Page 385 - Advanced Mine Ventilation

354 Advanced Mine Ventilation

C R ¼ Residual concentration of oxygen in the air inside the glass vessel after time, t; it
is given by:

O 2
C R ¼ as a fraction.
1 CO 2 CH 4
O 2 ,CO 2 , and CH 4 represent the concentration of the respective gases after 24 h
absorption. The higher the value of OAI for a coal, the more it is liable to spontaneous
combustion.

21.2 Detection of Spontaneous Combustion

Monitoring of mine atmosphere by either continuous monitors or by handheld instru-
ments can help detect spontaneous heating in its early stages and allow preventative
actions to be taken. Monitoring systems and instruments are already discussed in
Chapter 19. Besides monitoring air quality, there are some physical indicators of
spontaneous combustion.

21.2.1 Physical Indicators of Spontaneous Combustion

They can be classified as follows:
Sweating
Haze
Smoke
Heat, and
Smell.
Sweating or condensation of water on cooler surfaces is the earliest indication of a hot
spot in the mines. The next stage may be the appearance of haze, which is condensation
of volatile gas given out by coal in air. As the temperature rises, smoke may become
visible. Smoke is mostly ultrafine dust particles (less than 1 mm) or soot. The coal tem-
perature has to be above 300 C before smoke can be seen.

As heating produces smoke, it also creates an acrid smell. Expert nose can detect
this characteristic smell at parts in billion level. But it is a very individualistic criterion.
The smell is very similar to the smell given out by brick kilns.
In many cases, the smell is absent and spontaneous combustion is detected by heat. A
pillar with glowing fire can testify that there is a fire. Some infrared sensors can detect
incipient heating before the flame appears. Their use is strongly advocated.
21.2.2 Gas Analysis for Detection of Spontaneous Combustion

As coal heats many gases are given out in sequence. The following is an approximate
chronological list:

1. Carbon dioxide: may be difficult to draw a conclusion because mine air has CO 2 .
2. Carbon monoxide: this is accompanied by CH 4 ,H 2 ,C 2 H 4 , and C 2 H 6 .
3. Pyrolysis products of coal: higher hydrocarbons as propane and butane.