Petrographic studies of Nagaland coals

Abstract

Tertiary coals of Nagaland are found within the Barail group of rocks belonging to Oligocene age and were deposited along a linear belt of overthrusts known as ‘Belt of Schuppen’ trending ENE-WSW. The belt extends from Nagaland through Assam to Arunachal Pradesh. Coal seams were deposited to the northern flank of Naga-Patkai range and under a near shore, deltaic environment. Nagaland has a total reserve of about 22 million tonnes of Tertiary coals. These coals show characteristics of both low and high rank as it contains high volatile content and low ash, low moisture and high sulphur with high calorific value. Such abnormalities in these coals can be attributed to its peculiar condition of deposition or marine influence during deposition. The major drawback of Nagaland coals is the presence of relatively higher amount of sulphur which restricts its use industrially. But they can be utilized for other industrial purposes like cement, brick and fertilizer industries or in domestic purposes and power plants. Sulphur in these coals mainly occurs in the form of organic sulphur with moderate proportion of pyritic and sulphate sulphur. In the present investigation, physico-chemical characteristics and petrographic analysis of Nagaland coals have been reported. Petrographically, it shows dominance of vitrinite macerals with moderate percentage of exinite and inertinite. As the coal is vitrinite rich, it has a high oxygen absorbing capacity which enhances the rate of oxidation and is highly susceptible to spontaneous combustion. High oxygen content leads to the non-caking nature of these coals.