Effect of Gamma Irradiation on the Thermal Decomposition Behaviour and Electrical Resistivity of Some Indian Lignite and Bituminous Coals

Abstract

It is known that irradiation of coal by high-energy and ionising gamma rays affects the structure of coal, as a result of which its physical and chemical properties are also likely to be - and indeed are, affected or modified. Very little work has been on this aspect of Indian coals, which are quite different from their foreign counterparts. From this point of view, over the past some years, the effect of gamma irradiation on different types of Indian coals and lignites have been extensively investigated at the Central Fuel Research Institute, Dhanbad. The work reported here is a part of those studies. Since thermal decomposition behaviour of coal is quite important for the selection of a suitable type of coal to be used in the coal hydrogenation process and also since the electrical resistivity of coal too is an important property which is imperatively considered in the non-fuel use of coal - especially for the manufacture of carbon artifacts (e.g. graphite, electrodes etc.), it prompted us to investigate as to whether the irradiation of coal with gamma rays affects its thermal decomposition behaviour and its electrical resistivity and, if so, to what extent and how will it be useful technologically. Presented in this paper are the results of investigations on the effect of gamma irradiation on the thermal decomposition/transformation of Rajasthan lignite and bituminous coal of Raniganj (West Bengal) coalfield and also on the electrical resistivity of high volatile, non-coking Hariajam (Gopinathpur seam) coal of Raniganj coalfields.<br>

The results of these investigations have evidenced that at a maximum gamma dose of 120 Mrad, the thermal decomposition behaviour of both the Rajasthan lignite and bituminous Raniganj coal is significantly affected and eventually their transformation takes place. A comparison of the differential thermal analysis (DTA) curves of the unirradiated and irradiated coal and lignite samples reveals that in the DTA curves of irrradiated lignite and bituminous coal, additional endothermic and exothermic peaks appear.Whereas exothermic peaks correspond to solidifying temperature, the exothermic peaks connote temperature of maximum fluidity and decomposition and vapourisation of lower molecular weight constituents. Likewise, the results on the effect of gamma irradiation on high volatile, non-coking Hariajam coal clearly shows that the electrical resistivity of this coal is greatly

affected. It has been found that in the beginning, upto a gamma dose of 150 Mrad, the electrical resistivity decreases, but at doses higher than this, the electrical resistivity gradually increases at all the temperatures studied (viz. 200, 500. and 800^oC) and at 800^oC it shoots up abruptly, attaining maximum value. The results of these investigations have been explained on the basis of degradation and polycondensation as also the cross-linking of molecules that take place in coal under the influence of gamma rays. In conclusion, it is inferred from these studies that whereas the changes/modifications brought out in the thermal decomposition

behaviour of Rajasthan lignite and bituminous Raniganj coal after gamma irradiation can be advantageously exploited in the use of these coals in the coal hydrogenation process, the increased electrical resistivity of non-coking Hariajam coal will he potentially useful in the manufacture of coke and other carbon artifacts like graphite and carbon electrodes.