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SCIENTIFIC PAPERS Exposure Monitoring and Control--Coal Mines I OVERVIEW OF RESPIRABLE DOST CONTROL FOR UNDERGROUND COAL MINES IN THE UNITED STATES R. HAMEY R. Ondrey R. Stoltz, A Chiz Mine Safety and Health Administration Pittsburgh, Pennsylvania, USA ABSTRACT Control of respirable dust is an important consideration in the design ofdie production cycle of an underground coalmine, hi order to create an effective and efficient system, the mining engineer must integrate die regulatory requirements with the specific conditions that exist in a coal mine. Typical mine development is by room and pillar. Second mining is by mining rooms, extracting pillars or by retreating longwalls. Each of the mining systems can have specific constraints depending on die type of equipment used. Continuous miners and conventional mining systems (cut, shoot and load) are used for room and pillar application. Single and double drum shearers primarily are used for retreating longwall systems. This paper provides a review of the specific federal regulations affecting dust control and description of the various dust control systems commonly used to supplement those regulations for the various mining systems. INTRODUCTION There are over 2,000 mechanized mining sections in underground coal mines in the United States. Each of these sections must utilize a dust control system capable of main taining their dust levels below the specified standard. The purpose of this paper is to provide an overview of the specific federal regulations affecting dust control and a description of various respirable dust control systems cur rently used in underground coal mines. Utilization of these systems has been successful in controlling workers exposure to coal mine dust. FEDERAL REGULATIONS Current authority to establish and enforce a respirable coal mine dust standard was given to the Mine Safety and Health Administration (MSHA) ofthe Department ofLabor through die Federal Mine Safety and Health Act of 1977. Primary responsibility of enforcing the respirable dust standard rests at the federal level as state laws generally do not specify a respirable dust standard. Specific regulations pertaining to the dust standard and dust control are contained in Title 30, Code of Federal Regulations. Part 70--Mandatory Health Standards--Underground Coal Mines, contains die dust standards and die sampling pro cedures that must be followed by the coal mine operators. Part 70 establishes a respirable coal mine dust exposure stan dard of 2.0 milligrams per cubic meter (mg/m3. If the dust contains more than five percent quartz, die dust standard is computed by dividing the percentage quartz into the number 10. Additionally, Part 70 establishes a dust standard for in take air of 1.0 mg/m3. Part 70 also requires mine operators to collect and submit five dust samples from a designated occupation during each bimonthly sampling period. Part 75--Mandatory Safety Standards--Underground Coal Mines, contains various ventilation regulations that pertain to the control of respirable coal mine dust. Part 75 contains various regulations pertaining to the design and perform ance of a mine's ventilation system which also have an im pact on dust control. Specifically, each mechanized mining unit must be ventilated on a separate split of intake air. This prohibits series ventilation of working sections so that the return of one section cannot be used to ventilate another section. To provide dilution, the ventilation system must deliver 9,000 cubic feet of air per minute (cftn) to the last open cross cut of a set of developing entries and to the intake entries of a retreating section. The system must also supply 3,000 cfm to each working face where coal is being cut, mined or loaded. Unless otherwise approved by the local enforcement official, the line brattice or face ventilation device must be main tained within 10 feet of the face. For exhausting face ven tilation systems, the minimum mean entry air velocity in working places where coal is being cut, mined or loaded is 60 feet per minute (fpm). Each coal mine operator must also submit for approval a ven tilation system and methane and dust control plan. The plan must show in detail the methane and dust control practices along all haulageways and travelways, at all transfer points, at underground crushers and dumps, in all active working 43 Exposure Monitoring and Control--Coal Mines I places and in any other areas which may be required by MSHA's local enforcement official. Prior to approval, dust samples are collected by inspection personnel to verify system performance. The dust control plan concept was developed to provide flexibility, yet en sure that appropriate measures were being taken to control respirable dust. The following discussions provide more in formation on specific dust control systems used for various mining systems. DOST CONTROL ON CONTINUOUS MINER SECTIONS Approximately two-thirds ofthe mining sections in the United States utilize continuous mining machines. Continuous miners are used to both develop and retreat room and pillar mining sections. Dust generated on a drum type continuous miner is controlled by two primary means, ventilation and water. The two basic types of face ventilation are exhausting and blowing. In an exhausting ventilation system, air is brought to the face at a lower velocity, captures the dust cloud and then extracts it from die face at a higher velocity. For a blowing face ventilation system the return air passes over the mining machine. This situation necessitates the use of additional controls such as machine mounted dust collectors (scrubbers) to maintain adequate dust control. Water sprays are used in addition to ventilation to suppress and direct die dust cloud generated at the free. Typical sup pression sprays are mounted on die miner as close to die an ting drum and gathering arms as possible. These systems are designed to deliver water to strategic dusty locations around the machine. Directional sprays (spray fan systems) are mounted on die body of die miner up to 10 to 15 feet from the face. These sprays are designed to use die momentum of die water to direct the dust cloud away from die machine operator. Spray fan systems are normally used in conjunc tion with exhaust line brattice. Each continuous mining section utilizes one or more roof bolters to install roof support in the entries mined. Dust con trol on roofbolters is especially important because the drilled strata can contain high levels of quartz. The two primary methods of controlling dust generated during roof bolting operations are through proper use and maintenance of the machine dust collection system and proper ventilation ofthe working place. DUST CONTROL ON CONVENTIONAL MINING SECTIONS In a conventional mining system die coal is extracted in a series ofoperations each performed in proper sequence. The operations in a conventional mining system are: cutting, drill ing, blasting, loading and hauling. Each operation in die cy cle employs a specialized piece ofequipment to perform that operation. The cutting operation is performed with a mobile cutting machine which most nearly resembles a large chain saw on wheels. Dust from die cutting operation is controlled by die use of a "wet" cutter bar and external water sprays mounted above the cutter bar as well as proper ventilation. The wet 44 cutter bar is made by plumbing a water pipe inside the cut ter bar which terminates in a small opening at the end of the bar. The movement of die cutting chain around die bar distributes the water along die length of the cut. External water sprays should be directed towards the ingoing and outgoing bits and also toward the pile of cuttings being deposited on the mine floor. The drilling operation employs a mobile drilling machine with a single movable drill capable of drilling to the same depth as die cutting machine. The number of holes drilled depends on die height of the coal seam, width of die face, hardness of the coal and the desired size of die coal lumps. The period of highest dust concentration is when the drill is first sumped into die coal. Once the drill has penetrated the coal, the hole itself helps contain die (hist. The use of a wet auger (drill steel) is the preferable method of control ling dust on a coal drill. Water is directed through the hollow auger to the bit and is then forced out of the hole after it has mixed with the cuttings and dust. The coal cuttings and dust are thoroughly wet and come out of the hole in the form of a slurry, thus producing very little dust. Blasting is done chiefly with permissible explosives. An ex plosive charge is placed in each hole and then stemmed with an inert material (either water or clay dummies). The charges are wired together and then detonated. The rapid release of energy by the explosives breaks the coal and also generates a large amount of dust. However, the dust is rapidly dissipated if the free is properly ventilated. If foe blasting is done on the return air side of foe other mining operation, then personnel will not be exposed to foe dust generated by blasting. The next operation is the loading of foe coal by either a loading machine or a scoop. Loading machines have mechanical gathering arms which pull foe coal onto a chain conveyor located along the centerline of foe machine. The movement of foe gathering arms and chain conveyor pro duces dust. This dust is controlled by the face ventilation system and by external water sprays mounted on foe body oftiie loading machine. Prior to loading, the coal pile should be thoroughly wetted. Wetting foe coal pile is particularly important since subsequent loading of the coal is done with scoops that are not equipped with water spray systems. DUST CONTROL ON LONGWALL MINING SECTIONS In general longwall mining systems in the United States use single or double drum shearers to retreat mine a block of coal. Longwall faces range from 400 to 1,000 feet wide with total panel length often in excess of 4,000 feet. There are approximately 100 operating longwalls which produce ap proximately 15 percent of the underground coal mined. Nor mally seven people are required to operate the longwall face equipment. When identifying and attempting to control a longwall system's dust source(s), the longwall can be divided