Modelling of volatile product evolution in coal pyrolysis. The role of aerial oxidation

Non-isothermal thermogravimetry has been applied to study the pyrolysis behaviour of coal and its changes as a consequence of aerial oxidation. A mathematical model has been developed to calculate the kinetic parameters of the coal pyrolysis process, which takes into account three different groups of thermal decomposition reactions for fresh coal, and two groups for oxidised coals. The agreement between the model predictions, for any heating rate, and the experimental values was fairly good. The volatile matter release profile of fresh coal can be described using three stages: Stage I (250-475 degrees C), mainly light species are liberated; Stage II (475-575 degrees C), characteristic of bituminous coals, high molecular weight species (tar) and hydrocarbons (primary gases) are evolved, which may lead to melting (metaplast); Stage III (>575 degrees C), secondary gases are produced while undergoing ring condensation and leading to the formation of coke. The decrease in the amount of aliphatic hydrocarbons released in the pyrolysis of oxidised coals implies that stage II is not observed, 'mobile' phase is insufficient to allow aromatic planar units to slide over each other, the melting cannot begin, and oxidised coals pass directly to stage III. This causes loss of plasticity and is therefore, responsible for the degradation of coking properties, as a consequence of oxidation. The aromatic fractions, that cannot rearrange, form a poor ordered structure and yield a powdered char. (C) 1998 Elsevier Science B.V.