Technology News

No. 465 November 1997

U.S. Department of Health and Human Services

Public Health Service

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Method for Predicting Methane Emissions

on Extended Longwall Faces

Objective

To provide longwall operators with a method to predict methane

emissions from panels with increased face width.

Background

The explosion or ignition of methane has been a major cause of

multiple fatalities in underground coal mines in the United States.

Several factors are known to influence methane emissions during

longwall mining, including (1) coal extraction rate, (2) gas content

of the coal mined, (3) methane drainage practices, (4) gas in

surrounding strata, particularly the proximity of these gas-bearing

strata to the mined coalbed, and (5) the development of mininginduced fractures that provide pathways for gas migration to the

active face or gob areas. One trend in longwall mining for which

the methane gas emission consequences were largely unknown

was the mining of larger panels. In an effort to increase mining efficiency, average longwall panel size increased from approximately

183 by 1,411 m (600 by 4,630 ft) in 1986 to 247 by 2,292 m (810 by

7,520 ft) in 1996. Unfortunately, gains in mining efficiency, along

with the extraction of deeper, gassier coalbeds, may be

accomp anied by an increased flow of methane into the

underground workplace.

The ability to predict the methane emission consequences of

altering mine design parameters such as longwall panel dimensions,

particularly face width, is desirable for maintaining a safe work

environment. If higher emissions rates are the result of changing

mining practices, it is preferable from a safety perspective to be

prepared in advance, either with increased ventilation airflow or

with appropriate methane drainage capabilities.

Approach

Two adjacent mines (1,600 m (1 mile) apart) operating in the

Pocahontas No. 3 Coalbed in Virginia provided the test sites to

investigate the methane emission consequence of extending

longwall faces to greater width. Longwall face widths were to be

increased from 229 to 305 m (750 to 1,000 ft) at these mines.

However, because historically high methane emissions from the

longwall face and gob were already being experienced, there was a

concern for further increases in emission rates.

To predict the methane emission consequences of mining

longwall panels of greater face width, it was first necessary to

measure current emission rates to determine if a general increase in

methane emission levels occurs progressively during the mining of

a longwall pass. Methane emissions on the active faces were

measured with sensors attached to shields over the pan line. One

instrument was located close to the headgate; the other, close to

the tailgate. Airflow quantities were periodically measured at the

methane monitoring locations with a handheld anemometer. A

production time study was conducted to correlate methane

emission data to mining activity and shearer location on the face.

Characterizing methane emission trends across the longwall face

was facilitated by dividing each pass on the 229-m (750-ft) wide

faces into three equal-length segments of 76 m (250 ft) and

determining the average face emission rate during the mining of

each pass segment (figure 1).

Results

At the first mine site, methane emissions on the longwall face

increased by 17.4% (0.02 m3/sec (41 cfm)) to 0.13 m3/sec (276 cfm)

from the first to the second 76-m (250-ft) pass segment (figure 2).

Emissions increased by only 3.1% from the second to the third

pass segment. At the second mine site, methane emissions also

increased by 17.4% from the first to the second pass segment

(figure 2); however, the actual average emission rate for the second

pass segment was substantially higher at 0.21 m3/sec (438 cfm). In

contrast to the first mine site, the methane emission rate at the

second site increased significantly (14.6%) from the second to the

third pass segment. The average methane emission rate for the

third 76-m (250-ft) pass segment was 0.24 m3/sec (502 cfm).
 Figure 1.—Schematic plan view of longwall face divided into

equal-length pass segments for analysis and prediction of

methane emission trends during mining.

 Figure 2.—Methane emission prediction curves for 305-m

(1,000-ft) longwall faces in two mines operating in the

Pocahontas No. 3 Coalbed in Virginia.

Predictions of methane emission levels for faces extended to

305 m (1,000 ft) were made by regression analysis of the average

longwall pass segment emission values from the 229-m (750-ft) wide

faces at each mine. A power curve in the form Y ' 236.889X0.179889,

where Y ' average measured pass segment methane emission rate

and X ' individual 76-m (250-ft) pass segment number, fit the

measured emission data from the first mine best (R2 ' 0.95).

Projecting the regression curve to a fourth 76-m (250-ft) longwall

pass segment yields an average methane emission rate of 0.14

m3/sec (304 cfm), or a 7% (0.01-m3/sec (20-cfm)) increase for a 305-m

(1,000-ft) wide face (figure 2). 

A linear regression curve in the form Y ' 309.6 % 64.25X, where

Y ' average measured pass segment methane emission rate and

X' individual 76-m (250-ft) pass segment number, fit the measured

emission data from the second mine best (R2 ' 0.99). Projecting the

regression curve to a fourth 76-m (250-ft) longwall pass segment

yields an average methane emission rate of 0.27 m3/sec (567 cfm),

or a 13% (0.03-m3/sec (65-cfm)) increase for a 305-m (1,000-ft) wide

face (figure 2).

The relatively low predicted increase in methane emissions at

the face for the first mine site would probably not result in

additional methane control problems. However, it should be noted

that the predicted average emission value for the additional 76 m

(250 ft) of face width is not necessarily the "worst-case" scenario,

which would include high methane emission events such as the

floor gas bleeders common at this mine. In contrast, the predicted

average face emissions for 305-m (1,000-ft) wide faces at the second

mine site (0.27 m3/sec (567 cfm)) are within the general range where

methane-related mining delays were experienced during the study.

Face widths of 305 m (1,000 ft) have not been developed at the

first mine site; therefore, the accuracy of the prediction could not

be tested. Larger panels with 305-m (1,000-ft) wide faces were

developed at the second mine site without experiencing the

predicted higher methane emission levels. The mine's engineering

staff credits an enhanced methane drainage program that provided

both increased hole length and additional time for horizontal

boreholes to drain gas in advance of mining with maintaining

methane emissions at a lower than predicted level.

For More Information

To obtain a free copy of a technical paper on the prediction of

longwall methane gas emissions or for more information on other

longwall methane emission characterization studies, contact W. P.

Diamond, National Institute for Occupational Safety and Health

(NIOSH), Pittsburgh Research Laboratory, Cochrans Mill Rd., P.O.

Box 18070, Pittsburgh, PA 15236-0070, phone: (412) 892-6551, fax:

(412) 892-6891, e-mail: wbd5@cdc.gov Mine operators using the

prediction technique outlined in this Technology News are

encouraged to discuss their application and results with the NIOSH

contact listed above.

Mention of any company name or product does not constitute

endorsement by the National Institute for Occupational Safety and

Health.

To receive additional information about mining issues or

other occupational safety and health problems, call 1-800-

35-NIOSH (1-800-356-4674), or visit the

NIOSH Home Page on the World Wide Web at

http://www.cdc .

gov/niosh/homepage.html 

As of October 1996 the safety and health research

functions of the former U.S. Bureau of Mines are

now located in the National Institute for

Occupational Safety and Health (NIOSH).