into three primary sources of dust generation. These sources are foe machinery in the headgate area, foe shearer and foe shields. The dust generated in foe headgate area affects personnel on foe entire longwall face since it contaminates the intake air before it traverses foe face. The headgate sources are foe stageloader, crusher and product transfer points. The com mon practice employed for dust control is to enclose the stageloader and crusher on the sides and top and to install flat jet water sprays across the product inlet and outlet. To assist the water sprays in creating a tighter enclosure on the product inlet and outlet, a strip of mine conveyor belting or brattice is installed on both ends. Usually flatjet water sprays are located in the crusher and along the length of the stageloader. To control dust at transfer points, various types of water sprays are used. The shearer's primary dust source is the cutting of die coal by the bits on die drum(s). To combat this dust source, four control methods are normally used. The four dust control methods are: internal water sprays, external water sprays, remote control and work practices. Internal water sprays are the water sprays in/on the shearer cutting drum. The internal sprays are used to suppress die dust at the source and provide a cooling effect for the cut ting bits. The number of sprays range from 25 to 45 with die orifice ranging from 1/8 to 3/16-inch. The operating water pressure measured at the spray nozzle ranges from 40 to 100 pounds per square inch (psi). The external water sprays are die water sprays located on the shearer body or on any attached bar and/or arm. The best practice is to use these sprays to direct die dust laden air over the shearer body so that the shearer operator is main tained in a clean split of intake air not contaminated by die dust generated by the shearer. The operating water pressure measured at die spray nozzle ranges from 40 to 120 psi. To assist the external water sprays in directing die dust, passive barriers (usually made of mine conveyor belting) are sometimes attached to the shearer body, bars and/or arms. A remote control unit(s) is a device that allows the shearer operators) to control the shearer from various locations. It is used to remove the shearer operators) from the dust be ing generated by the shearer. Radio control or umbilical cord are the two types of remote control units available. Radio control is more versatile but not as durable as an umbilical cord unit. Approximately 50 percent of the shearers are equipped with a remote control system. Administratively controlled work practices are also used on longwalls to lower the dust exposure ofpersonnel. The most common work practice employed to lower exposure is to reduce the amount of time personnel spend on the face. Exposure Monitoring and Control--Coal Mines I This is accomplished by having personnel move to the up wind side of the shearer after they have completed their primary tasks. Also changing the cutting sequence of the shearer can reduce the exposure of face personnel. A com mon practice employed is to cut unidirectional, cutting twothirds of the face height in one direction and cutting the re maining one-third coming back. The shields (roof supports) are then pulled on the upwind side ofthe shearer. This prac tice keeps the shield setters out of the dust that is created by tiie shearer. However, the shearer operators are exposed to the dust generated by the shields. Bidirectional cutting, cutting fullface height in both directions, exposes shield set ters to the dust generated by the shearer for half the mining cycle and the shearer operators to the shield dust for half a mining cycle. The movement of the shield top creates a dust problem because the crushed and ground material on top ofthe shield falls. The severity of the dust problem will vary depending on the amount of this falling material. The dust problem can range from negligible to very severe. To circumvent this problem, the industry is phasing in electrohydraulic shields. The electrohydraulic shields have controls connected to a computer on the shields that allow a set of shields (1 to 15) to be electronically controlled. This allows shield setters to achieve an upwind position from this dust source. SUMMARY Prior to the 1969 Act respirable dust levels of 9 mg/m3 were commonly reported. Today the industry average ex posure for the designated occupation is approximately 1.0 mg/m3. These dust levels have been mainly achieved through the application of the various dust control methods previously discussed which include: 1. A supply of uncontaminated intake air. 2. Suppression through the use of machine cutting head design and water. 3. Containment through the use of properly designed and maintained face ventilation systems, water sprays or barriers. 4. Dilution from an adequate supply of fresh air. 5. Avoidance through the use of remotely operated cutting and loading machines. 6. Administratively controlled work practices. With continued application of these techniques, respirable dust levels can be maintained at acceptable limits. 45 Exposure Monitoring and Cootroi--Coal Mines I EXTRACTION DRUMS AND AIR CURTAINS FOR INTEGRATED CONTROL OF DUST AND METHANE ON MINING MACHINES VICTOR H.W. FORD, RSc.( Ph.Di T. Brierley, B.A. aj. HOLE, RSc. British Coal, Headquarters Technical Department Burton-on-Trent, UK INTRODUCTION Over the past 30 years British Coal has expended a con siderable amount of research effort on solving the problems of environmental control at and around the production machines in coal mines. That effort has borne much fruit in die field of respirable dust control, with the levels of pneumoconiosis falling from over 10% of the workforce in 1970 to the current level of0.9% for mineworkers ofall ages. This improvement has been achieved despite a doubling of productivity at die coalface. However, the current rapid rise in output demands even more efficient dust control systems for the future. The other major environmental hazard at the production machine is die frictional ignition of methane, caused by cut ting tools striking quartzitic or pyritic strata in the presence ofexplosive mixtures ofmethane. There has been little reduc tion in the incidence of frictional ignitions over the last 20 years, with an average of 14 ignitions reported each year,1 despite improvements in die ventilation of the cutting zone to dilute dangerous concentrations of methane and die more recent use of water sprays to cool die ignition source. There is often a conflict between the requirements for good dust control and those for effective dilution of methane in the cutting zone or dispersal of methane layers in die roof of drivages. Excessive amounts of dust are often dispersed by die high air velocities blown into die cutting zone or roof area to get rid of methane. On longwall shearers the hollowshaft ventilator does this job,2 while in drivages where ex haust ventilation is used to control dust, machine-mounted fans can be fitted to disperse methane layers. The high air velocities these fans produce can result in roll-back (or back up) of dust to the operator's position on the machine. To overcome die conflicting requirements for dust and methane control, and also to provide the improvements need ed to ensure that the vital productivity increases being gained by British Coal are not jeopardized by dust sanctions or in creasing numbers of ignitions, two new control technologies have been developed at Headquarters Technical Department, the Extraction Drum for longwall shearers, and Air Curtains for use in exhaust ventilated drivages. DUST EXTRACTION ON SHEARERS Numerous attempts have been made in various countries to provide effective dust extraction systems on shearers, using fans and dust collectors. All failed, because ofproblems with blockage of ducting by coarse material, or die large size of equipment needed to supply adequate extracted airflows. However, work on small, water-powered dust capture tubes in the early 1970's10 led to the development in die UK of effective dust extraction systems for use on shearers with cutting drums well shielded from face ventilation.8 In these systems die non-blocking, open-ended tubes were integrated with the coal loading doors or cowls around the cutting zone. Efficient dust control on ranging-drum shearers and those with unshielded drums was not possible until the concept of the extraction drum was devised in 1981.7 Description of Extraction Drum The extraction drum was developed after laboratory tests showed that the best place to extract dust was from the face side of die drum. A number of dust capture tubes are built into die drum barrel, with the tube inlets at die face side remote from face ventilation. Dusty air is drawn from the cutting zone, cleaned by the tubes, and blown out at the goaf side, from where it is turned back into the cutting zone, together with die water spray and debris, by an angled deflec tor plate fitted to the gearhead. Figure 1 shows a version commonly used on medium-sized drums. It has nine, 100 mm diameter, tubes which extract 1.5 mVs of air us ing 60 1/min of water, released from hollow-cone, wearresistant spray nozzles at a pressure of 100 bar. Even though up to 70% of the air is recirculated, nearly 0.5 mVs of fresh air is provided to dilute methane. On smaller drums rec tangular section tubes are used to minimize drum diameter, while up to twelve 100 mm tubes have been fitted to drums above 1.5 m in diameter in order to maintain air velocities across the cutting zone. High pressure water is fed through die drum shaft to nozzles on the face-side spray ring by a dual pressure water distribu tion system, which also delivers up to 45 1/min of water, at approximately 7 bar pressure, to sprays on die drum to wet die cut coal before it is loaded out. It is essential to 46 FACE SIDE SPRAY RING Exposure Monitoring and Control--Coal Mines I Figure 1. Schematic view and cross-section of a 9-tube extraction drum. 47 Exposure Monitoring and Control--Coal Mines I operate die tubes at high water pressures to provide high airflows and freedom from blockage, together with a respirable dust capture efficiency exceeding 95%.3 Operational problems experienced with die extraction drum have primarily resulted from inadequacies in die water sup ply. It is therefore