Control strategies for coal dust andmethane explosions in undergroundcoal mines: current South Africanresearch and development initiativesby G.L. Smith. and J.J. do Plesslst

Introduction

Catastrophic coal dust and methane explosionsare an unacceptable, but real consequence ofunderground coal mining operations.Conventional hazard amelioration, typically,involves a strategy of risk management basedon a reduction of event probability throughpro-active hazard awareness and controlprocesses. Central to this approach is a focuson explosion mechanisms and controlsnecessary to prevent their occurrence and tominimize associated consequences.

Formal research into coal dust andmethane explosion control strategies has beenongoing in South Africa for the last decadewith significant recent advances in theunderstanding of explosion mechanisms andthe development of control systems. Most ofthis work has been conducted by CSIR-MiningTechnology, the mining research division ofCSIRSouth Africa; this paper briefly reviewsrecent research and development conducted bythe organization.

An overview of the South African coalmining industry1,2

South Africa is the fifth-largest black coalproducer in the world and the third-largestexporter. In 1996 some 261,5 million run of

The Joumal of The South African Institute of Mining and Metallurgy

mine tons were produced, up from 258,6million tons the previous year. Total salesamounted to R14,9 bn (:l:US$3bn) of whichexports accounted for Ra, 1 bn (:l:US$1,65 bn)in foreign exchange. The coal industry is avital component of the South African economyin terms of employment, foreign earnings,power supply, fuel, chemical by-products,heating, industrial manufacturing and supplyindustries. Total employment in the industry is:I: 60 000 with each wage earner supporting anestimated 'extended family' of seven.

During 1996 approximately 56% ofproduction came from underground workings.Underground mining methods comprised bord-and-pillar using continuous miners/roadheaders (36,8%), drill and blast bord-and-pillar (11,2%) and 7,6% from longwall,shortwall,andlongwalVshortwalldevelopment. Surface mining using draglines(32,4%) and truck and shovel methods(12,1%) accounted for the remainder ofproduction. Forty-four per cent of productioncomes from three mines producing more thanone million tons per month. Productivity interms of tons/manlyear has increased fromapproximately 1800 in 1990, to 3400 in 1996.

Coal industry safety trends3

Although fatality and reportable injury rates(per 1 000 employed) show a markeddownward trend from 1,54 and 51,5 respec-tively in 1950 to 0,4 and 4,2 respectively in1995, this trend is marred by the incidence ofdisasters in which large numbers of lives arelost. These disasters are usually associatedwith methane or coal dusUm~thaneexplosions.

* snowden Mining Industry Consultants P(y Ltd,P.O. Box 2613, Parklands 2121, Johannesburg,South 4fHca.

t CSIR-Mining Technology, Johannesburg, South4fHca. P.O. Box 91230, Auckland Park, 2006,Johannesburg, Gauteng, South 4fHca.

Cl The South Mican Institute if Mining andMetallwgy, 1999. &t lSSN 0038-223X/3.00 +0.00. First presented at the coTJIerence:Mining InAfrica '98, Sep. 1998.

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Control strategies for coal dust and methane explosions in underground coal mines

A key driver of change in the South African mineralsindustry has been the promulgation of the Mine Health andSafety Act (29/1996). The Act arose from the findings of theLeon Commission of Enquiry into health and safety and as aresult South Africa has entered a period of renewed socialresponsibility towards workforce safety and health.

The Leon Commission4 identified four main areas ofconcern:

~ falls of ground (including rock bursts)~ accidents in underground haulage and transportation

systems~ the incidence of occupational ill-health among miners~ the continuing incidence of disasters from fires and

coal dust in coal mines.

These aspects of mine health and safety performance,although specifically identified within the context of theSouth African minerals industry, can be seen to be fairlygeneric to the global mining industry.

Similarly several key concepts can be identified as beingcentral to the Mine Health and Safety ActS:

~ Health and safety are the joint responsibility of theemployer, employee and the State.

~ Employers and employees are required to identifyhazards and minimize related risks.

~ Equipment manufacturers, suppliers and maintainersare responsible for the supply of 'fit for purpose'equipment.

~ Routine measurement of hazard exposure is crucial tothe management of health and safety.

~ Health and safety training is essential to be able toassume responsibility for the identification andminimization of risks.

