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346                                               Advanced Mine Ventilation

         Table 21.1 Number of Fires in Underground Coal Mines [1]


          Source                           Number of Fires  Percent (%)
          Electrical Short/Arcing          28                32
          Flame Cutting/Welding            17                20
          Spontaneous Combustion           15                17
          Conveyor Belt Friction           15                17
          Frictional Ignitions and Other Sources  12         14
          Total                            87               100



         equipment minimizing the risk of fire. Flame cutting and welding is mostly a house-
         keeping issue that can be made safe by training and enforcement of laws. Belt fires
         are mainly due to friction and will be discussed under frictional ignitions. Major
         remaining issues are the following:

         1. Spontaneous combustion.
         2. Frictional ignitions.
         3. Gas and dust explosions.

            These subjects will be discussed in detail in this and the following chapters.


         21.1   Spontaneous Combustion of Coal

         It is generally accepted that the spontaneous combustion propensity is related to the
         rank of coal: the lower the rank, the more the coal is liable to spontaneous heating.
         Other factors that may impact this phenomenon include heat of wetting, temperature
         of coal, ventilation pressure differential, oxygen content of coal, particle sizes, geolog-
         ical anomalies, and mining methods. It is always advisable to determine the liability to
         spontaneous heating in the laboratory prior to planning the mining method for the coal
         seam.
            At present, there are three different techniques to measure a coal seam’s liability to
         spontaneous combustion:
         1. Adiabatic heating oven.
         2. Temperature differential methods (also called crossing-point temperature method).
         3. Oxygen sorption methods.

         21.1.1   Adiabatic Heating Oven

         Smith and Lazzara [2] ran an experiment on 24 coal samples, ranging from anthracite
         to lignite, to determine the minimum temperature necessary to start the process of
         spontaneous combustion and called it, “minimum self-heating temperature” or SHT.
         Fig. 21.1 shows a schematic of the experimental setup.
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