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Effect of coalification on spontaneous combustion of coals

Authors
Journal
International Journal of Coal Geology
0166-5162
Publisher
Elsevier
Publication Date
Volume
16
Identifiers
DOI: 10.1016/0166-5162(90)90047-3
Disciplines
  • Earth Science

Abstract

Abstract A preliminary survey of the frequency of occurrence of fire due to spontaneous combustion in the different seams of the Raniganj Coalfield showed the possibility of a relationship between coalification and spontaneous combustion of coals. In this context, a systematic study was undertaken. For the purpose of this investigation, samples were collected from six coal seams (numbered B-I to B-IV, in ascending order) of the Barakar Formation (Lower Permian), and ten coal seams (numbered R-I to R-X, in ascending order) of the Raniganj Formation (Upper Permian) of the Raniganj coalfield of India. To measure the coalification, the rank was determined in terms of maximum reflectance in oil of vitrinite ( R 0max) of all the coal samples. Maceral composition and proximate composition of the coal samples were also determined. The crossing-point temperature was taken as a parameter for measuring spontaneous combustion of coals. R 0max was found to decrease almost progressively from the lowermost coal seam, i.e., from the oldest coal seam to the youngest coal seam (Fig. 1). The volatile-matter (d.m.f.) content of the coal seams was also observed to increase progressively from the lower coal horizons to upper coal horizons (Fig. 1). From the relationship between volatile-matter content and R 0max of the coal samples, the coal seams may be grouped, depending on the coalification, into three horizons: lower (less than 36% V.M.; R 0max. 0.90–1.25%), middle (36–41% V.M.; R 0max. 0.90–0.70%), and upper (41–45%; R 0max. 0.70–0.50%). The coal seams of the lower horizon are in the Barakar Formation, whereas the middle and upper coal horizons are in the Raniganj Formation. When the crossing points are plotted against the volatile-matter contents of all the coal seams, the same groupings appear to persist (Fig. 2). The results indicate that the coal seams of the lower coal horizons (B-I to B-VI) have a high crossing point between 155°C and 185°C (Fig. 2). Therefore, these coals can be predicted to be least susceptible to spontaneous combustion. The coal seams of middle horizons (R-I to R-IV) have a range of crossing points between 140°C and 155°C (Fig. 2). These seams may be taken as moderately prone to spontaneous heating. The crossing points of the coal seams of the upper horizons (R-VII to R-X) have a substantially low crossing point between 125°C and 140°C (Fig. 2). These coal seams appeared to be highly susceptible to spontaneous combustion. These three types of grouping, as noted from the results obtained in the laboratory, can be corroborated with the field studies of frequency of occurrence of fire due to spontaneous combustion in different seams of the Raniganj Coalfield over a period of eight years (1973 to 1980). It will be evident from the frequency of occurrence of fire as shown in Figure 3 that fire did not occur due to spontaneous combustion in the coal seams of the lower coal horizon, i.e., in the coal seams of the Barakar Formation. These coal seams appeared to be least susceptible to spontaneous combustion (Fig. 2). Maximum number of fires occurred in coal seams of the upper horizons (R-VII to R-X, Fig. 3). The crossing points of these coal seams are low between 140°C and 125°C. Thus, they are highly susceptible to spontaneous heating, which can be surmised from Figure 2. The coal seams I-R to R-VI appeared to be moderately prone to spontaneous combustion as indicated in Figure 2. This is evident also from the frequency of fire occurrence as shown in Figure 3. Besides rank, as evidenced from reflectance studies, the amount of vitrinite and exinite contents of the coal seams also influeced the spontaneous combustibility of the coal seams. This is evident from the gradual increase of vitrinite and exinite contents in the coal seams from bottom to top, following the stratigraphic sequence of the Raniganj Coalfield (Firg. 4). Out of the four seams (R-VIII to R-X) of the upper horizon that are highly susceptible to spontaneous combustion, the frequency of occurrence of fire has been noted to be maximum in seam R-VIII (Fig. 3). A study of this coal seam using the scanning electron microscope (SEM) revealed the highly porous nature of the seam, which contributed to the maximum occurrences of fire in the seam due to spontaneous combustion. Earlier, a few workers suggested that aerial oxidationn of pyrite is the cause of spontaneous heating of coal. An attempt was made to study the effect of pyrite on the spontaneous heating of the coal seams. The results showed that pyrite of the Raniganj Coalfield had no influence on the combustibility of the coal seams. It may also be that as the pyrite contents were low (0.23 to 2.43%) it had noo effect on the spontaneous combustibility of the Raniganj coals. From the foregoing results, it isi concluded that the proneness to spontaneous combustion of the coals is related to coalification. As the coalification increases, the intensity of spontaneous combustibility decreases gradually from highly susceptible to moderately susceptible to least susceptible to the spontaneous combustion stage.

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