Coal-Fired Power Generation Handbook. James G. Speight

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).

      4.4.2 Pyrite and Other Minerals

      Sulfur, once considered a major factor, is now thought to be a minor factor in the spontaneous heating of coal. There are many very low-sulfur western subbituminous coals and lignite that have high oxidizing characteristics and there are high-sulfur coals that exhibit relatively low oxidizing characteristics.

      However, pyrite (FeS₂) evolves heat from aerial oxidation and was believed to be the cause of the spontaneous heating of coal. The heat generation locally promotes the self-heating process of coal but the reaction products have a greater volume than the original pyrite, with the result of breaking open any coal in which they are embedded and thus exposing a greater surface of coal to the air.

      The interaction of pyrite (FeS₂) with water and oxygen is also an exothermic reaction and results in the formation of iron sulfate (FeSO₄) and sulfuric acid (H₂SO₄). Thus, if coal is stored in the open, rain will most likely increase the rate of this reaction and for the same reason water-flooding (to extinguish fires) may also increase the rate of the reaction.

      Pyrite, through its transformation to bulkier materials, has also been cited as responsible in some cases for slacking and the resultant production of fines. For these reasons, coal users are generally reluctant to stockpile high-sulfur coal for extended periods of time (Berkowitz and Schein, 1951). However, stockpiling low-sulfur content of coal is no guarantee of safe storage – coal with low sulfur content can also spontaneously ignite.

      Many minerals affect the oxidation rate to some extent, either accelerating or inhibiting it. Alkali chemicals are capable of accelerating the rate of the oxidation reaction while borates and calcium chloride can act as retardants of the reaction rate. The oxidation process is also promoted if ankerite [a calcium, iron, magnesium, manganese carbonate mineral of the group of rhombohedral-shaped carbonates, i.e., Ca(Fe.Mg.Mn)(CO₃)₂] is a constituent of the coal mineral matter. In contrast to ankerite, the presence of silica and alumina minerals tends to retard the oxidation reaction.

      4.4.3 Coal Size and Stockpile Ventilation

Oxidation increases with increasing fineness (decreasing size) of the coal pieces and the rate of oxidation of coal with oxygen of air is proportional to the specific internal surface (or external surface area). For the internal surface, the proportional coefficient at low temperatures is the cube root but analysis also shows that both rate and extent of oxidation increase with the decrease in particle size, until a critical particle diameter is reached, below which the rate remains fairly constant. For the external surface area, the surface area of a ton of half-inch particles is greater that the surface area of a ton of one-inch particles of coal.

      The natural ventilation in coal storage piles is generally adequate to remove sensible heat as fast as it is liberated in the oxidation process. However, in situations where the ventilation is adequate to maintain oxidation but inadequate to dissipate the heat produced, the coal absorbs the heat, causing a rise in the internal temperature of the stockpile. A chain reaction follows in which the oxidation rate increases with increasing temperature and, if the temperature rise is allowed to proceed unchecked in the stockpile, the ignition temperature of the coal will eventually be reached and the stockpile will begin to burn. To an external observer, it will at first appear that the coal is smoldering die to emission of light barely visible smoke but in reality, the fire inside the stockpile may be vicious and vigorous.

      Run-of-mine coal (ROM coal) is difficult to store because of the large percentages of fines mixed with the lump, of which some may be minerals that promote the oxidation and, thus, spontaneous ignition. On the other hand, there is usually less danger in storing lump coals that have been double-screened or closely sized. The uniform pieces of coal are honeycombed with passages through which air can circulate freely and carry off the heat generated. However, in stockpiles of coal fines sufficiently compacted so as to exclude air, the potential for spontaneous ignition is diminished. But it must be recognized that stockpiles of coal (whatever the size of the coal) are subject to some degree of oxidation and, when the auspices are correct, spontaneous ignition.

      If coal is to be stored for prolonged periods of time, the pile should be constructed so that air (in the case of fine coal or mixed sizes such as run-of-mine coal) is excluded. On the other hand, if the coal is to be stored as lump coal, air should be allowed to circulate freely through the pile.

      The total exposed surface area of the coal is of importance in that the more area exposed, the better the chance of oxygen interacting or reacting uniting with the coal and any heat liberated in a given time for a given weight of coal will be higher (Elder et al., 1945; Berkowitz and Speight, 1973).

      When coal stockpiles are constructed by allowing mixed varied size coal to fall, roll, or slide, the larger pieces tend to collect at the bottom outside of the pile and the fines will collect at the top and inside of the stockpile. As a result, air will move easily through the outer parts of the stockpile but with much less freedom in the interior of the stockpile. Such a pile will allow the development of hot spots which can (or will) lead to spontaneous ignition of the coal with subsequent combustion of stockpile.

      4.4.4 Moisture Content

Moisture present in the coal is known to influence spontaneous heating in a stockpile insofar as the moisture affects ventilation (air flow) and pyrite reactivity. The higher the inherent (equilibrium) moisture content, the higher the heating tendency. The lower the ash free Btu, the higher the heating tendency. Coal with high oxygen content typically has a higher tendency to self-heat than coal of lower oxygen content. Thus, there appears to be an interaction between oxygen functions in the coal and aerial oxygen leading to a higher potential for formation of the coal-oxygen complex as the first stage of the self-heating process leading to a higher tendency for spontaneous ignition of the high-oxygen coal.

      The effect of moisture on the self-ignition is twofold, thus (i) the vaporization of moisture consumes energy and hence the ignition process is impeded, (ii) promotion of self- ignition by the wetting of materials prone to evolution of heat during the moisture adsorption has been observed (Gray, 1990).

      In addition to the heat of wetting, moisture simply blocks the access of oxygen through the coal pores. The water vapor diffusing outwards through the pores reduces the oxygen partial pressure and hence lowers the rate of the reaction or the polar water molecules attach to the reactive sites in coal (Jones, 1998).

      The heat of condensation of coal in a stockpile can cause a rise in temperature in the pile, which is dependent upon the coal rank (Berkowitz and Schein, 1951). In addition, if dry screened coal is used as a storage-pile base for a shipment of wet coal, ignition can (or will) occur at the wet-dry interface of the two loads (Berkowitz and Speight, 1973). However, the more rapid oxidation occurring in high-moisture coals may be basically a function of coal rank rather than moisture content, since low-rank (high-oxygen) coal is usually also higher in moisture content.

      Wetting and drying coal repeatedly may make it more susceptible to combustion. The actions of water may break up