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The spontaneous ignition of coal (also variously referred to as the spontaneous combustion or autogenous heating of coal) has been recognized as a hazard for some time to the extent that, in the early years of the 20^th century, guidelines were laid down for the strict purpose of minimizing the self-heating process (Haslam and Russell, 1926) and have been revised since that time (Allen and Parry, 1954).

Self-heating in coal stockpiles occurs naturally, especially in low-grade coal with a high content of volatile matter, although several contributory proeprties have been identified (Table 4.1). These properties primarily influence the rate of heat generation during the self-heating of coal. Since most of the combustible matter in coal is carbon, when coal is stored in an atmospheric environment, the carbon slowly oxidizes to form carbon dioxide and carbon monoxide. The oxidation reaction with hydrogen in the coal forms water and the production of both water and carbon gases in the coal will contribute to the self-heating. These reactions produce heat; since coal is a relatively good insulator, much of this heat is trapped, increasing both the temperature and the rate of oxidation. Depending on how the coal is stored, heat production may substantially exceed heat loss to the environment, and the coal can self-ignite.

The self-heating occurs when the rate of heat generation exceeds the rate of heat dissipation. Two mechanisms contribute to the rate of heat generation, coal oxidation and the adsorption of moisture. The reactivity of coal is a measure of its potential to oxidize when exposed to air. The moisture content of a coal is also an important parameter in the rate of heat generation of the coal. Drying coal is an endothermic process, in which heat is absorbed, and the temperature of the coal is lowered. The adsorption of moisture on a dry coal surface is an exothermic process, with a heat producing reaction. If it is partially dried during its mining, storage, or processing, coal has the potential to re-adsorb moisture, thus producing heat. Therefore, the higher the moisture content of the coal, the greater the potential for this to occur. The most dangerous scenario for spontaneous combustion is when wet and dry coals are combined; the interface between wet and dry coal becomes a heat exchanger. If coal is either completely wet or completely dry, the risk is substantially reduced. In general, the moisture content of coal increases with decreasing rank.

Table 4.1 General properties that contribute to spontaneous combustion.

Property	Comment
Moisture content	Related to the amount of drying and rewetting occurs during handling.
Friability	Related to the extent of size degradation occurs.
Particle size	Related to the exposed surface reaction area.
Rank	Related to the percentage of reactive components that tend to decompose as the coal rank increases to bituminous coal and anthracite.
Pyrite	Concentrations greater than 2% w/w have high effect.

Friability and previous oxidation of the coal are also important factors in the self-heating process. The friability of the coal is a measure of the coal’s ability to break apart into smaller pieces. This exposes fresh coal surfaces to air and moisture, where oxidation and moisture adsorption can occur. Previous oxidation makes coal more friable. Although the oxidized matter is less reactive, the porous nature of the oxidized coal makes the coal more susceptible to air and water leakage when exposed to higher pressure differentials, such as in a pile or bunker. The oxidation of sulfur in pyrite is also a heat producing reaction. The heat generated can cause the temperature of the surrounding coal to increase, thus increasing the rate of oxidation. Also, as it oxidizes, the sulfur expands, causing coal degradation to occur.

The actual chemical process that results in self-heating is the low temperature oxidation, which is an irreversible exothermic reaction. The negative effect of self-heating is the decrease of coal quality (calorific value). If the self-heating is not controlled then a thermal avalanche type process occurs since increased temperature leads to a higher reaction rate. Spontaneous self-heating is a major problem during the transportation and storage of coal since the process, if not controlled, results in fire and important production loss.

Indeed, the phenomenon of spontaneous ignition is not limited to coal but has also been observed in other piles of organic debris (1983; Gray et al., 1984; Jones, 1990; Jones et al., 1990). However, By understanding how and why coal spontaneously combusts, coal users can plan, predict, and avoid accidents which could be costly in terms of coal lost, emissions of pollutants, and, ultimately, risk to the health and safety of those involved in the industry (Sloss, 2015).

Large coal stockpiles, especially those stored for long periods, may develop hot spots due to self-heating which, in some cases can lead to spontaneous combustion. The self-heating process depends on many factors including coal rank, temperature, airflow rate, the porosity of the coal pile, ash and moisture content of the coal, humidity as well as particle size of coal. Emissions of molecular hydrogen, carbon monoxide and low molecular weight hydrocarbons can also accompany the oxidation process. These processes raise environmental and economic problems for coal producers and consumers, who transport and store large coal piles (Nalbandian, 2010).

Thus, in the process, coal reacts with ambient oxygen, even at ambient temperatures and the reaction is exothermic. If the heat liberated during the process is allowed to accumulate within a stockpile due to inadequate ventilation, the rate of the oxidation reaction increases exponentially leading to an even more rapid rise in temperature. When the temperature within the stockpile reaches the ignition temperature of coal – typically on the order of 420 to 480°C (790 to 900°F) but under adiabatic conditions where all heat generated is retained in the sample, the minimum temperature at which a coal will self-heat is 35 to 140°C (95 to 285°F) (Smith and Lazzara, 1987) – the coal ignites (spontaneous ignition). This represents the onset of an exothermic chemical reaction and a subsequent temperature rise within the combustible material, without the action of an additional ignition source (spontaneous combustion) (US DOE, 1994; Medek and Weishauptová, 1999; Lyman and Volkmer, 2001).

Chemically, combustion falls into a class of chemical reactions categorized as oxidation, which is the chemical combination of a substance with oxygen or, more generally, the removal of electrons from an atom or molecule. Oxidation reactions are almost always exothermic, or release heat. Many materials react with oxygen to some degree. However, the rates of reactions differ between materials. The difference between slow and rapid oxidation reactions is that the latter occurs so rapidly that heat is generated faster than it is dissipated, causing the material being oxidized (coal) to reach its ignition temperature. Once the ignition temperature of coal is reached, it will continue to burn until it or the available oxygen is consumed.

Self-heating occurs when the rate of heat generation exceeds the rate of heat dissipation. Two mechanisms contribute to the rate of heat generation, coal oxidation and the adsorption of moisture. The reactivity of coal is a measure of its potential to oxidize when exposed to air. The mechanism of coal oxidation is not completely understood. The minimum self-heating temperature of the coal is sometimes used as a relative indication of the reactivity of the coal. There are various methods used to determine a minimum self-heating temperature of the coal, but determinations of the data all require running a test in real time and monitoring the temperature of the coal as any reaction occurs. These tests are typically 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 ignite following the sorption of water – dry coal in storage should not be kept in a damp place because this can promote self-heating. Therefore, it is recommended that dry and wet coal be stored separately.

Complications may also arise in the case of coals with high moisture and sulfur content and those with tendencies to degrade when exposed to aerial oxygen. This is a critical issue in the case of low-rank, high-sulfur coals. Lignite and subbituminous coal are difficult to store without occurrence of spontaneous combustion, in contrast to anthracite where the potential for spontaneous ignition to occur is minimal.

Thus, oxidation of the coal substance proper is the primary cause of spontaneous heating. This heating, however slight, is caused by slow oxidation of coal in an air supply which is sufficient to support oxidation but not sufficient to carry away all heat formed and proceeds whenever a fresh coal surface is exposed to air (Berkowitz and Speight, 1973).

Thus, coal presents hazards between the time it is mined and its eventual consumption in boilers and furnaces. Below are listed some of the characteristics of the factors that contribute to spontaneous ignition/combustion in coal stockpiles and which can be used to evaluate the potential for coal fires and as guidelines for minimizing the probability of a fire.