Handbook for Methane Control in Mining - AMMSA

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Ways to confirm the presence of gas underground. Given that low emissions of methane canbe hazardous, how does one determine if there is a potential methane problem, assuming thatthere were no clues from exploration boreholes? There are three possible ways.1. Look for gas in the parts-per-million range. When testing for methane gas, return air samplesshould be collected in a bag or bottle specially designed for gas sampling. A laboratory analysisthat uses a chromatograph to look for gas in the parts-per-million range is then conducted.Applying this method to the scenario in Figure 14–1, the return air sample would have shown100 ppm, a definite indicator of gas in low quantities. 4 When sampling, the ambient air on thesurface should also be measured, as it generally contains a few parts per million of methane.2. Hunt for gas when ventilation is temporarily off. It is common for tunnel ventilation systemsto be down for short periods while fan changes are being made or ductwork extended. Becauseeven low gas emissions accumulate to measurable levels quickly, this is an opportune time tohunt for gas accumulations with a handheld methane detector. If gas has already been shown toexist, this hunt is an important safety measure.3. Look for gas in those places where it is most likely to accumulate. Gas emitted at the facewill accumulate in unventilated corners near the face. Emissions from small cracks or fissuresnear the crown may produce a methane layer there because methane is much lighter than air.In operations using a tunnel boring machine (TBM), gas accumulations near the muck dischargepoint are likely. If the tunnel is unlined, any location along the entire length is a potential site fora methane layer at the crown.As with surface samples, the initial presence of gas underground must be confirmed to a higherlevel of accuracy by laboratory analysis. If a field instrument shows that gas is present, an airsample must be collected in a bag or bottle specifically designed for gas sampling. The analysisis normally conducted with a carefully calibrated gas chromatograph. To be effective, the huntfor gas in tunnels must be conducted at frequent intervals. This is the only way to detect gas inisolated pockets. If the presence of gas is suspected but not yet confirmed, the tunnel air shouldbe tested for methane with a handheld instrument at least twice per shift.171PROVIDING ADEQUATE VENTILATIONAmple dilution to safe levels. Enough ventilation air must be provided to immediately dilutethe methane gas to safe levels as soon as the gas enters the tunnel. Methane is combustible whenmixed with air in the range between 5 and 15 vol % of gas. The 5 vol % value is the lowerexplosive limit (LEL). Methane concentrations in air that are below the LEL are not explosive.The 15 vol % value is the upper explosive limit. Gas mixtures with concentrations above thislimit are not explosive, but may become so if mixed with more air.4 Low levels of methane gas have been found in a wide variety of hard-rock and noncoal mines. For mines classifiedas gassy by the Mine Safety and Health Administration, Thimons et al. [1979] found that the return air methaneconcentrations were 70 ppm or higher. No similar research on methane has been conducted in tunnels, but there islittle doubt that a comparable value in a return airflow of 10,000 cfm would show hazardous quantities of gas.
172Simultaneous application of three basic elementsreduces the methane hazard:• Adequate ventilation.• Regular monitoring of air quantities and gasconcentrations, with automatic equipment shutoffat high gas concentrations.• Elimination of ignition sources, including thosethat are worker-related.The simultaneous application of several elements isnecessary because if one fails, the others continue toensure safety.When methane is emitted from the strata, it is usually at high concentration. As it progressivelymixes with air, the concentration will pass through the explosive range and down below the LEL.A good ventilation system will supply enough fresh air to reduce all of the gas to far below theLEL as soon as the gas is emitted from the strata.In referring to gas concentrations, different government agencies may use different terminology.For example, in regulating coal mines, the Mine Safety and Health Administration (MSHA)specifies that the concentrations of methane at coal mine working faces remain below 1.0 vol %.This is the same as 20% of the LEL. With the LEL of methane in air at 5 vol %, 20% of 5 vol %is 1.0 vol %. Specifying a percentage of the LEL is advantageous when mixtures of flammablegases are emitted.Main ventilation systems. Main ventilation systems carry air from the portal into the TBMtrailing gear. These are classified as either blowing or exhausting. In blowing systems, fanslocated on the surface and along the ductwork push air through the ductwork into the tunnel.In exhaust systems, air in the ductwork flows out of the tunnel. Each system has its advantages.Selection of either an exhaust or a blowing main ventilation system will depend on whether aface shield and scrubber are used, on whether or not a scavenger system is used, and on the typeof ductwork used. 5Face ventilation systems. Face ventilation systems carry air from the trailing gear to the face ofthe tunnel where rock is broken and removed. In most instances, the primary source of gas is atthe face, so it is vital to provide adequate ventilation air all the way to the face, that is, to thelast foot. 6 For this reason, the ventilation focus of this chapter is on face ventilation.5 When planning a tunnel ventilation system, make a simple diagram of all ductwork and airflow movement toensure that the ventilation mistakes described in this section are not incorporated into your plans.6 In some instances it is also necessary to focus attention on the muck discharge point. For example, in earth pressurebalance machines most of the methane may be released at the end of the screw conveyor. A nearby fan or compressedair venturi can be used to dilute this gas, but does not relieve the need to provide adequate ventilation all theway to the last foot.
