INFLUENCE OF THE OXIDATION OF COALS OF DIFFERENT RANK ON COKE MICROTEXTURE AND OTHER RELEVANT PROPERTIES

Oxidation causes an important decrease of the plastic properties of co al during pyrolysis. Consequently, microtexture (direct imaging by tra nsmission electron microscopy of molecular orientation domains) and pr operties of the resulting cokes are modified to a considerable extent: the mean size of the domains is smaller, whereas the mechanical stren gth decreases and carboxyreactivity (i.e. the reactivity of coke towar ds CO2) increases. It is shown that the effect of oxidation on the for mation of coke microtexture depends on the mechanism followed by the c oal during its pyrolysis. For typical, good plastic, coking coals, oxi dation could play three roles: it could favour cross-links between the polyaromatic structural units, limit content in the suspensive medium and decrease thermostability of the latter. Consequently, as coal oxi dation increases, coal plasticity during pyrolysis, the size of the do mains and the mechanical strength of the cokes, strongly decrease; whe reas coke carboxyreactivity increases. In contrast, for higher rank co king coals, the positive role of an initial preferential planar orient ation of the units becomes predominant and the effect of coal oxidatio n on coal plasticity, microtexture and carboxyreactivity of the coke i s less drastic.