Spontaneous Combustion in Open Cut Coal Mines

Open-cut coal mining produces large quantities of waste material that must be disposed of around the mine site. Some of this material may be sufficiently reactive to begin to self-heat which can ultimately lead to spontaneous combustion in the spoil piles at some mines. Although a great deal of research into spontaneous combustion in coal and other carbonaceous materials has been conducted over many years, much of this information is not readily available or is too detailed to be of much use to the environmental managers who need to deal with spontaneous combustion in open-cut mines on a daily basis. To address this situation, ACARP Project C17006 was conceived with the goal of summarising the essential information from relevant research into a single, comprehensive reference document that can be readily accessed by personnel working in the open-cut coal mining industry.

This report is the end result of that project. Topics covered in the report include:

• The causes of and the major factors which affect spontaneous combustion in coal stock piles and spoil piles.
• Greenhouse gas emissions from spontaneous combustion.
• Other emissions, especially those relating to occupational exposure of workers.
• Management practices for avoiding spontaneous combustion.
• Techniques for controlling existing outbreaks of spontaneous combustion.

Causes of Spontaneous Combustion

Spontaneous combustion results from self-heating which is caused mainly by the oxidation of coal and other carbonaceous materials. If the heat generated by this reaction is trapped, such as in a spoil pile, the temperature of the material will begin to rise and if unchecked may ultimately ignite; i.e. spontaneously combust.   

The reactivity of the materials sent to spoil varies considerably; coal is the most reactive material whereas materials that contain no carbon, e.g. sandstone, rocks and soil and clay, are inert. In general, the reactivity of a material depends on its carbon content. Large amounts of reactive carbonaceous materials in spoil increases the risk of spontaneous combustion.

Problems of Uncontrolled Spontaneous Combustion

Spontaneous combustion in open-cut coal mines poses a number of potentially serious safety and environmental problems which include:

• Acute safety hazards to mine personnel working near actively burning ground.
• Toxic emissions such as particulates, CO, SO₂, H₂S, polynuclear aromatic hydrocarbons and volatile organic compounds.
• Greenhouse gas emissions of CO₂ and methane.
• Odours
• Destabilisation of spoil piles and long-term problems with rehabilitation.

Greenhouse Gas Emissions

Spontaneous combustion is a potential source of greenhouse gas emissions from open-cut coal mines. Emissions, however, are very variable across the industry; many mines have no instances of spontaneous combustion and therefore no emissions from this source. Some mines, on the other hand have widespread heating and would therefore be expected to have significant emissions. Estimates of emissions from spontaneous combustion are difficult and subject to large uncertainties. Although a number of research projects have been undertaken in this regard, a viable method for measuring greenhouse emissions from spontaneous combustion is not yet available.

Control of Spontaneous Combustion

As with any combustion, spontaneous combustion can be controlled by removing any one of the following:

• Fuel
• Heat
• Oxygen

In spoil piles it is normally unrealistic to remove the fuel and similarly, removal of heat by ventilation or water sprays, for example, is usually impractical. Hence, most strategies for control and prevention of spontaneous combustion focus on removing oxygen, or rather, preventing its access the fuel. This is usually best accomplished by the applying cover layers of inert material which reduce the rate at which oxygen can penetrate the spoil pile. The best cover materials are clays which can retain residual moisture. Moisture is important because it acts to seal off oxygen pathways, thus preventing the oxidation reaction (and hence heating) from proceeding.

Management of Spontaneous Combustion

Spontaneous combustion is best prevented by carefully managing the placement of high carbon content material within spoil piles. It is therefore important that the overall management plan for a mine include appropriate measures for dealing with spontaneous combustion and identify reactive materials and inert materials that can be used as cover layers. Research sponsored by ACARP has led to the development of guidelines that are available to mine operators to assist them to reduce the risk of spontaneous combustion occurring in spoil piles.

Spoil piles should be designed so that the reactive materials are isolated within thick non-reactive layers which prevent the ingress of oxygen. The thickness of the inert layers required depends on the inert material being used.  For clay, layers between 1 and 2 m thick may be enough to stop self-heating whereas for more porous material such as blocky sandstone, much thicker layers of 10 m or more are needed. It is also important to construct spoil piles so that the cover layers are not subject to excessive erosion or other processes that can lead to the formation of cracks and thus allow entry of oxygen.

Mines that consider themselves prone to spontaneous combustion should have a regular program of inspection of spoil piles, where trained operators seek out the telltale signs. These signs include the presence of steam and smoke, surface discolouration and the distinctive odour of spontaneous combustion.