Show spontaneous HEATING OF COAL WHEN IN STORAGE A general review of the results of cher chemical and physical research relative to the causes and prevention of the spontaneous heating of coal in has been published bythe united states bureau of mines as technical paper the subject is one calling 6 for attention at this season of the year when plans are under consideration for winter storage of coal supplies all coals but anthracite undergo 6 some spontaneous heating according to joseph D davis and D A reynolds the authors the liability to self heating is greatest among coals of lowest rank greater care therefore must be used in storing a bituminous sub coal than in storing a high rank bituminous coking coal more pains should be taken to prevent segregation of fine coal in the storage pile and foreign matter should be excluded more carefully oxidation of the coal substance itself is the main cause of spontaneous combustion some of the organic constituents may contribute more to the heating than others but it has not been shown that any one constituent exerts a preponderant influence to the exclusion of the rest how spontaneous heating develops the process of spontaneous heating may be considered to take place in two stages the first stage 6 is operative at room temperature as soon as freshly broken coal is exposed to the air it begins with the physical absorption of oxygen and is continued by the formation of a solid chemical compound of coal and oxygen which is gradually decomposed as the temperature rises decomposition is not complete however until the temperature reaches about degrees P F this first stage of the process generates heat but not as much as the second stage the coal increases in in weight by the amount of oxygen retained the second stage involves the breaking up of the solid compound of coal with oxygen and the formation of the final nal oxidation prod products carbon dioxide carbon monoxide and water with the appalachian coals this stage begins to be appreciable at about 85 degrees F and increases in intensity as the temperature rises with lower rank coals the ginning beginning temperature is lower both stages of the process go on simultaneously therefore from about 85 degrees to degrees F there is no sharp transition from one stage ge to the other the curve showing the relation between duration ration of heating and temperature is continuous the rate of af heating increases logarithmically with the temperature that is no coal has as an inherent property a critical critical temperature at which there is a sharp transition from arom relatively slow to rapid heating when the term critical temperature is used it should be clearly defined as the temperature at which the rate of generation of heat g 0 beccles be cOles greater than its rate of dissipation under certain fixed axed conditions clit ions for other conditions this critical temper attire e will be different for example in a storage pile overheating usually starts aarts in fine coal at a point about 6 feet from the surface lf combustion mb ultimately develops at this point one may regard the temperature of the coal when placed in storage as its critical temperature in so far as critical temperature is determined by conditions that prevail at this particular point at any other point where conditions fineness of coal y rate of air supply etc are different the critical temperature pera ture will be higher results of laboratory studies in which critical of different coals are compared as a measure of relative ease of oxidation are valid only when the test conditions are strictly comparative effects of pyrite and moisture etc pyrite when finely divided can increase the tendency of a coal to heat spontaneously although it has been shown that coals containing virtually no pyrite have fires spontaneously it has also been quite finitely definitely re shown that fine pyrite when present in the coal does increase the rate of oxidation one would not therefore select for storage a coal containing fine pyrite opinions differ as to what effect moisture in coal has on spontaneous heating probably the effect of moisture is determined by the conditions of storage if one could be sure of wetting only those parts of the storage pile where spontaneous heating would otherwise develop he could very probably prevent tit it the heat required to vaporize the water would be more than the oxidizing coal could supply however wetting down the surface of the whole pile changes the conditions of ventilation and may favor heating at points not reached by the water where dangerous heating would not otherwise occur chemical factors other than those touched upon above have little or no influence on the spontaneous heating of coal laboratory studies have indi indicated caled what kinds of coal heat the most readily and their relative tendencies to heat can be measured by several methods the effect of fine coal pyrites etc has also been shown the fact remains however that all bituminous coals have heated in storage when conditions were favorable it is believed therefore that the most fruitful studies of the future will deal with storage conditions work is particularly needed on the effect of air supply for example how should coal be stored to make sure that all parts of the storage pile will lack just enough air to support oxidation or shall have sufficient air circulation to carry off the heat generated |