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Measurement and Monitoring of Mine Gases
Measurement and Monitoring of Mine Gases
19
Chapter Outline 19.1
Detection Methods 19.1.1 19.1.2 19.1.3 19.1.4 19.1.5
19.2
313
Methane Measurement 314 Oxygen Detectors 315 Carbon Monoxide Detectors 315 Oxides of Nitrogen 315 Radon 316
Monitoring of Mine Gas
318
19.2.1 US Mine Survey Results 319 19.2.2 System Manufacturers 319
19.3
Wireless Communication and Monitoring System 320
19.4
Special Arrangements for Monitoring in Mines Liable to Spontaneous Combustion 320
19.3.1 Intrinsically Safe CO Detectors 320
19.4.1 Real-Time Monitoring 321 19.4.2 Tube Bundle 321 19.4.3 Gas Chromatography 322
References
323
Properties of mine gases were discussed in Chapter 13 of the book and their threshold limit values were listed in Table 1.3. Correct measurement of the concentrations of these gases and, in some cases, continuous monitoring of the concentrations is essential for the health and safety of coal mine workers. In some coal mining countries, it is also a legal requirement.
19.1
Detection Methods
Detection techniques can be classified into the following categories: 1. 2. 3. 4. 5.
Catalytic-oxidation detectors. Electrochemical sensors. Optical detectors. Electrical conductivity using semiconductors. Stain tubes: The concentration is usually read on a linear scale on the tube.
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Table 19.1 Gas Detection Methods Gas
Detection Methods
Methane
Flame safety lamp Catalytic oxidation Thermal conductivity Optical (infrared and interferometer)
Oxygen
Liquid absorption Stain tubes Paramagnetic analyzers Electrochemical sensors
Carbon dioxide
Liquid absorption Stain tubes Optical interferometer
Carbon monoxide
Electrochemical sensors Catalytic oxidation Optical and infrared Metal oxide semiconductor Stain tubes
Oxides of nitrogen
Electrochemical sensors Stain tubes
Hydrogen sulfide
Electrochemical sensors Metal oxide semiconductors Stain tubes
Sulfur dioxide
Electrochemical sensors Stain tubes
Hydrogen
Stain tubes
Radon
Radiation detectors
Detection (more correctly spot readings) methods for different gases are listed in Table 19.1 [1]. Many instruments have been developed over the past 100 years to measure the instantaneous concentrations of all important gases listed in Table 19.1. The oldest is the flame safety lampda symbol of safety in mines. It detected both the lack of oxygen (above 13%) and small concentrations of methane (below 5%) and provided light as well for miners to work safely. It is almost obsolete by now. Instruments specially suited to measure the concentration of important gases are discussed below.
19.1.1
Methane Measurement
Two handheld instruments are most commonly used. The first one, which is cheaper, uses the catalytic-oxidation technique. It works on the Wheatstone bridge principle:
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one leg of the bridge is used to burn methane catalytically raising its temperature. The imbalance in the current flow is a measure of methane concentration. Such detectors are also sensitive to the presence of higher hydrocarbons, hydrogen, and carbon monoxide, but fortunately, these interfering gases are present in mine air only in parts per million (ppm) and do not introduce serious error in methane measurements. Catalyticoxidation type instruments are liable to get damaged if methane concentration exceeds 5% because of excess heat. Optical detectors for methane are called “interferometer.” These detectors compare the speed of light through pure air with that in air contaminated by methane. Light traveling through both media is combined again producing “interference fringes.” The position of these fringes indicates methane concentrations. These instruments are also sensitive to other hydrocarbons and hydrogen. Ethane and propane present in air indicate a higher concentration of methane, but CO and H2 have an effect of lowering the actual methane concentrations. It is also sensitive to water vapor and carbon dioxide, but these gases are scrubbed out by passing the inlet gas through a column that absorbs both of them. One percent of CO2 in air will indicate 1% methane even if there is no methane there. Lack of oxygen also impacts an interferometer. Each 1% decrease below 20.95% (normal O2 in air) results in 2% methane reading even if there is no methane in air. As such, it is not a very reliable instrument for 0%e5% methane. It is much more useful to measure higher concentrations, 5%e100% of methane.
19.1.2 Oxygen Detectors A flame safety lamp was the most commonly used device to measure oxygen deficiency in past, but it is now replaced by new instruments that work on various principles, such as liquid absorption, paramagnetic, or electrochemical cells. Many instruments measure both methane and oxygen concentrations over the range 0%e5% and 0%e21%, respectively. Stain tubes are also available to measure O2 concentration in mine air.
19.1.3 Carbon Monoxide Detectors As listed in Table 19.1, five different techniques are available to measure CO in mine air. Ambient CO concentration is normally checked by a handheld instrument that uses catalytic oxidation, but the readings can be seriously impacted by other gases present in air, especially higher hydrocarbons. For detecting spontaneous combustion, reliable measurements are given only by gas chromatographs (GCs). Stain tubes are good indicators, but it must be followed by at least two samples of the mine air for GC analysis. In many cases, when the handheld CO monitor read 100e200 ppm, the actual CO concentration as measured by a GC was only 5e10 ppm.
