TECHNICAL SUPPORT DOCUMENT FOR
UNDERGROUND COAL MINES: PROPOSED
RULE FOR MANDATORY REPORTING OF
GREENHOUSE GASES
Office of Air and Radiation
U.S. Environmental Protection Agency
February 4, 2009
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CONTENTS
1 Source Description
a Total Emissions (based on Inventory)
b. Types of Emissions to be Reported
2. Options for Reporting Thresholds
a Options Considered
b Emissions and Facilities Covered Per Option
3 . Existing Relevant Reporting Programs/Methodologies
4. Proposed Monitoring Methods
Monitoring Methods Considered
5 Procedures for Estimating Missing Data
6. QA/QC and Verification Requirements
7. Reporting Procedures
REFERENCES
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1. Source Description
Coal production in the United States reached a record level in 2006, with production of 1,163
million short tons, according to the Energy Information Administration (EIA 2007). This was
produced from 1,424 mines, of which, 612 were underground mines and 812 were surface
mines.1
Two primary sources of greenhouse gas (GHG) emissions are of concern in the coal mining
sector. The first and largest source of emissions is fugitive emissions of CH4 released from the
coal and surrounding rock strata due to mining activities. There are five primary sources of
fugitive CH4 emissions from coal mining operations:
Ventilation air from underground mines, which contains dilute concentrations of CH4
Degasification systems at underground coal mines (also commonly referred to as
drainage systems). These systems may employ vertical and/or horizontal wells to recover
CH4 in advance of mining (known as "pre-mine drainage") or after mining (called "gob"
or "goaf wells)
Fugitive emissions from post-mining operations, during which coal continues to emit
CH4 as it is stored in piles, processed, and transported
Surface mines, from which CFLi in the coal seams is directly exposed to the atmosphere
Abandoned or closed mines, from which CFLi may seep out through vent holes or through
fissures or cracks in the ground.
The second source of GHG emissions combustion-related CC>2 emissions are associated with
the use of energy in mining operations, both from stationary and mobile sources, for both surface
and underground mines. In some cases, CFLi produced from the coal mine itself is combusted as
a source of energy.
Total Emissions (based on Inventory)
The annual Inventory of U.S. GHG Emissions and Sinks considers active coal mining-related
fugitive CH4 emissions to the atmosphere from three sources: underground mining, surface
mining, and post-mining (i.e., coal-handling) activities (EPA 2008). Total CFLi emissions from
active mining operations in 2006 were estimated to be 58.5 million metric tons of CC>2 equivalent
(CO2e) from these sources, a decline of 30% since 1990. Of this, underground mines accounted
for 35.9 million metric tons of CC^e, surface mines accounted for 14.0 million metric tons of
CO26, and post-mining emissions accounted for 8.6 million metric tons of CC^e. Fugitive CFLi
emissions from inactive, closed (or abandoned) coal mines were estimated to contribute another
5.4 million metric tons of CC^e.
The relative contribution of the various sources of coal mine emissions is illustrated in Figure 1.
1 An underground mine is where coal is produced by tunneling into the earth to the coalbed, which is then mined
with equipment such as cutting machines and continuous, long wall, and shortfall mining machines. Underground
mines are classified according to the type of opening used to reach the coal, i.e., drift (level tunnel), slope (inclined
tunnel), or shaft (vertical tunnel). In contrast, surface mines are mines that are usually within a few hundred feet of
the surface. Earth and rock above or around the coal (overburden) is removed to expose the coalbed, which is then
mined with surface excavation equipment such as draglines, power shovels, bulldozers, loaders, and augers. Surface
mines include: area, contour, open-pit, strip, and auger mines.
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Figure 1
Estimated Fugitive CH4 Emissions from U.S. Coal Mines (2006)
5.40
35.9
(56%)
n Underground mines
(\«ntilation and
degasification)
Surface mines
D Post mining
(underground)
D Post mining (surface)
I Abandoned underground
mines
Source: Environmental Protection Agency, Inventory of U.S. Greenhouse Gas Emissions and
Sinks: 1990-2006, USEPA#430-R-08-005, April 2008
Coal mining operations require a wide variety of equipment that uses coal, distillate, residual fuel
oil, natural gas, gasoline, and purchased electricity, and hence, results in emissions from
combustion. An estimated 9 million metric tons of CC>2 equivalent emissions were associated
with energy use in coal mining operations in the United States in 2006 (see discussion below).
Types of Emissions to be Reported
i) Process Emissions
Currently, only fugitive CH4 emissions from coal mining operations are addressed separately in
the U.S. national inventory for the coal mining sector. Five categories of fugitive CFLi emissions
are considered:
Ventilation air from underground mines
Degasification systems at underground coal mines
Fugitive emissions from post-mining operations
Surface mines
Abandoned and/or closed mines.
The nature of each of these sources of emissions, and considerations associated with various
options for inclusion in the proposed rulemaking, are discussed in more detail below.
Underground mines - mine ventilation emissions
Mine ventilation emissions from underground coal mines account for the largest share of fugitive
CH4 emissions 32.8 million metric tons of CC^e, or 56% of all U.S. coal mining fugitive
emissions. In underground mines, CFLi can create an explosive hazard to coal miners. To ensure
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mine safety, fresh air is circulated through underground coal mines using ventilation systems to
dilute in-mine concentrations of CH4 to well below explosive levels. In the U.S., mine safety
authorities at the Mine Safety and Health Administration (MSHA) regulate these concentrations.
Typically, CH4 concentrations in ventilation air range from 0.1% to 1.0%. These ventilation
systems can exhaust significant amounts of CH4 to the atmosphere in relatively low
concentrations.
MSHA field inspectors, as part of periodic mine safety inspections, conduct air sampling
according to well-defined procedures. The results of this sampling currently provide the basis for
estimating CH4 emission rates from ventilation systems at underground coal mines for the
Inventory of U.S. Greenhouse Gas Emissions and Sinks.
Underground mines - Degasification
In many cases, it is necessary to supplement the mine ventilation with a CH4 degasification
system. Methane drainage must be performed when the ventilation system cannot dilute CH4
emissions from the mine to below statutory levels. Mines that employ degasification systems
also liberate large quantities of CH4 in relatively high concentrations. Degasification systems
use wells drilled from the surface or boreholes drilled inside the mine to remove CH4 before,
during, or after mining operations. These degasification systems usually consist of a network of
boreholes, pumping systems, and gas gathering pipelines. The majority of the CH4 liberated
from degasification systems is combusted by flaring or at energy projects. Of the 21.8 million
metric tons CC^e of CH4 liberated by degasification systems in 2006, 18.7 million metric tons
CO26 of CH4 was combusted, and 3.1 million metric tons were emitted to the atmosphere.
