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404 Advanced Mine Ventilation

composition that can predict the status of mine fire, including the final question: “Is the
fire dead yet?”

24.3.1 Indices Used to Predict the Status of Fire Behind Seals

There are many indices that help decide if the fire is dead or not. The most commonly
used are the following five:

1. CO Index
2. CO 2 Index
3. JoneseTrickett Ratio
4. Litton Ratio
5. Hydrocarbon ratio
The CO Index and JoneseTrickett Ratio are already discussed in Chapter 21, Spon-
taneous Combustion of Coal. They are normally used when a gas sample is collected
from a ventilated airway but in the author’s experience (about 20 mine fires), they are
also useful when the sample comes from the sealed area. Besides these indices, a
“trend analysis” for all gases at each sampling location is also very important. The con-
centration of gases, as well as the resulting indices listed above, are plotted against time
to determine if the fire is dying or dead and if mine recovery should start.

24.3.2 Sampling and Gas Analysis Procedure
Where possible, real-time instruments are used to provide instantaneous results. These
instruments are handheld stain tube chemical sensors, infrared sensors, or electrochem-
ical sensors permanently installed in the mine airways.
The other and more reliable technique is to withdraw gas samples from boreholes
drilled all around the fire area, mine shafts, vent holes, etc. A gas laboratory is imme-
diately established at the mine to analyze gas samples.
A gas chromatograph (GC) is the most commonly used apparatus for gas analysis.
Instantaneous data obtained with different instruments should be confirmed by GC
analysis because the latter is more reliable.
There are two types of GCs based on the gas concentrations. The flame ionization
detector-type GC is used for low gas concentrations. It is very sensitive and can detect
very low (up to 1 ppm of CO and H 2 ) concentrations of many gases. It, however, de-
stroys the sample. The thermal conductivity detectoretype GC is used when gas con-
centrations are high. It is not very precise, but it does not destroy the gas sample.
The gas samples are collected by either pulling the gas into an air-evacuated cylin-
der or by forcing the gas sample into a container with an air pump. Air-evacuated
cylinders are made of plastic or glass and have a volume of 10e22 cc. The air evac-
uation is usually 95%. They usually deteriorate with age and retain some CO. Their
shelf life is 1e2 months [5]. Some operators use syringes that can be cleaned and
reused. Even Mylar bags have been used to collect large volumes of samples.
Some good rules for proper gas sampling procedure are listed below: