Beamish Instantaneous Outbursts in Underground Coal Mines A

Ž .International Journal of Coal Geology 35 1998 27–55

Instantaneous outbursts in underground coal mines:An overview and association with coal type

B. Basil Beamish a,), Peter J. Crosdale b

a Department of Geology, The UniÕersity of Auckland, PriÕate Bag 92019, Auckland, New Zealandb Department of Earth Sciences, James Cook UniÕersity, TownsÕille, Qld 4811, Australia

Received 18 December 1996; accepted 16 May 1997

Abstract

Instantaneous outbursts in underground coal mines have occurred in at least 16 countries,Ž . Ž .involving both methane CH and carbon dioxide CO . The precise mechanisms of an4 2

instantaneous outburst are still unresolved but must consider the effects of stress, gas content andŽphysico-mechanical properties of the coal. Other factors such as mining methods e.g., develop-

. Žment heading into the coal seam and geological features e.g., coal seam disruptions from.faulting can combine to exacerbate the problem. Prediction techniques continue to be unreliable

and unexpected outburst incidents resulting in fatalities are a major concern for underground coaloperations. Gas content thresholds of 9 m3rt for CH and 6 m3rt for CO are used in the Sydney4 2

Basin, to indicate outburst-prone conditions, but are reviewed on an individual mine basis and inmixed gas situations. Data on the sorption behaviour of Bowen Basin coals from Australia haveprovided an explanation for the conflicting results obtained by coal face desorption indices usedfor outburst-proneness assessment. A key factor appears to be different desorption rates displayedby banded coals, which is supported by both laboratory and mine-site investigations. Dull coalbands with high fusinite and semifusinite contents tend to display rapid desorption from solid coal,for a given pressure drop. The opposite is true for bright coal bands with high vitrinite contentsand dull coal bands with high inertodetrinite contents. Consequently, when face samples of dull,fusinite- or semifusinite-rich coal of small particle size are taken for desorption testing, much gashas already escaped and low readings result. The converse applies for samples taken from coalbands with high vitrinite andror inertodetrinite contents. In terms of outburst potential, it is thebright, vitrinite-rich and the dull, inertodetrinite-rich sections of a coal seam that appear to bemore outburst-prone. This is due to the ability of the solid coal to retain gas, even after pressurereduction, creating a gas content gradient across the coal face sufficient to initiate an outburst.

) Corresponding author.

0166-5162r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0166-5162 97 00036-0

( )B.B. Beamish, P.J. Crosdaler International Journal of Coal Geology 35 1998 27–5528

Once the particle size of the coal is reduced, rapid gas desorption can then take place. q 1998Elsevier Science B.V.

Keywords: instantaneous outbursts; coal mine; gas emission; coal lithotype; differential desorption; gas contentgradient

1. Introduction

Two gases are predominantly associated with coal seams, methane and carbonŽdioxide. Methane is generated as a result of the coalification process Juntgen and

.Karweil, 1966a,b . Carbon dioxide is often introduced to the coal seam as a result of theŽ .presence of igneous intrusions Stutzer, 1936; Smith and Gould, 1980 , which may or

may not be in contact with the coal seam. Both gases pose a hazard when encountered insufficient quantities in the coal seam. Methane mixtures are explosive in the range of5–15% in air. Carbon dioxide is not a life supporting gas and at concentrations above1–2% in air, it begins to have major detrimental physiological effects.

Fundamental investigations concerning the gases found in coal seams can be relatedŽback to the earliest coal mining in Europe von Meyer, 1872, 1873; Thomas, 1876;

.Fischer et al., 1932 . The primary concern has always been safety, whether to assess gasŽ .content and composition for: 1 ventilation purposes in order to reduce the hazard of

Ž . Ž .methane emissions and possible ignitions Curl, 1978; Dunmore, 1981 or 2 indica-Ž .tions of outburst-proneness Hargraves, 1958; Jackson, 1984 . This work is continuing

as high production operations must combat large quantities of gas into workingsŽ . ŽHargraves, 1993 , and the complexity of the gasrcoal system is recognized Levine,

.1992 .Ž .The discharge of gas from coal may take place in three ways: 1 it may flow evenly

Ž .from the pores and fractures of the coal, 2 it may escape from the coal in the form ofmore or less persistent ‘blowers’, issuing from some fractures or major seam disruptions

Ž .or 3 it may burst out suddenly in great quantities into the mine workings.The two latter cases are rare, but the most dangerous to deal with in underground

workings and are generally referred to as outburst phenomena. The outburst problemarises from the effects of three main factors: stress, gas content and physical andmechanical properties of the coal. An unfavourable combination of these factors withmining methods can lead to a recipe for disaster if not recognized at an early stage ofmine development.

The global threat that the outburst problem continues to pose to the future ofunderground mining was acknowledged by the ‘‘symposium-cum-workshop on themanagement and control of high gas emissions and outbursts in underground coal

Ž .mines’’ held in Wollongong in March 1995 Lama, 1995 . This paper will use much ofthe Australian experience to provide a summary of old and new concepts on instanta-neous outbursts in coal mines. In particular, due to the banded nature of the Bowen andSydney Basin coals of Australia, more emphasis will be placed on coal lithotype effectswhere these have been recognized and documented.