19.1.4 Oxides of Nitrogen With the introduction of diesel engines in coal mines some 50 years ago, the need to measure both NO and NO2 has become urgent. Most commonly stain tubes with a
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handheld pump were used for this purpose, but now electrochemical cells have become available for this purpose. When NO, NO2, or even CO enter a special cell; they react with the electrolyte and create a light signal, “a photon.” A bank of photomultiplier tubes picks up the signal and translates it into specific gas concentrations. The precision of such instruments is quite good at 1 ppm. Earlier instruments were bulky, such as Ecolysers [2], and were difficult to carry, but new instruments are light and can be handheld. An instrument named “Passport” can measure five gases in a single unit as listed below. CO (1e50 ppm) O2 (1%e20%) CH4 (1%e5%) NO (1e25 ppm) and NO2 (1e5 ppm).
It is strongly advised that for conclusive results, all gas samples must be analyzed in a laboratory by trained technicians using a GC.
19.1.5
Radon
Radon is a gaseous, chemically inert, radioactive product of the disintegration of radium. Found primarily in uranium mines, although present in trace amounts in several coal mines, radon diffuses from the rock strata into the mine environment, where the decay process continues. Table 19.2 gives the disintegration process for uranium-238 to become lead-206. Shown in this table is the type of radiation given off by each decay process and the half-life of each element in the series. The halflife of a radioactive substance is the time required for a given amount of that substance to lose one half of its radioactivity. The half-life of uranium-238 is approximately 4.5 billion years; radium, 1622 years; and radon, 3.8 days. Once radon is released into the mine environment, the decay process continues with the formation of radium A, which decays to radium B, which produces radium C, and so forth. The products formed by the decay of radon are referred to as radon daughters. The radon daughter products are atoms of solid matter having relatively short half-lives. During the decay process, either alpha or beta particles are emitted. These emissions may also be accompanied by gamma ray activity. It is the shortlived alpha particles and potential alpha emitters such as radon and its daughters that are of prime concern to the ventilation engineer. Because it is a gas and has a relatively long half-life, inhaled radon is exhaled before large amounts of alpha particles are emitted. The daughter products, however, attach themselves to the dust that is present in the environment and when inhaled, tend to be deposited in the respiratory system. It has been estimated that when both radon and radon daughters are inhaled, only about 5% of the alpha radiation received is contributed by the radon [3]. During radioactive decay, the individual members in the series are decaying and being formed at the same time. At some point in time, equilibrium is reached, and the quantity of each member in the series remains constant. At this time, each member
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317
Table 19.2 Uranium Disintegration Process Common Name or Symbol
Isotope
Type of Radiation
Half-Life
Uranium
238 92
Uranium
Alpha
4.49 109 year
UX1
234 90
Thorium
Beta
24.1 days
UX2
234 91
Protactinium
Beta
1.17 min
Uranium-234
234 92
Uranium
Alpha
2.48 105 year
Ionium
230 90
Thorium
Alpha
8 104 year
Radium
226 88
Radium
Alpha
1622 year
Radon
222 86
Radon
Alpha
3.825 days
Radium A
218 84
Polonium
Alpha
3.05 min
Radium B
214 82
Lead
Beta, gamma
26.8 min
Radium C
214 83
Bismuth
Beta, gamma
19.7 min
Radium C0
214 84
Polonium
Alpha
2.73 106 min
Radium D
210 82
Lead
Beta, gamma
22 year
Radium E
210 83
Bismuth
Beta
5.02 days
Radium F
210 84
Polonium
Alpha
138.3 days
Radium G
206 82
Lead
–
Stable
in the series is being generated at the same rate which it is decaying. The time required for the radon daughters through radium C0 to reach equilibrium from a given quantity of radon is approximately 3 h. In approximately 40 min, the alpha energy reaches approximately 50% of maximum.
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Exposure to excessive concentrations of radon and radon daughters has been linked with a high incidence of lung cancer. The maximum exposure limit for radon daughters has been set at 1.0 working level (WL), with a yearly cumulative exposure of 4 working level months (WLM). A working level is defined as that concentration of shortlived radon daughter products in a liter of air that will yield 1.3 105 million electron volts of alpha energy in decaying through radium C’. WLM is a cumulative measure of exposure that is calculated by multiplying the average WL of exposure during a given timed period by the time of exposure and dividing by 173 (the number of working level hours per WLM) [4]. An example calculation is as follows: Given the following exposures during a shift: 4 h 0.4 WL 2 h 0.2 WL 2 h 0.1 WL
Find the WLMs of exposure. Solution: Average WL ¼
4 0:4 þ 2 0:2 þ 2 0:1 8 ¼ 0:275 WL
WLM ¼ ð0:275 WLÞð8 hÞ=ð173Þ ¼ 0:013 WLM
19.2
Monitoring of Mine Gas
In the United States, CO monitoring in the belt entry is required by law for fire detection. This was the beginning of mine monitoring. It slowly developed into atmospheric monitoring system (AMS). While the CO-monitoring system monitors only CO concentrations in strategic locations, the AMS monitors many other parameters and locations. Parameters that are monitored comprise CO, CH4, O2, NO, H2 concentrations, smoke, air velocity, and air temperature. The locations that are monitored comprise mine airway, battery charging station, fans, fan houses, electric equipment, pumps, and coal storage. A monitoring system, as defined by the Mine Safety and Health Administration, is a network of hardware and software meeting the requirements of 30CFR 75.301 and capable of performing the following functions: 1. 2. 3. 4. 5.
