Effects of preoxidation on the swelling and softening of bituminous coals

Abstract

A study was conducted to evaluate the applicability of a DuPont 942-TMA (Thermomechanical Analyzer) dilatometer system for the characterization of the swelling and softening properties of caking coals. The results of this investigation demonstrate the utility of this dilatometer system. Several parameters clearly defined with this unit are characteristic of the plastic transitions occurring upon carbonization. These include softening, dilation and resolidification temperature and volume contraction, expansion and total dilation values. The transition parameters measured with the 942-TMA unit are influenced significantly by coal particle size, dilatometer load, and heating rate. All transition temperatures exhibit marked increases with increasing heating rate. Softening temperatures decrease with increasing load and particle size. Dilation temperatures are essentially independent of dilatometer load effects and resolidification temperatures are independent of load and particle size over the range of conditions used in this study. Volume contraction and expansion values show an increase with decreasing coal particle size. Volume expansion values also exhibit strong heating rate and load dependencies. The results point up the great difficulty involved in obtaining truly fresh unoxidized coal samples. Storage of fine coal particles for extended periods of time in atmospheres with even slight oxygen concentrations show signs of weathering. Exposure of an HVA coal to dry air at ambient temperature has much less effect on subsequent swelling and softening properties than does exposure to moist air.