Kinetics of Volatile Product Evolution in Coal Pyrolysis: Experiment and Theory

Abstract

Relatively few studies have addressed the kinetics of individual volatile species evolution, particularly at high-temperature, high-heating-rate conditions. In addition to the sparsity of species evolution data, substantial controversy surrounds the wide variation (factors of 1000) in reported kinetic rates for both overall weight loss and species evolution. The aim of this study was to use data from three types of reactors, each with different heating characteristics, to develop a more accurate reactor-independent, heating-rate-independent, and coal-independent set of kinetic parameters. Toward this end, several steps were taken to obtain better measurements of the pyrolysis rates and heat-transfer rates for coal. In addition to improvements to the experiments, improvements were also made to a previously described functional group (FG) model for coal pyrolysis. Two submodels were added to describe (a) the cracking of hydrocarbon species released in primary pyrolysis and (b) the equilibration of oxygen-, hydrogen-, and carbon-containing species at high temperatures. Comparisons of data obtained in the three reactors with the predictions of the improved FG model are presented for six coals. In general, the agreement of the FG model and the data is quite good for all the pyrolysis products at temperatures below 1100 °C. As the temperature increases above 1100 °C, secondary reactions, including soot formation and gasification, begin to play an important role. This leads to overprediction of olefins, CH4, H20, CO2, and tar and underprediction of CO, H2, C2H2, and benzene. The results for weight loss during primary pyrolysis are in reasonable agreement with predictions of a single first-order model for primary pyrolysis weight loss that uses a rate constant k = 4.28 X 1014exp (-54570/RT) s-1. This indicates that the rate of primary pyrolysis is much higher at elevated temperatures (>700 °C) than predicted by commonly used rate expressions. © 1987, American Chemical Society. All rights reserved.