Engineering Controls Database

Guidelines for the Control and Monitoring of Methane Gas on Continuous Mining Operations – Effects of Scrubber Operations – Direction of Scrubber Exhaust, Effect on Intake and Recirculation

Problem:	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.
What this means to you:	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.
Description of control:	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 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. Most air from the scrubber exhaust moves straight back through the entry toward the return, but some of this air can recirculate back toward the face. The amount of recirculation depends on several factors including the direction and location of the scrubber exhaust relative to the mouth of the ventilation curtain. Tests examined the effects of the direction of the scrubber exhaust air on recirculation. The blowing curtain setback distance was 35 ft, and intake flow quantity was 7,000 ft3/min. A louver panel placed over the scrubber exhaust allowed the direction of the scrubber exhaust to be varied in the vertical and horizontal directions (Figure 1). Figure - 1 - Directions of scrubber exhaust. Methane gas was released from the face manifold. The average concentration was determined for six locations near the face as shown in Figure 2. Figure 3 shows the average concentrations for each of the exhaust directions. Figure - 2 - Methane sampling locations. Figure - 3 - Effect of scrubber exhaust direction on face concentration. • Methane concentrations were highest when the scrubber exhaust was directed toward the left (intake) side of the entry. • Varying the vertical direction of the scrubber exhaust had little effect on methane concentrations. 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.
References:	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. [http://www.cdc.gov/niosh/mining/highlights/programareahighlights16.html] USBM [1958]. Auxiliary ventilation of continuous miner places. By Stahl RW. Washington, DC: U.S. Bureau of Mines, Report of Investigations, No. 5414.
Keywords:	coal mining continuous mining operations deep-cut mining miners
How well does this control work?	• To reduce face methane concentrations o Do not direct scrubber exhaust toward the intake side of the entry. Directing flow toward the intake side of the entry interferes with and reduces flow moving toward the face. o Whenever possible, the scrubber exhaust and intake curtain should be placed on opposite sides of the entry.