New Respirable Dust Suppression Systems for Coal Mines

Sept, 2007Jourmal of China University of Mining& TechnologyVol 17 No. 3Availableonlineatwww.sciencedirect.comSCIENCEDIRECT·J China Univ Mining Technol 2007, 17(3): 0321-0325New Respirable dust suppressionSystems for Coal MinesXIE Yao-she. FAN Gao-xian. DAI Jun-wei, SONG Xiao-boSchool of Mining and Safety Engineering, China University of Mining Technology, Xuzhou, Jiangsu 221008, ChinaJIning Mining Industry Group Co Ltd, Jining, Shandong 272100,ChinaAbstract: Dust suppression in coal mines is a worldwide problem which has not been solved effectively. The applica-tion of negative pressure secondary dust removal(NPSDR)is a breakthrough in the coal mine safety field. In this paper,NPSDR technology and ultrasonic dust suppression systems are introduced. High pressure water is supplied to thenPSDR device which is mounted on the shearer. A negative pressure field is formed in the device. At the same time, thedusty air around the shearer drum will be sucked into, and purged from, the NPSDR device by the negative pressurefield. An ultrasonic dust suppression system uses water and compressed air to produce micron sized droplets whichsuppress respirable coal dust effectively. The NPSDR technology can be used for shearer dust suppression while ultrasonic dust suppression can be applied in areas such as the transportation positions. These dust suppression methods havethe following advantages: high efficiency, wide applicability, simple structure, high reliability and low cost.Key words: dust suppression; negative pressure; ultrasonic; compressed air; respirable dustCLC number: TD 7141 Introductiondisease caused by coal dust deposition in the lungs,was observed to be highly common in coal minA dust suppression system should suppress the coalersdust generated during coal cutting and transfer, in-cluding dust emitted by the various feed/discharge 2 Existing Dust Suppression Systemspoints of conveyors used in the coal mine. There areseveral existing methods of controlling dust but manyThe existing dust suppression system in coal minesof them are ineffective, costly and have detrimental is a wet dust-suppression system. This system coneffects on machinery. An effective system for the sists of two main partscontrol of fugitive dust in coal mines should meet the1)Spray headers with pipelines and pumpingfollowing needs: 1)it must be efficient to meet health equipment,and safety requirements; 2)it must be practical and2)Control units with related electrical systemssimple in operation; 3)it must have low initial cost;The spray headers and pipelines include isolating4) it must have low operating costs, and; 5)it must valves, spray nozzles, and the pipe lines themselveshave no adverse effects on product quality or produc- Water pressurized by a gravity head is supplied to thetion machinery.spray nozzles. a number of nozzles may be used toThe system used at the coal working face does not spray water as required.meet all these requirements; especially it does notThe control units consist of sensing units and con-meet the health and safety requirements. Coal miners trol panels. The coal flow is monitored by a sensor.are typically exposed to mixed coal dust in the work The control panels consist of various relays and trans-place. Significant exposure to coal dust may occur formersespecially during underground coal mining. The in-2.1中国煤化工 isting systemhaled coal dust can deposit in lungs and can lead tothe development of several diseases in exposedCNMH May be used toworkers.Coal workers'pneumoconiosis(CWP), a govern tne sysine main LanK is filled with waterReceived 06 November 2006: accepted 11 January 2007orrespondingauthorTel+86-13952115898;E-mailaddressxyscumt@163.comJournal of China University of Mining TechnologyVol 17 No. 3which is supplied from this main tank to the spray mine atmosphere. This problem prevents more im-nozzles. The feed water tank operates automatically provement in this directionunder float switch control. The float switch picks up 2.3 Efficiency of systemat the low level and stops at the high level, therebyregulating the water level in the main tankThe use of water sprays over the years has estab-2.2 Existing system drawbackslished the following facts1)For a given spray nozzle, the collectionThe existing system does not work effectively on ciency for small dust particles increases as therespirable dust. Since water is pressurized by the sure increaseforce of gravity and is then supplied to the spray noz2) At a given pressure, the efficiency increases aszles, the typical water droplet sizes are 200 to 600 the nozzle design is changed to produce smallemicrons. These dimensions are much greater than the dropletssize of the respirable dust.The conclusion is clear: smaller droplets are moreA At the same time, the flow volume is insufficient. effective in knocking small dust particles out of theus, the water sprays can be improved by increasing air-y.the line pressure or by designing different noozzieConsider a water droplet about to impinge on awhich can produce smaller, more numerous droplets. dust particle or, what is aerodynamically equivalent, aMost of the improvements in this direction already dust particle about to impinge on a water droplet ashave been made. The difficulty with this approach is shown in Fig. 1the introduction of very small water droplets into theDust particle follows stream lineDust particle impacts and agglom eratesDust particleStream line(b)Small dropletig. I Comparison of big droplet with small