Читать книгу Coal-Fired Power Generation Handbook - James Speight G., James G. Speight - Страница 107

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 reacts with oxygen (of the 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.