 MPa, and from approximately 850 to 1233K at 1 MPa. It also decreased moderately with increasing pressure from 0.1 to 1 M Pa at all temperatures from 850 to 1200 K, except in the neighbourhood of 970K, where no change with pressure was apparent. A mathematical model was developed to predict tar and extractables yields from rapid pyrolysis of softening coals. The model accounts for intraparticle transport and secondary reactions of liquids generated by coal pyrolysis, and satisfyingly captures important features of the current experimental data, e.g. temperature and pressure effects on tar yields, and rapid changes in extractables yields from 760 to 900 K at 0.1 and 1 MPa. The data and modelling results indicate that secondary reactions of liquids within the coal significantly affect tar release at temperatures greater-than-or-equal-to 1000 K.