In 2006, twenty U.S. coal mines supplemented their ventilation systems with active
degasification systems. Thirteen coal mines collected CH4 from degasification systems and sold
this gas to a pipeline. One coal mine used CH4 from its degasification system onsite to heat mine
ventilation air. Two of the coal mines that sold gas to pipelines also used CH4 to generate
electricity or fuel a thermal coal dryer on site (EPA 2005).
Before mining, drainage wells are drilled through a coal seam or seams and cased to pre-drain
the CH4 prior to mining. The wells are normally placed in operation 2 to 7 years ahead of
mining and the coal seam is often hydraulically fractured to remove much of the CH4 and water
from the seam. These pre-mine drainage wells can recover relatively high-quality gas from the
coal seam and the surrounding strata in most cases because the CH4 is not diluted by ventilation
from the mine. The total amount of CH4 recovered depends on site-specific conditions such as
the gas content of the coal seams and surrounding strata, permeability of the geologic materials,
the drainage time, the amount of pressure differential applied, and other characteristics of the
geologic and extractive systems. Such pre-mine drainage wells can recover 50% to 90% of the
gas content of the coal.
Horizontal holes can also be drilled into the coal seam from development entries in the mine to
drain CH4 from the unmined areas shortly before mining, reducing the flow of CH4 into the
mining section. In this case, because CH4 drainage occurs only from the mined coal seam, and
the period of drainage is relatively short, and the recovery efficiency of this technique is
relatively low normally, about 10% to 20% of the CH4 is recovered from the drilled area.
Higher recoveries can generally be achieved in room-and-pillar sections by drilling longer holes
farther in advance of mining.
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Gob wells and cross-measure bore holes recover CH4 from the overburden (i.e., gob area) after
mining of the seam, primarily in longwall mines. The gob area is a fractured zone in a mine seam
that is the result of the relaxation and collapse of strata surrounding the mined coal seam. Such
"gobs" can be significant sources of CFLt emissions. Gob gas vent holes drilled into mine gobs
after mining generally vent the degasified CH4 directly to the atmosphere. These are typically
equipped with exhausters on the surface to draw the gas from the gob. Monitoring equipment
may or may not be equipped as part of the surface infrastructure for these gob gas vent wells, but
would be in place where the collected gas is sold or used onsite.
In many cases, the CH4 recovered from degasification systems can be productively utilized.
Methane emitted from coal mine ventilation systems constitutes an unused potential source of
energy. Utilization technologies for recovered coal mine CH4 fall into three broad categories: 1)
natural gas substitution, 2) direct use at or near the mine site, and 3) electricity generation and/or
cogeneration. High quality coal mine gas may require only modest processing before
compression and pipeline injection, while medium-quality coal mine gas may require some
processing or upgrading, but then can be used in nearby boilers or furnaces, or in internal
combustion engines and gas turbines.
In cases where the sale and/or use of coal mine gas would not be profitable or feasible, an
alternative to utilization for mitigating coal mine CH4 emissions is to destroy the CH4 via
flaring, thus converting the higher global warming intensity CH4 into lower intensity CC>2.
Procedures to estimate CH4 emissions from degasification will include estimating "emissions
avoided." Estimating these "emissions avoided" has value in several respects. First, it provides
a method for verifying emissions estimates for gassy mines. Second, it provides a mechanism
for documenting current actions by operators to reduce emissions, even if these reductions were
primarily implemented for safety, rather than GHG reduction, reasons.
Emissions from post mining operations
Other sources of CFLt emissions from the mining sector relate to activities after the coal is mined,
and include coal piles and coal storage areas, coal transport operations, and the coal preparation
plant. Estimated post-mining emissions accounted for 14% of all coal mining sector fugitive
CFLt emissions in 2006, representing 8.6 million metric tons of CC^e. These emissions apply to
both underground and surface coal mines. For these sources of fugitive emissions, emissions
estimates are generally developed for emissions inventory purposes by using standard region or
basin-specific emission factors, multiplied by rates of coal production, applied to all "post-
mining" activities. Measuring and/or monitoring emissions from these operations would be
difficult, since no robust monitoring methodology has been developed for these emissions.
Moreover, existing emission factors are highly uncertain; and are established based on very
sporadic data, if based on any emissions data at all.
One category of post-mining emissions that perhaps could be considered for monitoring would
be emissions from coal preparation plants. On April 16, 2008 (Federal Register: April 28, 2008,
Vol. 73, No. 82, pp. 22901-22913), EPA proposed revisions to emissions control requirements
for new coal preparation plants. These proposed new source performance standards (NSPS)
would apply to new coal preparation plants that process more than 200 tons of coal per day. The
proposed NSPS would apply to new, modified, and reconstructed coal preparation plants and
reflect improvements in emission control technologies for particulate matter (PM) that have been
developed since the original NSPS for these sources were issued in 1976.
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As part of these new proposed NSPS requirements, EPA is proposing to require
owners/operators of thermal dryers and pneumatic coal-cleaning equipment at constructed,
modified, or reconstructed coal preparation plants to either install and operate a PM continuous
emissions monitoring system (CEMS), or to conduct annual PM performance tests. A possible
logical extension of this proposal would be to also include monitoring for fugitive CH4 emissions
from these coal preparation plants. For the installation of a new CEMS, the incremental costs of
adding CR4 monitoring capability to the PM monitoring capability would be quite small, since
the dual monitoring capability can be designed within the same system. Retrofitting after the fact
to add CH4 monitoring would cost more.
It is important to note that fugitive emissions from coal preparation plants represent a small
portion of the total emissions from the coal mining sector. In fact, the vast majority of GHG
emissions from a coal preparation plant correspond to the fuel consumed for various operations
such as coal drying. Fugitive CH4 emissions will only correspond to any remaining CH4 emitted
from the coal during processing.
Emissions from surface mines
Surface coal mines release CH4 as the overburden is removed and the coal is exposed. The level
of estimated emissions from surface mines is much lower than that from underground mines,
representing 22% of all coal mine fugitive emissions, amounting to 14.0 million metric tons of
CO26. Moreover, similar to that for post mining operations, emissions for surface mining are
generally estimated for national inventory purposes by using standard region or basin-specific
emission factors, multiplied by rates of coal production, applied to all surface mining activities.
