New Classification Scheme for Microtextures of Natural Cokes: A Case Study from Heat Affected Coking Coals of Jharia Coalfield, India

摘要

In Jharia coalfield, approx. 1250 Mt of coking coal has been devolatilized due to igneous intrusives (dolerite dykes & mica peridotite sills) and mine fires. An effort has been made to investigate the effect of carbonization vertically and laterally in two severely heat affected coal seams in Ena & Alkusa collieries. Through micropetrographic studies, characterisation of components of normal & heat affected coals have been done, enabling to deduce the level of changes leading to carbonization and subsequent enhancement of rank, etc. The microstructures and microtextures deduced through the above study of heat altered maceral products in-situ, have revealed the nature & extent of heat effect, direction of heating source, quality and quantity of natural coke produced. Based on this approach and survey of existing literature, an effort has been made to classify the microtexture of natural cokes generated due to carbonisation in the coal seams. Changes, due to heat effect on coking coals, whether in-situ or laboratory carbonization are almost similar, barring the effect of pressure. Organic and inorganic constituents undergoing changes at particular temperature and pressure are almost similar in both the above conditions. Several studies have been made on microtextures in coke, but for the natural cokes very scanty effort has been made. As a result still a universally acceptable classification system for classifying the microtextures generated in the coking coal seams due to in-situ carbonization is lacking. Through this work an effort has been made to study the microtextures of natural cokes and devise a new acceptable classification system. Three broad categories i.e. isotropic, semi-isotropic and anisotropic, encompassing several sub-classes have been taken into account. Organic and inorganic inerts, different forms (mosaic & flow) of anisotropic natural coke components, pyrolytic carbon, domains (isochromatic & isometric) and intermediate stage (mesophase spheres) have also been taken into account. Most of the components and microtextures produced in heat affected coking coals due to transformation have been covered under this classification system.