Petrographic characteristics of the solid fuels of India with particular reference to the coking coals

Abstract

A summary is given of the available petrographic data on Indian lignites and coals, the latter categorised into coking, semi-caking and non-coking fuels, as delimited by vitrinite oil reflectances. The coking coals form one-fifth of the total solid fuel reserves in Tertiary, Mesozoic and Gondwana rock formations. An attempt is made to establish the geological factors responsible for their genesis. The Tertiary lignites occur in the Nichahom (Upper Siwaliks), Neyveli (Miocene), Panandhro (Eocene), Kalol (Eocene) and Palana (Paleocene) lignite-fields. They are of meso-lignitous rank and composed mainly of huminite ( R m < 0.40% ), only the Panandhro lignite being mainly of inertinite. The Tertiary coals of the Jammu coalfields (Eocene) are of hypoanthracitous rank and composed essentially of vitrinite ( R m :2.3% ) and fusinite. The Mesozoic coal deposit at Ghuneri (Cretaceous) mainly comprises vitrinite and subordinate inertinite and is of meta-lignitous rank ( R m :0.52% ). The Gondwana (Permian) coals are composed of variable proportions of vitrinite and inertinite in the main and exinite in very minor amounts, showing marginal variations from one coalfield to another and significant variations from one depositional basin to another. The rank of the coals varies between meta-lignitous and meso-bituminous ( R m :0.42−1.45% ). Evaluation of the Permian coals indicates that the vitrinite oil reflectance determined in vitrinite-rich coal is the main parameter for delineating, predicting and assigning the coking properties. Many of the non-coking coals have a maceral/microlithotype composition which prevents the formation of good quality coke, even from coal of suitable rank. The proportion of vitrite and clarite, together, is higher than that of durite and fusite in the coking coals. In the non-coking coals, the proportion of durite and fusite together exceeds that of vitrite and clarite, and of the “intermediates”. The semi-caking coals are intermediate between the coking and non-coking coals. The vitrinite oil reflectance ranges from 0.85% to 1.50% in the coking coals, from 0.70% to 0.85% in the semi-caking coals, and is less than 0.70% in the non-coking coals. The coking coals were formed at paleodepths exceeding 3 km, other factors such as geological time being equal. It is assumed that the petrographic composition of the coal is a function of the rate of subsidence of the basin. Thus the coking potential can be predicted from the knowledge of the geological history of the Gondwana coal basins.