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
