Product compositions and formation kinetics in rapid pyrolysis of pulverized coal—Implications for combustion

Abstract

Previous work in this laboratory on the rapid pyrolysis of a lignite and a bituminous coal has been extended to include time-resolved measurements of volatiles composition. Approximately monolayer samples of small coal particles supported in a metal screen were rapidly heated in helium to a desired temperature, held there for a desired time, and then rapidly cooled. Volatiles were rapidly diluted and quenched, and all products were collected. Sample weight loss and product compositions were determined as a function of heating rate (270-10,000°C/s), peak temperature (up to 1100°C), holding time at peak temperature (0–30s), pressure (0.0001–100 atm), and particle, size (74–1000 μm). Volatile yields increase montonically as temperature increases, with the evolution of different products or groups of products occurring in sequential but overlapping temperature intervals. The lignite volatiles are dominated by CO, CO2 and H2O, while the main volatiles from the bituminous coal are tar and light hydrocarbons. Heating rate has negligible effect on total yields or product distributions from either coal over the range studied. Pressure and particle size have little effect on product yields from the lignite. In contrast, for the bituminous coal, increases in pressure or particle size result in less tar and more char and hydrocarbon gases, apparently reflecting mass transport limitations and the occurrence of secondary reactions within or on the coal particles. The kinetics of the evolution of different products from the lignite is successfully modeled by one, two or three first-order decomposition reactions. For bituminous coal, a similar model which includes evaporation and diffusion of tar along with pyrolytic and secondary reactions is found to be consistent with the observed behavior. Use of the data to calculate the heating value and surface flux of volatiles under conditions pertinent to pulverized coal combustion indicates, in agreement with previous work, that combustion on the particle surface may in some cases precede the occurrence of a volatiles flame away from the surface.