Page 434 - Advanced Mine Ventilation

Mine Sealing and Recovery 403

On the negative side, usually a vast amount of water is needed, and pumping costs
for dewatering may be very high. In some cases, there is a slightly increased danger of
rekindling of fire, especially if the coal is of low rank and more liable to spontaneous
combustion.

24.2.2 Inertization With Nonreactive Gas

As discussed in Chapter 21, Spontaneous Combustion of Coal, inertization with N 2
and CO 2 is a very old, established technique to extinguish a sealed fire.
Nitrogen is available commercially, but it can be produced much more cheaply by a
PSA type generator at the mine site. Liquid nitrogen is transported in cryogenic tanker

trucks that convert the liquid N 2 (at 196 C) to a gas at temperatures higher than 0 C.
The volumetric change from liquid to gas creates a 700 times higher expansion and it
drives the gas into the sealed area through steel pipes. The PSA type N 2 generators are
becoming popular. It strips nitrogen out of the atmosphere using molecular sieves and
can generate a flow of 500e1000 CFM at 98% purity. The air has to be compressed but
the delivered nitrogen remains pressurized and can be easily pumped into the sealed
area. A plant of this capacity is not expensive, costing about $500,000. It is designed
to run almost unattended using solid-state electronics and Programmable Logic Con-
trol (PLC) controls for the process.
Advantages of using nitrogen include easy availability, nontoxic nature, and ease of
transport. On the negative side, liquid nitrogen may present some transport and main-
tenance problems. PSA N 2 generators have no such limitations.
Carbon dioxide is widely used for extinguishing fire. It is available as liquid or solid
form (dry ice). Just like N 2 , it can be stored and transported with ease and it is very
effective in quenching the fire. Disadvantages of using CO 2 are as follows:
1. It is a noxious gas and can be problematic in the recovery of the mine.
2. Liquid CO 2 presents fire hazards in the actual operation.
3. Dry ice can cause explosion when it comes in contact with fire.
For these reasons, N 2 is preferred for all inertization work. In most cases, N 2 is
cheaper than CO 2 .

24.3 Sampling the Sealed Mine Atmosphere and
Interpretation of Data

Sampling the mine atmosphere before and after sealing can provide very useful data to
manage a mine fire. The gas composition prior to sealing can provide a reliable base-
line, such as the normal ambient CO concentration. Once the mine or a section of the
mine is sealed, air samples should be withdrawn on a routine basis. It could be every 4
hours or at less frequent intervals. There are a number of indices derived from the gas