Monitoring emissions from these operations would be difficult, since; again, no robust
methodology has been developed. Moreover, existing emission factors are highly uncertain; and
are established based on very sporadic data, if based on any emissions data at all.
Emissions from abandoned mines
When coal mines are no longer operated to produce coal, they are known as closed or
"abandoned" mines. Even though active mining no longer occurs, these abandoned mines can
still produce CH4 emissions from diffuse vents, fissures, or boreholes. This CH4 can be
deliberately extracted and used to generate power or for other end uses.
Methane liberated from abandoned mines is estimated to be 6.8 million metric tons of CC^e per
year, of which an estimated 1.4 million metric tons of CC^e per year is recovered and used (EPA
2008). Therefore, an estimated 5.4 million metric tons of CC^e is emitted per year, representing
8% of all fugitive CH4 emissions from coal mines.
There are several thousand abandoned coal mines in the United States. Of these, EPA has
identified some 400 abandoned mines that are considered "gassy," and has developed profiles of
potential projects at abandoned mines that may be good candidates for project development
(EPA 2004).
Measuring and/or monitoring emissions from abandoned mines would be difficult. There is
currently no robust way to measure fugitive emissions from abandoned mines. Measuring at
discrete borehole/wellhead sites would not adequately capture the emissions seeping out of
various other fissures and leakage points. Moreover, in many cases, it can be quite difficult to
identify owners of abandoned mine sites, i.e., it would be difficult to identify the responsible
parties to monitor and report. Even where the owner is known, abandoned sites are unmanned
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and often remote; and it would be burdensome to require company personnel to travel to each
abandoned mine site to measure emissions, along with being expensive and resource-intensive,
especially given the relatively small contribution abandoned mines make to total GHG emissions
in the coal mining sector.
ii) Combustion Emissions
Estimates of fuel consumption in the mining sector are reported by the Bureau of the Census
(Census, 2004a, 2004b, 2004c, 2004d) for 2002, which include coal consumed directly in mining
operations, distillate, residual fuel oil, natural gas, gasoline, and purchased electricity. The 2002
fuel consumption estimates were updated to 2006 based on 2006 levels of coal production, and
based on this, an estimated 9 million metric tons of CC>2 emissions were associated with energy
use in coal mining operations in the United States in 2006. The majority of these emissions
result from the generation of electricity purchased to support mining operations. The CC>2
emissions from only on-site combustion (the value used in this analysis) are estimated to be 3.62
million metric tons.
2. Options for Reporting Thresholds
In general, MSHA samples (quarterly or more frequently) CH4 emissions for mines liberating
more than 100,000 cubic feet of CH4 per day from ventilation systems,2 which is equivalent to
about 15,000 metric tons CC^e per year. However, emissions rates from some mines emitting
less than that threshold are included in the MSHA database. Of the over 600 underground coal
mines operating in the U.S. according to EIA, ventilation air emissions based on MSHA
inspections were reported and electronically accounted for in 128 mines in 2006.
a. Options Considered
Several different emissions thresholds were considered for fugitive CH4 emissions from
underground coal mines based on the data in the MSHA databases and the Inventory of U.S.
Greenhouse Gas Emissions and Sinks: 1990-2006. The estimated emissions and facilities
covered are summarized in Table 1 for different emissions threshold levels, based on 2006
emissions levels.
Table 1
Threshold Analysis for Underground Coal Mining-
Source
Category
Underground
Coal Mining
Underground
Coal Mining
Threshold
Level
(Metric
tons CO2e
per year)
100,000
50,000
Total
National
Emissions3
(Metric
tons
C02e)
39,520,000
39,520,000
Number
of
Facilities'1
612
612
Emissions Covered0
Process
Emissions
(Metric
tons
CO2e/yr)
30,649,815
32,316,726
Estimated
Combustion
CO2
Emissions
(Metric
tons/yr)
405,040
438,262
Metric
tons
(C02e/yr)
31,054,856
32,754,988
Percent
79%
83%
Facilities Covered
Number
53
77
Percent
9%
13%
Personal communication, Fred H. Menke, Jr., Supervisory IT Specialist, Mine Safety and Health Administration,
to Michael Codec, Advanced Resources International, April 25, 2008.
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Underground
Coal Mining
Underground
Coal Mining
Underground
Coal Mining
Underground
Coal Mining
25,000
MSHA
quarterly
reporting
threshold
10,000
1,000
39,520,000
39,520,000
39,520,000
39,520,000
612
612
612
612
33,082,854
33,364,265
33,465,371
33,483,922
453,531
459,140
461,155
461,524
33,536,385
33,823,404
33,926,526
33,945,446
85%
86%
86%
86%
100
114
122
125
16%
19%
20%
20%
a Estimated national fugitive CH4 emissions for underground mining from 2006 Annual Inventory (Table 3-
26) (http://epa.gov/climatechange/emissions/downloads/08 Energv.pdf): plus estimated combustion
emissions of 3.62 million metric tons, excluding those associated with purchased electricity.
b Number of active underground coal mines in the U.S. in 2006 according to the Energy Information
Administration (EIA 2007).
0 Based on the fugitive CH4 emissions volumes reported for individual mines in the MSHA database
"Analysis of 2006 Emissions Data for Active Underground Mines," along with estimated combustion-related
emissions from these mine sites, but not including emissions associated with purchased electricity.
Estimates made for fuel combustion emissions based on Bureau of Census data on fuel use as a % of
coal mined (for gassiest mines) or as % of vented emissions for less gassy mines. Combustion emissions
calculated as approximately 1% to 3% of vented emissions, on CO2e basis.
b. Emissions and Facilities Covered Per Option
The fugitive CH4 emissions from the 114 of the 128 underground coal mines reporting
ventilation emissions in 2006 of over approximately 15,000 metric tons of CC^e per year
(equivalent to the current MSHA threshold for accounting for emissions from gassy mines)
accounts for 86% of the 39.5 million metric tons estimated to be emitted by underground coal
mines (35.9 million metric tons from fugitive CH4 emissions and 3.6 million metric tons from
combustion emissions, excluding those emissions associated with purchased electricity). These
114 mines represent 19% of the active underground mines in the United States. Increasing the
reporting threshold to 25,000 metric tons of CC^e per year would reduce the number of reporting
mines by 14 to 100 and would reduce the amount of total emissions reported from this sector by
3%. If no threshold is established, and all mines must report, this will add nearly 500 mines to
the number required to report, but these 500 mines would only represent another 14% of the total
GHG emissions from underground coal mines.
