189-193It 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.
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
800oC) and at 800oC 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