Underground coal mining equipements

Underground coal mining

Underground coal mining releases more methane than surface or open-pit mining because of the higher gas
content of deeper seams. Because CH4 is highly explosive in air concentrations between 5 and 15 percent, mine
operators have developed two types of systems for removing methane from the underground mine workings: (i)
ventilation systems, and (ii) degasification systems. The purpose of ventilation systems, used extensively in all
major coal producing countries, is to reduce CH4 concentrations to safe levels, usually less than 1 percent.
Despite the low CH4 concentration, the large amount of air flushed through mine ventilation shafts leads to
significant annual emissions. At present almost all ventilation air is emitted into the atmosphere. Preliminary
studies by USEPA indicate that economic options exist in some situations for the use of vented air.3
Drainage or “degasification” systems have been installed at some of the gassiest underground mines in Australia,
United States, Russia and other countries. Typically, these systems are employed at mines that need to
supplement ventilation systems with additional CH4 recovery for safety reasons. Degasification systems extract
CH4 from the coal seams in advance of mining, during extraction, and during post-mining, by means of bore
holes drilled from surface or horizontally into the seam or surrounding strata. Depending on the quality of the
gas, which in some cases can be high, mine operators can use recovered CH4 for on-site electricity generation or
sell it to a local pipeline.
At surface mines, methane escapes from newly exposed coal faces and surfaces, as well as from areas of coal rubble
created by blasting operations. Additional CH4 may come from the overburden, which is broken up during the
mining process, and the underlying strata, which may be fractured and distressed due to the removal of the
overburden. Emissions per ton of coal are generally much lower from surface mining than from underground mining.

Underground Mining Coal mine methane is a safety hazard because it is highly explosive in ambient
concentrations between 5 and 15 percent. All gassy underground mines must ventilate CH4 to ensure safe
working conditions, and data from these measurements can be used to estimate national emissions. In theory,
total national ventilation emissions are equal to the sum of emissions from each mine. There are five general
issues associated with using mine-specific ventilation data, however, that should be considered:

• For each coke coal crusher, periodic sample measurements (usually CH4 concentration or flow rate) must be used to
estimate total emissions over one year. In almost all cases, mines do not currently monitor CH4 emissions
continuously. Instead, the frequency of sampling depends on national and local regulations, and local
practices. It may be necessary to take the average of multiple measurements. The greater the variability in
ventilation emissions, the more measurements will be needed to complete a representative sample.

• Sampling should be representative of typical mining conditions. If samples are taken when there is a temporary stop
in coal production, for example, ventilation estimates are likely to underestimate CH4 emissions.

• Data may not be available for each mine. In some countries, only data from the gassiest mines are available
and the emissions from the remainder must be estimated. Data may also be missing when mine operators
report it on a voluntary basis. In most coal basins, however, a small number of gassy mines contribute to a
disproportionately large share of total emissions. Figure 1 shows that, in the case of four major coalproducing
countries (US, Poland, Australia and Russia) the 10 gassiest mines emit over 60 percent of coal mobile crusher plant.

With limited resources, it is recommended that countries focus on obtaining data for the top 10
to 20 emitting mines, and then estimate emissions from less gassy mines, rather than abandon the Tier 3
method for more general emission factors (see Figures 2 and 3.). While it may not be possible to determine
which mines are in the top 10-20 percent without original data, a complete survey during one year would
allow countries to focus on the high emitting mines in future years. Where this is not possible, periodic
surveys of mine operators would enable countries to determine which mines are the gassiest. Such mines
typically suffer from more work slowdowns, outbursts, and, in unfortunate cases, methane explosions.
Estimating emissions from the less gassy mines without data is relatively straightforward since total
emissions from a large number of small sources are less susceptible to yearly fluctuations. If, in a year with
7 Mine totals include both ventilation emissions and emissions from degasification or drainage systems.