dropletdroplet diameter is much greater than the nozzle;dust particle, the dust particle simply follows the air2)optimizing the energy transfer between spraystream lines around the droplet, and little or no con- droplets and the dust-ladentact occurs. In fact, it is difficult to impact micronImproving water droplet and dust particle interac-size particles against anything which is why inertial tion requires optimized spray nozzle technologiesseparators do not work well at these particle sizes.If, on the other hand, the water droplet is of a sizecomparable to that of the dust particle contact occurs 3 Negative Pressure Secondary dusas the dust particle tries to follow the stream linesmoval (NPSDR) Technology -/ Re-Thus, the probability of particle to droplet contactincreases as the size of the water droplets decreasesSeveral studies have shown that the shearer is theThis explains why water sprays are not very effec- major source of dust in a longwall working face minetive on respirable dust: typical droplet sizes are 200 to Negative pressure secondary dust removal technology600 um-much greater than the size of respirable is mainly designed for shearer dust control. Based ondust which is less than 5 um. Thus, water sprays can the above principle a high pressure water system isbe improved by designing nozzles which produce used for shearer dust suppressionsmaller droplets.The followings should be taken into consideratioFactors affecting the efficiency of water droplet when the NPsformation are particle size, particle solubility, particle,First, the device must be miniaturized to suit thecharge, temperature, relative humidity, pressure, elec- working conditionstric fields. etcproduce enough air suction volumeCertainly, the efficiency of spray dust- capture will to pu中国煤化工 shearer. Based onbe increased by increasing the probability of contact fieldbetween water droplets and respirable dust particles. inducCNMHGound the shearerThis probability may be increased byis about 40-50 m/min In order to ingest all the dustya I)increasing the number of smaller sized spray is designed for 80-100 migair created by the shearer, the NdDR device capacitylets per unit volume of water passed through theXIE Yao.she etNew Respirable Dust Suppression Systems for Coal MinesThird, the contaminated air should not escape from From the test results we can see that dust can ef-he spraying shield which is produced by the NPSDr fectively be controlled and suppressed by the NPSDRdevice.The negative pressure field produced by the technologynPSDR device cannot control all the dusty air aroundWhere: the coal seam height is 3. 1 m; cut depth isthe shearer's locaton due to the devices small volume 0.6 m; shearer speed is 4.0 m/min, f =3.9 H to Tand installed position. Hence, the device must pro- means headgate to tailgate, T to H means tailgate toduce a tight spraying shield to prevent the dusty air headgate. FD means full dust concentration. ARDfrom moving away from the face and getting out of means airborne respirable dust concentrationeld and thenegative pressure zone.4 Ultrasonic Dust Suppression SystemsThe ultrasonic dust suppression system uses waterand compressed air to produce micron sized dropletsthat can suppress respirable dust effectively. This design is ideal for spray curtains intended to contaiNdust within hoppers. Generally the system consists ofhree main parts:D)air compressor units;2)spray headers with pipe lines and a pumpinsystem, and3)control units with electrical systems, as shownFig 2 Distribution of the spraying shield andin Fig 4the negative pressure zone1. Shearer drum; 2. Conveyor; 3. Spraying shieldWater lineWaterHigh pressure water(about 12-17 MPa)is suppliedto the npSdr device allowing the production ofhigh speed water spray jet. This allows the formationof a negative pressure field in the device suction areaThus the dusty air can be sucked into the device forcleaning.Fig. 3 shows the how the equipment used for thismethod are installedFig 4 Ultrasonic dust suppression systemOf course, there will be two lines for each header,one for the compressed air and one for waterEither auto or manual control will govem the sys-tem. First, it must be insured that the main tank isfilled with water. The water is then pumped by a feedFig 3 Device installation metwater pump from the main tank to a feed water tank.1. Shearer drum; 2. NPSDR device: 3. Pump station; 4. Water lineThe feed water pump is controlled automatically by afloat switch. The pump picks up at low level and isTests evaluating the nPSdr device effectiveness stopped at high level. The interlock with the low levelhave been conducted at a test facility (Fig. 3)in sensor of the main tank will not allow the feed waterBaodian Coal Mine. The results are shown in Table 1pump to run with a low main tank levelTable 1 Summary test results for NPSDR deviceThe air compressors fill an air tank to the desirepressure. There are two pressure levels for the airCondition TypeDust concentration(mg/tank. The high level tripping the air compressor whileTto H. FD 12.52532544.511.7the low level is for pickup of the compressor. ThereARD 4must be a safety valve which vents the tank ifTLoH中1195pressure increases above a set pressure. This pressure中国煤化工 r trip and wi303251309834H to t+onlyompressor-control32327.2faCN MH GS for each headera.NOHone for the airline and other for the water lineThe sensing system for controlling the spray ats stands for with the NPSDr: without the NPSDrach header ensures spraying when a conveyor isJournal of China University of Mining