Measure the required atmospheric parameters. Transmit the data to a surface location. Provide alert and alarm signals. Process and store measured data. Create reports by analyzing the data.
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A recent report [5] surveyed 235 US mines and major findings are presented below.
19.2.1 US Mine Survey Results These mines had 204 CO systems and 33 AMS systems. The parameters monitored and number of sensors are listed in Table 19.3 [5]. It is apparent that the CO systems make up the majority. It may be because it is cheaper and has fewer regulatory requirements for inspection, operation, and maintenance.
19.2.2 System Manufacturers There are several manufacturers of the monitoring systems in the United States, but the four major manufacturers are (1) Pyott-Boone, (2) AMR, (3) Conspec, and (4) Matrix. Table 19.4 shows their share of the market.
Table 19.3 Distribution of Monitors in US Mines Parameter Monitored
Percent of Mines
Number of Sensors
CO
100
2e300
CH4
17
1e20
O2
6
1e20
NO
1
1e3
H2
2
1e3
Smoke
2
1e14
Air velocity
9
1e20
Heat
2
4e191
Table 19.4 Manufacturers of the Monitoring Systems Manufacturer
Number of Systems Installed
Average Number of CO Sensors/System
Pyott-Boone
161
27
AMR
32
54
Conspec
25
84
Matrix
15
45
Others
9
NA
320
19.3
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Wireless Communication and Monitoring System
After the Sago Mine disaster in 2006 where 12 miners died, MINER Act was passed by the US Congress. It required underground coal mines to install two-way communication and tracking systems. The need to communicate is universal but essential in an emergency in underground coal mines. Several manufacturers have developed communication and tracking systems but one of them, the Innovative Wireless Technologies (IWT), shows the promise to transmit gas concentration data also [6]. IWT earlier developed the SENTINEL system which is comprises line-/battery-powered network mesh modes. It supports voice, text, tracking, data, personnel and vehicle tracking tags, two voice handset models, dispatch and tracking stations. In 2017, they developed an HDRMesh system that is wireless extension or booster for a fiber optic network. It can provide long-range, reliable communications in hard-to-reach working areas with no fiber interconnect between them. The ability of this system is claimed to be better than radio-frequency identification tracking systems. In early 2017, IWT expanded the use of their system to carry CO concentration data from various locations without any cable; that is wirelessly. The CO monitor has a 6-month battery and a 1000 ft range. These units can be placed every 1000 ft along the belt, and they can transfer the data to the next sensor, 1000 ft away until the surface is reached. It is also claimed to be cheaper than the cabled system. Extensive field testing is necessary to confirm the claims of the manufacturers.
19.3.1
Intrinsically Safe CO Detectors
One such detector, called Sentro 1, is manufactured by Trolex Ltd. (UK) and marketed in the United States by Strata Worldwide. The latter has a wireless communication system that works on batteries. A combination of the intrinsically safe CO sensors with a wireless communication system provides a very good alternative to the present cabled system. A wireless system is defined as one that needs no external power, cables, repeaters, or splitters. Battery life is 40e60 days. Monthly calibration is needed.
19.4
Special Arrangements for Monitoring in Mines Liable to Spontaneous Combustion
Mine AMS becomes especially important when the coal seam being mined is liable to spontaneous combustion. Brady [7] recommends a combination of three kinds of systems, namely (1) real-time monitoring, (2) tube bundles, and (3) on-site ultrafast GCs. Even an aggressive approach like this cannot prevent a fire, but it does offer means to identify the problem early and a chance to contain the fire before it becomes too big to control and the mine has to be shut down.
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19.4.1 Real-Time Monitoring Real-time sensor systems are ideal for telling us what is happening now. The sensors must be located where the gas needs to be measured, and the measurement signal is sent to the surface. This means having multiple sensors underground. These sensors are exposed to the harsh underground environment which is not ideal for precise analytical measurements. This is not really a major problem as these systems are used to detect step changes, such as the onset of a fire, a sudden increase in a mine gas in the general body, or reduction in oxygen. They offer real-time warning and are the best system for identifying a sudden event such as a belt fire. The situation is reported when it happens. Generally, sensors included are for methane, carbon monoxide, carbon dioxide, and oxygen. These types of sensors employed underground tend to have limited measuring ranges: carbon monoxide is often only capable of being measured up to 50 ppm, methane to 5%, and carbon dioxide to several percent. This range is fine while no problems exist and indeed to alert the onset of a problem. But if a fire or other major incident involving generated gases occurs, these sensors may quickly reach full scale and become unable to return a true indication of the concentrations. Most of these sensors require the presence of oxygen to work and are therefore unsuitable for monitoring areas of low oxygen concentration such as sealed or nonventilated gobs. As each individual sensor needs to be calibrated regularly (at least monthly), they are not suited to being located for long-term monitoring in inaccessible areas such as the gob. Some of these sensors also suffer from cross sensitivities, as the reactions they rely on to give a response that can be common to other gases found underground, such as carbon monoxide sensors being cross sensitive to hydrogen sulphide and hydrogen. In the case of an explosion, it is likely that the real-time monitoring system will be rendered inoperable, requiring other techniques for the determination of the status of the underground environment.