essential to install water pumps with suf ficient capacity, together with the correct water control and monitoring equipment. The high pressure water pump can be sited in die roadway at die end of the free, or integrated with die shearer. These pumps are expensive (for a double drum machine, 32,000 for a roadway pump and 14,000 for a shearer-mounted pump) and represent the major cost for a system. The additional cost of fitting the two extrac tion drums, water distribution equipment, and deflector plates is only about 5,500. Dust Control Efficiency Results from underground trials on a range of shearers, see Table I, have indicated dust reductions during cutting opera tions of between 40% and 80% when extraction drums replaced drums incorporating die normal pick-face-flushing water spray systems. In most cases water flows were similar for each system. At one site in the UK, installation of ex traction drums enabled output to be raised from 1000 to 1600 tonnes per shift without exceeding the statutory dust limits. This result shows die size of the benefits to be gained from die use of the extraction drum. Results quoted are for dust levels in die face return air. Evidence from die USA11 indicates somewhat less improve ment at the operator's position, possibly due to the effect ofdie high air velocities leaving die exit annulus ofdie drum. Effect on Methane Dilution Extensive surfaces and underground trials3 have shown that at least 30% of die air drawn in by die extraction drum is fresh air which dilutes methane in the cutting zone. Thus, for a nine-tube drum approximately 0.5 mVs of fresh air is provided for methane dilution, which is more than twice the airflow given by die hollow-shaft ventilator normally used at ignition risk sites. In laboratory tests on a shearer in an artificial coalface,4 a hollow-shaft ventilator prevented fric tional ignitions up to a methane emission rate of 5.51/s. Us ing the extraction drum ignitions did not occur until methane emission reached 15 1/s, which is above die emission rate on most UK coalfaces. Measurements taken during the underground trials,3 of methane emission rates at the shearer and methane concen trations in the cutting zone, confirmed die superiority ofthe extraction drum for ventilation. Consequently, British Coal now considers the extraction drum to be the best device for methane dilution, and is installing diem at a number of sites primarily for ignition control. In such cases, attempts are being made to continuously monitor die extracted airflow by measuring die air pressure developed across die outlet annulus between the edge of the drum barrel and die deflec tor plate.5 Alarms are activated when the airflow fells below a preset level. Systems have been fitted to a number of machines, and development is continuing to improve their reliability. Utilization of Extraction Drums Since 1985, when 15 drums were in use, there has been a rapid increase in numbers, with more than 85 drums in opera tion on some 20% of feces in the UK. In addition, drums Table I Reduction in Dust Produced During Cutting with Extraction Drums as Compared to Normal Water Sprays 1 | Machine 1 Type 1 1 1 1 | Fixed Height 1 1 1 | Single-Ended j Ranging Drum 1 1 | Double-Ended j Ranging Drum 1 1 Drum Diameter m 1.3 1.5 1.8 1.4 1.4 1.5 1.7 1.8 1 | Number j of Tubes 1 1 1 19 19 f9 1 1 19 19 1 1 1 io 1 io 1 12 1 Face Air Flow ra^/p 12 15 5 14 18 18 12 13 Reduction in Dust % 80 78 62 40 72 53 60 55 1 1 | 1 1 1 1 1 1 | 1 1 1 | 1 | 1 1 1 48 have been installed both in the USA and Australia. Hie drums have been fitted to most types of shearer, operating on faces ranging from 1.07 m to 3.0 m in height. HQTD have produced a comprehensive training package on the extraction drum system to aid the transfer of this technology to the collieries. This includes interactive video to cover die fault-finding and maintenance aspects. Future developments include the use ofhigher water pressure to increase efficiency. AIR CURTAINS FOR DRIVAGE MACHINES Exhaust ventilation gives effective dust control in drivates, providing the exhaust duct entry is kept in front of the machine operator and a forward air velocity of 0.5 m/s is maintained around the machine. In practice, these re quirements are often not met, and even when they are, ex haust ventilation alone cannot provide high enough air velocities to disperse methane. The air curtain system was developed to generate these velocities without dispersing dust, and also to increase dust control efficiency at sites where the ventilation criteria for preventing dust back-up were not being met.6 Air Curtain System The air curtain system directs `sheets' offast moving air for ward from the top and side of the machine body into zones of the drivage where air velocities are low and dust therefore backs up, as illustrated in Figure 2. An additional tube is usually fitted above die machine's conveyor to prevent dust from being pulled back to the operator's position by die outgoing debris. Air curtains are produced from 100 mm diameter steel tubes, fitted with cover plates from which the air is released tangentially to the tube surface through 2.5 mm deep slots running the length of the tube. The `Coanda Effect' causes the discharged air to cling to the tube surface until directed off in the required direction by a `split ter' bar on the tube, as shown in the tube cross-section il lustrated in Figure 3. Exposure Monitoring and Control--Coal Mines 1 Air is fed to the tubes at pressures of between 0.75 and 2.0 kPa by a small centrifugal fen powered from die machine's hydraulic supply at a flow of 401/min. The total airflow to a system depends on the length of air curtain tube used. It ranges from 0.15 m3/s on a small boom-type machine, like the Dosco 2A, to about 0.30 m3/s on a continuous miner, such as the BJD/Dresser Heliminer. On some machines the exhaust duct can be installed on either side of the heading, whilst on others, aircooled motors are fitted which draw dust back beneath die exhaust duct. For such cases, tubes are sited on both sides ofthe machine, and the air pressures in the tubes are balanced to provide the cor rect flow of air around the front of the drivage towards the duct inlet. At present systems are available for ten different boom-type machines, and three continuous miners, with equipment for a further two of the latter soon to follow. Figure 4 shows a typical system, fitted on a Dosco LH1300 machine. Prices range from 4,500 to 7,500 for a complete system, depen dent upon the number and complexity ofthe air curtain tubes. Airflow monitoring systems are currently under development to ensure that adequate airflows are provided for methane dispersal whenever the machine starts to cut. A new tech nique is at present also under development as an addition to the air curtains, to give integrated ventilation of the cut ting zone for continuous miners. Air for this system would be taken from the same fen as the air curtain, and it is hoped that use of both systems will provide effective ventilation of the cutting zone and the roof, whilst maintaining effec tive dust control. Dust Control Benefits Underground trials6 have shown that the air curtains significantly reduce dust back-up, see Figure 5. Over the range of forward airspeeds and duct entry positions used, the proportion ofdust from cutting that reached the operator was reduced by at least 70% when the air curtains were Figure 2. Plan view of drivage showing air curtains containing dust cloud. 49 Exposure Monitoring and Control--Coal Mines I switched on. The visual improvements when using air cur tains are dramatic on most types of machines, and operators are loath to work with them turned off. Time lost in waiting for dust to clear is reduced, with consequent improvement in production. As a consequence, to date more than 80 systems have been installed. Effectiveness of Methane Layer Dispersal Full-scale laboratory tests were carried out to ascertain the air velocity profiles around Dosco 2A and LH1300 machines in an arched section drivage.9 These tests showed that the air curtains directed air into the roof area at velocities well above die 1 m/s required for the dispersal ofmethane layers. In addition, they were just as effective as a marhinp-mmmtpd blower fan for removing the `dead zones' present at the front of die drivage when exhaust ventilation was used alone. Underground evaluation confirmed these results. It is now British Coal policy to fit air curtains to all drivage machines. CONCLUSIONS The extraction drum and air curtain systems both provide effective control of dust and methane on longwall coalfaces and in exhaust ventilated drivages respectively. Each system can be easily integrated with mining machines without detri ment to operational performance, and offer solutions to the problems of environmental control on high performance coalfaces and in rapidly advancing drivages. Figure 3. Cross-sectional view of air curtain tube. Acknowledgements: The authors wish to thank Mr C.T. Massey, British Coal's Head ofTechnical Department for permission to publish this paper, and to acknowledge the wok carried out by their colleagues at Headquarters Technical Department. The European Community gave financial assistance for this research. Any opinions expressed are those ofthe authors and not necessarily those of British Coal. Figure 4. Air curtain system for Dosco LH1300 Drivage Machine. 50 DUCT ENTRY 2 m FROM Exposure Monitoring and Control--Coal Mines I 0 2 m/s AIR SPEED AROUND MACHINE AVERAGE AIR SPEED MOVING FORWARD AROUND MACHINE (m/sl 0 1 2 34 5 DISTANCE OF DUCT ENTRY FROM LAST SET ARCH (m) Figure 5. Underground results with air curtains on Dosco MK2A Machine. REFERENCES 1. Browning, E.J.: Frictional Ignitions. Fourth Internationa} Mine Ven tilation Congress. Brisbane, Queensland (1988). 2. Browning, EJ.: Ventilation of the Cut. Proc. 16th International Con ference ofSafetyin Mines Research Institute. Washington DC, (1975). 3. Clarke, R.D.C.: The Extraction Drum as a Ventilation Device. Col liery Guardian 235:392-394 (1987). 4. Creedy, D.P.,etal.: The Use ofSurface Rigs to StudyFrictionalIgni tions. Final Report cm ECSC Research Project 7258-03/08/106, Head quarters Technical Department (HQTD), British Coal (1988). 5. Ford, V.W.H., etal.: Continued Work on the Extraction ofDustfrom Longwall Face Machines. Final Report on ECSC Research Project 7260-02/012/08, HQTD, British Coal (1987). 