TechnologyVol 17 No. 3running with coal but will not allow spraying if a converging section and expands into a diverging sec-conveyor is running empty.tion and resonator chamber. This produces a powerfuThe solenoid valves(airline and waterline)of the sonic shock wave. Water or other liquids deliveredheaders are energized when the conveyor is running into this sonic area are shattered into droplets. Drop-with coal. If all, or some, of the water solenoid valves lets produced by sonic atomization are small andare unenergized water will discharge through a pres- relatively uniform in size, on the order of 10 um, withsure relief valve. This protects the system. a solenoid a low mass and low forward velocity. The nozzlesvalve that operates when none of the spray header have large liquid ports which are not prone to block-valves are operative is provided as a bypass for the age. Fig. 5 illustrates the design of the ultrasonic noz-water line4.1 Operating theoryThe theory behind the ultrasonic dust suppressionsystem is based on research considering water dropletimpingement on a dust particles. The probability ofcontact increases as the size of the water droplet decreases. This explains why conventional hydraulicwater sprays are not effective on respirable dust. withtypical diameters of 200-600 um the droplets areluch larger than the dusts they are attempting toFig. 5 Ultrasonic atomizing nozzlesuppressThe ultrasonic dust suppression system generates 4.3 Application of the ultrasonic dust suppres-droplets of 10 um average size and is capable of sup-pressing sub-micron dust particlesdditional research indicates that another phe-Fig. 6 shows the ultrasonic dust suppression sys-tems. This effect can be compared to an electrostatic Mine Preparation Plant position in the Baodian Coalnomenon occurs in ultrasonic dust suppression sys- tem used at the transportprecipitator in which dust particles are charged andthen collected on plates of opposite charge. It wasfound that dust particles generally carry a negativepotential which depends on the nature of the dust andthe ambient conditions. Water droplets produced bythe nozzles carry a charge that is strongly positive inrelation to the dust particles.The result is that the probability of collision between a droplet and particle is greatly increasedFig. 6 Ultrasonic dust suppression system used atthe transport positionimplying that fewer water droplets are required to 1.Conveyor bel: 2. Control unit; 3. Ultrasonic nozzle;4.Suppression boxensure the desired efficiency.If the diameter of the droplet is much greater thanDust removal effectiveness tests for the ultrasonicthe dust particle, the dust particle simply follows the dust suppression system have been conducted at theair streamlines around the droplet and little or no 179# transport position". The results are shown incontact occurs. If, on the other hand, the water droplet Table 2is of a comparable size to the dust particle, contactoccurs as the dust particle tries to follow the airTable 2 Test results using the ultrasonic dust suppressionstreamline. Thus, a nozzle generating a dense fog ofTest position1-10 um size droplets can be used to envelop andAverage dust concentration FD 148.6 141.4 186.8smother dust particles at their source and preventFD中1817.8252them from becoming airbormeAverage dust concentration4.2 Design of the ultrasonic nozzleNote:*(")stands for without(with)using the ultrasonic dust suppressionStudies show that ultrasonic nozzles are best forystem FD means full dust concentration; ARD means airbone respirabledust concentrationdust scrubbing, although they are susceptible todamage and foul easily in the mine environment. The中国煤化工 ust can be effec.ultrasonic nozzles use hydraulic pressure or com- tivelypressed air to produce very small, fog-like, water dustYHCNMHdropletsThe ultrasonic nozzle takes the form of a whistle It4.4 Advantagesuses compressed air which accelerates through a The inherent design features make ultrasonic dustXIE Yao-she et alNew Respirable Dust Suppression Systems for Coal Minescontrol extremely reliable from a maintenance stand- 5 Conclusionspoint. As the fogger does not rely on high pressurewater to achieve maximum atomization, wear prob-1)The NPSDr technology can be used for shearerlems are virtually eliminated, as is the need for dust suppression, and ultrasonic dust suppression canhigh-pressure water pumpsbe applied to dust suppression such as at the transportThe nozzle has no moving parts and is constructed positionof 100% stainless steel as compared to some com- 2) The NPSDR technology has the following ad-petitors who use brass parts. This eliminates wear and vantages: wide applicability, simple structure, highcorrosion and insures years of maintenance free serreliability and low cost.vice even with poor water quality, as many material 3)By using NPSDR technology, the dusty airhandling plants havearound the shearer can be well controlled and sup-the advantages of using ultrasonic dust suppres. pressed, which can effectively decrease work facereduced health hazards; 2) decreased atmospheric4)The ultrasonic dust suppression systerem has ad-pollution; 3)improved working conditions, and 4) vantages over the other suppression methods. Theseowered water consumptionrelate to the use of water and the dust suppressionefficiency.References[1] Chapman J S, Ruckley V A Microanalysis of lesions and lymph nodes from coal miners'lungs. Br J Ind Med, 1985, 42:551-555[2] Leigh J, Driscoll T R, Cole B D, et al. 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