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Engineering Controls Database

Guidelines for the Control and Monitoring of Methane Gas on Continuous Mining Operations – Effects of Scrubber Operations – Effects of Scrubber use on Methane Levels above the Machine

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 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.

Effect of Scrubber Operation on Methane Concentrations above the Machine

Scrubbers are used to remove dust from the air in the environment of the mining face. Dusty air from the face passes through and captured on a wetted filter and the cleaner air is exhausted at the rear of the mining machine. The scrubber moves a large quantity of air in the face area. Earlier work had shown that this air movement can improve the dilution and removal of methane gas from the face area.

Earlier work examined the effects of face airflow patterns on methane distributions in an empty entry. Tests were conducted to determine how scrubber operation affects methane levels above the mining machine, in the area between the face and blowing curtain [Taylor et al. 2006]. Methane levels were measured at 21 locations above the mining machine (Figure 1), which was located at the center of the 13-ft wide entry. The sampling locations were 4 to 4½ ft from one another and 2 ft from the sides and face of the entry. Curtain setback distance was 35 ft. Intake flows were 4,000 and 6,000 ft3/min. The scrubber was either off or operated at flows of 4,000 or 6,000 ft3/min.
Figure - 1 - Methane sampling locations above mining machine (scrubber tests).

Figure - 1 - Methane sampling locations above mining machine (scrubber tests).

Methane was released from the face manifold. The distributions of the methane over the mining machine are shown in Figure 2 for intake flows of 4,000 and 6,000 ft3/min.
Figure - 2 - Methane concentrations above mining machine.

Figure - 2 - Methane concentrations above mining machine.

As methane gas moves further from a mining face it is diluted by the intake air. At some distance from the face, intake air and methane will be mixed uniformly and no further dilution will occur. To illustrate this progressive dilution of gas, the average concentration for each horizontal row of three samples was averaged and plotted versus distance from the face (Figure 3).
Figure - 3 - Methane concentrations above mining machine.

Figure - 3 - Methane concentrations above mining machine.

• With 4,000 ft3/min intake flow and the scrubber off, gas dilution takes place slowly.
• With 6,000 ft3/min intake flow, dilution takes place more quickly with and without the scrubber.
• With 4,000 and 6,000 ft3/min intake flow, scrubber use increased the rate of methane dilution, especially within the first 15 ft of the face.

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, Chilton JE, Hall E, Timko RJ [2006]. Effect of scrubber operation on airflow and methane patterns at the mining face. In: Mutmansky JM, Ramani RV, eds. 11th U.S./North American Mine Ventilation Symposium. Leiden, The Netherlands: Taylor & Francis/Balkema, pp. 393–399.

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
• Increasing scrubber flow decreased methane concentrations at most locations above the machine.
• Increasing intake flow decreased methane concentrations at most locations above the machine.
• The highest methane concentrations generally occurred in the corner of the face opposite the blowing curtain