Other options for establishing thresholds could be based on other factors, such as coal
production. Alternative threshold levels for reporting could also be considered. However, since
all underground coal mines are subject to MSHA inspections where CH4 concentration samples
and ventilation air rate measurements are taken; and thresholds for reporting emissions above
defined levels are already established by MSHA policies and procedures, maintaining this
threshold is likely the most logical and least burdensome method for establishing GHG
monitoring and reporting thresholds for the coal mining sector, while capturing a majority of
GHG emissions from coal mining.
3. Existing Relevant Reporting Programs/Methodologies
For this proposal, EPA reviewed several protocols and programs with guidance monitoring
and/or estimating GHG from this source, including the 2006 IPCC GL, U.S. GHG Inventory,
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California AB32 ("CARS rule"), EIA's 1605(b) program, EPA's Coalbed Methane Outreach
Program (CMOP), MSHA, BLM, and Australia's National Mandatory GHG Reporting Program
(draft).
In general, there are three methods for monitoring emissions: the use of emission factors,
periodic sampling of CH4 concentrations and flow, and direct measurement.
This section presents the existing relevant monitoring methods by coal mine CH4 sources.
Underground mines - mine ventilation
To calculate emissions from ventilation at underground mines in the Inventory of U.S.
Greenhouse Gas Emissions and Sinks, EPA uses CH4 emissions data collected through an
MSHA sampling program, which also serves as the basis for EPA's CMOP emissions estimates
from this source. MSHA regulatory requirements specify that qualified and trained MSHA field
inspectors perform periodic mine safety inspections at all underground coal mines in the United
States, and as part of these inspections, test CH4 emissions rates at each coal mine according to
MSHA-approved sampling procedures.3 Air sampling is conducted by MSHA inspectors
collecting air bottle samples at a mine's main fans, along with a total quantity air ventilation
volume reading. Sampling procedures and equipment are approved by MSHA (NIOSH 2006).
The sample bottles are sent to the MSHA lab for analysis and the results are provided back to the
MSHA district offices for inclusion in the inspection report. The results are also maintained
separately by the district ventilation group.
In general, air sample readings with CH4 concentrations below 50 ppm are considered non-
detectable. Annual air samples and ventilation readings are taken annually unless emission rates
exceed 100,000 standard cubic feet per day, where quarterly sampling is conducted. MSHA
electronically keeps track of emissions for mines liberating more than 100,000 cubic feet of CH4
per day,4 which is equivalent to about 15,000 metric tons CO26 per year. If emissions levels are
greater that 200,000 cubic feet per day, according to Section 103 (i) of the Federal Mine Safety
& Health Act of 1977 (Public Law 95-164), more frequent inspections are mandated, with the
frequency determined by the daily CH4 liberation rate calculated for the mine.
For the mines with emissions exceeding 100,000 cubic feet per day, annual ventilation emissions
estimates are developed by averaging the four quarterly tests, and are accurate to the extent that
the data collected are representative of actual emissions.
In addition, for federal coal leases where degasification systems are in place, the Bureau of Land
Management (BLM) requests that mine operators provide CH4 emissions rates from both
degasification and ventilation systems to BLM. Operators currently provide this information
voluntarily; it is not a requirement either by regulation or under their federal coal lease terms.5
3 MSHA requires that monitoring be performed by a qualified person using CH4 monitoring devices approved by
MSHA (under 30 CFR Parts 18,21,22,23,27, and 29); and that are maintained in permissible and proper operating
condition and calibrated with a known CH4-air mixture at least once every 31 days.
4 Personal communication, Fred H. Menke, Jr., Supervisory IT Specialist, Mine Safety and Health Administration,
to Michael Codec, Advanced Resources International, April 25, 2008.
5 Personnel communication, Desty Dyer, U.S. Bureau of Land Management (BLM), Montrose Field Office, to
Michael Codec, Advanced Resources International, Inc., June 6, 2008
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For this source, the IPCC Guidelines recommend that where mine-specific measurement data are
available, it is good practice to use a method comparable to that based on the MSHA sampling
results. In situations when mine-specific measurement data are available only for a subset of
underground mines (which generally should not be the case in the United States), the Guidelines
specify that emissions from the remaining mines can be calculated with emission factors based
on the mines with measurement data. These emission factors can be based on specific emission
rates derived from operating mines within the same basin, or on the basis of mine-specific
properties, such as the average depth of the coal mines. When no mine-specific data are
available, but country- or basin-specific data are, the IPCC Guidelines suggest that it is good
practice to employ a Tier 2 method. Where no data (or very limited data) are available, the
Guidelines specify that it is good practice to use a Tier 1 approach, provided underground coal
mining is not a key sub source category.
DOE 1605(b) inventory reporting guidelines specify that mines with undetectable CH4 levels can
calculate ventilation emissions by multiplying a flow rate of 3,000 cubic feet per minute by an
assumed CH4 concentration of 0.05% to derive a CH4 emissions rate (DOE 2007). DOE
1605(b) guidelines otherwise provide the same methodology as the Inventory of U.S.
Greenhouse Gas Emissions and Sinks and IPCC.
Australia's National Mandatory GHG Reporting Program (draft) (Australia 2008) recommends a
methodology comparable to that in the 2006 IPCC Guidelines.
Recent draft regulations proposed by the California Air Resources Board (CARB 2008) do not
prescribe any approach for estimating emissions from coal mining operations; these only propose
the use of emission factors to estimate emissions from post-mining coal storage and handling
activities (see discussion below).
Underground mines - degasification
As discussed above, of the 20 U.S. coal mines deploying degasification systems in 2006, 13
mines sold this gas to a pipeline. It is common practice for these mines to monitor the volume of
gas produced and delivered to the pipeline (which also monitored these volumes). Coal mines
using CH4 from degasification systems on site to heat mine ventilation air are also likely to
monitor the volumes of CH4 produced.
In contrast to CH4 emissions from ventilation systems, no agency requires mines to report the
amount of CH4 they drain from degasification systems. MSHA collects information about the
presence and type of degasification systems in some mines, but does not collect quantitative data
on the amount of CH4 liberated. As mentioned above, BLM requests that mine operators on
federal coal leases provide information on CH4 emissions from both ventilation and
degasifications systems.