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TMIC 9486Information Circular/2006H
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ORDERING INFORMATIONCopies of Natio
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ILLUSTRATIONS—ContinuedPage4-6. U
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HANDBOOK FOR METHANE CONTROL IN MIN
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4Below 5%, called the lower explosi
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6reduced pressure, except at very l
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8Static electricity. Protection aga
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10Figure 1-4.—Estimated methane c
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12LAYERING OF METHANE AT THE MINE R
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14good eyesight. 24methane level.Ot
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16a material balance indicated that
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18As an example, assume that themet
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20Figure 1-10.—Relative frequency
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22Davies AW, Isaac AK, Cook PM [200
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24Margerson SNA, Robinson H, Wilkin
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CHAPTER 2.—SAMPLING FOR METHANE I
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29USING PORTABLE METHANE DETECTORST
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Out-of-range gas concentrations in
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Figure 2-3.—Recorder chart from a
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35Industrial Scientific Corp. [2004
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38peaks, not the overallmethane lev
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40hung on J-hook assemblies, which
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42Methane dilution effectiveness.Th
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44found that effective scrubber ope
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46When the scrubber exhaust is not
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48Methane monitors are usually moun
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50to use radial bits instead of con
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52Mott ML, Chuhta EJ [1991]. Face v
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54Service, Centers for Disease Cont
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56Methane accumulationsaround thesh
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58corner and by 43% at supportNo. 4
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60When using water sprays to reduce
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62Cecala AB, Zimmer JA, Thimons ED
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64DESIGNING BLEEDER SYSTEMSAs part
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66Caved area characteristics. The c
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68then move this gas into the activ
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70perform tests to determine whethe
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72A major purpose of the bleeder sy
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74• Inlets to the pillared area n
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76REFERENCESCFR. Code of federal re
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78Methane is released into each min
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80Figure 6-1.—Gas content of coal
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82Figure 6-3.—Simplified illustra
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842. In-mine inclined or vertical b
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861. Packed cavity method and its v
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88Table 6-3.—Methane capture rati
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90Early experiences with this metho
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9211. At the surface installation (
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94• Estimated cost for moderately
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96Thakur PC [1981]. Methane control
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98Anomalous, unanticipated methane
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100Vertical methane drainage boreho
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102Figure 7-2 shows a mine entry ap
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104obvious solution to this problem
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106Figure 7-8.—Hypothetical gas c
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108Lama and Bodziony [1998] compile
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110In-mine methane drainage systems
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112Iannacchione AT, Ulery JP, Hyman
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114More sophisticated reservoir eng
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116coal lithotype on gas content is
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118FORECASTING REMAINING GAS-IN-PLA
Page 125 and 126: 120⎛ y⎞⎜⎛⎞ ⎛ ⎞= ⎜
Page 127 and 128: 122emissions. The geometry and size
Page 129 and 130: 124Reservoir models require a subst
Page 131 and 132: 126King GR, Ertekin T [1989a]. A su
Page 133 and 134: 128an area of 314 ft 2 would requir
Page 135 and 136: 130In the case of the abovementione
Page 137 and 138: 132FILLING SHAFTS AT CLOSED MINESFi
Page 139 and 140: 134Hinderfeld G [1995]. Ventilation
Page 141 and 142: 136To calculate the effectiveinert,
Page 143 and 144: 138exhaust. The remaining diesel ex
Page 145 and 146: 140required only 4 min. As a result
Page 147 and 148: 142Figure 11-1.—Desorption test a
Page 149 and 150: 144enclosed in a tunnel-like struct
Page 151 and 152: 146Kolada RJ [1985]. Investigation
Page 153 and 154: 148air in a 6-ft by 9-ft by 6.5-ft
Page 155 and 156: 150represents flammable mixtures. F
Page 157 and 158: 152• In Eastern Europe, petroleum
Page 159 and 160: 154Category II applies to domal sal
Page 161 and 162: 1562. Monitoring for gas and taking
Page 163 and 164: 158These mines typically have large
Page 165 and 166: 160Dave Graham is the safety and he
Page 167 and 168: 162Figure 13-2.—Examples of metha
Page 169 and 170: 164REFERENCESAndrews JN [1987]. Nob
Page 171 and 172: 166APPENDIX A.—ONTARIO OCCUPATION
Page 174 and 175: 169CHAPTER 14.—PREVENTING METHANE
Page 178 and 179: 173The tunnel face is usually venti
Page 180 and 181: 175Figure 14-5.—TBM ventilation s
Page 182 and 183: face. While one of these elements a
Page 184 and 185: 179ELIMINATING IGNITION SOURCESElec
Page 186 and 187: 181INDEXAAbnormally gassy faces....
Page 188 and 189: 183NNatural ventilation, coal silos
Page 190 and 191: Delivering on the Nation’s Promis
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