Coal-Fired Power Generation Handbook. James G. Speight

after freezing and thawing. Wet coal should not be piled or mixed with dry coal. Nor should coal be stored on a damp base. After heavy rains and snows (with accompanying snow melt) the stockpile should be inspected and observed for potential fires.

      Thus, the moisture content of 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 coal 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 contents of the coal, the greater the potential for self-heating 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 (Berkowitz and Schein, 1951; Smith et al., 1991). 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.

      4.4.5 Time Factor

      The oxidation process commences once a fresh coal surface is exposed to air; however, the oxygen absorption rate is inversely proportional to time if the temperature remains constant. Therefore, if the coal is stockpiled so that the temperature in the pile does not rise appreciably insofar as the heat is removed at least as fast as it is generated by the oxidation process, the oxidation rate and, thus, the deterioration or weathering rate of the coal will lessen with time, but nevertheless, deterioration of coal properties during storage may be a major issue for the ultimate use of the coal (Porter and Ovitz, 1917; Vaughn and Nichols, 1985).

4.5 Mechanism of Spontaneous Ignition

      Spontaneous combustion of coal is an important problem in its mining, long-distance transportation, and storage, in terms of both safety and economics. This is because coal reacts with oxygen in the air and an exothermic reaction occurs, even in ambient conditions. A problem arises when the rate of heat release produced by this process is more than dissipated by heat transfer to the surroundings. The heat of reaction accumulates, the reaction becomes progressively faster, and thermal runaway may take place to the point of ignition. It is for these reasons that the phenomenon of spontaneous combustion of coal has been of fundamental and practical importance to scientists.

      There have been considerable difficulties in understanding the mechanism of the spontaneous ignition and spontaneous combustion of coal because of the involvement of many internal and external factors which affect the initiation and development of the phenomenon (Kröger and Beier, 1962; Güney, 1968; Beier, 1973; Chamberlain and Hall, 1973; Didari and Ökten, 1994; Kim, 1997; Kaymakçi and Didari, 2002).

      However, large-scale and laboratory studies of the spontaneous ignition and combustion of coal have shown that high-volatile C bituminous coals exhibited high spontaneous combustion potentials in laboratory-scale tests. The results of these tests showed that the self-heating of a large coal mass depends not just on the reactivity of the coal, but also on the particle size of the coal, the freshness of the coal surfaces, the heat-of-wetting effect, and the availability of oxygen at optimum ventilation rates (Smith et al., 1991; Kim, 1997).

      In addition, several theoretical and experimental studies have been performed on coal spontaneous combustion (Van Doornum, 1954; Nordon, 1979; Schmal et al., 1985; Brooks and Glasser, 1986; Arisoy and Akgun, 1994; Akgun and Arisoy, 1994; Krishnaswamy et al., 1996; Monazam et al., 1998; Arisoy and Akgun, 2000; Akgun and Essenhigh, 2001; Diaconu et al., 2011). The main purposes of modeling studies has been to develop methods for determining the conditions at which the coal pile could undergo spontaneous combustion, to predict the safe storage time under those conditions, and to determine the influences of factors contributing to the spontaneous ignition. However commendable such studies are, it is always necessary that, in order to achieve dependable results, theoretical models can only be successfully used to investigate coal self-heating and self-ignition if the theoretical models are supported by experimental investigations and by field investigations (Arisoy et al., 2006).

      First and foremost, the oxidation of coal is a solid-gas reaction, which happens initially when air passes over the coal surface. Attempts to model this phenomenon have met with some success (Akgun and Essenhigh, 2001; Sensogut and Ozdeniz, 2005). However, there is often the failure to recognize that the phenomenon of self-ignition followed by combustion is site specific and is dependent upon several criteria such as (i) the coal type, (ii) the construction of the stockpile, and, last but not least, (iii) the atmospheric conditions. Indeed, there is no reason to conclude that the self-ignition of coal in a surface stockpile has the same initiation mechanism as self-ignition of coal in an underground coal mine.

      In the process, oxygen from the air combines with the coal, raising the temperature of the coal. As the reaction proceeds, the moisture in the coal is liberated as a vapor and then some of the volatile matter that normally has a distinct odor is released. The amount of surface area of the coal that is exposed is a direct factor in its heating tendency. The finer the size of the coal, the greater the surface area exposed to the air and the greater the tendency for spontaneous ignition.

      Thus, the spontaneous ignition of coal is believed to center around the basic concept of the oxidation of carbon to carbon dioxide:

      This particular reaction is exothermic (94 kcal/mole) and will be self-perpetuating especially since the rates of organic chemical reactions usually double for every 10°C (18°F) rise in temperature. 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 B, 1951). Similar effects have been noted during the storage of hay in the conventional haystacks and ignition has been noted to occur. Thus, any heat generated by climatic changes will also contribute to an increase in the rate of the overall oxidation process. Obviously, if there are no means by which this heat can be dissipated, the continued oxidation will eventually become self- supporting and will ultimately result in the onset of burning.

      Spontaneous ignition and the ensuing combustion of coal is usually the culmination of several separate chemical events and although precise knowledge of the phenomenon is still somewhat incomplete it is gradually becoming known (Kreulen, 1948; Dryden, 1963; Gray et al., 1971; Faveri et al., 1989; Vilyunov and Zarko, 1989; Jones and Wake, 1990; Shrivastava et al., 1992); there are means by which the liability of a coal to spontaneously ignite can be tested (Schmeling et al., 1978; Chakravorty, 1984; Chakravorty and Kar, 1986; Jones and Vais, 1991; Ogunsola and Mikula, 1991; Chen, 1992; Carras and Young, 1994).

      The main factors which have significant effects on the process are (i) the pyrite content of the coal may accelerate spontaneous combustion, (ii) changes in moisture content; i.e., the drying or wetting of coal, have apparent effects, (iii) as the particle size decreases and the exposed surface area increases, the tendency of coal towards, spontaneous combustion increases, iv) lower-rank coals are more susceptible to spontaneous combustion than higher-rank coals – the abnormalities in this relationship may be attributed to the petrographic constituents of coal, and (v) mineral matter content generally decreases the liability of coal to spontaneous heating – certain constituents of the mineral matter, such as lime, soda and iron compounds, may have an accelerating effect, while others, such as alumina and silica, produce a retarding effect (Kaymakçi and Didari, 2002).

      For example, exposure of coal (freshly mined) to air will bring about not only loss of moisture but also oxidation. The latter process, often referred to as auto-oxidation or autoxidation (Joseph and Mahajan, 1991), commences when the coal