Page 392 - Advanced Mine Ventilation

Spontaneous Combustion of Coal 361

feet per day of methane and gob air contains 80% or higher methane-reducing oxygen
content to less than 4%. With such high emission of methane, gob methane drainage is
almost essential. The gob wells must be monitored to make sure that excessive air is
not produced.

21.3.4.2 Induced Inertization
Most mildly gassy and moderately gassy coal seams do not produce enough methane
to inert the gob area. Induced inertization becomes necessary in these mines. Nitrogen
is most commonly used for this purpose. Carbon dioxide can also be used but it is
expensive and requires expert handling when in liquid form. Nitrogen is cheaper
and much safer in underground mines.
Compressed or liquid nitrogen was purchased and service companies would inject
them in mines from surface. Lately, a much cheaper source of nitrogen has been devel-
oped. It is called “pressure swing adsorption” or PSA technique. Atmospheric air is
compressed to about 100 psi and made to pass through especially designed molecular
sieves that can separate oxygen from nitrogen because of a slight difference in the size
of their molecules. Successive filtration can yield 98% nitrogen at practically the same
pressure as the inlet pressure. The low pressure nitrogen is conducted in PVC pipelines
and delivered 1000 to 1500 feet behind the active longwall faces. Vertical gob wells on
older panels can also be used to monitor the gob gas composition and inject nitrogen, if
necessary.

Problems

1. Calculate the SHT for three different ranking coals using the data in Table 21.1 and comment
on the effect of rank on SHT.
2. Calculated the Graham Index and comment on the status of spontaneous combustion using
the air analysis data given below:
CO ¼ 500 ppm
CO 2 ¼ 3%
O 2 ¼ 17.5%
N 2 (þAR) ¼ 79.5%
H 2 ¼ 200 ppm
Also calculate JoneseTrickett ratio and determine what is burning.

References

[1] NIOSH. Analysis of mine fires for all US underground and surface coal mining categories
(1990-1999). IC 9470. 2004. p. 36.
[2] Smith A, Lazzara CP. Spontaneous combustion studies of US coal, USBM R.I. 9079.
1987. p. 28.