Spontaneous-combustion propensity of New Zealand coals under adiabatic conditions

Pulverised New Zealand coal samples have been tested from an initial temper ature of 40 degreesC and reacted adiabatisally in an oven with oxygen to pr ovide a full temperature history of auto-oxidation up to the self-sustained process of combustion. This procedure produces a self-heating rate index, R-70, calculated as the ratio of the time taken to reach 70 degreesC (degre esC/h). The R-70 index is a measure of the coal's propensity to spontaneous combustion. R-70 values for New Zealand coals are much higher than any pre viously published results. They show a rank dependence, whereby subbitumino us coals have the highest propensity to spontaneous combustion (14.91-17.23 degreesC/h). A lignite sample has an R-70 value of 7.76 degreesC/h, and hi gh-volatile bituminous B coals have R-70 values of 0.31-2.23 degreesC/h. Sa mples stored for 2 years show the same rank trend. The nature of this trend is most likely a function of the internal surface area of the coal that go verns the available sites for oxidation. Calculating the Suggate rank; for any New Zealand coal can be used to rare its propensity to spontaneous comb ustion. Resin bodies in the subbituminous coal show no propensity to sponta neous combustion. (C) 2001 Elsevier Science B.V. All rights reserved.