19.4.2 Tube Bundle Tube bundle systems draw gas samples from designated sampling locations underground to the surface through plastic tubes using vacuum pumps and analyzed sequentially using infrared and paramagnetic techniques. Gases measured are carbon monoxide, carbon dioxide, methane, and oxygen. Because the analyzers are on the surface, tubes can be located in the gob as once positioned there is no requirement to access the end sampling point. Tube bundle systems are suited to long-term trend analysis. Very good analytical equipment is available and can be housed in dedicated air-conditioned rooms on the surface with the samples dried and passed through particulate filters prior to entering the analyzer. Generally tube bundle systems are set up to measure oxygen, carbon monoxide, carbon dioxide, and methane. Given their ability to measure carbon monoxide down to
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1 ppm, the long-term stability of these analyzers, and the frequent sampling, this technique is best for long-term trending of carbon monoxide to identify a spontaneous combustion event. With respect to measuring range, it is normally only carbon monoxide that presents problems, with most systems capable of measuring to only 1000 ppm. Because methane and oxygen concentrations can be measured over all expected concentrations ranges, this technique is the best for automated monitoring of explosibility of an area so long as a fire or heating does not exist. To get this improved stability and analytical capability, the immediate availability of the results is sacrificed. The samples need to be drawn to the surface prior to being analyzed, meaning the data being generated can be from samples collected from over an hour before. There is only one bank of analyzers, so only one sample is analyzed at a time. Depending on the number of tubes in the system and the programmed sampling sequence, each point may only be sampled once every 30e60 minutes. Add this to the time taken to draw the sample from underground, which may be as long as an hour, and it is obvious that this technique is not suitable for the instantaneous detection of an incident such as a fire. Because the analyzers in these systems rely on infrared absorbance and paramagnetic attraction, the gas matrix is not important, making this technique suitable for the analysis of gases from oxygen-depleted areas such as the gob. The measurement of oxygen using paramagnetic analyzers is flow rate dependent, and the flow from each tube must be balanced to be the same, including any calibration gases used. Otherwise, it is possible that two locations could in fact have the same oxygen concentration, but because of more resistance in one of the tubes, the flow through the analyzer is at a lower flow rate and as such results in a lower reading than a location with the same concentration but flowing through the instrument at a faster rate. In the event of a mine explosion, the tube bundle monitoring system may still appear to be functional, but the location from which tubes are sampling may not be the same, due to damage to the tubes. A good tube bundle system will include monitoring of the vacuum pressure in each of the tubes, so following an explosion this data can be used to determine whether a tube has been compromised or not. It is also useful during routine operation for identifying increased restriction or sudden leakage in a tube, both of which can compromise the operation of the system. If the tubes are damaged and not providing any valuable information, it may be possible to make use of boreholes and connect new tubes to locations of interest as the surface equipment will still be operational. This is the preferred technique in the United States, although quite expensive.
19.4.3
Gas Chromatography
Gas chromatography, with regard to gas analysis, involves the separation of all sample components followed by their measurement on relatively nonspecific detectors. Specificity is obtained by virtue of the separation process rather than detection. The use of a GC expands analytical capabilities to include gases crucial in the interpretation of spontaneous combustion events, particularly ethylene and hydrogen. The GC provides a complete analysis of the gases expected underground and is the only
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one of the three techniques capable of measuring hydrogen, nitrogen, ethylene, and ethane. Determination of nitrogen is particularly important for determining oxygen deficiency in some spontaneous combustion indicating ratios (refer to Chapter 21). Similar to the tube bundle, problems exist with bringing the samples to the GC. The significance of time delays in getting results is dependent on what the results are being used for. GC is not going to be suitable for detection of a belt fire because of the time delay between collection of the sample and analysis, but the delay is acceptable for confirmation of other results or for evidence and trending of spontaneous combustion indicators. Like the tube bundle system, the gas matrix of the sample does not affect GC analysis. So long as appropriate calibration gases are available, this technique is capable of measuring gases at any concentration above their detection limit. This eliminates the problems seen with the other techniques, particularly for carbon monoxide concentrations greater than 1000 ppm. The ultrafast GCs in use in Australian mines allow the analysis of most of the components expected underground in approximately 2 minutes. This increased speed of analysis is invaluable during emergency situations, particularly when assessing the safety of the underground atmosphere for reentry or during reentry by mine rescue teams. In these cases, what makes this assessment more effective is that GC is on-site and can be operated by mine personnel. There is no delay in determining the status underground, while waiting for external providers to arrive or transporting samples away from site for laboratory analysis.
References [1] Hartman HL, et al. Mine ventilation and air conditioning. 2nd ed. John Wiley and Sons; 1982. p. 59e60. [2] Thakur PC. Computer-aided analysis of diesel exhaust contamination of mine ventilation systems. Ph.D. thesis. The Pennsylvania State University; 1974. p. 234. [3] Holaday DA. Control of radon and daughters in uranium mines and calculations of biologic effects. Washington, DC: Publication No. 494, U.S. Department of Health, Education and Welfare; 1957. [4] Thakur PC. In: Darling P, editor. Gas and dust control in SME mining engineers handbook; 2011. p. 1595e609. [5] Rowland JH, Harteis SP, Yuan L. A survey of atmospheric monitoring systems in US underground coal mines. Mining Engineering 2018;70(No. 2):37e40. [6] Morton J. New wireless tech for underground mines could save lives, costs. Coal Age April 2018:20e3. [7] Brady D. The role of gas monitoring in the prevention and treatment of mine fires, Coal Operator’s Conference, the AusIMM. February 2008. p. 202e8.
