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The relationship between the friability of coal and its rank has a bearing on its tendency to undergo spontaneous heating and ignition (Chakravorty, 1984; Chakravorty and Kar, 1986). The friable, low-volatile coals, because of their high rank, do not oxidize readily despite the excessive fines and the attendant increased surface they produce on handling. Coals of somewhat lower rank, which oxidize more readily, usually are relatively non-friable; hence they resist degradation in size with its accompanying increase in the amount of surface exposed to oxidation. But above all, the primary factor in coal stockpile instability is unquestionably oxidation by atmospheric oxygen whilst the role of any secondary factors such as friability is to exacerbate the primary oxidation effect (Jones and Vais, 1991).

Thus, spontaneous combustion is a rank-related phenomenon. The tendency of coal for self-heating decreases as the rank increases, with lignite and subbituminous coals being more susceptible to self-heating than bituminous coals and anthracite (Pis, 1996). As rank decreases, inherent moisture, volatile matter and oxygen and hydrogen contents increase. Medium- to high-volatile coal with the ability to produce yields of volatile matter content in excess of 18% w/w daf perform a faster oxidation rate coal that produces a lower yield of volatile matter and are therefore more prone to spontaneous combustion. Furthermore, low-rank coals often have a greater porosity than higher-rank coal and therefore more surface area is available for oxidation. Low-rank coals also contain long chain hydrocarbon derivatives, thereby rendering the coal less stable than, for example, the high-rank anthracite coal which has a lower hydrocarbon component. However, the oxidation rate for coals of the same rank may show variety within a wide range.

Coal is highly variable (due to the rank of the coal) in the ability to absorb oxygen (thereby weathering or causing combustion) and oxygen absorption generally decreases with increasing rank, i.e., low for anthracite and high for subbituminous coal and lignite (Fieldner et al., 1945). Oxygen absorption is also higher for those coals with high bed moisture (natural bed moisture, determined as capacity moisture, natural bed moisture, equilibrium moisture (ASTM D1412), oxygen content, and volatile content, i.e., the low-rank coals (Speight, 2005, 2008, 2013).

It is generally (but not totally) accepted that the mechanism of the oxidation of coal oxidation takes place in five steps, each one chemically dependent upon the temperature. These steps are (i) the coal begins to oxidize slowly until a temperature of approximately 50°C (122°F) is reached, (ii) at this point, the oxidation reaction increases at an increasing rate until the temperature of the coal is approximately 100 to 140°C (212 to 285°F), (iii) at approximately 140°C (285°F), carbon dioxide and water vapor are produced and expelled from the coal, (iv) liberation of carbon dioxide increases rapidly until a temperature of 230°C (445°F) is reached, at which stage spontaneous ignition may occur and spontaneous combustion may take place, and (v) at 350°C (660°F), the coal spontaneously ignites and vigorous combustion occurs (Barkley, 1942; Parry, 1942; Roll 1963).

At low temperature, the first step is developed faster than others and is often recognized as the rate determining step. Oxygen molecules are connected to the coal surface physically (adsorption) and reaches to the passing pores by diffusion. In this stage, since the oxide layer formed with the exposure of coal surface to the air prevents the diffusion of oxygen partially, oxidation rate is decreased in time.

The overall reaction is exothermic (releasing 94 kcal/mole of thermal energy) and the heat produced is generally (or should be) carried away from the reaction site by airflow and there is not any significant change in ambient temperature. However, in some cases formed heat cannot be carried away from the environment and the temperature begins to increase. The reaction gets accelerated and spread over with the increasing temperature; produced heat takes the coal to ignition temperature (approximately 175°C, 345°F) in suitable conditions and open flamed fire begins. Thus, without removal of the heat from the stockpile, the oxidation and heat generation can be (and will be) self-perpetuating especially since the rates of organic chemical reactions usually double for every 10°C (18°F) rise in temperature. The time passed from the beginning of oxidation to reaching the ignition temperature is the incubation period.

Furthermore there has also been the suggestion that the heat release which accompanies the wetting of dried (or partially dried) coal may be a significant contributory factor in the onset of burning. Support for such a concept is derived from the observations that stored coal tends to heat up when exposed to rain after a sunny period (during which the coal has been allowed to dry) or when wet coal is placed on a dry pile (Berkowitz and Schein, 1951). Therefore, it may be unwise to stockpile wet coal or to store coal on a damp base if it can be avoided. After a rain or snowstorm a coal pile should be carefully inspected.

In general, the critical temperature for bituminous coal in storage is approximately 50 to 66°C (122 to 150°F). From this temperature, heating will usually increase rapidly and may be unstable after which ignition occurs, unless preventive steps are taken. The basic chemical premise is that for every 10°C (18°F) the rate of a chemical reaction approximately doubles (for coal oxidation, the factor is 2.2). Hence oxidation leading to spontaneous ignition may appear to be (and often is) irreversible unless steps are taken to modify the oxidation reaction and the ensuing liberation of heat.

The petrographic composition of a coal is determined by the nature of the original plant material from which it was formed and the environment in which it was deposited rather than the degree of coalification (i.e., rank). The homogenous microscopic constituents of coal (macerals, named by analogy of minerals in inorganic rocks) can be distinguished in three groups: (i) vitrinite consists of the remains of woody material, (ii) liptinite – formerly called exinite – consists of the remains of spores, resins and cuticles, and (iii) inertinite consists of the remains of oxidized plant material.

At constant rank, as the inertinite content of a coal increases, the self-heating propensity of the coal decreases. The general trend also indicates an increase in self-heating propensity with increasing vitrinite and/or liptinite content. Thus, the ease of oxidation of coal macerals is:

However, coal rank seems to play a more significant role in self-heating than the petrographic composition of coal (Speight, 2013).

Finally, spontaneous ignition and spontaneous combustion of coals also causes a serious problem for coal producers and users during transportation and storage (Chapters 3, 4) (Nugroho et al., 2000). Improvements to low-rank coal are made by either thermal drying or through blend with higher-rank coals. Thermal drying of moist lower-rank coals could increase the calorific value of a product whilst blending of coals of different types offers a greater flexibility and economic benefit. However, the problem of spontaneous ignition and combustion assumes even greater significance since the removal of moisture can enhance the potential for spontaneous ignition and combustion. The risk of spontaneous combustion is also made greater during blending and when storage of such lower-rank coals takes place. This is particularly the case with low-sulfur subbituminous coals which are now used to meet emission limits.

The primary source of heat generation within coal stockpiles is the exothermic low- temperature oxidation reaction, while mass and heat transport play a major role in determining the magnitude of the temperature rise in a given situation. Despite the extensive previous works on spontaneous ignition of coal using various techniques, the effect of particle size in the case of single-type coals on the rate of low-temperature oxidation, remains controversial (Nugroho et al., 2000).