Engineering Controls Database

Guidelines for the Control and Monitoring of Methane Gas on Continuous Mining Operations – Moving Air to the Mining Face – Blowing Curtain and Tubing

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. Moving Air to the Mining Face Effective face ventilation requires that a sufficient quantity of intake air be delivered to the mining face in order to dilute liberated methane to a safe level. Federal regulations include the following requirements: • Face ventilation control devices shall be used to provide ventilation to dilute, render harmless, and to carry away flammable, explosive, noxious, and harmful gases, dusts, smoke, and fumes [30 CFR 75.330]. • A minimum quantity of air (3,000 ft3/min) is required at each face area [30 CFR 75.325]. A mine operation must specify in its ventilation plan the minimum quantity of air required to maintain methane levels below 1% at their working faces. Early USBM research examined ways to deliver air to the end of the curtain or tubing with minimal losses. Recent NIOSH research has examined more effective ways to move air from the end of the curtain to the face. New monitoring instruments and sampling techniques made it possible to examine how operating conditions affect airflow inby the curtain or tubing. Blowing Curtain and Tubing Either curtain or tubing is used to direct intake air toward a mining face. The area behind a curtain is usually much larger than the cross sectional area of tubing. Therefore, for a given flow quantity, the velocity of air provided by tubing is much higher than that provided by a curtain. Tests were conducted to evaluate how this difference in velocities from curtain and tubing affects methane liberated from a mining face. A series of tests were conducted with two methods of directing air to the face: 18-in diameter solid tubing and a 7-ft high curtain placed 2 ft from the side of the entry [Taylor et al. 1997]. Setback distances of 10, 30, and 50 ft with an intake flow of 5,500 ft3/min were evaluated. Velocities at the mouth of the tubing and curtain were 3,125 and 393 ft/min, respectively. The model mining machine was placed at the center of the face. Methane was released from the face manifold and measured at six locations near the face (Figure 1). The results are displayed in Figure 2. Figure 1 - Methane sampling locations for curtain/tubing tests. Figure 2 - Average face methane concentration with tubing or curtain. Methane levels increased as the curtain setback distance was increased from 10 to 50 ft. At the 50-ft setback, the test with curtain had to be terminated because methane concentrations at one or more of the sampling locations exceeded safe operating limits (2.5%) for the test gallery. 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] Taylor CD, Rider JP, Thimons ED [1997]. Impact of unbalanced intake and scrubber flows on methane concentrations. In: Ramani RV, ed. Proceedings of the 6th International Mine Ventilation Congress, Chapter 27. Pittsburgh, PA: SME/AIME, pp. 169–172. 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?	(1) Methane levels did not change significantly as tubing setback distance increased from 10 to 50 ft. (2) The higher tubing velocity had a significant effect on lowering face methane levels. (3) Methane levels increased as curtain setback distance increased from 10 to 50 ft. (4) The curtain was less effective than the tubing at controlling methane levels at greater setback distances.