19
Chapter Outline 19.1
Detection Methods 19.1.1 19.1.2 19.1.3 19.1.4 19.1.5
19.2
313
Methane Measurement 314 Oxygen Detectors 315 Carbon Monoxide Detectors 315 Oxides of Nitrogen 315 Radon 316
Monitoring of Mine Gas
318
19.2.1 US Mine Survey Results 319 19.2.2 System Manufacturers 319
19.3
Wireless Communication and Monitoring System 320
19.4
Special Arrangements for Monitoring in Mines Liable to Spontaneous Combustion 320
19.3.1 Intrinsically Safe CO Detectors 320
19.4.1 Real-Time Monitoring 321 19.4.2 Tube Bundle 321 19.4.3 Gas Chromatography 322
References
323
Properties of mine gases were discussed in Chapter 13 of the book and their threshold limit values were listed in Table 1.3. Correct measurement of the concentrations of these gases and, in some cases, continuous monitoring of the concentrations is essential for the health and safety of coal mine workers. In some coal mining countries, it is also a legal requirement.
19.1
Detection Methods
Detection techniques can be classified into the following categories: 1. 2. 3. 4. 5.
Catalytic-oxidation detectors. Electrochemical sensors. Optical detectors. Electrical conductivity using semiconductors. Stain tubes: The concentration is usually read on a linear scale on the tube.
Advanced Mine Ventilation. https://doi.org/10.1016/B978-0-08-100457-9.00019-5 Copyright © 2019 Elsevier Ltd. All rights reserved.
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Table 19.1 Gas Detection Methods Gas
Detection Methods
Methane
Flame safety lamp Catalytic oxidation Thermal conductivity Optical (infrared and interferometer)
Oxygen
Liquid absorption Stain tubes Paramagnetic analyzers Electrochemical sensors
Carbon dioxide
Liquid absorption Stain tubes Optical interferometer
Carbon monoxide
Electrochemical sensors Catalytic oxidation Optical and infrared Metal oxide semiconductor Stain tubes
Oxides of nitrogen
Electrochemical sensors Stain tubes
Hydrogen sulfide
Electrochemical sensors Metal oxide semiconductors Stain tubes
Sulfur dioxide
Electrochemical sensors Stain tubes
Hydrogen
Stain tubes
Radon
Radiation detectors
Detection (more correctly spot readings) methods for different gases are listed in Table 19.1 [1]. Many instruments have been developed over the past 100 years to measure the instantaneous concentrations of all important gases listed in Table 19.1. The oldest is the flame safety lampda symbol of safety in mines. It detected both the lack of oxygen (above 13%) and small concentrations of methane (below 5%) and provided light as well for miners to work safely. It is almost obsolete by now. Instruments specially suited to measure the concentration of important gases are discussed below.
19.1.1
Methane Measurement
Two handheld instruments are most commonly used. The first one, which is cheaper, uses the catalytic-oxidation technique. It works on the Wheatstone bridge principle:
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one leg of the bridge is used to burn methane catalytically raising its temperature. The imbalance in the current flow is a measure of methane concentration. Such detectors are also sensitive to the presence of higher hydrocarbons, hydrogen, and carbon monoxide, but fortunately, these interfering gases are present in mine air only in parts per million (ppm) and do not introduce serious error in methane measurements. Catalyticoxidation type instruments are liable to get damaged if methane concentration exceeds 5% because of excess heat. Optical detectors for methane are called “interferometer.” These detectors compare the speed of light through pure air with that in air contaminated by methane. Light traveling through both media is combined again producing “interference fringes.” The position of these fringes indicates methane concentrations. These instruments are also sensitive to other hydrocarbons and hydrogen. Ethane and propane present in air indicate a higher concentration of methane, but CO and H2 have an effect of lowering the actual methane concentrations. It is also sensitive to water vapor and carbon dioxide, but these gases are scrubbed out by passing the inlet gas through a column that absorbs both of them. One percent of CO2 in air will indicate 1% methane even if there is no methane there. Lack of oxygen also impacts an interferometer. Each 1% decrease below 20.95% (normal O2 in air) results in 2% methane reading even if there is no methane in air. As such, it is not a very reliable instrument for 0%e5% methane. It is much more useful to measure higher concentrations, 5%e100% of methane.
19.1.2 Oxygen Detectors A flame safety lamp was the most commonly used device to measure oxygen deficiency in past, but it is now replaced by new instruments that work on various principles, such as liquid absorption, paramagnetic, or electrochemical cells. Many instruments measure both methane and oxygen concentrations over the range 0%e5% and 0%e21%, respectively. Stain tubes are also available to measure O2 concentration in mine air.
19.1.3 Carbon Monoxide Detectors As listed in Table 19.1, five different techniques are available to measure CO in mine air. Ambient CO concentration is normally checked by a handheld instrument that uses catalytic oxidation, but the readings can be seriously impacted by other gases present in air, especially higher hydrocarbons. For detecting spontaneous combustion, reliable measurements are given only by gas chromatographs (GCs). Stain tubes are good indicators, but it must be followed by at least two samples of the mine air for GC analysis. In many cases, when the handheld CO monitor read 100e200 ppm, the actual CO concentration as measured by a GC was only 5e10 ppm.