In most instances, it would be relatively straightforward for operators to report the annual
volumes of CH4 liberated from mine degasification systems, along with the disposition of the
degasified CH4 (e.g.; vented, flared, sold, and/or consumed on-site). This is based on the fact
that most mines already provide such information to EPA's CMOP program (and, in a few cases,
to BLM), though perhaps not always on a regular basis. If not, they are still likely to have good
measurements or estimates on hand.
As part of the annual U.S. inventory (EPA 2008), EPA estimates the volume of CH4 drained
assuming drainage efficiencies based on information obtained from MSHA district offices or
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from information provided by operators. Some of the coal mines employing degasification
systems have provided EPA with information regarding CH4 liberated from their degasification
systems. In cases in which mines sell CH4 recovered from degasification systems to a pipeline,
gas sales are used to estimate CH4 liberated from degasification systems. For those mines that do
not sell CH4 to a pipeline and/or do not currently provide information to EPA, CH4 liberated
from degasification systems is estimated by EPA based on the type of system employed. For
example, for coal mines employing gob wells and horizontal boreholes, the methodology
assumes that CH4 volumes liberated from degasification systems account for 40% of total CH4
liberated from the mine. This is also the recommended method presented in the DOE 1605(b)
Guidelines.
The IPCC Guidelines suggest a similar approach. Where ventilation can be measured (or can be
estimated from measurements from similar mines in the same basin or region), but not
degasification volumes, they suggest that reporters estimate the amount degasified (although no
methodology is specified) and use appropriate methods for accounting for the disposition of the
degasified CFLt (e.g.; vented, flared, sold, consumed on-site). Several suggested approaches are
provided in the Guidelines. Australia's National Mandatory GHG Reporting Program (draft)
(Australia 2008) recommends a methodology comparable to that in IPCC Guidelines.
Post mining operations
For post-mining operations, since mine-specific emissions measurements are not available,
emissions estimates for the U.S. inventory are developed by using basin-specific coal production
multiplied by a basin-specific emission factor. Emission factors for surface mined coal were
developed from the in situ CFLt content of the surface coal in each basin. Furthermore, the post-
mining emission factors used were estimated to be 25 to 40% of the average in situ CFLt content
in the basin. Thus, for the current inventory, the post-mining emission factor was determined to
be 32.5% of the in situ CH4 content in the basin.
The IPCC Guidelines and the DOE 1605(b) guidelines also recommend this approach for
inventory development purposes, and CARB recommends this approach (for coal storage) as part
of its mandatory greenhouse gas reporting program. Australia's National Mandatory GHG
Reporting Program (draft) (Australia 2008) recommends a methodology comparable to that in
IPCC Guidelines.
Surface mines
Similarly, surface mining emissions estimates for the U.S. inventory are also developed by using
basin-specific coal production multiplied by a basin-specific emission factor. Emission factors
for surface mined coal were developed from the in situ CH4 content of the surface coal in each
basin. Surface mining emission factors were estimated to be from 1 to 3 times the average in situ
CFLt content in the basin. For the current inventory, the surface mining emission factor was
determined to be twice the in situ CFLt content in the basin.
Again, the IPCC Guidelines and the DOE 1605(b) guidelines also recommend this approach for
inventory development purposes, as does Australia's National Mandatory GHG Reporting
Program (draft).
Abandoned mines
EPA (EPA 2004) and the IPCC (IPCC 2006) have developed emissions estimation
methodologies for abandoned underground mines for emissions inventory purposes. EPA (EPA
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2004) has developed emissions estimation methodologies for abandoned or closed underground
mines for emissions inventory purposes based on the time since abandonment; gas content and
adsorption characteristics of the coal, CH4 flow capacity of the mine; mine flooding; the
presence of vent holes, and mine seals. The IPCC methodology is based on the elapsed time
since abandonment, the mine's initial gassiness, and the extent to which the mines have become
flooded. Similarly, the EPA methodology is based on the time since abandonment; gas content
and adsorption characteristics of coal, CH4 flow capacity of the mine; mine flooding; the
presence of vent holes, and mine seals.
Both methodologies recommend default assumptions and approaches for calculating emission
factors for abandoned underground and surface mines and the portion of mines that are gassy.
The current methodologies (EPA and IPCC) use an estimation technique based on modeling;
thus it is not very accurate unless considerable data are available to make site-specific estimates.
Australia's National Mandatory GHG Reporting Program (draft) (Australia 2008) recommends a
methodology comparable to that in IPCC Guidelines.
DOE 1605(b) guidelines contain no specific recommendations for estimating emissions from
abandoned coal mines.
4. Proposed Monitoring Methods
a. Monitoring Methods Considered
Emissions in Ventilation Air
i. Option 1: Direct Measurement (Annual Reporting)
The direct measurement option considered would involve implementing a continuous monitoring
program in underground U.S. coal mines, focused specifically on monitoring the emissions from
underground coal mine ventilation shafts. This option builds upon the fact that continuous
monitors are already in place throughout all underground coal mines. For compliance with this
GHG emissions monitoring rule, such monitoring devices could be placed at or near the mine
vent outflows where the air samples are taken my MSHA inspectors.
Federal safety standards6 mandate that, "when 1.0 percent or more CH4 is present in a working
place or an intake air course [...] electrically powered equipment in the affected area shall be de-
energized, and other mechanized equipment shall be shut off." A flammable mixture of CH4 and
air can be ignited by electric arcs and sparks, open flames, or by the heat of friction between the
cutting bits of mining equipment and the mine rock immediately above or below the coal.
To ensure that CH4 levels in underground coal mines remain below combustible levels,
continuous monitoring devices are already in operation within underground coal mines. MSHA
regulations require that wall and/or machine-mounted CH4 monitors are mounted at various
locations and on certain types of machinery to continuously monitor mine air quality. These
6 Code of Federal Regulations. Title 30, Part 75: Mandatory safety standards-underground coal mines
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monitors are certified under 30 CFR Part 27,7 which requires monitoring system design and
operation that prevents the mining equipment from operating unless the CH4 monitoring system
is functioning. The CH4 monitoring system has a warning device that activates when the CH4
concentration is above 1.0% to 1.5%, and has a means to shut off power to the equipment when
the CH4 concentration is above 2.0%.
The advantage of continuous monitoring is that it takes into consideration any variability in
emissions from mining operations that may not be represented in periodic sampling. Moreover,
since such devices are already used within the mine, mine operator personnel are familiar with
their operation.
