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Engineering Controls Database

Guidelines for the Control and Monitoring of Methane Gas on Continuous Mining Operations – Moving Air to the Mining Face – Effects of Airflow on Methane Concentrations

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.

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.

Effects of Airflow on Methane Concentrations

Methane and airflow readings were taken at the same locations with the same operating conditions in order to compare the effects of airflow on methane concentrations. Methane was released from the face manifold. Profiles were drawn (Figure 1) to show the distribution of methane between the mouth of the curtain and the face for a 10,000-ft3/min intake flow. Similar profiles were obtained with the 6,000-ft3/min intake flow, but the concentrations were higher.

The distribution of the methane between the curtain and the face and at the face locations 1 to 4 generally corresponded to the direction of the airflow.

• For left to right face airflow, the highest concentrations were generally on the right side of the entry.
• For right to left face airflow, the highest concentrations were generally on the left side of the entry.

The methane concentrations (Figure 1) appeared to be similar for the 16½- and 13-ft wide entries. However, methane flow rates through the manifold had to be reduced 80% during tests in the 13-ft wide entry in order to maintain safe methane levels in the ventilation test gallery. If methane flows had been the same for all tests, concentrations would have been much higher in the 13-ft wide entry due to the lower face flow velocities.
Figure - 1 - Methane profiles with 10,000 ft3/min curtain flow for 13- and 16½-ft entries at a 35-ft curtain setback (A), 25-ft curtain setback (B), and 15-ft curtain setback (C).

Figure - 1 - Methane profiles with 10,000 ft3/min curtain flow for 13- and 16½-ft entries at a 35-ft curtain setback (A), 25-ft curtain setback (B), and 15-ft curtain setback (C).

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. []

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
Maintaining adequate intake airflow to the face is one of the most important factors for controlling methane gas at the face. Decreasing entry width can have a major effect on flow patterns by reducing air quantities reaching the face.