These concepts embody the principles of co-responsibilityand elements of self regulation within a frameworkdetermined largely by workplace risk assessment andmanagement without which explosion hazard controlstrategies are meaningless.

CSIR-Mining Technology6

The CSIR,with nine operational divisions ranging fromaeronautical engineering to transport technology, andemploying over 3 000 people, is the largest community andindustry directed scientific and technological research anddevelopment organization in Africa. Its Mining Technologydivision works in close collaboration and strategicpartnership with the mining industry, employee organi-zations and government institutions in acquiring, developingand transferring technologies to improve the safety andhealth of employees, and to improve the profitability of themining industry and contribute to the long-term wealth andwell-being of the region.

CSIR-Mining Technology is structured into three researchprogrammes: Rock Engineering, Mining Systems, andEnvironmental Safety and Health, which mirror industryconcerns in the areas of safety, productivity and healthrespectively. The recent demise of state-funded mining R&Dinstitutions around the world has emphasized the importanceof conducting R&D on sound business principles. CSIR-Mining Technology derives 80% of income from contract

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R&D activities with individual research programmesconducting regular market needs analyses and developingbusiness strategies in response to anticipated five-and ten-year scenarios.

The Environmental Safety and Health Programme(ES&H) provides multi-disciplinary research, developmentand technology transfer expertise in the solution of environ-mental problems, with specialists in a wide range of fieldsfrom human physiology and occupational hygiene topollutant control, mine fires and explosions, ventilation,cooling and escape and rescue strategies. The ES&Hprogramme comprises four functional thrust areas:Occupational Hygiene Services; Environmental Engineering;Coal, Fires, Dust & Explosions and Surface Environment.Activities across thrust areas are consolidated at programmelevel to ensure an integrated environm~ntal safety and healthoffering.

Coal dust and methane explosion ameliorationresearch strategy

Coal dust and methane explosion research and developmentis conducted in the Coal, Fires, Dust and Explosions researchthrust area. Activities are centred at the Kloppersbosexplosion research facility approximately 110 km north ofJohannesburg. The facility was established in 1987 primarilyto determine the explosibility of South African coals. Sincethen the facility has been expanded to include:

~ the 200 m mild steel 'Badenhorst' tunnel of 2,5 mdiameter used primarily for explosion and barrierevaluation

~ a 20 m tunnel, 7 m wide with variable height to 6 m, tosimulate roadway face explosions and evaluatemachine mounted active suppression systems

~ a rectangulartest tunnel,6 mwidex3,5 mhighx140 m long arranged in a last through road, headingand pillar split configuration, to allow full-scalemodelling of ventilation systems under varying dustand methane loading conditions

~ a 10 m tunnel of 2 m diameter used for limited ignitionsuppression system testing and demonstrationpurposes

~ a 40 litre explosion vessel for the characterization ofcoal dusts both with and without methane

~ sophisticated laboratory equipment for thermal and gasanalysis

~ data collection, transmission and analysis equipment~ electronic and mechanical workshops.

Activities are directed towards the development oftechnology (products, processes, knowledge and skills) toreduce the incidence of catastrophic risk events in the coalmining industry, and typically occur in four areas:

~ knowledge generation through industry-based researchand development activities

~ creation of appropriate laboratory service capabilities~ training in health and safety skills and awareness~ application of specialist knowledge through

consultancy services.

Critical technologies that are anticipated to have asignificant effect on coal industry safety, in the area of

The Joumal of The South African Institute of Mining and Metallurgy

Control strategies for coal dust and methane explosions in underground coal mines

catastrophic risk are: active machine mounted coal dusUmethane explosion suppression systems, passive 'bag based'stone dust barriers systems, integrated escape and rescuestrategies, and neural network based coal dusUmethaneexplosion risk assessment. Current developments in each ofthese areas are considered in turn.

Active machine mounted (continuous miner/roadheader) coal dust/methane explosion suppressionsystems

Most of the coal mined in underground sections in SouthAfrica is extracted using continuous miners. Owing to theincidence of pyrite and sandstone lenses and the regularoccurrence of sandstone roof, great potential exists forfrictional ignition of methane. In addition to face and cutterhead ventilation strategies, the use of water sprays and theevaluation of 'wet head' cutter drums, effort has beendirected toward the use of active on-board explosionsuppression systems. These systems are mounted on boardcontinuous mining machines and detect the presence of amethane ignition by means of light sensors. The electronicsignal from the sensors is used to trigger the suppressionsystem, creating a barrier of flame-suppressing material, thuscontaining the flame in the immediate vicinity of the ignitionand so preventing further development and the propagationof a coal dusUmethane explosion.