Methane detectors fall into two categories: portable CH4 detectors and wall or machine-mounted
monitors. Most CH4 detectors used in mining use a catalytic heat of combustion sensor to detect
CH4 and other combustible gases. These have been proven reliable through many years of
operation. For detection of CH4, proper operation of catalytic heat of combustion sensors
requires both a CH4 concentration below 8% (by volume) and oxygen content above 10% -
requirements that are usually satisfied in mining applications.
Some detectors measure the CH4 concentration by using infrared absorption as an operating
method. These infrared detectors can measure accurately without oxygen and at concentrations
up to 100% CH4. However, water vapor and dust can cause operating difficulties. In some
mines, the CH4 may be accompanied by ethane, which can produce an exaggerated infrared
detector response.
Machine-mounted CH4 monitors are usually mounted on mining and tunnel-boring machines.
They are designed to have their readout display separated from the sensing head so that the
readout is visible to the machine operator and the sensing head is placed in a location where CH4
is most likely to accumulate. Machine and wall-mounted monitors operate continuously and can
identify emission peaks. Using machine or wall-mounted monitors could likely improve the
accuracy of emissions measurements in underground mines, when operated and calibrated
properly, compared to just periodic sampling. The accuracy of such devices is acceptable for
emissions monitoring purposes, since they are sufficient for purposes of ensuring mine safety.
The resolution of these monitors can be specified - in general, the higher the resolution, the
higher the cost. Resolutions at levels lower than the 50 ppm limit, which characterizes the
current MSHA sampling protocol, are certainly achievable.
Nonetheless, emissions at levels below the resolution of the monitors, if assumed to be zero,
whatever the threshold, could correspond to an underestimation of emissions. However, as
shown above in Table 1, 85% of the GHG emissions from U.S. coal mines (including
combustion emissions) occur in just 100 of the over 600 coal mines in the United States, so
increased accuracy does not necessarily provide much improvement in capturing GHG emissions
from this sector.
A disadvantage would be the larger costs associated with purchasing and maintaining these
continuous emissions monitoring systems (CEMS) devices for this application.
ii. Option 2: Quarterly Sampling of CH4 Content and Gas Flow
7 A list of approved gas monitors for U.S. mines is available from MSHA's Approval and Certification Center
(http://www.msha.gov/TECHSUPP/ACC/lists/27mthmon.pdf)
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Under this option, coal mine operators would be required to duplicate the MSHA process of
collecting air samples and ventilation rates, submitting the samples to a lab for analysis, and
developing estimates of emissions on their own. The advantage of this option would be that the
responsibility for monitoring would rest with the mine operator, which is consistent with most, if
not all, of the other industrial sectors likely to be affected by the rule. Moreover, it would not
involve EPA imposing regulatory responsibilities on another federal agency. However, it would
require mine operators to purchase sampling equipment, train personnel in their use, develop
emissions estimates based on the data collected, and report the results. If this option is pursued,
the operator would have to incur the capital and O&M costs involved in purchasing and
maintaining the air sampling equipment, and in sending air samples to an independent lab for
analysis.
Alternatively, the results of MSHA inspections could be used to estimate ventilation air
emissions. In this case, either the mines would obtain data from MSHA (it is not currently
shared with the mine), or sampling and reporting would be the responsibility of MSHA.
An advantage of MSHA sampling and reporting is that MSHA inspectors have procedures in
place, and a high level of QA/QC established for this process. No additional training of operator
personnel would be required, the implementation period could be almost instantaneous, and the
accuracy of emissions estimates would most likely be higher.
iii. Option 2: More Frequent Sampling
Sampling could be done more frequently than quarterly. This could provide a more consistent,
reliable, and representative characterization of fugitive CH4 mine emissions, but at a higher cost.
iv. Option 3: Simplified Emission Calculation
This option would involve utilizing the same procedures as recommended by the IPCC and DOE
1605(b) Guidelines, assuming no mine-specific data were available. For inventory reporting
purposes, the IPCC Guidelines recommend that when ventilation emissions cannot be measured,
they can be estimated from measurements from similar mines in the same basin or region. If
analogous mine-specific data are unavailable, they recommend using basin/region-specific
emission factor multiplied by production, and representative emissions factors are provided in
the Guidelines.
Since mine-specific data is available in the U.S. from MSHA inspections, Option 3 represents a
more simplified, and considerably less accurate, methodology compared to that currently used in
U.S. underground coal mines.
Degasification Systems
(i) Option 1: Direct Measurement
This option involves continuous monitoring of gas recovered from mine degasification systems.
This option would apply to all degasification wells, including gob gas vent holes and other
degasification wells that are currently not monitored.
Approaches for monitoring the volumes of CH4 produced from coal mine degasification systems
prior to mining use the same general techniques for monitoring gas flows from natural gas
production wells. Recommendations for improved approaches for both degasification systems
and monitoring gas flow specifically from degasification wells date back to the mid-1970s
(Lambert and Trevits, 1978; Diamond, 1994). Degasification systems operate in conjunction
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with the ventilation system and overall mining operations, generally requiring coordinated
management (using measuring instruments, monitoring, controls, and good communications) to
optimize overall operations.8
The advantages to this approach are that continuous monitoring provides accurate data and can
captures seasonal changes in gas flow and concentrations, and that most degasification systems
already have continuous monitoring in place. The disadvantage would be the costs incurred by
mines that do not currently have these systems in place.
ii) Option 2: Sampling of CFU Content and Gas Flow
This option would involve continuous monitoring for those wells already deploying such
systems, but would allow periodic sampling and reporting for those that do not. For example,
gob gas vent holes that are currently not monitored could conduct periodic sampling, as could
perhaps any other degasification borehole, rather than installing continuous monitoring. For
example, weekly or monthly sampling at each gob gas vent hole/well could be conducted. While
such an approach would involve less capital costs, greater labor costs would be involved with
traveling to each (often remote) well/vent site to take samples.
If direct measurements of gas volumes are not available for degasification wells, other options
could be to: (1) use gas sales data, and/or (2) apply an assumed degasification efficiency based
on the technology used. This is the procedure suggested in the IPCC and DOE 1605(b)
Guidelines, and is the method currently used in the U.S. inventory if underground coal mine
degasification volumes are not available. For example, for coal mines employing gob wells and
horizontal boreholes, the methodology assumes that degasification emissions account for 40% of
total CH4 liberated from the mine. Gas sales data, if available, should be a direct proxy for the
volume of methane produced from degasification systems, and is clearly better than a assumed
degasification efficiency should sales data be available. While simple in application, the use of
an assumed degasification efficiency is highly uncertain, since such an efficiency is clearly a
function of many mine-specific parameters that can vary considerably from mine-to-mine.