6. Ford, V.W.H., and Hole, B.J.: Air Curtains for Reducing Exposure of Heading Machine Operators to Dust in Coal-Mines. Ann. Occup. Hyg. 28:93-106 (1984). 7. French, A.G.: The Extraction ofRespirable Dust from Machines Wok ing on LongwaD Faces. Reporton CEC Conference on Health in Mines. Luxembourg (1983). 8. Hamilton, R.J., French, A.G., James, G.C.: Dust Control Using Ex haust Ventilation Techniques in UK Coalmines. Second bit. Mine Ven tilation Congress. Reno, Nevada (1979). 9. James, G.C., et al.: Further Work on Dust Control in Drivages. Final Report on ECSC Research Project 7260-02/004/08, HQTD, British Coal (1986). 10. Jones, A.D. and James, G.C.: Air Movement and Cleaning by Water Sprays. Ann. Occup. Hyg. 31:161-179 (1987). 11. Kelly, J.S. and Muldoon, T.L.: Shearer Mounted Dust Collector; Evaluation of Ventilated Cutting Drums. Final Report cm Contract J0387222, US Bureau of Mines (1987). 51 Exposure Monitoring and Control--Coal Mines I INCREASING COAL OUTPUT WILL REQUIRE BETTER DUST CONTROL RICHARD S. GILLETTE* * Robert A. Jankowsktt Fred N. Kissell* Mineral Economist, Division of Policy Analysis, Bureau of Mines, Washington, DC 20241 fSupervisory Physical Scientist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA 15236 fResearach Supervisor, Pittsburgh Research Center, Bureau of Mines, Pittsburgh PA 15236 BACKGROUND In 1969, die Federal Coal Mine Health and Safety Act (FCMHSA) was passed for die purpose of reducing die in cidence of Coal Workers* Pneumoconiosis (CWP), or black lung, a chronic lung disease caused by coal dust inhalation. The FCMHSA limited the average exposure of coal miners over an eight hour working shift to 3.0 mg/m3 (milligrams of respirable dust per cubic meter of air); this maximum dust level was reduced to 2.0 mg/m3 in late 1972, effective in 1973. Additionally, in order to reduce die incidence of silicosis, a lung disease caused by the inhalation of silica dust, the FCMHSA requires that the Mine Safety and Health Ad ministration (MSHA) enforce a more stringent standard if dust samples contain silica in excess of 5.0 percent. (Dust standard = 10/(percent SiOj in sample); the standard is less than 2.0 mg/m3 if die silica content of the sample exceeds 5.0 percent.) The annual costs of the black lung program, which include compensation payments to retired miners or their survivors and die program costs of the Departments of Labor and Health & Human Services, have leveled off in the $1.6-1.7 billion range since 1979. The cumulative cost of die pro gram from 1970 through 1985 is estimated at $18.4 billion.2'5 In constant 1970 dollars using the Consumer Price Index (CPI) to adjust for inflation, however, the cumulative cost of die program was $10.0 billion, and an nual costs have declined every year since 1979, from $834 million to $585 million in 1985. Due to die time lag between initial exposure of miners to respirable coal dust and the filing of black lung claims, sometimes as long as 25-30 years, it is likely that future com pensation payments will decline, if compliance with the stan dard is maintained, as miners who worked in dustier condi tions prior to passage of die FCMHSA leave the compensa tion rolls. Based on a British study predicting the incidence and progression of CWP over a ten year period as a func tion of mean dust concentration and assuming compliance with the 2.0 mg/m3 dust standard, Attfield forecasted die future incidence ofCWP Category 1, a less debilitating form ofdie disease, to be about 9 percent ofthe underground work force and the incidence of CWP Category 2/Progressive Massive Fibrosis, a disabling form of die disease, at 1-2 percent.1'10 Throughout die remainder of this analysis, it is accepted as given that there is a direct relationship between lower dust levels and reduced worker morbidity and mortality. Therefore, this paper evaluates die relationship between dust control and mine worker health indirectly through its im pact on mine dust levels rather than directly on incidence of dust related disease. UNDERGROUND COAL MINING METHODS The three major underground mining methods employed by die domestic coal industry are conventional, continuous, and longwall mining. Since conventional mining currendy ac counts for only 11.7 percent of underground coal produc tion and is predicted to decline to 4.2 percent by 1995 it will not be further considered in this analysis.3,8,11.17 Longwall mining is more productive than continuous min ing and generates more coal dust. 12>13 The silica dust prob lem, however, is currendy almost entirely restricted to con tinuous mining due to die cutting pattern used in this mining method. DUST LEVELS AND COMPLIANCE Due to improvements in dust control technology, average dust levels of continuous and longwall mining sections are currendy at or below the required dust levels (Figure 1). These data are average values, implying that not all mines operate in compliance with the dust standard. This is evi dent when die standard deviations of these average data are examined (Table I)- Furthermore, compliance data indicate that die problem is far from having been solved--through May 1987, 70 percent of longwall sections were in com pliance and only 59 percent of continuous mining sections could comply with more stringent dust standards due to the presence of silica in excess of 5 percent (Figure 2). As an example of die remaining problem, several U.S. longwall mining sections having the highest output per shift recorded an average dust exposure value of 3.8 mg/m3, more than two standard deviations above the longwall average.15 The costs to die underground coal mining industry of the decline in die average dust level foil into two categories: (1) direct costs, and (2) opportunity (i.e., lost production) costs. In fiscal year 1986, for example, mine operators submitted 83,985 samples at a cost of $10.3 million.14 The General 52 Exposure Monitoring and Control--Coal Mines I KEY -Longwall mining ------Continuous mining ............... ftderdl Dust Standard, post 1972 --------------Averoge more stringent standard in l87 due to excess sttca dust (almost exclusively oppficoble to . 2.5 continuous mining) 1990 Figure 1. Average dust levels of operator samples from selected underground mining methods. Table I Dust Levels, by Underground Mining Method (mg/m3) Year 1975 1980 1985 1987 Source Continuous Minina Lonawall Minina Ave. Std. Dev. Ave. Std. Dev. 1.5 0.62 2.3 1740 1.3 0.53 2.1 0.71 1.3 0.42 2.0 0.52 1.3 0.48 2.0 1 0.87 (16); Bureau of Mi nes records accounting Office cited a National Coal Association claim in 1977 that 15-20 percent of the total payroll in large underground coal mines is paid to employees involved with MSHA-related tasks; it is uncertain whether this figure is still accurate.9 The opportunity costs associated with lowering dust levels include: (1) the present value ofproduction lost due to reduc tions in production rates to generate less dust per eight hour shift and thereby maintain compliance, and (2) the present value of production lost as a result ofclosure of mines unable to meet die standard. Longwall operators employ unidirec tional cutting methods instead of bidirectional cutting solely to comply with dust regulations, resulting in an estimated production loss of 12 percent per working face. (Estimated based on personal communications with Consolidation Coal, Old Ben Coal, Jim Walters Resources, and island Creek Coal Corp.) In 1985 this translated into a loss in potential revenues of approximately $200 million. (Revenue Loss -- {[(350.8 million tons mined underground in 1985) x (14.7 pet longwall mining underground)/(100--12 pet)]--[(350.8 million tons) x (14.7 pet)]] x [$28.18 per ton average underground coal price in 1985) = $198.2 million.) EFFECT OF COAL OUTPUT ON DUST LEVELS A fundamental fact of coal mining is that as coal is mined at a faster rate, more dust is generated. Coal producers must balance increased production per eight hour shift against die reduction of average dust levels per eight hour shift.4 This has become more difficult in recent years since: (1) the use of longwall mining, a more productive yet dustier mining method than continuous mining, has increased from only 3.6 percent of underground coal production in 1975 to 20.8 per cent in 1987 (Figure 3), and (2) longwall mining technology has advanced dramatically. The average production of longwall sections per shift was approximately 850 short tons in 1978 and has increased to 1,968 short tons in early 1987, an increase of 132 percent.14 vl Mine section compiance with Federal dust standard x&Mine section compliance with more stringent dust standord due to excess silica dust 314 Figure 2. Compliance of selected underground mining methods with dust standards. YEAR ton-ea is Figure 3. Production by underground mining type. 53 Exposure Monitoring and Control--Cos] Mines I Due to the direct positive relationship between output and dust generated by longwall mining and its growing share of underground coal production, {dots ofdust levels against time (Figure 1) are extremely misleading. It is evident that for a given amount of dust control technology, dust levels will rise as coal output per eight hour shift rises. Average dust levels have decreased through time despite die fact that coal output per hour has increased considerably, but not as much as they would have, given the dust control technology im plemented, if output per hour had remained constant. In Figure 4, the observed path of dust reduction is indicated by the round markers. Had output per shift remained at "out put level 1,** dust would have been reduced even further, as indicated by die square markers. KEY Raw data ------------Adjested to 1986 output-par- hotir level ..............Oust level given (breasted 1995 output per hour ----------- Federal dust standard, post 1972 ------------Average more stringent standard in <987 . Longwall . _ ,,- E- ?- ^ . UJ > UJ !