Although commercial systems are widely applied inEurope, differences in mining geometry and practice betweenSouth African and European collieries prompted thedefinition of a South African test protocol, adapted for localconditions from established German protocols, anddevelopment of the 20 m rectangular test tunnel atKloppersbos. Following construction and equipping of thetest tunnel, and under the auspices of a SIMRAC(Safety inMines Research Advisory Committee) project two systemtests were devised and conducted on two available systems; aprotocol test and a system evaluation test. The objective ofthe protocol test is to determine if a system or configurationis able to fulfil the following acceptance criteria for variouscutter head positions, methane concentrations and roadwaywidths:

~ flame propagation must not reach the operator'sposition

~ pressure shock at the operator's position must bewithin acceptable tolerance levels

~ temperature rise at the operator's position should beless than 100°C over 2 milliseconds

~ no false triggering may occur as a result of otherequipment in use in underground operations

~ equipment downtime associated with a discharge mustbe less than eight hours.

Evaluation testing is concerned with the performance ofthe system and individual components, and centres primarilyaround risk aspects of engineering 'reliability'.

A large number of tests have been conducted at variousmethane concentrations ranging from 7,5% to 12%, with andwithout equipment and the second pass shoulder in thetunnel. The implication of multiple pass mining is that whenthe second cut is taken, there is a substantial volume ofmethane immediately adjacent to the continuous miner whichmust be shielded from possible ignition. Indications are that

The Journal of The South African Institute of Mining and Metallurgy

active machine mounted explosion suppression systems are aviable means of explosion control. Key aspects are sensorsystems, nozzle orientation and the need to extinguish theignition as rapidly as possible.

Passive 'bag based' stone dust barriers systems

Traditionally, the last line of defence against methane or coaldusUmethane explosions has been the stone dust or waterbarrier system. Although the 'Polish' type barrier hasInspectorate approval and is widely implemented in the SouthAfrican coal industry, some doubts were raised about itseffectiveness following investigations into several ignitions.These doubts, coupled with the fact that erection andmaintenance of the system is labour intensive andincreasingly costly, resulted in efforts being directed into thedevelopment of an alternative passive stone dust barriersystem. .

The new stone dust bag system in various configurationsmakes use of a previous concept of containing stone dust in abag but incorporates a new method of rupturing the bagbased on materials properties, relative volumes and apatented suspension device. Evaluation tests have beenconducted over a period of two years to establish design andmaterials specifications, and installation configurations. Twoinstallation configurations, namely, dispersed and concen-trated have been devised, with associated stone dust loadingsand system spacings. Tests at Kloppersbos and theunderground test facility at DMT-Tremonia have indicatedthat a minimum dynamic pressure of 3 kPa is required forbag rupture and that coal dusUmethane explosions areeffectively stopped with a stone dust loading of 100 kg/m2cross-sectional area.

Currently more than 80% of collieries have implementedat least one set of bag barriers on a trial basis, with theadoption of the bagged barrier being closely related to theimplementation of the Guidelines for a Code of Practice forExplosion Prevention. Khutala; Twistdraai East, West andCentral: Syferfontein: DNC;Matla and Delmas Collieries havefully implemented systems.7 Implementation has beenfacilitated by a number of contractors that deliver a fullsystem service to the industry.

Integrated escape and rescue strategies

OWing to the high concentrations of carbon monoxide, escape

in the irrespirable atmosphere that results from anunderground explosion or fire is dependent on the use ofartificial breathing apparatus. Self-contained self rescuers(SCSRs) thus form the basis of any escape and rescuestrategy. SCSRs, although being the core technology, arelargely ineffective unless integrated into an overall escapeand rescue strategy comprising components of escape routes,guidance systems, refuge bays, hazard identification andcommunication systems.

