Pyrolysis of coal at high temperatures

Pyrolysis of coal and of tar produced by the rapid pyrolysis of coal has been studied in small fluidized-bed reactors and in a shock tube. Kinetic parameters determined for the formation of light hydrocarbon gases from the rapid pyrolysis of coal suggest that rate-limiting coupled processes, such as internal mass transfer, have an influence on the formation rates. However, when this limitation is removed, by studying the gas-phase cracking of tar free from the influences of the original coal or char, only the light olefins that apparently arise from a single type of functional group give activation energies indicative of bond-breaking reactions. Other products, which appear to have several different chemical sources, exhibit low activation energies of formation (110-140 kJ/mol). Results of a detailed study of tar composition as a function of temperature in the fluid-bed reactor were consistent with the proposal that long-chain polymethylene groups are the source of the olefins and that secondary cracking reactions of the tar are an important source of simple aromatics, polycyclic aromatic hydrocarbons and carbon oxides. Results are also presented to show the effect of preparation conditions on the combustion kinetics of chars produced by rapid pyrolysis. At combustion temperatures of 700°C, the reactivities of the chars show an inverse relationship to their preparation temperatures but at higher combustion temperatures, these differences largely disappear.