Page 383 - Advanced Mine Ventilation

352 Advanced Mine Ventilation

Table 21.3 Coal in Eq. (21.2)

Rank of Coal SHT (8C) E (K Cal/mol) A (K/S)
Lignite:
Beulah-Zap 60 16.8 5.1 10 6
Hvc
No. 6 Illinois 70 14.5 9.2 10 5
F Seam 45 12.6 3.1 10 5
hvA
Clarion, Ohio 75 20.4 3.4 10 8
Lower Sunnyside 85 21.1 5.9 10 8
Pittsburgh 90 21.1 4.4 10 8
mvb
Coal Basind1 120 e e
lvb
Pocahontas#3 110 20.8 1.1 10 7
Mary Lee 135 20.2 1.4 10 7
Blue Creek 135 22.6 4.6 10 8
Anthracite >140 e e

SHT, self-heating temperatures.
Modified from Smith A, Lazzara CP. Spontaneous combustion studies of US coal, USBM R.I. 9079 1987:28.

Typical values of SHT, E, and A for various coals are given in Table 21.3. R70
values, E, and A all can be used to predict the liability of coal seam to spontaneous
combustion.

21.1.2 Crossing-Point Temperature Index

A typical apparatus used to obtain this index is described by Gouws [5]. The apparatus
consists of three cells containing finely sieved coal and three cells containing a ther-
mally inert reference material (calcined alumina) immersed in an oil bath that is heated

at a constant rate of 1 C/minute. Oxygen is supplied to the coal cells by means of an air
compressor at the rate of 400 cc/min of air. The temperatures of the coal and the inert
reference material are measured at 15 s intervals.
Initially, the reference material tends to heat up faster than the coal, which loses
heat due to the evaporation of moisture and desorption of gases. At higher tempera-
tures the coal heats up faster than the reference material and a “cross-over tempera-
ture” is reached. A typical crossing-point temperature plot for a coal sample is shown
in Fig. 21.5. In general, higher-ranked coals have a higher crossing-point
temperature.