_________________________ I___________ : <s> o CONTROL TECHNOLOGY, dollars Ml 609 Figure 4. Effect of shifting output level cm dust versus expenditures on control technology. Dust levels of longwall and continuous mining sections ad* justed for output per hour are presented in Figure 5. These adjustments were made as follows: output per hour data for the years 1970,1978, and 1986 were indexed to 1986 levels and these ratios were used to adjust the raw dust data. The adjusted curves, then, show die dust level assuming output per hour had been held constant at die 1986 level, ceteris paribus. The adjusted average dust lew! in longwall sections declined from 7.29 mg/m3 in 1970 to 5.50 mg/m3 in 1978 to 2.0 mg/m3 in 1986. Raw data indicate a decrease from 2.3 mg/m3 to 2.1 mg/m3 to 2.0 mg/m3 in these years, respectively. Thus, these curves indicate that, particularly in longwall sections, average dust levels have been lowered more drastically since 1970 than is apparent from die raw data. The 1986 average dust level was then adjusted to the year 1995 given forecasted output per hour of die two mining methods. Output per hour data for 1986 were indexed to forecasted 1995 levels and these ratios were used to adjust die 1986 dust data. (Output per hour is forecasted to increase by 28 percent for longwall mining and by 25 percent for con tinuous mining by 1995.) Under this scenario, if output per hour were allowed to increase to die forecasted values, by 1995 dust levels would exceed die current dust standards by 28 percent in longwall sections and by 2 percent in continuous mining sections (Figure 5). YEAR Figure 5. Dust levels of longwall and continuous mining sections adjusted for output per hour. Unless new dust control technology is developed which enables compliance to be reached at these higher production rates, it is likely that output per hour will be significantly constrained in die future due to required compliance with the dust standard. Indeed, because the average dust level of longwall mining sections is already at the 2.0 mg/m3 stan dard, future increases in output per hour are already con strained, on average. Barring the introduction of new dust control technology, the lost 28 percent increase in longwall mining output per hour forecasted fra* 1995 translates into a loss in potential revenues in 1995 of $584 million from currently existing longwall sec tions. (Coal production from longwall mining is expected to total 74 million tons in 1987 (based on calculations from data in 3, 8, 11)). Revenue Loss = {[1.28 x (74 million tons)] -- [74 million tons]) X [$28.18 per ton average underground coal price in 1985) = $583.9 million.) This estimate is a maximum figure because even if no new dust control technology is developed by 1995, it is expected that more of die existing technology will be implemented by die industry before 1995. COMPETITIVENESS The United States is a major coal exporting nation; exports totalled 85.5 million short tons in 1986, 50 percent going to Europe and 17 percent to Canada.6 There are numerous indications, however, that die U.S. is losing market share to foreign competitors despite die transition to more efficient 54 underground mining technology. Coal exports have dropped significantly from the 1981 high of 112.5 million tons. The Energy Information Administration reported that the U.S. share of the European market declined from 42 percent in 1981 to 31 percent in 1985; Australia and South Africa ap pear to have gained market share at die expense of die U.S.7 The reason for this loss in competitiveness is apparent from a comparison of die price ofdelivered coal to Europe (Figure 6)--the U.S. price is by far the highest of the major coal exporting nations to this market. The U.S. has been losing market share even though European coal imports have been rising. And European coal imports have been forecasted to increase from 139 million tons in 1985 to 174 million tons in 1995. Thus, unless the U.S. is able to improve its com petitiveness, a continued loss of market share in Europe can be expected. Figure 6. C.l.F prices of non-EEC coal delivered to Europe. CONCLUSION To reduce unit costs and thereby ameliorate its competitive position in world markets, the domestic coal industry must continue to increase output while holding the line on pro duction costs. Output from longwall mining sections is forecasted to increase to 45.0 percent of underground coal production, from 20.8 percent currendy as the industry at tempts to achieve this goal. The silica dust problem, presently uncommon in longwall sections, is anticipated to become more prevalent as a con sequence of increased longwall production because con tinuous mining machines are used to develop coal panels for extraction by longwall methods. In addition, due to geologi Exposure Monitoring and Control--Coal Mines I cal conditions--mining of thinner and more heavily faulted and fractured coal seams--the amount of silica dust in air borne respirable dust is expected to increase. In light of the industry trend toward longwall mining, ad vancement of dust control technology is necessary to enable associated increases in production while maintaining com pliance with the mandated standard. If no new control technology is made available, the dust standard will act as a binding constraint on future output per hour. This is especially pertinent to longwall mining where the average dust level is already 2.0 mg/m3. REFERENCES 1. Attficld, M.D. etal.: Past. Present and Predicted Future Levels ofCoal Workers Pneumoconiosis in Working U.S. Coal Mines. NIOSH Ap palachian Laboratory, Morgantown, WV, (1987). 2. Breslin, /.A., and Niewiadomski, G.E.: Improving Dust Control Technology for U.S. Mines. BuMines Impact Report, (1982), 40 pp. 3. Britton, S.G.: Financial Risk Analysis of Longwall Mining. Paper presented at Longwall USA, Pittsburgh, PA, (1987). 4. Coal Age. UMW Identifies Chronically Dusty Mines. V. 24, No. 8, (1987), p. 11. 5. Daniel, J.H., Jr.: Personal Communication. BuMines, DivisionofHeahh and Safety Technology, (1987). 6. Energy Information Administration. Annual Energy Review 1986. Deft. Energy, (1987), 293 pp. 7. Coal-Exporting Countries: The European Market Dept Energy, (1987), 38 pp. 8. Coal Production Trends. Dept. Energy, (1986). 9. General Accounting Office. Low Productivity in American Coal Min ing: Causes and Cures. Report to the Congress, (1981). 10. Jacobsen, M. et al.: Progression of Coal Workers* Pneumoconiosis in Britain in Relation to Environmental Conditions Underground. Pro ceedings ofthe Conference on Technical Measures of Dust Prevention and Suppression in Mines. Luxembourg, Commission ofthe European Communities, (1972), pp. 77-93. 11. Kost, J.A.: Personal Communication. BCR Longwall Production Com mittee, BCR National Laboratories, (1984). 12. Mundell, R.L. et al.: Respirable Du& Control on Longwall Mining Operations in the United States. MSHA IR-1151, (1984). 13. Niewiadomski, G.E. et al.: Respirable Dust and Noise Compliance Trends at Longwall Operations. Published in the proceedings of Longwall USA, June 2-4, 1987, Pittsburgh, PA, pp. 165-172. 14. Personal Communication. MSHA, Coal Mine Safety and Health, (1987). 15. Niewiadomski, G.E., and Nesbit, R.E.: Personal Communication. MSHA, Coal Mine Safety and Health, (1987). 16. Parobeck, P. et al.: Assessment of the Respirable Dust Levels in the Nation's Underground and Surface Coal Mining Operations. AIHAJ, Vol. 40, No. 10, (1979). 17. Richardson, C.A.: 1986 Keystone Coal Industry Manual. McGraw Hill, (1986), 662 pp. 55 Exposure Monitoring and Control--Coal Mines / ON THE TRANSPORT OF AIRBORNE DUST IN MINE AIRWAYS R.V. RAMANI* R. Bhaskarf Department of Mineral Engineering, The Pennsylvania State University University Park, PA 16802 tDepartment of Mining Engineering, University of Utah Salt Lake City, UT 84112 ABSTRACT One of the primary means of control of health hazards from respirable contaminants in mine atmospheres is through design and operation of mines to meet mine health and safety regulations and recommended prac tices. A U.S. National Academy of Sciences study concluded that for significant progress in coal mine dust control, research should be directed more toward obtaining fundamental understanding ofthe origin, transport and characteristics of respirable coal mine dust. Theoretical and experimental studies on transport of dust in mine airways, particularly coordinated efforts to validate theory with practice, are scarce. Some em pirical models, developed on die basis of experimental data, are available but these models cannot be ap plied to new conditions. The purpose of this paper is to present the results of theoretical and experimental studies on the transport and deposition of dust in mine airways. This study is a part of an ongoing research project in the Generic Mineral Technology Center on Respirable Dust. In die paper, the assumptions of die modeling phase of die project and die development of a convectiondiffusion equation for (hist transport in mine airways are outlined. The important aspect of die modeling effort is the capture ofthe deposition phenomenon. The experiments performed under controlled conditions in a typical mine airway, as well as under normal mine operating conditions, are discussed. The comparison of die model predictions with experimental results are made to identify critical areas of agreements and deviations. The implications of die findings and areas for further research and development are presented. LIST OF SYMBOLS b class of size distribution c concentration at center of duct d particle diameter Dp Brownian diffusivity K,j collision frequency function L length of airway under consideration % number of particles in size class k N deposition rate ri radius of particle in i* class R radius of dust Sq molecular Schmidt number Sq turbulent Schmidt number u* friction velocity vt terminal velocity V deposition velocity of particles y distance from surface of deposition e eddy diffusivity o dimensionless particle INTRODUCTION The objective of this study was to aid in the control of dust in underground mines through an improved understanding of die behavior of dust clouds in mine airways. The results of the study presented in this paper span three phases. S6 Phase I involved the development of a mathematical model; Phase II related to experimental studies in underground mine airways; and Phase m dealt with comparative analyses of the mathematical model predictions with experimental data. A summary of the three phases is presented in this paper. MATHEMATICAL MODEL The dispersion and deposition of dust in underground mine airways was modeled as a convective-diffusion problem. To achieve this, the constituents of the model