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Natural Oxidation of Coal

AUTHOR: Robert M. Davidson
DATE: September 1990

One of the fascinating problems of coal oxidation and weathering is the curious mixture of subtle and dramatic effects. That such a reaction, scarcely detectable by some chemical techniques, can lead to spontaneous combustion or severe degradation of properties still provides a considerable challenge to coal science. Indeed, it has been claimed that "The best option for dealing with oxidized coal is to avoid it."

This review examines the effects of natural weathering and low temperature (less than 50 ~ C) oxidation on the chemistry, structure and properties of coal. The extent of oxidation of a coal sample is difficult to assess. The techniques used to test whether a coal is oxidised and the extent of any oxidation are discussed. Many of these tests are empirical, perhaps even arbitrary, and provide little understanding of the chemistry.

Because coal is a heterogeneous material, factors other than the coal's organic matter and oxygen can affect the oxidation reaction. These factors, including minerals, moisture, thermal chemistry and particle size and surface area, are considered. However, it is obvious that the parameters affecting the natural oxidation are far from fully understood. This is followed by an examination of the structural changes oxidation causes in the organic matter. Here there is more agreement but some unresolved questions remain.

Finally, the way in which oxidation affects the value of coal is assessed. Oxidation generally has a deleterious effect on the way coal behaves in combustion, beneficiation and processing (especially carbonisation). Among the conclusions, the importance of coal's mineralogy and physical structure are noted. Some suggestions for further research are made.