The effect of oxidation on the thermoplastic and coking properties of coal at elevated pressures

Abstract

The effects of oxidation on coal thermoplastic and coke properties have been investigated using a variety of techniques. The methods used include thermogravimetric analysis, high pressure dilatometry, constant shear rate plastometry and optical anisotropy. The apparent effects of oxidation are strongly influenced by the temperature and time of oxidation as well as the experimental conditions under which the coal carbonization behaviour is studied. Oxidation decreases swelling, the maximum rate of volatile evolution and optical anisotropy except under very mild conditions where usually little or no effect was observed. Oxidation can cause the shape of the dilatation versus pressure curves to change dramatically. This may result in the dilatation of an oxidized coal being higher at elevated pressure than at atmospheric pressure in contrast to the results for the fresh coal. However, under the same experimental conditions, the dilatation of the oxidized coal is either identical within experimental error or less than that of the original fresh coal. Comparison of coals with varying extents of oxidation has shown that an increase in the carbonization pressure tends to decrease the apparent effect of oxidation on the coal swelling and coke optical anisotropy index. There are good correlations between the changes in the swelling, maximum rate of devolatilization and coke optical anisotropy parameters, resulting from oxidation indicating the relationship between these concurrent processes during carbonization. The perturbation of the structure of a coal by oxidation in conjunction with the investigation of the dependence of caking, swelling, devolatilization and coke structure factors on experimental conditions provides a clear insight into the development of coal thermoplasticity and coke structure during carbonization and gasification.