Spontaneous combustion of coal is a phenomenon that happens when the rate of heat generated exceeds the rate of heat loss in heterogeneous and porous coal particles. This propagates internal chemical reactions in the inter-particle channels and microstructure leading to self-heating and spontaneous combustion. For example, in coal stockpiles, spontaneous combustion is a dynamic thermal hazard that results in the loss of coal as a resource and possible caking which creates health and safety issues. These include open fires and the production of noxious gases such as carbon monoxide and nitrous oxide in addition to harmful elements such as arsenic, selenium, mercury and lead. In this GRT industry article, we focus on the causes, effects, preventative measures and best practice dust control for spontaneous combustion of coal. 

What about coal fuels’ spontaneous combustion?

Firstly, let’s mention and focus on what coal fuels spontaneous combustion and self-heating:

  • Coal consists of carbonaceous material. 
  • Heat evolves when moisture bonds with dry coal particles. 
  • Coal has poor thermal conductivity as a result it stores excessive heat.

What causes spontaneous combustion in coal?

The mechanism of stockpiled coal self-heating is mainly propagated by low-temperature oxidation. At low temperatures, coal is oxidized in the presence of oxygen-rich air. Coal and oxygen reaction at low temperature is exothermic. Some reaction sequences are endothermic. Low-temperature oxidation is the main source of heat. After long periods of time, the interior temperature can reach a sufficient level to produce the critical flow of volatiles required for ignition even when the ambient temperature is still relatively low. Other exothermic processes such as microbial metabolism, the interaction of coal with water and oxidation of pyrite also contribute to the self-heating of coal. The stepwise propagation of spontaneous combustion of coal is as follows:

  1. heating and accumulation by oxidation (internal temperature increase)
  2. exhalation of heat 
  3. internal temperature increase and maintain a fixed temperature 
  4. drying of internal moisture and rapidly increasing temperature 
  5. spontaneous combustion.

What are the effects of spontaneous combustion of stockpiled coal at a mine site?

A typical coal mine site runs operations that involve material haulage via conveyor belts to a stockpile after coal is mined from underground operations. These stockpiles consist of gaseous species like oxygen, water vapour and gaseous product liberated by oxidation reactions. The oxidation of coal at different temperatures results in the generation of different gases and soot and loss of the economic value of coal. Take your peak from the table below to find out the effects of spontaneous combustion of stockpiled coal at a mine site. The temperature is the main determinant factor of the effects of spontaneous combustion of stockpiled coal. The higher the temperature the more likely the severity of the effects of spontaneous combustion of stockpiled coal. 

Duration  Temp (ºC) Phenomenon Gas generated 
General Oxidation period  25 – 30 Partial temperature increase  Exhalation of moisture & gas in coal 
30 – 40 Generation of moisture 
40 – 50  Dry, decomposition of wood 
50 – 69 Generating paraffin smell
Early Heating period  60 – 100 Quickly increasing temperature (especially around 84ºC) H2O, CO about 80ºC
100 – 150 Rapid increase in temperature (spontaneous combustion gas) CO2 about 150ºC
150 – 200  Rapid temperature increase  CH4 about 250ºC 
200 – 300 Slight decomposition of coal & more strong simulative smell Ethane & ethylene gas relative to temp increase 
Initial Heating period  300 – 500  Ignition & carbonization of coal Smoke from various gases, H2 gas generated
Later Heating period 500 – 800  2nd decomposition of coal, coalition & gas combustion.  Huge amounts of gas & smoke, Stock coal fire

What are some of the measures used to prevent spontaneous combustion?

In our solution-finding mission, we focus on available methods to prevent the spontaneous combustion of stockpiles. Here as some of the measures: 

Are environmental regulations, health and safety concerns or potential profit loss a concern right now?

Prevention of combustion by air isolation:

  • making low incline plane, compaction of stock by bulldozer and vibrating roller
  • stock altitude below 12 m
  • avoid blending lump coal and fine coal

Don’t stock long-term high volatile coals 

  • The elevated temperature of coal received, or receiving coal in season prone to easy temperature rises (dry season) should be given priority reclaiming over other coals 

Best practice dust suppression 

It is important to implement best practices for dust control in coal mines. Dust suppression on coal stockpiles using water alone accelerates the chances of spontaneous combustion in coal which creates more problems. The first thought is to use water to reduce the temperature of the coal stockpile, but the organic nature of coal and its metamorphosis from plant matter creates a fertile substrate for oxidation which can be propagated by the addition of water. In addition, the temperature differential between the water meant to suppress dust and the coal particles themselves renders the intended outcome of suppressing dust null and void. The water droplets from the fog cannon quickly evaporate on contact with the coal particles on the stockpile. So, in addition to the prevention measures given above, it is best to use a dust suppression product which:

  • resists evaporation  
  • adsorbs available heat
  • has antioxidant properties 
  • is stable at different stockpile temperatures 
  • can be dosed or sprayed onto the stockpile
  • has no effect on the calorific value of coal 
  • binds to soot particles from spontaneous combustion 
  • further isolates air which propagates spontaneous combustion 

In closing, let’s reflect on the requirements for the spontaneous combustion of coal

The stages involved in this process are well represented by the requirements for:

  • a degree of porosity sufficient for air (oxidant) to enter the pile and initiate self-healing via a chemical reaction 
  • the ensuing generation of heat within the stockpile at a rate faster than the loss of heat by mechanisms of conduction within the stockpile, convection from the pile surface to the atmosphere, and radiation from the stockpile surface 
  • the attainment of a critical temperature at which thermal runaway and ignition occur

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