Study on petrographic characteristics of devolatilization char/coke related to coal rank and coal maceral

Abstract

Char/coke is the medium product of coal gasification, combustion, and metallurgy, whose morphological and optical characteristics are determined by coal rank and pyrolysis conditions. Twelve coals varying in coal rank from lignite to anthracite, collected in China, were used to study morphological and optical characteristics of char/coke upon different pyrolysis conditions systematically, specifically the char/coke structure and texture influenced by coal rank and coal maceral. Coke and char are distinguished by their caking ability. The morphology of the chars derived from low rank coals and high rank coals are closely related to coal maceral composition, whereas the morphology of cokes derived from high volatile bituminous A, medium, and low volatile bituminous is closely related to coal rank. In lignite, subbituminous, high volatile bituminous C, semianthracite, and anthracite, macerals are infusible during the heating process, but cavities generated in the low rank coals are related to the shapes of the liptinite macerals in the raw coals. Morphological changes of chars derived from high rank coals are difficultly to identify microscopically using an oil-immersion objective with a 1-lambda gypsum plate. in addition, the random reflectance (Rr) range of the chars becomes narrower as pyrolysis temperature increases. Overall, the Rr range of cokes is much larger than that of the chars, which is a result of the optical anisotropy of the caking coals. Although the Rr of vitrinite/huminite in the low and high ranks is distinct, the Rr of their chars is always similar when the pyrolysis conditions are the same. The Rr of char/coke is closely related to both coal rank and the final pyrolysis temperature. Longer residence time of the char/coke at the same pyrolysis temperature did not show a relevant difference in the reflectance of char/coke.