Body-worn SCSRswere first introduced into the SouthAfrican mining industry in the late 1980s, following theKinross disaster, as a proven but novel means of providingprotection to mineworkers in the event of a fire or explosion.A considerable amount of research was required to addressvarious issues associated with their introduction. Aspectscovered were:

~ the formulation of appropriate rejection criteria for the

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Control strategies for coal dust and methane explosions in underground coal mines

selection of SCSRs, these centred around the determi-nation of physiological limits for the inhalation ofcarbon dioxide, definition of breathing resistance limitsand establishment of unacceptable inhalation airtemperatures

~ the development of a non-destructive leak testingprocedure to assess the functional integrity of theSCSRs in daily deployment in the industry

~ the development of performance evaluation protocolsutilizing breathing simulator testing as well as persontests in the laboratory and in underground trials

~ compilation of guidelines to assist mines with theimplementation of SCSRs, including trainingprogrammes and record keeping systems

~ the introduction of an ongoing national monitoringsystem to assess the functional performance of SCSRsdeployed within the industry with the objective ofdetermining long-term performance trends.

However, the primary means of escape is through wellmaintained, properly designed escape routes. Apart frominherent technology features, the performance of SCSRs isdetermined by a variety of factors including the physiologicalcharacteristics of the wearer and the nature of the escaperoute. In this regard studies have been conducted todetermine the effect of inclination, reduced roof height andzero visibility on the speed at which escape routes can benegotiated as well as safe travelling distances. It wasascertained that, typically, safe escape distances are reducedfrom 1500 m, based on a breathing duration criterion, to amaximum of 750 m, when adverse condition criteria areapplied. This information is currently used by mines todetermine the location of back-up facilities and refuge bays.Currenttechniques involvethe use of long duration SCSRcaches and the use of an intermediate breathing stationunder development by CSIR-Mining Technology. The mobileair rescue station (MARS) is an easy to operate mobile lifesupport system for underground workers, developed inconjunction with MSA- South Africa. Other initiatives havebeen directed into SCSRergonomics and the development ofnoseclip design to cater for the low ridge noses typical of themajority of the South African workforce. This nose clip iscurrentlybeing retro-fittedon severalbrands of SCSR.

Current R&D initiatives are directed towards improvedguidance systems which integrate electronic location devices,gas detection and monitoring systems, and communicationsystems.

Design and positioning of refuge bays

Refuge bays are an integral part of escape and rescuestrategies and are a regulated requirement in South Africancollieries. The main purpose of refuge bays is to keepworkers safe from the effects of poisonous gases and fumesfollowing a fire and/or explosion, prior to rescue initiatives.Whilst refuge bays are not intended to protect workers froman explosion they should have sufficient structural integrityto remain intact following an event so as to provideprotection to any surviving workers. Given that the greaterthe strength of the explosion, the lower the likelihood of anyworker surviving, the practical strength requirement forrefuge bays should not be significantly higher than thepressure at which the probability of workers surviving and

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using the refuge bay is fairly good. Coupled with this is thefundamental requirement that a refuge bay should at leastprevent the ingress of gases and fumes following anexplosion. Within this rationale, considerable effort has beendirected into defining the overall strength and viability ofrefuge bays in the event of fires and explosions. Currentrefuge bay designs are more than adequate in the event offire. However, it is doubtful that the strength of structures, asdefined in various mines' codes of practice, could withstandthe force of an explosion. Based on probability of humansurvival at various explosion over-pressures, investigationshave revealed that a criterion of 140 kPa should be applied inthe design of refuge bay bulkheads.

Of considerable importance are the practical consider-ations of refuge bay placement and construction. In decidingrefuge bay placement the first criterion is that it should bewithin practicable reach of the workers that it serves. Basedon evaluation of SCSRperformance under zero visibilityconditions, refuge bays should not be further than 750 mfrom the workplace. This implies, dependent on seam height,that refuge bays would need to be constructed at intervals ofbetween 36 shifts (0,75 m seam height) and about 185 shifts(5 m seam height). Given the robust nature of theconstruction required for structural integrity of a refuge bayat distances less than 750 m from an explosion, constructionof permanent refuge bays at these intervals is impractical.The development of the MARS system, previously mentioned,resulted from the need to create an economic, intermediatestaging point and breathing station at distances of less than750 m from the workplace, that would readily integrate withexisting refuge bay designs and positioning.

In the context of fires, aspects investigated include thebuild-up of heat and carbon monoxide when occupied.Results to date indicate that, without ventilation and with anoutside carbon monoxide level of 1,5%, carbon monoxidelevels in a refuge bay could reach the threshold limit valuewithin 8,5 hours due to contamination from door openingsand leakages. Creation of positive pressure inside the bay istherefore critical. Various systems for oxYgen generation andcarbon monoxide scrubbing are under development bycommercial organizations in South Africa.