were identified, relationships developed, and assumptions made that closely approximate the physical conditions in a mine airway. A brief dekrription of the major components are presented in this section. PARTICLE DEPOSITION The three major mechanisms of deposition in turbulent airflow in mine airways are Brownian diffusion, convective diffusion, and sedimentation. Deposition due to other mechanisms such as electrostatic and thermal force, and in ertial impaction were considered not significant compared to tiie mechanisms considered. The equation for the turbulent diffusion of particles to the sides ofthe airways may be written as (Friedlander, 1977): (equation 1) N - (D + e) dc P cty while die flux towards the floor and roofare (Sehmel, 1973): (equation 2) N - (D+e) dc v c P dy t The value of the eddy diffusivity e varies within the bound ary layer. Therefore, different values of e have to be used when integrating die flux equation from the deposition sur face to the core of the airflow. An empirical relation is used for describing deposition due to turbulent diffusion in the inertial range, given by (Wood, 1981): (equation 3) V+ - - - 0.13 for 17 < o < 265 cu. and: (equation 4) + v Skyrme 2^6 a a 5-2, for a < 265 The total deposition due to all mechanisms is given by: (equation 5) 5. " Jo1 n<r dr vdiff + f2 Jci 0.13 n(r) dr + f'3 v! n(r) dr Jr- SSkyrme The deposition due to gravity is a function of the terminal velocity and can be written as: (equation 6) Vgravity vto* COAGULATION Coagulation of airborne particles was represented in the mathematical model by a modified rate equation (Chung, 1981) and is given as: (equation 7) dn (t) k dt 1 k-1 z X. n n 2 i-1 13 i 3 b +n X K n k i-l ik i nX Xn k i-b+1 ik i Exposure Monitoring and Control--Coal Mines I where the first term represents the gain in particles in size class k due to the collision of particles of size i and j. The second term represents the loss of particles from size class k due to collision of class k particles with other particles. The last term represents those k class collisions occurring with particles of class less than b, the resultant size being less than the upper boundary of size class k. Ky is the col lision frequency term that takes into account the motion of the particles with the air, relative motion due to the air and relative motion due to sedimentation. The formula proposed by Saffman and Turner (1956) was used in the model. The governing equation is a convective-diffusion equation. A one-dimensional equation was adopted and is represented by the relation: (equation 8) 3c . Ji._ - u -- + sources - sinks at x 3x* dx The equation was solved for a range of particle sizes obtained by discretizing the particle size distribution ofdie source dust. The behavior of the total dust cloud is a weighted average ofthe contribution from the various sizes. The initial condi tion to solve the equation is of the form: (equation 9) c(x, t) = 0 for t = 0, 0 < x < L where L is the length of the region of interest. The boun dary condition was developed by assuming that the concen tration of the dust becomes asymptotic at the end ofthe region of interest. It is represented as: (equation 10) dc 0 dx The source term (S(t)) was developed as a step function and is given as: (equation 11) n S(t) - X A (5(t - t) i-1 x i when n = the number of operating modes. A; is die amount of dust released in mode i, and 8 is the dirac delta function. The model was solved numerically, using an implicit scheme (Bandopadhyay, 1982) and programmed in WATFIV. EXPERIMENTAL STUDIES To obtain a better understanding of the spatial and temporal behavior of dust clouds in underground mines and data to compare with the predictions of the mathematical model, a set of mine experiments were conducted. They were per formed in the Lake Lynn Laboratory of the U.S. Bureau of Mines. The laboratory was formerly a limestone mine. Six experiments were conducted. The salient parameters are listed in Table I. The experiments provided data on ambient concentration, floor deposition, particle size distribution, and cross-sectional variation of dust at various stations along the length of the airway. In addition, two experiments were per formed in the return airway of a longwall section. 57 Exposure Monitoring and Control--Coal Mines l Experiment 1 Experiment 2 Experiment 3 Experiment 4 Experiment 5 Experiment 6 Table I Salient Data cm Controlled Experiments Dust Tvne Semianthracite Bituminous Semianthracite Bituminous Semianthracite Bituminous Velocity, m/s 0.838 0.838 1.855 1.855 1.525 1.525 for airborne concentration and deposidon is shown in Figure 1. Centerline and cross-sectional air borne dust samples were collected as shown in the figure. Twelve samples were collected at each of the three cross sectional sampling stations. The sampling systems were designed for isokinetic sampling, using specially shaped sharp-edged nozzles. Corrections as suggested by Belyaev and Levin (1974) were applied to those data for which isokinetic sampling conditions were not achieved. Floor samples were collected at about 13 stations, 100 feet apart, along the airway. Samples were collected along die center and across the width of die airway. Flat deposition plates covered with preweighed, lightweight, "sharkskin" filter papers were used to collect die dust. The dust was dispersed by a fluidized bed-type trickle duster through a four-port system of tubes. Each port was located at die center ofdie four quadrants ofthe airway cross-section. Semi-anthracite and bituminous dust, with top size of 25 pm and median size in die 4.96 to 7 pm range were used as source dusts. COMPARISON OF MODEL OUTPUT WITH EXPERIMENTAL DATA The experimental data were compared with the output from the mathematical model for similar physical conditions. Com parisons were made for ambient concentration, deposition, particle deposition rates, dispersion coefficient and cross sectional concentration of the dust. The inputs to the model were based on die physical conditions prevailing during the experiments. These included die airway, source dust characteristics, and airflow conditions. For reasons ofbrevi ty, comparisons for only a select set of experiments are presented. The results of comparison of model output and experimental data for experiments 1 and 6 are presented in Figures 2 and 3 for ambient concentration, and in Figures 4 and 5 for deposition. The comparison of predicted and actual concentrations for experiment 1 (Figure 2) shows that predicted concentration falls rapidly with distance from the source tending to an asymptote towards the end of the region of interest. The ex perimental data also shows a rapid decrease in concentra tion from the source, in fact, more than that predicted by the model. However, part of this deci agglomeration induced increase in deposition rate. The two data sets closely follow each other after 120 m from die source. The respirable dust data show that while the predicted and experimental data are generally in agreement, the ex perimental data show a more consistent deposition along die airway. The concentration data for experiment 6 (Figure 3) shows a closer match between die predicted and experimental data up to 180 m, after which the experimental data tends to assume a less steeper decline in concentration. This pattern is also true in die case of respirable dust data for die experi ment. The experiment was conducted at 1.55 m/s. It appears that some of the differences between the concentration data sets may be due to the greater sensitivity and scope for er rors in concentration data measurement. The deviation be tween predicted and experimental data at the first two sta tions near the source may possibly be due to inadequate dispersion of the source dust. The deposition data for experiment 1 is presented in Figure 4. The data shows good agreement between die predicted and experimental data. The agreement is especially close be tween 60 and 400 m. Hie deposition data for experiment 6 (Figure 5) also show good correlation between die two data sets between 100 and 420 m. In addition to comparison ofthe predicted and experimental ambient concentration and deposition, comparisons were also made between the deposition rate per unit concentration, per unit time, and die dispersion coefficient of the dust cloud. The comparisons could be made for floor deposition only, as the amount of dust deposited on the sides and roof could not be collected with an acceptable degree of accuracy. Very little dust, compared to floor deposition, could be collected on the sides and roof. The theoretical deposition rate was assumed to be dependent on only the physical parameters relating to die particle and flow properties. The volume concentration of the dust was assumed to be low enough to be considered a `dilute' flow. Therefore, the particles were assumed not to affect the fluid flow properties and the theoretical deposition rate was con sidered to be independent of concentration or location of the dust cloud in the mine airway. However, the experimental data showed that deposition rate decreased with distance from the dust source, becoming fairly constant towards die end of the airway (Figure 6). 58 Exposure Monitoring and Control--Coal Mines 1 DISTANCE IN METERS 19.50 - 30.48 - t AIRFLOW * "V SOURCE 60.97 - 0 0 Q 0 C/S 91.46 ' 0 121.95 -0 0 0 0 0 C/S 152.40 + 0 182.90 4-0 0 0 0 0 C/S 213.40 + 243.90 4-D C/S . CROSS-SECTIONAL CONC. SAMPLING 0 - FLOOR DEPOSITION SAMPLING POINT 0 FLOOR AND AMBIENT CONC. SAMPLING - SPRINKLER SYSTEM A . TRICKLE DUSTER AND DISPERSER ASSEMBLY 274.40 T 304.90 4 O 335.40 + 365.80 0 396.30 426.82 4o 0 0 Q 487.80 4 Figure 1. Ambient concentration and deposition sampling plan (Controlled Experiment 6). 