19.1.4 Oxides of Nitrogen With the introduction of diesel engines in coal mines some 50 years ago, the need to measure both NO and NO2 has become urgent. Most commonly stain tubes with a
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handheld pump were used for this purpose, but now electrochemical cells have become available for this purpose. When NO, NO2, or even CO enter a special cell; they react with the electrolyte and create a light signal, “a photon.” A bank of photomultiplier tubes picks up the signal and translates it into specific gas concentrations. The precision of such instruments is quite good at 1 ppm. Earlier instruments were bulky, such as Ecolysers [2], and were difficult to carry, but new instruments are light and can be handheld. An instrument named “Passport” can measure five gases in a single unit as listed below. CO (1e50 ppm) O2 (1%e20%) CH4 (1%e5%) NO (1e25 ppm) and NO2 (1e5 ppm).
It is strongly advised that for conclusive results, all gas samples must be analyzed in a laboratory by trained technicians using a GC.
19.1.5
Radon
Radon is a gaseous, chemically inert, radioactive product of the disintegration of radium. Found primarily in uranium mines, although present in trace amounts in several coal mines, radon diffuses from the rock strata into the mine environment, where the decay process continues. Table 19.2 gives the disintegration process for uranium-238 to become lead-206. Shown in this table is the type of radiation given off by each decay process and the half-life of each element in the series. The halflife of a radioactive substance is the time required for a given amount of that substance to lose one half of its radioactivity. The half-life of uranium-238 is approximately 4.5 billion years; radium, 1622 years; and radon, 3.8 days. Once radon is released into the mine environment, the decay process continues with the formation of radium A, which decays to radium B, which produces radium C, and so forth. The products formed by the decay of radon are referred to as radon daughters. The radon daughter products are atoms of solid matter having relatively short half-lives. During the decay process, either alpha or beta particles are emitted. These emissions may also be accompanied by gamma ray activity. It is the shortlived alpha particles and potential alpha emitters such as radon and its daughters that are of prime concern to the ventilation engineer. Because it is a gas and has a relatively long half-life, inhaled radon is exhaled before large amounts of alpha particles are emitted. The daughter products, however, attach themselves to the dust that is present in the environment and when inhaled, tend to be deposited in the respiratory system. It has been estimated that when both radon and radon daughters are inhaled, only about 5% of the alpha radiation received is contributed by the radon [3]. During radioactive decay, the individual members in the series are decaying and being formed at the same time. At some point in time, equilibrium is reached, and the quantity of each member in the series remains constant. At this time, each member
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Table 19.2 Uranium Disintegration Process Common Name or Symbol
Isotope
Type of Radiation
Half-Life
Uranium
238 92
Uranium
Alpha
4.49 109 year
UX1
234 90
Thorium
Beta
24.1 days
UX2
234 91
Protactinium
Beta
1.17 min
Uranium-234
234 92
Uranium
Alpha
2.48 105 year
Ionium
230 90
Thorium
Alpha
8 104 year
Radium
226 88
Radium
Alpha
1622 year
Radon
222 86
Radon
Alpha
3.825 days
Radium A
218 84
Polonium
Alpha
3.05 min
Radium B
214 82
Lead
Beta, gamma
26.8 min
Radium C
214 83
Bismuth
Beta, gamma
19.7 min
Radium C0
214 84
Polonium
Alpha
2.73 106 min
Radium D
210 82
Lead
Beta, gamma
22 year
Radium E
210 83
Bismuth
Beta
5.02 days
Radium F
210 84
Polonium
Alpha
138.3 days
Radium G
206 82
Lead
–
Stable
in the series is being generated at the same rate which it is decaying. The time required for the radon daughters through radium C0 to reach equilibrium from a given quantity of radon is approximately 3 h. In approximately 40 min, the alpha energy reaches approximately 50% of maximum.
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Exposure to excessive concentrations of radon and radon daughters has been linked with a high incidence of lung cancer. The maximum exposure limit for radon daughters has been set at 1.0 working level (WL), with a yearly cumulative exposure of 4 working level months (WLM). A working level is defined as that concentration of shortlived radon daughter products in a liter of air that will yield 1.3 105 million electron volts of alpha energy in decaying through radium C’. WLM is a cumulative measure of exposure that is calculated by multiplying the average WL of exposure during a given timed period by the time of exposure and dividing by 173 (the number of working level hours per WLM) [4]. An example calculation is as follows: Given the following exposures during a shift: 4 h 0.4 WL 2 h 0.2 WL 2 h 0.1 WL
Find the WLMs of exposure. Solution: Average WL ¼
4 0:4 þ 2 0:2 þ 2 0:1 8 ¼ 0:275 WL
WLM ¼ ð0:275 WLÞð8 hÞ=ð173Þ ¼ 0:013 WLM
19.2
Monitoring of Mine Gas
In the United States, CO monitoring in the belt entry is required by law for fire detection. This was the beginning of mine monitoring. It slowly developed into atmospheric monitoring system (AMS). While the CO-monitoring system monitors only CO concentrations in strategic locations, the AMS monitors many other parameters and locations. Parameters that are monitored comprise CO, CH4, O2, NO, H2 concentrations, smoke, air velocity, and air temperature. The locations that are monitored comprise mine airway, battery charging station, fans, fan houses, electric equipment, pumps, and coal storage. A monitoring system, as defined by the Mine Safety and Health Administration, is a network of hardware and software meeting the requirements of 30CFR 75.301 and capable of performing the following functions: 1. 2. 3. 4. 5.
