Basil Beamish

Spontaneous-combustion propensity of New Zealand coals under adiabatic conditions

Pulverised New Zealand coal samples have been tested from an initial temperature of 40 degreesC and reacted adiabatisally in an oven with oxygen to provide 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 (degreesC/h). The R-70 index is a measure of the coals propensity to spontaneous combustion. R-70 values for New Zealand coals are much higher than any previously published results. They show a rank dependence, whereby subbituminous 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 high-volatile bituminous B coals have R-70 values of 0.31-2.23 degreesC/h. Samples 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 governs the available sites for oxidation. Calculating the Suggate rank; for any New Zealand coal can be used to rare its propensity to spontaneous combustion. Resin bodies in the subbituminous coal show no propensity to spontaneous combustion

Adiabatic testing procedures for determining the self-heating propensity of coal and sample ageing effects

An adiabatic testing procedure for determining the self-heating propensity of coal has been evaluated using New Zealand coals. Repeatability of the self-heating rate (R-70), Obtained by the method, is less than +/-5% of the average value for tests performed consecutively over 3-4 days. For longer time periods, even using samples under controlled storage, the self-heating rate value decreases noticeably due to pre-oxidation effects. This pre-oxidation effect can be described by the equation R-70(t) = at(b), where t is the time in days, b a constant dependent on storage method and particle size and a the R-70 value of fresh coal. When stored samples are used for analysis, knowledge of this relationship can be used to extrapolate a fresh coal R-70 value by doing repeated tests over time

Combustion behaviour of Bowen Basin coals

Tightly constrained experimental thermogravimetry proves to be effective for characterising the effects of rank and maceral variations on Bowen Basin coal combustion behaviour. These coals show an increase in char burnout temperature with rank for both dull, inertinite-rich and bright, vitrinite-rich coals. Most dull coals have higher char burnout and peak combustion temperatures than their bright rank equivalents. The maximum rates of combustion for dull coals are lower than their bright counterparts, with the difference between the two varying with rank.

Kinetic parameters associated with self-heating of New Zealand coals under adiabatic conditions

Abstract Adiabatic self-heating tests were carried out on five New Zealand coal samples ranging in rank from lignite to high-volatile bituminous. Kinetic parameters of oxidation were obtained from the self-heating curves assuming Arrhenius behaviour. The activation energy E (kJ mol−1) and the pre-exponential factor A (s−1) were determined in the temperature range of 701−140ºC. The activation energy exhibited a definite rank relationship with a minimum E of 55 kJ mol−1 occurring at a Suggate rank of ~6.2 corresponding to subbituminous C. Either side of this rank there was a noticeable increase in the activation energy indicating lower reactivity of the coal. A similar rank trend was also observed in the R70 self-heating rate index values that were taken from the initial portion of the self-heating curve from 40 to 70ºC. From these results it is clear that the adiabatic method is capable of providing reliable kinetic parameters of coal oxidation.