Coal-Fired Power Generation Handbook. James G. Speight
atmosphere). Both processes result in an alteration of the properties of the coal, that is, there is a decrease in the calorific value of the coal through the introduction of oxygen functions while there is also a very marked, adverse, effect on the caking properties of the coal.
There are indications that the tendency for spontaneous ignition is reduced by thermal upgrading and further decreased with increase in treatment temperature (Ogunsola and Mikula, 1992). The decrease in the tendency to spontaneously ignite appears to be due to the loss of the equilibrium moisture as well as the loss of oxygen functional groups. The loss of the equilibrium moisture is an interesting comment because of the previous comment that the presence of indigenous moisture appears to enhance (i.e., increase the rate of) the oxidation reaction.
Coal tends to spontaneously ignite when the moisture within the pore system is removed, leaving the pores susceptible to various chemical and physical interactions (Berkowitz and Speight, 1973) that can lead to spontaneous ignition. It is a question of degree and the correct order of reactions being in place. It is obvious that the system is complex and, as noted earlier, spontaneous ignition is the culmination of several interrelated chemical and physical events. Finally, it has been estimated that under specific conditions considered subbituminous coal in a stockpile can reach thermal runaway in 4.5 days (Arisoy et al., 2006).
Thus, the results of spontaneous combustion are serious and negative because of (i) damaging economic effects, (ii) detrimental environmental consequences, and (iii) unwanted costs in health problems and, in some cases, human life (Nalbandian, 2010; Sloss, 2015). To prevent such events, the processes that lead to coal self-heating must be understood and precautions must be taken to avoid fires caused by spontaneous combustion. There is general agreement that there is a strong relationship between self-heating rate and coal rank – as coal rank decreases the self-heating rate increases. Thus, spontaneous combustion, or self-ignition, is most common in low-rank coals and is a potential problem in storing and transporting coal for extended periods. Major factors involved in spontaneous combustion include volatile content, the size of the coal (smaller sizes are more susceptible) and the moisture content.
The chemical reaction between coal and oxygen at low temperature is complex and remains not well understood despite many years of research. The gaseous reaction products, evolved during coal oxidation, are primarily carbon monoxide (CO), carbon dioxide (CO2), and water (H2O, as water vapor). Typically, three types of process are believed to occur including physical adsorption, chemical adsorption (which leads to the formation of coal-oxygen complexes and oxygenated carbon species), and oxidation (in which the coal and oxygen react with the release of gaseous products, typically carbon monoxide, carbon dioxide and water vapor). Oxidation is the most exothermic of these processes.
Physical adsorption can begin at ambient temperature where coal is exposed to oxygen whereas chemical adsorption takes place from ambient temperature up to 70°C (158°F). Initial release of oxygenated reaction products starts from 70 to 150°C (158 to 302°F), while more fully oxygenated reaction products occur between 150 and 230°C (302 and 446°F). Rapid combustion takes places over 230°C (446°F). The start of this rapid temperature rise is also known as thermal runaway. The time it takes to reach a thermal runaway stage is called induction time. The induction time can be used to indicate the potential hazard of coal self-heating. The temperature rise from ambient to 230°C (446°F) is a slow process compared to the fast temperature increase after 230°C (446°F), which can lead to major fire hazards and even explosions. In stockpiles, parametric model analysis indicates that parameters such as pile slope, the availability and movement of air through the pile, material segregation, coal reactivity, particle size, temperature and moisture play important roles in the occurrence of spontaneous combustion.
The significance of the greenhouse gas emissions resulting from the oxidation during transport and/or storage, especially CO2 were investigated. However there appears to be no emphasis in research work or published material specifically quantifying these emissions. In summary, heat build-up in coal stockpiles can (i) degrade the quality of coal, (ii) cause the coal to smolder, and (iii) lead to a fire.
4.6 Preventing Spontaneous Ignition
Put simply, coal should be stored in specifically designed bunkers, silos, bins, or in outside piles (CFR, 2012). The most important aspects of coal storage are minimizing the flow of air through the pile, using the first-in, first-out rule of thumb, and minimizing the amount of finely divided coal in the pile. Hot spots should be removed or exposed to the atmosphere to allow cooling. Coal should be compacted if possible to reduce the amount of air in the pile. Water may be used to cool hot spots, but should be used with caution on large areas of hot coal to present accumulations of hazardous amounts of water. Coal should not be stored in outside piles located over utility lines, such as water lines and gas lines.
In order to prevent spontaneous ignition and combustion of coal, it is (first) necessary to understand coal properties and their influence on self-heating and ignition. Next (second), there is a group of additional factors that also play a major role in spontaneous ignition and combustion and these are (i) climatic conditions (temperature, relative humidity, barometric pressure and oxygen concentration), (ii) stockpile compaction, as related to height and method of stockpiling, and (iii) stockpile consolidation, which is influenced by height, the method of formation, and the equipment used for the stockpiling operation.
Spontaneous combustion resulting from spontaneous ignition can be detected fairly early in the development of the fire, i.e., before any obvious smoke and/or flame. Any of the following may assist in early detection, depending upon the particular circumstances. For example, the temperature difference – heat haze and steam/vapor plumes – may be observed on cold mornings and in times of high humidity. Efflorescence caused by the decomposition of pyrites and sublimation of sulfur is a strong indication of heating in pyritic (high-sulfur) coals. Also, hot spots may also be detected by infrared monitoring instruments or photography. Routine surveying of stockpiles using infrared scanning devices is an excellent precaution in situations where spontaneous combustion may be likely to occur.
Spontaneous ignition is a time-dependent phenomenon. Early attention to the potential sources of problems may prevent occurrences of heating progressing to full-scale spontaneous combustion. Examples of commonly used methods of dealing with spontaneous combustion in different circumstances are detailed: (i) tailings, which are the power plant rejects, (ii) dams should be capped with at least three feet of inert non-carbonaceous material, (iii) top soil should be added and the whole area vegetated, (iv) the coarse reject 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 total layer thickness should be no greater than 15 feet and each layer should be covered by a 3-foot thick layer of inert (non-carbonaceous) material and the final landform should be such that erosion and runoff is minimized and new areas of discard coal are not exposed to the atmosphere, (v) spoil heaps in strip-mining should result in accumulations of coal material, particularly if pyritic, being buried under inert spoil – although difficult to achieve, the most reactive material should be enclosed within less reactive material but if this is not possible, 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, (vi) product stockpiles and coal inventory in the cut should not be left longer than the incipient heating period – there is considerable variation in the time taken for heating to occur, but most mines have an understanding, based on experience, of the time limits for the product, and (v) the shape and orientation of stockpiles and dumps is often a critical criterion and a site-specific consideration – when the technique is feasible, considerable benefit can be obtained by building dumps in relatively thin compacted layers and longer-term stockpiles can be further safeguarded by spraying the surfaces with a thin (bituminous) coating to exclude air.
In summary, stockpile management to mitigate spontaneous ignition and combustion can be achieved by actions such as (i) cooling by ventilation or by water spraying to avoid increase of coal stock temperature, (ii) storing the coal in smaller stockpile lots to enable better cooling to prevent heating up of the coal in the stockpile, (iii) reducing access to air, i.e., by storage in compressed