59 Exposure Monitoring and Control--Goa/ Mines / Experiment 1 60 Deposition (g/m 2) Exposure Monitoring and Control--Coal Mines I Experiment 1 Figure 4. Comparison of model predicted floor deposition with experimental data (Controlled Experiment 1). Experiment 6 61 Exposure Monitoring and Control--Coal Mines I 1X 1.4 Co *2c* 1.2 0oc) o 1.0 O c 0.8 x X a> Q. a 0.6 ca cc o 0.4 X X x x 0o) Q. a> 0.2 Q x X 0.0 J--i------1-------- 1--------1-------- r 12 3 4 5 7 Station T 9 Figure 6. Normalized deposition rates of dust along mine airways (Experiment 5, size 3.73 microns). The dispersion coefficient relationship used in the model was developed by Skubunov (1974) and is given by: (equation 12) E - 15.8 UDSc70*6 Sch /T/x" X 1 c The values obtained by this relation was compared with die experimental data. The experimental dispersion coefficient was calculated using die procedure outlined by Klebanov and Martynyuk (1974). The results are presented in Table I. The comparison of the calculated and experimental data show that both are in the same order of magnitude. The experimental values vary from 11.79 to 45.06 nF/s while die model assumed value was 61.46 mVs. Cross-sectional concentration data were also collected dur ing die experiments. The data showed that die average con centration across die cross-section is 75 % of die concentra tion at die center of die airway, with all the points in the cross-section given equal weights. The concentration decreases from the roof to die floor, with die top third of the airway having a concentration 72% of that in the lower third, while die concentration in die middle third being 89% of that in die lower third of die airway. Complete details of the theoretical and experimental study are presented in Bhaskar (1987). SUMMARY A mathematical model describing die behavior of dust clouds in mine atmospheres was developed with special reference to the condition prevailing in a mine. The model was pro grammed for die computer and outputs ambient concentra tion and deposition data as a function of time and location. The output includes both the total and respirable size ranges. In addition to mathematical modeling, experimental studies were performed in mine airways for two types ofdust at three velocities. The experimental data were compared with die output of die mathematical model for similar conditions. The results show that there are areas of agreement and devia tion between the two data sets. The comparison highlighted areas, such as deposition rate, reentrainment and diffusion coefficient, where additional studies have to be performed. Studies in these areas have been initiated and are continuing. 62 REFERENCES 1. Bandopadhyay, S., 1982, "Planning with Diesel Powered Equipment in Underground Mines," Pb.D., Thesis, The Pennsylvania State University. 2. Belyaev, S.P. and Levin, L.M., 1974, "Techniques for Collection of Representative Aerosol Samples,"/. .Aero. Sci., Vol. 5, pp. 325-338. 3. Bhaskar, R., 1987, "Spatial and Temporal Behavior ofDuk in Mines-- Theoretical and Experimental Studies," Fh.D. Thesis, The Pennsylvania State University. 4 Chung, H.S., 1981, "Coagulation Processes for Fine Particles," Ph.D. Thesis, The Pennsylvania State University. 5. Friedlander, S.K., 1977, Smoke, Dust and Haze. Fundamentals of Aerosol Behavior, John Wiley and Sons, New York. Exposure Monitoring and Control--Coal Mines 1 6. Klebanov, F.S. and Martynyuk, G.K., 1974, "A Method for Ex perimental Determination ofthe Coefficient of Longitudinal Turbulent Diffusion in Ventilating Currents of Mine Workings," Sov. Min. Sci., pp. 413-416. 7. SafTman, P.G. and Turner, J.S., 1956, "On the Collision of Drops in Turbulent Clouds," J. Fluid Mechanics, Vol. 1, pp. 16-30. 8. Sehmel, G.A., 1973, "Particle Eddy Diffusivities and Deposition Velocities for Isothermal Flow and Smooth Surfaces," J. Aerosol Sciences, Vol. 4, No. 2, pp. 125-138. 9. Skubunov, V.V., 1973, "Turbulent Transport Coefficients for Mine Workings and Tunnels," Soviet Mining Science, Vol. 9, No. 4, pp. 402-417. 10. Wood, N.B., 1981, "The Mass Transfer of Particles and Acid Vapor to Cooled Surfaces," /. Inst. Energy, Vol. 76, No. 6, pp. 73-93. 63 Exposure Monitoring and Control--Coal Mines / DUST CONTROL ON LONGWALL SHEARERS USING WATER-JET-ASSISTED CUTTING C.D. TAYLOR P.D. Kovscek K. Neihaus E.D. Thimons Bureau of Mines, U.S. Department of the Interior* INTRODUCTION Since 1977 the number of U.S. longwaU mining sections using double-ended ranging-arm shearers has more than doubled. Improved productivity is a primary reason for us ing the longwall mining method. Average U.S. longwall pro duction is 700 to 1,200 tons/shift compared with 300 to 400 tons/shift for room and pillar mining. However, in some cases, production on longwall sections must be limited because die levels ofairborne respirable dust exceed die man datory standard. The best way to suppress dust generated by die shearer is to add water to die coal at a location near the cutting bit. The most effective way to accomplish this is to supply water through the rotating drum and distribute it to nozzles located in die bit block. All longwall shearers operating in die United States are equipped with this type of water spray system for dust control. Typically the water pressure measured at die nozzle is 100 to 200 psi. Increasing die water pressure delivered through the drum-mounted sprays will usually decrease dust levels. Water-jet-assisted cutting uses moderately high pressure, 2,000 to 10,000 psi (13.S to 67.5 MPa), solid streams of water, called water jets, that are directed to strike near the cutting bit tip. The Bureau of Mines and others have evaluated the potential advantages of using high-pressure streams of water for water-jet-assisted cutting. Water-jetassisted cutting was used with a roadheader. Energy sup plied by die water jets enabled the roadheader to cut hard rock that could not be cut when operating dry.1 Results of an earlier laboratory test program showed that airborne dust formed during cutting could be reduced by using water-jetassisted cutting.2 The objective of this research program was to determine what effect use of water-jet-assisted cut ting has on respirable dust levels generated during cutting with a longwall shearer. Testing was conducted on the surface at a simulated longwall face and on an operating underground longwall section. The initial study took place at the Bureau of Mines' surface test facility in Pittsburgh, PA. Operating parameters could be controlled more precisely at the surface site than underground. A 60-ft-long (18.5 m) by 6-ft-high (2 m) coalcrete block, composed of coal, fly ash, and concrete, was used to simulate a longwall face. Because coalcrete has a higher silica content, it is more abrasive than coal; however, when using conventional drag bits, its cutting properties are similar. Overall the coalcrete face was homogeneous. The shearer used to cut the coalcrete was a Joy 1-LS1* double-drum machine (Figure 1). For each test die shearer cut from right to left. Only the left hand, or leading drum, was supplied with high-pressure water and used for cutting during the tests. The right-hand drum was positioned so that it traveled within the cut made by the left-hand drum. A longwall face conveyor, located adjacent to the coalcrete block, provided continuous removal of the cut material, as well as functioning as a support along which the shearer moved. The diameter of the cutting drum (bit tip to bit tip) was 54 in. (137 cm), and the drum width was 28 in. (71 cm). During die tests, web width (thickness of the cut) varied from 25 to 29 in. (63.5 to 73.5 cm). The machine tram rate was maintained at approximately 5 ft/min (1.5 m/min). Drum rotation speed was 46 r/min with a bit tip speed of650 ft/min (200 m/min). Thirty-two radial attack bits were mounted on die drum. The site for the underground work was a longwall section in die Auguste Victoria Mine, which is located in Mari, West Germany. The face was 7.54 ft (2.3 m) thick, 919 ft (280 m) long, and mined on retreat. During the tests, one single-drum and one double-drum shearer were operated on the face. Figure 2 shows the relative locations of the shearers on the longwall face. The single drum machine, an Eickhoffmodel EW-200/170-L shearer, was supplied with high-pressure water (Figure 3). This shearer operated within 164 ft (50 m) of tiie longwall tailgate. While making dust measurements, the shearer cut only in die upper part of the face. Shearer tram rate and web width were maintained as constant as possible. Underground testing at low pressure was conducted using the cutting drum that was originally supplied with the shearer. This drum was not designed for use with high-pressure water and a new drum had to be designed and built for the waterjet-assisted cutting tests. Table I compares features of the original and new drums. Included with die high-pressure Reference to specific products does not imply endorsement by the Bureau of Mines. 64 Exposure Monitoring and Control--Coal Mines I 65 Exposure Monitoring and Control--Coal Mines I Table I Comparison of Cutting Drums Used During Underground Tests High-Pressure Drum Diameter(in)...................... 67 Web depth (in)........................ 33.4 R/min........................................ 23.6 Bits (No./type)................ 51/conical Bit tip speed (ft/min).. 413 Spray nozzles (No./type) 50/Sapphire Flow rate(gal/min).......... 10 to 21 Low-Pressure Drum 63 33.5 48 55/radial 791 41/conical 10 Figure 3. Shearer operating underground. drum was a newly designed ranging arm with double planetary gearing that provided a drum rotational speed of 23.6 r/min. The slower rotational speed allowed a more ef ficient distribution of fluid energy, i.e., more energy could be supplied per length of cut. However, another consequence of slower rotation speed was a deeper depth of cut. The bit lacing was modified to provide more efficient cutting and loading at deeper cutting depths. SURFACE WATER DELIVERY SYSTEM A 200-hp (112-kW) Aqua-Dyne triplex pump was used to supply the desired water pressures to the shearer. The pump was placed adjacent to the coalcrete face and water was transported to the shearer through a 2-in (5.1 cm) flexible hose. Water pressure during the low-pressure tests was main tained at 190 psi (1 mPa). During each water-jet-assisted cut ting test the pressure was maintained constant. High pressures between 1,000 and 6,000 psi (7 to 40 MPa) were used. Water entered the cutting drum through a high-pressure Aqua-Dyne rotary seal, located in the drum hub. Six hoses were at tached to toe rotary seal. Each one of die six hoses carried water to a sector of the cutting drum which contained ap proximately 1/6 of the water jet nozzles. A waterjet nozzle was located in front of each of the 32 cutting bits on the left cutting drum (Figure 4). All water nozzles in the drum operated continuously during die surface tests. Each nozzle used for these tests had a 13 degree Leach and Walker configuration (Figure 4). To maintain approximate ly the same flow rate during die high- and low-pressure tests, 0.024 in. (0.6 mm) and .07 in. (1.78 nun) orifices, respec tively, were used. Nozzle flow rates for each test pressure are given in Table Q. Each nozzle delivered a solid stream of water to a location about 0.1 in. (3 mm) in front of the bit tip. Distance from the nozzle to the bit tip averaged about 66 Exposure Monitoring and Control--Coal Mines I To provide the high-pressure water needed for water-jetassisted cutting, a five-piston pump was mounted on a trailer that was pulled by the shearer. The maximum capacity of the pump was 34 gal/min at 10,000 psi. Fifty of die 51 bit blocks were equipped with jet nozzles (Figure 5). Blockage of die 0.6 sapphire nozzle orifices was reduced by installing a 10 micrometer filter in the water line. The drum built for die high-pressure tests, was divided into 10 sectors. Water was directed to each sector through manifolds and high-pressure hoses (Figure 6). A phasing system was designed to feed the water to five of the ten sec tors at a time. The average angle of die arc of rotation that was supplied wife water was 195 degrees (see Figure 7). Us ing this phasing system reduced the water required by about 50 pet. Figure 4. Bit block and nozzle configuration for surface testing. Table H Flow Rate versus Water Pressure for Surface Tests Flow rate Pressure, psi qal/min High-pressure:1 6,000.............. 