Option 3: Simplified Emission Calculation
This option would involve utilizing the same procedures as recommended by the IPCC and/or
DOE 1605(b) Guidelines, assuming no mine-specific data were available. For GHG inventory
reporting purposes, the IPCC Guidelines recommend that when recovered gas from
degasification systems cannot be measured, they can be estimated from measurements from
similar mines in the same basin or region. If analogous mine-specific data are unavailable, they
recommend using basin/region-specific emission factor multiplied by production, and
representative emissions factors are provided in the Guidelines. Again, similar to that discussed
above, while simple in application, the use of simplified emissions calculation approach is highly
uncertain, since emission are a function of many mine-specific parameters that can vary
considerably from mine-to-mine.
Methane Combustion at Ventilation or Degasification Systems
(i) Option 1: Direct Measurement
1 http://www.epa. gov/cmop/docs/ggasrecpv.pdf
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This option involves continuous monitoring of the disposition of gas recovered from coal mines.
Although this measurement is continuous, reporting would only be annual. This option would
apply to all degasification wells, including gob gas vent holes and other degasification wells that
are currently not monitored.
Approaches for monitoring the volumes of CH4 produced from coal mine degasification systems
prior to mining use the same general techniques for monitoring gas flows from natural gas
production wells. Recommendations for improved approaches for both degasification systems
and monitoring gas flow specifically from degasification wells date back to the mid-1970s
(Lambert and Trevits, 1978; Diamond, 1994). Degasification systems operate in conjunction
with the ventilation system and overall mining operations, generally requiring coordinated
management (using measuring instruments, monitoring, controls, and good communications) to
optimize overall operations.9
The advantages to this approach are that continuous monitoring provides accurate data and can
captures seasonal changes in gas flow and concentrations, and that most degasification systems
already have continuous monitoring in place. The disadvantage would be the costs incurred by
mines that do not currently have these systems in place.
iii) Option 2: Sampling of CFU Content and Gas Flow
This option would involve continuous monitoring for those wells already deploying such
systems, but would allow periodic sampling and reporting for those that do not. For example,
gob gas vent holes that are currently not monitored could conduct periodic sampling, as could
perhaps any other degasification borehole, rather than installing continuous monitoring. For
example, weekly or monthly sampling at each gob gas vent hole/well could be conducted. While
such an approach would involve less capital costs, greater labor costs would be involved with
traveling to each (often remote) well/vent site to take samples.
If direct measurements of gas volumes are not available for degasification wells, other options
could be to: (1) use gas sales data, and/or (2) apply an assumed degasification efficiency based
on the technology used. This is the procedure suggested in the IPCC and DOE 1605(b)
Guidelines, and is the method currently used in the U.S. inventory if underground coal mine
degasification volumes are not available. For example, for coal mines employing gob wells and
horizontal boreholes, the methodology assumes that degasification emissions account for 40% of
total CH4 liberated from the mine.
iv) Option 3: Simplified Emission Calculation
This option would involve utilizing the same procedures as recommended by the IPCC and/or
DOE 1605(b) Guidelines, assuming no mine-specific data were available. For GHG inventory
reporting purposes, the IPCC Guidelines recommend that when recovered gas from
degasification systems cannot be measured, they can be estimated from measurements from
similar mines in the same basin or region. If analogous mine-specific data are unavailable, they
recommend using basin/region-specific emission factor multiplied by production, and
representative emissions factors are provided in the Guidelines.
5. Procedures for Estimating Missing Data
9 http://www.epa. gov/cmop/docs/ggasrecpv.pdf
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Options and considerations for missing data vary will vary depending on the proposed
monitoring method. In general, each option would require a complete record of all measured
parameters and parameters determined from company records that are used in the GHG
emissions calculations (e.g., carbon contents, monthly fuel consumption, etc.). Therefore,
whenever a quality-assured value of a required parameter is unavailable (e.g., if a monitor or
CEMS malfunctions during unit operation or if a required fuel input parameter is not obtained), a
substitute data value for the missing parameter must be used in the calculations.
In addition, since periodic inspections are performed on all underground coal mines, data on CH4
concentrations (above 50 ppm) and ventilation rates exist for all mines. For mines below the 50
ppm "detectable" threshold, estimates could be made for emission rates assuming some value for
CH4 concentrations (say 25 ppm). However, it again should be emphasized that the vast
majority of emissions from underground coal mines come from roughly 1/6 of the gassiest
underground coal mines.
For emissions avoided from degasification systems, methods are described above for estimating
volumes for CFLi recovered (Option 3) should recovered volumes not be known.
For missing gas flow or gas recovery data, the substitute data value could be the arithmetic
average of the quality-assured values of that parameter immediately preceding and immediately
following the missing data incident. If, for a particular parameter, no quality-assured data are
available prior to the missing data incident, the substitute data value shall be the first quality-
assured value obtained after the missing data period.
EPA considered not deducting CFLi combustion that was not recorded, but not including CFLi
recovery could greatly overestimate an entity's emissions. On the other hand, allowing extended
periods of missing data provides a disincentive to repairing the monitoring system.
6. QA/QC and Verification Requirements
In addition, mines should conduct quality assurance and quality control of emission estimates
reported. Facilities are encouraged to prepare an in-depth quality assurance and quality control
plan which would include checks on the data collected, and the calculations performed to
estimate GHG emissions.
MSHA has established specific regulatory requirements for maintaining and ensuring the
accuracy of mine sampling processes and for the operation of in-mine monitors. In particular, 30
CFR Part 75 specifies requirements for the maintenance and use of in-mine CFLt monitors and
for portable air quality detectors and measurement devices. The section also sets forth
requirements for ensuring that MSHA inspectors are trained and certified to take samples, and
operators responsible for in-mine monitoring systems must also be certified.
In general, since their primary current applications are for ensuring mine safety, the accuracy of
existing CH4 detectors and emission measurement devices provide a level of accuracy and
performance that is acceptable for GHG emissions monitoring applications. Similarly, since they
are used to register volumes for gas sales, existing monitors for measuring gas produced from
degasification systems are considered to be accurate, and both sellers and buyers of the gas
produced have a vested interest in ensuring that the volumes recovered are accurate.