Measure the required atmospheric parameters. Transmit the data to a surface location. Provide alert and alarm signals. Process and store measured data. Create reports by analyzing the data.
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A recent report [5] surveyed 235 US mines and major findings are presented below.
19.2.1 US Mine Survey Results These mines had 204 CO systems and 33 AMS systems. The parameters monitored and number of sensors are listed in Table 19.3 [5]. It is apparent that the CO systems make up the majority. It may be because it is cheaper and has fewer regulatory requirements for inspection, operation, and maintenance.
19.2.2 System Manufacturers There are several manufacturers of the monitoring systems in the United States, but the four major manufacturers are (1) Pyott-Boone, (2) AMR, (3) Conspec, and (4) Matrix. Table 19.4 shows their share of the market.
Table 19.3 Distribution of Monitors in US Mines Parameter Monitored
Percent of Mines
Number of Sensors
CO
100
2e300
CH4
17
1e20
O2
6
1e20
NO
1
1e3
H2
2
1e3
Smoke
2
1e14
Air velocity
9
1e20
Heat
2
4e191
Table 19.4 Manufacturers of the Monitoring Systems Manufacturer
Number of Systems Installed
Average Number of CO Sensors/System
Pyott-Boone
161
27
AMR
32
54
Conspec
25
84
Matrix
15
45
Others
9
NA
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Wireless Communication and Monitoring System
After the Sago Mine disaster in 2006 where 12 miners died, MINER Act was passed by the US Congress. It required underground coal mines to install two-way communication and tracking systems. The need to communicate is universal but essential in an emergency in underground coal mines. Several manufacturers have developed communication and tracking systems but one of them, the Innovative Wireless Technologies (IWT), shows the promise to transmit gas concentration data also [6]. IWT earlier developed the SENTINEL system which is comprises line-/battery-powered network mesh modes. It supports voice, text, tracking, data, personnel and vehicle tracking tags, two voice handset models, dispatch and tracking stations. In 2017, they developed an HDRMesh system that is wireless extension or booster for a fiber optic network. It can provide long-range, reliable communications in hard-to-reach working areas with no fiber interconnect between them. The ability of this system is claimed to be better than radio-frequency identification tracking systems. In early 2017, IWT expanded the use of their system to carry CO concentration data from various locations without any cable; that is wirelessly. The CO monitor has a 6-month battery and a 1000 ft range. These units can be placed every 1000 ft along the belt, and they can transfer the data to the next sensor, 1000 ft away until the surface is reached. It is also claimed to be cheaper than the cabled system. Extensive field testing is necessary to confirm the claims of the manufacturers.
19.3.1
Intrinsically Safe CO Detectors
One such detector, called Sentro 1, is manufactured by Trolex Ltd. (UK) and marketed in the United States by Strata Worldwide. The latter has a wireless communication system that works on batteries. A combination of the intrinsically safe CO sensors with a wireless communication system provides a very good alternative to the present cabled system. A wireless system is defined as one that needs no external power, cables, repeaters, or splitters. Battery life is 40e60 days. Monthly calibration is needed.
19.4
Special Arrangements for Monitoring in Mines Liable to Spontaneous Combustion
Mine AMS becomes especially important when the coal seam being mined is liable to spontaneous combustion. Brady [7] recommends a combination of three kinds of systems, namely (1) real-time monitoring, (2) tube bundles, and (3) on-site ultrafast GCs. Even an aggressive approach like this cannot prevent a fire, but it does offer means to identify the problem early and a chance to contain the fire before it becomes too big to control and the mine has to be shut down.
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19.4.1 Real-Time Monitoring Real-time sensor systems are ideal for telling us what is happening now. The sensors must be located where the gas needs to be measured, and the measurement signal is sent to the surface. This means having multiple sensors underground. These sensors are exposed to the harsh underground environment which is not ideal for precise analytical measurements. This is not really a major problem as these systems are used to detect step changes, such as the onset of a fire, a sudden increase in a mine gas in the general body, or reduction in oxygen. They offer real-time warning and are the best system for identifying a sudden event such as a belt fire. The situation is reported when it happens. Generally, sensors included are for methane, carbon monoxide, carbon dioxide, and oxygen. These types of sensors employed underground tend to have limited measuring ranges: carbon monoxide is often only capable of being measured up to 50 ppm, methane to 5%, and carbon dioxide to several percent. This range is fine while no problems exist and indeed to alert the onset of a problem. But if a fire or other major incident involving generated gases occurs, these sensors may quickly reach full scale and become unable to return a true indication of the concentrations. Most of these sensors require the presence of oxygen to work and are therefore unsuitable for monitoring areas of low oxygen concentration such as sealed or nonventilated gobs. As each individual sensor needs to be calibrated regularly (at least monthly), they are not suited to being located for long-term monitoring in inaccessible areas such as the gob. Some of these sensors also suffer from cross sensitivities, as the reactions they rely on to give a response that can be common to other gases found underground, such as carbon monoxide sensors being cross sensitive to hydrogen sulphide and hydrogen. In the case of an explosion, it is likely that the real-time monitoring system will be rendered inoperable, requiring other techniques for the determination of the status of the underground environment.
