Spontaneous Combustion of Coal : a South African Perspective

Abstract

Spontaneous combustion initiates as a result of low-temperature oxidation of organic coal components. The upper limit of the temperature range cited for oxidation is 70°C and the primary requirement is that combustion initiates without the use of external triggers. Oxidation results in the formation of oxygenated hydrocarbon complexes, decomposition of some of these complexes liberates a combination of heat and gaseous phases. In the instance where spontaneous combustion is triggered, this heat is insufficiently dissipated into the atmosphere such that the coal’s temperature surpasses the so-called ‘crossing point temperature’. The nature of the gases evolved is dependent on coal’s overall composition, the ability of oxygen to flow through the coal-body and the maximum temperature of the body. CO2 production requires that oxygen is able to circulate throughout the coal-body whereas CH4 production requires that parts of the body reach high temperatures simultaneous with oxygen starvation. Hydrocarbon complexes may be divided broadly into aromatic and aliphatic groups corresponding to benzene and related compounds, and alkanes (and alkenes and alkynes) respectively. The low-temperature oxidation of these two major groups is specific for the nature of the hydrocarbon involved. Oxidation of benzene and similar compounds require high temperature aqueous solutions as well as catalysation to progress. Conversely, aliphatic compounds readily oxidize liberating heat energy ultimately responsible for self-heating. Hydroxyls, carboxyls, carbonyls and inorganic carbonates thermally decompose at temperatures above those commonly accepted for low-temperature oxidation and subsequent spontaneous combustion. However, peroxides which are oxidation products of some ethers, decompose within the appropriate low-temperature range. Inertinite has been largely taken to be the main maceral group in South African Coalfields and those of other Gondwana Provinces. However, this appears to be untrue for the Waterberg Coalfield in general and several other seams in some of the other coalfields. It is these vitrinite-dominated coals, that based on the knowledge of organic chemistry of vitrinite appear to be most risk of spontaneous combustion. The Highveld Coalfield and the No. 2 and 4 seams of the Witbank Coalfield as well as the Vryheid Formation in the Waterberg atleast, are dominated by inertinite resulting in what has been interpreted to be similar organic chemical makeup. On the other hand, vitrinite is the principal maceral group of samples from the Waterberg specifically the Grootegeluk Formation and the No. 5 seam of the Witbank. This study concludes that the Grootegeluk Formation of the Waterberg, No. 5 seam of the Witbank as well as the Ermelo Coalfield are dominated by aliphatic hydrocarbons and have an increased propensity towards to spontaneous combustion.