a relative measure of the propensity of coal to self-ignite – in general, the reactivity of coal increases with decreasing rank.
Furthermore, the ability of coal to variously self-heat (spontaneous ignition), emit flammable gases, corrode, and deplete oxygen levels has made the ocean transport of this commodity a particularly hazardous exercise. This is particularly the case in situations where loading is staggered or delayed and the potentially disastrous consequences of a shipboard coal fire can be realized.
Generally, spontaneous ignition (often referred to as self-ignition) occurs when the thermal equilibrium between the two counteracting effects of heat release due to the oxidation reaction and heat loss due to the heat transfer to the ambient surroundings is disturbed. When the rate of heat production exceeds the heat loss, a temperature rise within the material will consequently take place including a further acceleration of the reaction.
The temperature at which the coal oxidation reaction becomes self-sustaining and at which spontaneous combustion occurs varies generally depending on the type (nature and rank) of coal and the dissipation (or lack thereof) of the heat. For low-quality coal and where the heat retention is high, the coal starts burning at temperatures as low as 30 to 40°C (86 to 104°F).
Spontaneous combustion, or self-heating, of coal is a naturally occurring process caused by the oxidation of coal. The self-heating of coal is dependent on a number of factors, some of which are controllable (Table 4.2). Controllable factors include close management in the power plant, of coal storage in stockpiles, silos/bunkers and mills and management during coal transport. Uncontrollable factors include the coal itself and ambient conditions.
Coal reacts with oxygen, even at ambient temperatures and the reaction is exothermic (Speight, 2013). If the heat liberated during the process is allowed to accumulate, the rate of the above reaction increases exponentially and there is a further rise in temperature. When this temperature reaches the ignition temperature of coal, the coal ignites (spontaneous ignition – the onset of an exothermic chemical reaction and a subsequent temperature rise within a combustible material, without the action of an additional ignition source) and starts to burn (spontaneous combustion).
Generally, self-ignition occurs when the thermal equilibrium between the two counteracting effects of heat release due to the oxidation reaction and heat loss due to the heat transfer to the ambient is disturbed. When the rate of heat production exceeds the heat loss, a temperature rise within the material will consequently take place including a further acceleration of the reaction.
Table 4.2 Examples of common methods of preventing spontaneous combustion.
| Factor | Method |
| Tailings (plant rejects) | Tailings dams should be capped with at least 3 feet of inert (non-carbonaceous) material, topsoil should be added and the whole area revegetated. |
| Coarse reject (discard) | Problem material should be placed in layers and compacted using a roller, particularly on the edges of the dump, so that the infiltration of oxygen is minimal. The final landform should be such that erosion and runoff is minimized and new areas of discard coal are not exposed to the atmosphere. |
| Spoil heaps in strip-mining | The sequence of spoiling should result in accumulations of coal material, particularly the coal contains pyrite being buried under inert spoil. Although difficult to achieve, the most reactive material should be enclosed within less reactive material. If this is not possible then rehabilitation of the spoil heaps should take place as soon as possible and a thick layer of softs should be used before topsoil is added. |
| Product (coal) | Product stockpiles and coal inventory in the cut should not be left longer than the incipient heating period. The situation is particularly aggravated by prevailing hot, moist winds and this may lead to a higher risk of spontaneous combustion in the summer months. |
| Stockpile shape | The height of stockpiles and dumps may be a critical site-specific consideration. When the technique is feasible, considerable benefit can be obtained by building dumps in relatively thin compacted layers. Longer-term stockpiles, particularly of product coal, can be further safeguarded by spraying the surfaces with a thin (bituminous) coating to exclude air. |
| Highwalls at surface mines | Coal spalling from the seams should not be allowed to remain against the highwall. If the coal is liable to spontaneous combust, loose coal should be cleared away promptly and/or the highwall reinforced with soft, spoil material if it is to be left for an extended period. At the end of the life of mine complete rehabilitation and closing of the final void should take place. If this is not undertaken the highwall should be effectively sealed with water, clay, or a thick blanket of inert spoil. |
The self-heating of coal is due to a number of complex exothermic reactions. Coal will continue to self-heat provided that there is a continuous air supply and the heat produced is not dissipated. The property of coal to self-heat is determined by many factors, which can be divided into two main types, properties of the coal (intrinsic factors) and environment/storage conditions (extrinsic factors). Self-heating results in degradation of the coal by changing its physical and chemical characteristics, factors that can seriously affect boiler performance.
The tendency of a coal to heat spontaneously in storage is primarily dependent upon the tendency of the coal to oxidize. This in turn is closely correlated with (i) coal rank (the higher the rank, the lower the tendency to oxidize), (ii) the size consistency or distribution of the coal in the pile (small pieces of coal have a higher surface area available for oxygen to react), (iii) the method by which the coal is stockpiled, (iv) the temperature at which the coal is stockpiled, (v) the amount and size of pyrite present, (vi) moisture content and ventilation conditions in the pile, (vii) time in storage, and (viii) the presence of foreign materials. In addition, the variability of coal, added to these factors does not allow accurate prediction of when spontaneous ignition (spontaneous combustion) will occur (Fieldner et al., 1945; Yoke, 1958; Feng, 1985; Medek and Weishauptová, 2004).
Oxidation is an exothermic reaction and, since the rate of a chemical reaction increases for each 10°C (18°F), the reaction will generate heat at a faster rate than can be dissipated or expelled from the stockpile by natural ventilation. Hence, the temperature will rise to a point where spontaneous ignition occurs and combustion ensues.
The risk of spontaneous combustion during final preparation such as in silos/bunkers and mills also presents concerns in some cases. Properties which influence the propensity of coal to self-heat include volatile content, coal particle size, rank, heat capacity, heat of reaction, the oxygen content of coal and pyrite content. The propensity of coal to self-heat and spontaneously combust tends to increase with decreasing rank. Thus, lignite and sub-bituminous coal are more prone to spontaneous combustion than bituminous coals and anthracites.
The temperature at which the coal oxidation reaction becomes self-sustaining and at which spontaneous combustion occurs varies generally depending on the type (nature and rank) of coal and the dissipation (or lack thereof) of the heat. For low-quality coal and where the heat retention is high, the coal start burning at temperatures as low as 30 to 40°C (86 to 104°F).
Thus, the temperature of coal increases due to self-heating until a plateau is reached, at which the temperature is temporarily stabilized. At this point, heat generated by oxidation is used to vaporize the moisture in the coal. Once all the moisture has been vaporized, the temperature increases rapidly. On the other hand, dry material can readily