19.4.2 Tube Bundle Tube bundle systems draw gas samples from designated sampling locations underground to the surface through plastic tubes using vacuum pumps and analyzed sequentially using infrared and paramagnetic techniques. Gases measured are carbon monoxide, carbon dioxide, methane, and oxygen. Because the analyzers are on the surface, tubes can be located in the gob as once positioned there is no requirement to access the end sampling point. Tube bundle systems are suited to long-term trend analysis. Very good analytical equipment is available and can be housed in dedicated air-conditioned rooms on the surface with the samples dried and passed through particulate filters prior to entering the analyzer. Generally tube bundle systems are set up to measure oxygen, carbon monoxide, carbon dioxide, and methane. Given their ability to measure carbon monoxide down to
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1 ppm, the long-term stability of these analyzers, and the frequent sampling, this technique is best for long-term trending of carbon monoxide to identify a spontaneous combustion event. With respect to measuring range, it is normally only carbon monoxide that presents problems, with most systems capable of measuring to only 1000 ppm. Because methane and oxygen concentrations can be measured over all expected concentrations ranges, this technique is the best for automated monitoring of explosibility of an area so long as a fire or heating does not exist. To get this improved stability and analytical capability, the immediate availability of the results is sacrificed. The samples need to be drawn to the surface prior to being analyzed, meaning the data being generated can be from samples collected from over an hour before. There is only one bank of analyzers, so only one sample is analyzed at a time. Depending on the number of tubes in the system and the programmed sampling sequence, each point may only be sampled once every 30e60 minutes. Add this to the time taken to draw the sample from underground, which may be as long as an hour, and it is obvious that this technique is not suitable for the instantaneous detection of an incident such as a fire. Because the analyzers in these systems rely on infrared absorbance and paramagnetic attraction, the gas matrix is not important, making this technique suitable for the analysis of gases from oxygen-depleted areas such as the gob. The measurement of oxygen using paramagnetic analyzers is flow rate dependent, and the flow from each tube must be balanced to be the same, including any calibration gases used. Otherwise, it is possible that two locations could in fact have the same oxygen concentration, but because of more resistance in one of the tubes, the flow through the analyzer is at a lower flow rate and as such results in a lower reading than a location with the same concentration but flowing through the instrument at a faster rate. In the event of a mine explosion, the tube bundle monitoring system may still appear to be functional, but the location from which tubes are sampling may not be the same, due to damage to the tubes. A good tube bundle system will include monitoring of the vacuum pressure in each of the tubes, so following an explosion this data can be used to determine whether a tube has been compromised or not. It is also useful during routine operation for identifying increased restriction or sudden leakage in a tube, both of which can compromise the operation of the system. If the tubes are damaged and not providing any valuable information, it may be possible to make use of boreholes and connect new tubes to locations of interest as the surface equipment will still be operational. This is the preferred technique in the United States, although quite expensive.
19.4.3
Gas Chromatography
Gas chromatography, with regard to gas analysis, involves the separation of all sample components followed by their measurement on relatively nonspecific detectors. Specificity is obtained by virtue of the separation process rather than detection. The use of a GC expands analytical capabilities to include gases crucial in the interpretation of spontaneous combustion events, particularly ethylene and hydrogen. The GC provides a complete analysis of the gases expected underground and is the only
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one of the three techniques capable of measuring hydrogen, nitrogen, ethylene, and ethane. Determination of nitrogen is particularly important for determining oxygen deficiency in some spontaneous combustion indicating ratios (refer to Chapter 21). Similar to the tube bundle, problems exist with bringing the samples to the GC. The significance of time delays in getting results is dependent on what the results are being used for. GC is not going to be suitable for detection of a belt fire because of the time delay between collection of the sample and analysis, but the delay is acceptable for confirmation of other results or for evidence and trending of spontaneous combustion indicators. Like the tube bundle system, the gas matrix of the sample does not affect GC analysis. So long as appropriate calibration gases are available, this technique is capable of measuring gases at any concentration above their detection limit. This eliminates the problems seen with the other techniques, particularly for carbon monoxide concentrations greater than 1000 ppm. The ultrafast GCs in use in Australian mines allow the analysis of most of the components expected underground in approximately 2 minutes. This increased speed of analysis is invaluable during emergency situations, particularly when assessing the safety of the underground atmosphere for reentry or during reentry by mine rescue teams. In these cases, what makes this assessment more effective is that GC is on-site and can be operated by mine personnel. There is no delay in determining the status underground, while waiting for external providers to arrive or transporting samples away from site for laboratory analysis.
References [1] Hartman HL, et al. Mine ventilation and air conditioning. 2nd ed. John Wiley and Sons; 1982. p. 59e60. [2] Thakur PC. Computer-aided analysis of diesel exhaust contamination of mine ventilation systems. Ph.D. thesis. The Pennsylvania State University; 1974. p. 234. [3] Holaday DA. Control of radon and daughters in uranium mines and calculations of biologic effects. Washington, DC: Publication No. 494, U.S. Department of Health, Education and Welfare; 1957. [4] Thakur PC. In: Darling P, editor. Gas and dust control in SME mining engineers handbook; 2011. p. 1595e609. [5] Rowland JH, Harteis SP, Yuan L. A survey of atmospheric monitoring systems in US underground coal mines. Mining Engineering 2018;70(No. 2):37e40. [6] Morton J. New wireless tech for underground mines could save lives, costs. Coal Age April 2018:20e3. [7] Brady D. The role of gas monitoring in the prevention and treatment of mine fires, Coal Operator’s Conference, the AusIMM. February 2008. p. 202e8.