NIOSH logo and tagline

Engineering Controls Database

Guidelines for the Control and Monitoring of Methane Gas on Continuous Mining Operations – Methane Monitoring during Roof Bolting – Laboratory Tests

The introduction of conventional mining methods, which increased the rate of mining, was an important step in the mechanization of mining. The intermittent nature of the conventional mining process halted the extraction process for coal-loading and usually allowed time for methane gas to be dispersed. However, the introduction of continuous mining machines in the 1940s produced a constant flow of coal from the working face of the mine and resulted in an increase in methane levels.

The number of face ignitions increased as more continuous mining machines were placed underground. Methane levels were found to be dangerously high. In some cases, methane concentrations measured 20 ft from the mining face exceeded the lower explosive limit (5% by volume) [USBM 1958]. The need for better face area ventilation was recognized to reduce the potential for explosions.
Excessive levels of methane gas can affect the safety of the underground work force. Available methane control systems have been challenged in recent years by mining developments which include the use of continuous mining machines.

In the past 10 years, explosions have led to 65 fatalities and 18 injuries with major explosions occurring at the Sago Mine in West Virginia in 2006 (12 fatalities and 1 injury), the Darby No. 1 Mine in Kentucky in 2006 (5 fatalities and 1 injury) and, most recently, at the Upper Big Branch Mine in West Virginia in 2010 (29 fatalities) [NIOSH 2011]. The occurrence of a methane gas explosion puts the lives of the entire underground workforce at risk.
The U.S. Bureau of Mines (USBM) was formed in 1910 following a series of underground explosions that resulted in many fatalities and injuries [Kirk 1996]. The agency was responsible for conducting scientific research and disseminating information on the extraction, processing, use, and conservation of mineral resources. The USBM research program for mining health and safety was transferred to NIOSH in 1996. Since that time, NIOSH has established a ventilation test gallery where techniques for methane control and monitoring are evaluated under a variety of conditions that simulate airflow near the working face of a continuous mining section. Airflow patterns and methane concentrations are studied in a detailed manner that is not possible in a working underground mine.

Methane Monitoring during Roof Bolting

Although most frictional methane ignitions occur during the mining of coal, ignitions can also occur during roof bolting. Most of these ignitions occur when the drill bit is heated during drilling in hard or abrasive rock and comes in contact with the methane liberated from roof strata near the drilling operation. Methane concentrations must be measured with a handheld detector at least once every 20 minutes to assure that methane levels do not exceed 1%.

Laboratory Tests

Workers can remain under a supported roof if extendable probes are used to position methane detectors at the face. However, the deeper the cutting depth, the more difficult it is to reach the face with the probe. None of the techniques currently available for making these face measurements during deep cutting are easy to use or have been widely accepted. NIOSH conducted a study to examine alternative ways for monitoring methane at the face during roof bolting [Taylor et al. 1999].

NIOSH conducted tests in the ventilation test gallery to simulate airflow patterns and methane distributions with a roof bolter operating. The model mining machine was located at four locations near the face of the test gallery (Figure 1). Methane was released from the face manifold to simulate uniform face emissions and from a hose nozzle placed against the roof near either the right or left drill booms to simulate methane release from the drill holes.

Methane concentrations were measured at 11 locations (Figure 1) which were divided into three areas:

• Face locations (1–3) were 1 ft from the roof and face.
• Sweep locations (4–7) were 1 ft from the roof and 20 ft inby the location where the bolter operators would stand (i.e., under supported roof).
• Machine locations (8–11) were 1 ft from the roof and adjacent to the T-bar. Location 11 was at the midline of the machine.

Machine locations (1 to 4), intake flows (4,000 and 7,000 ft3/min), and curtain setback distances (28 and 40 ft) were varied to simulate a variety of operating conditions parameters.

Concentrations from all tests for a given sampling area were averaged for each release location. The results (Figure 2) show that concentrations were:

• Highest at the face when gas was released from the manifold.
• Highest at the sweep location when the gas was released from the right or left drill locations.

For the manifold release tests, concentrations measured at the face were compared to concentrations measured at the sweep and machine locations. Measurements made at locations 4 (right side sweep) and 8 (right side machine) had the best correlation with the face concentrations.

During mining, most methane gas is liberated from the face. During roof bolting, generally less gas is released but most liberation occurs from both the face and the roof near the drill holes. The tests show that it is necessary to sample at the sweep and face locations to get the best estimates of gas from the face and roof.

Based on this study, an alternative sampling procedure for use during roof bolting was proposed. The procedure included use of a hand-held detector with extendable probe and a machine-mounted monitor. The detector and probe are used to make measurements no less than 16 ft inby the last area of permanently supported roof at least once every 20 minutes. The methane monitor is mounted on the roof bolter near the inby end of the automated roof support and used to continuously monitor methane levels. The final rule allowing this alternative procedure was published by MSHA in 2003.
Figure 1 - Methane sampling locations for roof bolter tests.

Figure 1 - Methane sampling locations for roof bolter tests.

Figure 2 - Effects of release location on concentration.

Figure 2 - Effects of release location on concentration.

NOTE: The above control information is taken directly from the following publication:
NIOSH [2010]. Information circular 9523. Guidelines for the control and monitoring of methane gas in continuous mining operations. Morgantown, WV: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2010-141.
Kirk WS [1996]. The history of the Bureau of Mines. In: U.S. Bureau of Mines Minerals Yearbook, 1994. Washington, DC: U.S. Bureau of Mines.

NIOSH [2011]. Ventilation and explosion prevention highlights.

Taylor CD, Thimons ED, Zimmer JA [1999]. Comparison of methane concentrations at a simulated coal mine face during bolting. Mine Vent. Soc. of SA 52(2):48–52.

USBM [1958]. Auxiliary ventilation of continuous miner places. By Stahl RW. Washington, DC: U.S. Bureau of Mines, Report of Investigations, No. 5414.
coal mining
continuous mining operations
deep-cut mining
If it is not practical to monitor methane levels at the face during bolting, such levels should be measured with a bolter machine-mounted monitor and with a detector held 16 ft inby the last row of bolts using an extensible pole.