Page 239 - Advanced Mine Ventilation

218 Advanced Mine Ventilation

In higher concentration, it creates throat irritation and coughing but if inhaled for
long time, it can cause death the next day by pulmonary edema (retention of water
in lungs). All diesel engine exhausts are continually monitored with hand-held instru-
ments to make sure no one is overexposed.
The properties of NO and NO 2 are listed in Table 13.1. The threshold limit values
(TLV) for NO is 25 ppm.

13.2.9 Sulfur Dioxide
It is colorless, nonflammable and a highly toxic gas. Mine fires (in coal seams contain-
ing sulfur), blasting, and some internal combustion engines produce SO 2 . It irritates the
eyes, nose, and throat. The TLV is set at 2 ppm but the STEL is at 5 ppm [2].
Properties of SO2 are listed in Table 13.1.

13.2.10 Variations in TLV
It will be appropriate here to define the variations in the TLV for gases. They are three
in kind.
1. TLVdTime-weighted average (TWA)
It is an average value over an exposure period of 8 hours per work-day or 40 h per work-
week that does not create any adverse health effects. In short, it is just called the TLV.
2. TLVeTWAdShort-term limit (STEL)
A higher concentration of gas above its TLVeTWA can be tolerated but only for a very
short period. Thus, the TLV for CO is only 50 ppm but the TLVeTWAeSTEL is at 400 ppm
for 15 min provided no more than four such exposures occurs in a day with at least 60 minutes
of normal air between two high CO exposures. The TLVeTWA will not be exceeded in spite
of short durations of higher exposures.
3. TVAdCeiling
Some gases are so toxic that their TLV has a ceiling limit that cannot be exceeded at any
time. For example, the TLV for NO 2 at 5 ppm is a ceiling limit or TLV-C. For methane, it
is 2%.

13.3 Characteristics of Coal

Coal is a heterogeneous and anisotropic material. As far as the gas storage in coal is
concerned, only the rank of the coal is of significance. To determine the rank, two
types of laboratory tests are done on all coal:

Proximate analysis and
Ultimate analysis.
The data thus obtained is used for estimating gas contents, respirable dust index,
and spontaneous heating characteristics of a particular coal seam. These subjects are
discussed in detail under appropriate headings in this text.