Handbook for Methane Control in Mining - AMMSA

38peaks, not the overallmethane level. Figure 3–2shows peak average 4methane emissions in severalU.S. coalbeds [Haney et al.1983]. Except for onecoalbed at 1,300 ft depth,peak average coalbed emissionsrange from 4 to 33 cfm,with an overall U.S. averageof 17 cfm. Any face ventilationsystem must be able tosafely handle gas flows ofthis magnitude.Figure 3–2.—Peak average methane emissions for several U.S.coalbeds.Although the values cited here are peak averages, the valuefor individual peaks can vary widely. For example, a studyby Smith and Stoltz [1991] has shown a variation 5 of 46% inemission peak values. Similarly, there was a variation 5 of50% in methane dilution capacity.VENTILATION WITH EXHAUST LINE CURTAIN OR DUCTPrior to the development of improved face ventilation systems, most coal mine faces wereventilated with exhaust line curtain or ventilation duct. For this reason, exhaust line curtain orventilation duct can serve as a baseline against which newer systems can be measured. Federalcoal mine regulations [30 CFR 6 75.330] mandate that exhaust systems have a maximum setbackdistance of 10 ft, i.e., the distance from the face being mined to the inlet of the line curtain orduct is 10 ft or less (Figure 3–3). However, if a mining machine starts a cutting cycle when thesetback is 10 ft and subsequently advances another 10 ft, then the curtain or duct ends up at 20 ftif it has not been moved forward during the cutting cycle.4 This is the average of the emission peak values.5 Strictly speaking, the coefficient of variation, which is the standard deviation divided by the mean.6 Code of Federal Regulations. See CFR in references.