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7. Reporting Procedures
For the coal mining industrial sector, the issues to consider with regard to reporting relate both to
the party responsible for reporting emissions to EPA, and to the frequency of reporting. In the
case of the reporting party, options to consider include whether MSHA does the reporting to
EPA (of emissions from ventilation systems), or if the operator should be responsible for
reporting.
Recovered CFLi volumes from degasifications systems (emissions avoided) will be reported by
the operator.
In terms of frequency, options to consider include annual, quarterly, and, perhaps, even more
frequent reporting. EPA considered quarterly reporting.
The issues (other than frequency, as noted above) to consider are somewhat different for
reporting of emissions from ventilation systems or emissions avoided from degasification
systems, so each of these two areas are discussed separately below.
Emissions in Ventilation Air
With regard to mine CFLi ventilation emissions, the information currently collected by MSHA
(CFLt concentration, ventilation flow rates, major changes in mine activity, etc.) should be
sufficient to support implementation of Options 2, 3, or 4, discussed above. However, current
CH4 emissions data collection and reporting are conducted by MSHA field inspectors, with the
results reported to and maintained by MSHA district offices. In general, mine operators are
currently not provided a copy of the results of the sampling or the estimates made for CH4
emissions, unless emission levels are of concern to MSHA.
As discussed above, MSHA currently voluntarily provides EPA with its mine-by-mine estimates
of annual CH4 emissions for the gassiest mines. This provides the basis for both EPA's estimates
of coal sector emissions in the national inventory (EPA 2008), as well as providing the Coalbed
Methane Outreach Program (CMOP) its basis for identifying opportunities for CH4 recovery at
U.S. coal mines (EPA 2005). However, in developing is annual emission estimates, MSHA
develops these estimates by averaging the sample data results from quarterly inspections.
Several options are available for reporting the results of GHG emissions monitoring in the coal
mining sector. These are:
MSHA continues to collect ventilation air samples and rates, and from these develop
estimates of GHG emissions for all mines that exceed a defined emissions threshold
(currently 100,000 cubic feet per day). Then, MSHA would be responsible for reporting
emissions estimates directly to EPA. Under this option, the incremental burden on both
coal mine operators and MSHA would be small.
MSHA continues to collect ventilation air samples and rates, and from these develop
estimates of GHG emissions for all mines that exceed the defined emissions threshold.
Then, MSHA would make these emissions estimates available to mine operators, and the
operators would be responsible for reporting emissions levels to EPA. Under this option,
the burden on MSHA would be the same - it would now just provide its emissions
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estimates back to the operator, rather than EPA. In this case, the burden on the operator
would be greater, since it would now need to report to EPA.
MSHA would continue to collect ventilation air samples and rates, but provide just the
sampling results to the operator, and the operator would develop its own estimates of
GHG emissions from these sample results, and report emissions estimates to EPA. Here,
the burden on MSHA would actually be reduced somewhat (though it would still need to
send the operators the sampling results), but the burden on the operator would be
considerably greater.
Coal mine operators would duplicate the MSHA process, by collecting air samples and
ventilation rates, submitting the samples to a commercial lab for analysis, developing
estimates of emissions, and reporting this information to EPA. This would impose a
burden on the operators. The extent of the burden reduction for MSHA will depend on
whether or not MSHA continues to do its own sampling under its mine inspection
program, which, presumably, it would still do.
Currently, although MSHA collects samples quarterly for the gassy mines, emissions estimates
are only developed on an annual basis. If quarterly reporting is implemented, this would require
that MSHA develop quarterly, rather than annual, CH4 emissions estimates, and report these
either to EPA or to the mine operator. This would result in an increase in responsibility for
MSHA as a result of regulatory requirements to mandate industrial GHG emissions monitoring,
under any of the reporting options listed above.
If mine air ventilation emissions are monitored using a CEMS, the operator would be responsible
for reporting, at the frequency specified (annual, quarterly, etc.).
Under any of the reporting options, it would also be expected that operators should also report
any CH4 emissions from ventilation air that are destroyed or utilized. This data is presumably
automatically collected by the installed equipment. It is also recommended that operators report
the number of days in a quarter that the mine is shut down.
Degasification Systems
With regard to coal mine degasification systems, even when continuous monitoring of CH4
recovered from degasification systems, reporting is done on an annual or quarterly basis.
Operators would be responsible for reporting the volumes of CH4 recovered from these systems,
along with the disposition of the CH4 (sold, used on site, and/or destroyed/flared. Reporting
would be directly to EPA.
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Australian Government, Department of Climate Change, National Greenhouse and Energy
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California Air Resources Board, Second 15-Day Modified Regulatory Language For Public
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2008 (http ://www.arb.ca.gov/cc/reporting/ghg-rep/ghg-rep.htm)
Diamond, William P., Methane Control for Underground Coal Mines, Bureau of Mines
Information Circular 9395, 1994
United States Department of Energy, Office of Policy and International Affairs, Technical
Guidelines Voluntary Reporting of Greenhouse Gases (1605(B)) Program, January 2007
http://www.pi. energy. gov/enhancingghgregistry/documents/january2007_1605btechnicalguideli
nes.pdf
Environmental Protection Agency, Methane Emissions from Abandoned Coal Mines in the
United States: Emission Inventory Methodology and 1990-2002 Emissions Estimates, Coalbed
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(http ://www. epa. gov/coalbed/docs/amm_final_report.pdf)
Environmental Protection Agency, Identifying Opportunities for Methane Recovery at U.S. Coal
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Environmental Protection Agency, Inventory of U.S. Greenhouse Gas Emissions and Sinks:
1990-2006, USEPA#430-R-08-005, April 2008
(http://www.epa.gov/climatechange/emissions/usinventoryreport.htmn
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Intergovernmental Panel on Climate Change (IPCC), 2006IPCC Guidelines for National
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nggip.iges.or.jp/public/2006gl/index.htm)
Lambert, S.W., and M.A. Trevits, "Improved Methods for Monitoring Production from Vertical
Degasification Wells, U.S. Bureau of Mines, Report of Investigations 8309, 1978
(http://www.cdc.gov/niosh/mining/pubs/pdfs/ri8309.pdf
National Institute for Occupational Safety and Health (NIOSH), Handbook for Methane Control
in Mining, CDC Information Circular (1C) 9486, June 2006
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Census, December 2004b
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