Coal Mine Methane
The
methane recovered from working mines can be grouped under the term Coal
Mine Methane (CMM). Two key drivers for CMM recovery are mine safety and
the opportunity to mitigate significant volumes of methane emissions
arising from coal mining activities. There is also strong potential to
utilise CMM for energy production.
Methane emissions in working mines arise at two key stages:
(1)
Methane is released as a direct result of the physical process of coal
extraction. In many modern underground mines, the coal is extracted
through longwall mining. Longwall mining, as with other sub-surface
techniques, releases methane previously trapped within the coal seam
into the air supply of the mine as layers of the coal face are removed,
thus creating a potential safety hazard.
(2) Methane emissions
arise from the collapse of the surrounding rock strata after a section
of the coal seam has been mined and the artificial roof and wall
supports are removed as mining progresses to another section. The debris
resulting from the collapse is known as gob and also releases methane
or ‘gob gas’ into the mine.
Recovery Techniques
Recovery techniques for CMM vary for each of the two stages of emissions.
(1)
Methane released from the worked coal face can be diluted and removed
by large ventilation systems designed to move vast quantities of air
through the mine. These systems dilute methane within the mine to
concentrations below the explosive range of 5-15%, with a target for
methane concentrations under 1%. The ventilation systems move the
diluted methane out of the working areas of the mine into shafts leading
to the surface. The methane removed from working mines via this
technique is known as Ventilation Air Methane (VAM).
The VAM is
released through the ventilation shafts and can then be destroyed or
captured for utilisation rather than allowing it to be released directly
into the atmosphere, as may have occurred in the past. VAM has the
lowest concentration levels of all forms of recoverable methane from
coal seams because of its high exposure to air; often displaying levels
of 0.05-0.8%.
(2) To pre-empt the release of gob gas from post
mining collapse, it is possible for vertical gob wells to be drilled
directly into the coal seam’s surrounding strata before mining
activities pass through that section. These pre-drilled wells can then
remove the gob gas once the collapse takes place, thus avoiding the
release of methane directly into the mine. The gob gas can then be
destroyed or captured for utilisation via the wells, rather than
allowing it to be released directly into the atmosphere. As gob gas is
exposed to significantly lower volumes of air than VAM, it displays much
higher methane concentration levels - typically between 35-75%.
Destruction & Utilisation
There are two main options available for the end utilisation of CMM.
(1) Power Generation
- If projects are seeking to take advantage of the benefits that CMM
can provide as an energy source, there are alternatives to simply
destroying the gas through flaring systems. Although both VAM and gob
gas provide much lower methane concentrations than methane recovered
from unmined coal seams, there are power generation technologies
available today that can harness the energy production potential of
these resources. VAM can not only be used for combustion dilution and
cooling purposes in standard gas turbines, but also as a primary fuel in
a number of ‘lean-burn’ gas turbine systems. These systems can utilise
VAM with methane concentrations as low as 1% (hence the term lean-burn)
and therefore can harness the energy potential of high percentages of
the VAM recovered from working mines.
VAM’s potential as an energy
source can also be harnessed by a number of oxidation systems available
on the market today. Methane can be converted to CO
2 by the
process of oxidation, thus reducing its global warming potential. This
process also creates energy which can be used to generate heat or power.
Oxidation systems can utilise VAM with methane concentration levels of
less than 1%. These systems are often deployed on-site to provide
auxiliary heat and power to the mine.
(2) Flaring -
Options exist for destroying gas that would otherwise be released
directly into the atmosphere. Flaring is an important technology for
disposing of the methane safely and efficiently and can help to
significantly reduce a major source of GHG emissions. The flared methane
is converted to CO
2, heat and water. Although flaring still leads to GHG emissions in the form of CO
2, because methane’s global warming potential is 23 times greater than that of CO
2, flaring actually reduces the overall greenhouse effect. However, the resulting CO
2
emissions still clearly present a huge challenge in terms of combating
global warming and flaring is therefore not regarded as the most
efficient or environmentally friendly of end use options.
Flaring
can be performed in either open or enclosed systems, and the technique
is similar to that deployed in the oil and gas industries. This method
of methane disposal is relatively cheap when compared to the extra costs
incurred in developing power generation infrastructure or incorporating
recovered methane into a region’s natural gas pipeline network.
Coal Mine Methane Potential
Methane
emissions from working underground mines make up the majority of
emissions from coal mining related activities - around 90% in 2006
according to figures from the US Environmental Protection Agency (US
EPA). VAM is widely found to make the greatest contribution to these
emissions, with US EPA figures suggesting that over 50% of all global
methane emissions from coal mining arise in this form.
At present,
there are more than 220 CMM projects worldwide in 14 countries. These
projects help to avoid around 3.8 billion cubic metres of methane
emissions every year.
Australia has been particularly active in
deploying the power generation and oxidation systems currently
available. The United States also has vast potential for utilising CMM
for energy purposes, but continues to primarily incorporate the gas
directly into its pipeline network rather than deploy power generation
systems specifically designed for CMM.
Outside of the developed
world, China is experiencing significant growth in interest in the
recovery and utilisation of CMM due to its high volume of methane
emissions from coal mining and the particularly gassy coal seams that
are found in the country. A number of projects utilising CMM for energy
purposes in China are currently approved or awaiting approval under the
Kyoto Protocol’s Clean Development Mechanism (CDM). Of these projects, a
number plan to utilise CMM as a fuel within power generation systems.
The greatest potential for CMM projects in the developing world lies
under the CDM due to the increased profitability that the generation of
emissions reduction credits can provide, which acts as an economic
driver.
The potential for the development of CMM projects is also
high in a number of other countries, including India and Mexico. Mexico
in particular is a key area for potential development as some of the
world’s gassiest mines are located there.