The swelling pressures developed by some New Zealand coals during constant volume carbonisation

A dilatometer type apparatus has been used to measure the swelling pressure developed in a small coal sample during carbonisation at constant volume. A standard test procedure is outlined and the apparatus specified. Two types of results are given. In the first no corrections are made for effects not associated with the coal. These results, characteristic of the apparatus and standard test conditions, can be used as a quantitative measure of the swelling properties of coals, and are reproducible to ± 5%. In the second set of results elimination of such effects is attempted. The values so obtained are for the pressure at a point in a coal change throughout carbonisation. These results show that swelling pressure occurs at lower temperatures than the associated property of volume increase, and is not due to volatile evolution through the plastic coal as the latter property is. The temperatures at which the maximum swelling occurs is rank dependent and has a range 200-350°C in the rank range investigated. The magnitude of this maximum swelling pressure also increases with rank. It is also know that considerable shrinkage occurs in the coal sample after the maximum swelling pressure is developed. The results are correlated with the BSS 1016 Crucible Swelling Test Scale and experimental data is extrapolated in an attempt to obtain equivalent swelling number values for the arbitrary 9⁺, 9⁺⁺, 9⁺⁺⁺ designations at present used for high-swelling New Zealand coals. Values of 11½, 13½, 16½ respectively are predicted. Prediction of equivalent swelling numbers is also made using a percentage swelling – ultimate analysis formula. Similar values are obtained. The correlations show swelling properties to have a second order dependence on swelling number. The results before correction for extraneous efforts show an exponential dependence on swelling number