Coals high in carbonate minerals show abnormally high VM on dmmf basis. This abnormality has been shown to be due to the presence of iron carbonate. Effects of the presence of different carbonate minerals on VM have been estimated, and a formula is advanced for estimating VM on dmmf basis of a coal having a particular distribution of carbonate minerals: VM on dmmf basis = As analysed VM -0.1 Ash -1.3 CO, x 100 100 -(MM + Moisture)
Volatile matter is an important parameter for assessing coal quality. Its correct estimation is also essential as it can help in correlating many other properties of coal, i.e. rank, caloritic value, total carbon and hydrogen, coke yield, petrographic composition, etc. Many formulae have already been developed based on these correlations.
While calculating I'M on unit-coal basis, mineral matter and moisture are considered as diluents and it is presumed that they do not affect the pyrolitic behaviour of a coal to the extent of altering the I'M expressed on unit-coal basis. Thus, the present concept is that the pyrolysis of coal and that of the associated mineral matter proceed independently and that the products of pyrolysis from these two do not interact.
Unless there is an unusual variation in petrographic composition, coal samples drawn from a seam at any particular place show more or less the same T/M when expressed on unit-coal basis. In a very thick seam (say 45 m) it may vary up to 2%. But it is observed that wherever there is high carbonate content in a coal, the unit-coal T/M is found to be much higher than that of other coal samples having little or no carbonate (Table 1). This cannot be properly explained with the help of the present concept outlined above.
So one is led to think that CO2 evolved owing to decomposition of carbonate minerals reacts with the coke during carbonization and possibly produces thereby some CO. The consequent result is that a lower yield of futed carbon and a correspondingly higher yield of VM are eventually recorded. However, according to the findings of Roy Bardhan and Guptar, this penomenon may as well be explained by the fact that a part of the Fe0 formed from the decomposition of ferrous carbonate is reduced to metallic iron.