Neural network based coal dust/methane explosionrisk assessment

Exploratory research has been initiated in the area ofartificial intelligence and neural networks in an attempt todevelop a real time explosion risk assessment andmanagement tool. The approach is based on the concept thatchanges in, and the rate of change of, certain critical criteriacan indicate the possibility of increased risk. In this approachthe probability of an ignition rather than an explosion isdetermined; when this probability exceeds a control value anemergency response is initiated. Current activities are focusedon the identification of critical factors such as diurnalpressure change, methane release rates and seam character-istics, the inter-relationship of factors, their influence on theexplosion hazard, and electronics hardware requirements. Ata later stage it is anticipated that other factors such asincreased production rates, shift duration and cycling, andlife style factors which may be linked to human error andnegligence would be incorporated. In due course the artificial

The Journal of The South African Institute of Mining and Metallurgy

Control strategies for coal dust and methane explosions in underground coal mines

intelligence system would be integrated with real time hazard

monitoring, personnel location and emergency communi-

cation systems.

Conclusions

The research and development initiatives described in thispaper are component parts of an overall control strategy forcoal dust and methane explosions in South African coalmines. Although described as individual technologies, thepotential success of their application lies in a fundamentalunderstanding of explosion hazards and the interaction ofpeople, processes and equipment that comprise miningoperations.

The critical learning that has been achieved during thisresearch and development process has not been around theindividual technologies but in the understanding thateffective risk management and mitigation is dependent onaddressing explosion risk from a systems perspective. Thedevelopment of individual technologies will provide short-term improvements in safety performance but longer termsignificant reductions in catastrophic risk events isdependent on understanding the interaction of miningsystem components. This approach means:

~ recognizing that problems arise from the interaction ofthe different parts within the system

~ integrating and synthesizing diverse components tounderstand the whole

~ focusing on the underlying structure of inter-relationships and inter -actions when devising solutions

~ recognizing actions to alleviate a specific problem canresult in unintended and undesirable consequenceselsewhere in the system

~ understanding that the relationship of cause and effectis often obscure

~ acknowledging that systems are most effectivelychanged at 'leverage' points.

The exploratory research into neural networks andexplosion risk assessment represents an attempt to develop asystemic understanding that will allow identification ofsystem 'leverage' points-those critical issues where smallchanges bring about massive benefits. Active and passivebarrier systems, escape and rescue strategies, and refuge baydesigns are reactive mitigation activities that will save livesin the short term but will not reduce the incidence of methaneand coal dust explosions.

Effective catastrophic risk management in the area of coaldust and methane explosions is thus dependent on anintegrated, systems approach that encompasses fundamentalunderstanding of hazard sources, qualitative and quantitativeprobability of occurrence, magnitude of consequences andefficacy of control and recovery measures.

References

1. Chamber of Mines of South Africa, Statistical Tables 1996, Johannesburg,

South Africa, 1996.

2. South African Institute of Mining and Metallurgy, Colloquium-(oal

Mining 2000 and Beyond, Johannesburg, South Africa, 1996.

3. Department of Mineral and Energy Affairs, Safety in Mines Research

Advisory Committee-Annual Report 1996, Pretoria, South Africa,

1997.

4. CSIR-Mining Technology, Leon Commission transcripts-internal

documentation.

5. Mine Health and Safety Act No. 29/1996, Lex Patria Publishers,

Doornfontein, South Africa, 1996.

6. CSIR-Mining Technology, CSIR: Mining Technology Annual Review

1996/97, Johannesburg, South Africa, 1997.

7. Personal communication, J.J. du Plessis, Johannesburg, 1998. .

Udelivered the paper on bedivisional metallurgist. Other team meCatherine McInnes, Wayne Stange,Dean.

The Nkomati joint Viand platinum group metals

The Journal of The South African Institute of Mining and Metallurgy MAY/JUNE 1999 121 ....

perday,was recently met. This clearly demonsttmarket acceptance, credibility of the prodUctefthe productivity and eachieves'.

~ 122 MAY/JUNE 1999 The Joumal of The South African Institute of Mining and Metallurgy