5.000.............. 4.000.............. 3.000.............. 2.000.............. 1.000........ . Low-pressure:^ 190.................. 1.26 1.15 1.03 .90 .75 .54 .90 10.024-in orifice ^0.071-in orifice 4 in (10 cm). The water lines in the cutting drum were flushed frequently, and the water passed through 10 micron filters to reduce the possibility of nozzle blockage. UNDERGROUND WATER DELIVERY SYSTEM Normal head pressure provided water to die shearer at 340 psi (2.4 MPa). Forty-one conical spray nozzles mounted in the cutting drum were used for dust control. Total flow rate for this normal operating pressure was approximately 10 gal/min (38 1/min). Figure 5. Bit block and nozzle configuration for underground testing. TEST PROCEDURE Cuts made in the coalcrete block were 5 to 40 feet (1.5 to 12.3 m) in length. Water pressure was monitored during each test cut to assure the water pressure did not vary. Dust levels were measured at two locations near the shearer. 1. About 6 ft (1.8 m) from the cutting drum at approximate ly the same height as the top of the cut. 2. About 24 in. (0.6 m) from the bottom of the lead drum. Real-time aerosol dust monitors (GCA RAM l's) and strip chart recorders were used to track the levels of airborne respirable dust. Dorr-Oliver 10 mm-nylon cyclones were used to separate the respirable dust from die larger particulates. Underground, one location upwind, and another downwind of the shearer were sampled. As much as possible, during underground testing, no other work that produced dust, such 67 Exposure Monitoring and Control--Coai Mines I pared with dust levels generated while operating at 190 psi (1 MPa). The percentage dust reductions achieved by using the higher water pressures are shown in Table IE. At a water pressure of3,000psi (20 MPa), the dust levels were 79.2 pet less than when operating at 190 psi (1 MPa). Raising the pressure further from 3,000 to 6,000 psi (20 to 40 MPa) resulted in only small additional dust reductions. Table HI Comparison of Dust Reduction During High- and Low-Pressure Operation Pressure, psi Dust reduction. High-pressure:* Et 6,000.............. 80.4 5,000.............. 84.8 4,000.............. 80.4 3,000.............. 79.2 2,000.............. 63.9 1,000........ .. 4.2 Low-pressure:* . 190.................. o 0 A0.024-in orifice *0.071-in orifice Figure 7. Phasing system for underground testing. as moving foe roof supports, was carried out upwind of foe shearer. The foist generated by foe second shearer, which operated on foe headgate side of foe test shearer, did not in fluence foe dust readings, because airflow was from tailgate to headgate. RESULTS For foe surface tests, foe average dust levels measured while using high-pressure water (1,000 to 6,000 psi) were com 68 The underground respirable dust results are shown in Figure 8. At a water pressure of 1,800 psi (12 MPa) and a water flow rate of 10 gal/m (381/min), average dust levels were reduced almost 80 pet compared to dust levels measured while operating at 340 psi (2 MPa) and 10 gal/min (381/min). Maintaining the water pressure at 1,800 (12 MPa) and in creasing foe flow rate to 21 gal/min (801/min), by increas ing foe nozzle orifice size, resulted in no further reduction in dust. Additional reductions in dust level due to increas ing foe pressure to 7,200 psi (SO MPa), with a flow rate of 21 gpm, (801/min) were not significant. DISCUSSION Dust Levels Use of water during longwall mining reduces the levels of airborne dust by: 1. Capturing airborne dust particles. 2. Wetting foe dust particles before they can become airborne. The surface study results showed that increasing foe water pressure from 190 to 1,000 psi (1 to 7 MPa) did not significantly reduce dust levels. Dust levels decreased rapidly as foe water pressure was raised from 1,000 to 3,000 psi (7 to 20 MPa). Any further decrease in dust level, as foe water pressure was raised from 3,000 to 6,000 psi (20 to 40 MPa), was small. Raising foe water pressure underground from 340 to 1800 psi (2 to 12 MPa) reduced airborne dust levels 70 to 80 pet. There was no significant additional reduction in dust level when foe pressure was raised from 1800 to 7,200 psi (12 to 50 MPa). The fact that there is a maximum pressure above 16 PRESSURE / FLOW Figure 8. Underground respirable dust results. which no further dust reductions take place further confirms the results ofthe surface longwall shearer study and die work performed by other researchers with roadheaders.3 The operation of the shearer during surface cutting of the coalcrete was similar to the operation of a shearer on an underground longwall section. However, die airflow patterns on an underground longwall face, which have a significant effect on the distribution ofthe airborne dust near die shearer, could not be simulated during surface testing. Also, the amount of dust generated by cutting coalcrete and coal would not be the same, due to physical differences between the two materials. Therefore, the dust levels measured during sur face testing cannot be directly related to the amount of dust generated underground. However, fee underground study results verify that the relative reductions in dust resulting from use of die high-pressure sprays are typical of what can be achieved underground. Mining conditions during underground testing were represen tative of a typical longwall operation although the amount of dust generated was extraordinarily high. This may have been due to cutting in a faulted zone. The same reductions in respirable dust obtained underground cannot be expected for all faces. Use of high-pressure water directed through drum mounted jet nozzles would be effective for dust sup pression on all longwall faces. Exposure Monitoring and Control--Coal Mines I Interpretation of the underground dust data is complicated by the fact that during the high-pressure tests, a different cutting dram was used and the dram r/min was reduced. Cut ting depth was increased because the tram rate was kept con stant. Reduced drum r/min and increased cutting depth has been shown to reduce airborne dust levels.4 It is not possi ble to determine how much each factor, reduced r/min, deeper cutting, or water-jet assist, contributed to the reduc tion in dust levels. For optimum dust control, it is recom mended that high-pressure water be used with reduced drum speed and deeper depth of cut. Supplying high-pressure water for water-jet-assisted cutting requires a large amount of fluid energy. The quantity of energy can be reduced if water is supplied only to that part of the cutting drum where the bits are in contact with the rock. Although a phasing system was used for the underground study, a suitable system wasn't available for the surface study. To more accurately reflect fee amount of energy directed to fee bits that were cutting during fee sur face tests, the total fluid energy supplied was divided by two. Using these calculations, at 190 psi (1 MPa) operating pressure, fee fluid energy accounted for less than 2 pet of fee total energy used during cutting. At 6,000 psi (40 MPa), almost 33 pet of fee total energy supplied during cutting was provided by fee water jets. During underground testing a similar proportion of fee total energy was supplied by fee water jets. CONCLUSIONS The results of fee surface and underground studies showed that use of water-jet-assisted cutting significantly reduces air borne dust generated by a longwall shearer. Optimum dust suppression was achieved using pressures between 1,000 and 3,000 psi (7 to 20 MPa). These reductions in respirable dust were obtained without increases in water flow rate. Underground a phasing system, used to direct water to only those bits feat were cutting, reduced water flow rate by 50 percent. The second underground trial called for under this research project will be conducted on a longwall face in the United States. During this test a double ranging arm shearer will be equipped wife a high-pressure water supply system. REFERENCES 1. Morris, A.H., Tomlin, M.G.: Experience wilh Boom-Type Roadheaders Equipped with High-Pressure Water Jet Systems for Roadway Drivage in British Coal Mines. Bureau of Mines Open Industry Meeting. U.S. Bureau of Mines. PA, (June 21, 1984). 2. Evans, R.J., Handewith, H.J., Taylor, C.D.: Analysis of Mechanical Tod Force Reductions When Using Water-Jet-Assisted Cutting. Bureau ofMines Open Industry Meeting. U.S. Bureau of Mines, PA, (June 21, 1984). 3. Haslett, G.A., Corbett, G.R., Young, D.A. An Investigation into the Effect of Varying Water Pressure and Flow Rates Upon the Release ofAirborne Respirable Dust by a Dosco MKIIB Roadheader Equipped with a Water Jet Assisted Cutting Head. 8th Internationa} Symposium on Jet Cutting Technology. BHRA, England, (September 1986). 4. Ludlow, J., Wilson, R.J.; Deep Cutting: Key to Dust Free Longwalling. Coal Mining and Processing, Vol. 19, No. 8, (August 1982). 69