( )B.B. Beamish, P.J. Crosdaler International Journal of Coal Geology 35 1998 27–55 29

2. Instantaneous outburst phenomena

Violent ejections of coal and gas from the working coal seam have plaguedunderground mining operations for over a century. These phenomena are referred to as

Ž .instantaneous outbursts and have been reviewed by Hargraves 1958, 1980, 1983, 1993 .Such outbursts range in size from a few tonnes to thousands of tonnes of coal withcorresponding gas volumes from tens of cubic metres to hundreds of thousands of cubicmetres. Gas composition is predominantly methane, carbon dioxide or their mixtures.Carbon dioxide outbursts tend to be more violent, although there is the added risk of asubsequent explosion accompanying a methane outburst. Fatalities continue to occur

Ž .from instantaneous outbursts e.g., West Cliff Colliery, New South Wales, 1994 and thephenomenon is not totally understood with respect to the gasrcoal system.

A classical instantaneous outburst event resembles someone opening the top of ashaken carbonated soft drink. The coal is often in pulverized form and appears to flow,although some incidents are merely large face slumps or floor heaves and associated gasrelease. A cavity results and in the vicinity of the outburst a gas haze is often seenassociated with temperature differential andror layering and refractive indices.

Ž .Noack et al. 1995 reported that: ‘‘The German guidelines of the Chief MinesInspectorate on gas outbursts differentiate between four types of occurrences, namelyoutbursts of gas and coal, outbursts of gas and rock, heavy floor gas emissions, andother sudden liberation of major amounts of gas’’.

3. Instantaneous outburst occurrence

3.1. Global experience

Ž .Jackson 1984 gives an historical account of the global incidence of instantaneousŽ .outbursts. His tables have been modified and updated Tables 1 and 2 , to reinforce the

extent of the problem in underground coal operations around the world.

3.2. Australian experience

In Australia there has been considerable research related to gas in coals. Severalsymposia have presented Australian research results and related overseas experiences

Ž .with respect to this topic Lama, 1995 . Summaries of instantaneous outburst occur-Ž .rences and consequent fatalities in Australian coal mines Tables 3 and 4 show both the

major coal producing Bowen and Sydney Basins have suffered from this phenomenon atdepth. However, instantaneous outbursts in the Bowen Basin have occurred at much

Ž .shallower depths Table 3 . The violence often associated with carbon dioxide outburstsŽ .is highlighted by the corresponding number of fatalities Table 4 , which continue to

occur despite many years of research into the problem.Instantaneous outbursts in the Bowen Basin have been nonexistent since the early

1980’s, primarily due to the lack of mine development in gassy areas. However, as PeterŽ .Allonby, general mine manager at Broken Hill Proprietary Co. Ltd. BHP Appin


Table 1Ž .Outburst-prone areas of the world in part from Jackson, 1984

Country Area Numbers Gasof involvedoutbursts

Australia Sydney and Bowen Basins over 650 to 1995 CO rCH2 4

Belgium Charleroi–Mons 1,190 from 1956 to 1963 CH4

Bulgaria Balkans 105 to 1974 CH4

Canada AlbertarBritish Columbia over 400 to 1995 CH 4

rNova ScotiaChina widespread over 14,000 to 1995 CH 4

Commonwealth Donetz Basin over 3,500 to 1995 CH4

of Independent StatesCzechoslovakia Ostrava–Karvina 279 to 1974 CO rCH2 4

France Cevennes Basin 6,245 from 1899 to 1964 CO rCH2 4

Germany North Rhine–Westphalia 338 from 1970 to 1993 CH 4

Hungary Pecs Basin 565 to 1982 CH4

Japan widespread 1,000 from 1925 to 1964; CH4

21 from 1970 to 1980 CH4

Poland Lower Silesia over 2,000 to 1995 CO rCH2 4

South Africa Karoo Basin several from 1993 to 1994 CH4

Turkey Zonguldak Coalfield 57 from 1962 to 1993 CO rCH2 4

UK West Wales 250 from 1907 to 1981 CH 4

USA Colorado a few since mid-1970’s CH 4

Table 2Ž .Major outburst incidents in part from Jackson, 1984 and Hargraves, 1980

Country Year Area Mine Outburst Outburst FatalitiesŽ .coal t gas

3Ž .m

Australia 1954 Bowen Basin Collinsville State 500 14,000 CO 72

MineAustralia 1991 Sydney Basin South Bulli 120 3Belgium 1879 Agrappe 340,000 CH 1414

Canada 1904 British Columbia No. 1 Morrissey 3,500 140,000 CH 144

China Sichuan Province Tianfu 12,780Commonwealth 1969 Donbass Yu.A. Gagarin 14,000 250,000of Independent StatesFrance 1938 Ricard 1,270 400,000 CH 24

Germany 1981 Westphalia Ibbenbueren 750 21,240 8Hungary 1957 Istvan 1,400 273,000 CH4

3Ž .Japan 1981 Hokkaido Yubari–Shin 4,000 m 600,000 CH 934

Poland 1930 Silesia Wenceslaus 5,000 28,000 CO 1512

Poland Silesia Nowa Ruda 5,000 800,000 CO2

Turkey 1992 Zonguldak Kozlu 263Coalfield

UK 1971 West Wales Cynheidre 400 60,000 CH 64

rPentremawr


Table 3Ž .Summary of Australian outburst occurrences in part from Hargraves, 1983 and Harvey, 1995

Ž . Ž . Ž .Colliery Year s Seam Depth m Number Maximum size t Gas

Sydney BasinAppin 1969–1995 Bulli 520 11 88 CH 4

Brimstone 1995 Bulli 2 30 CO2

Bulli 1972 Bulli 380 1 100 CH & CO4 2

Coal Cliff 1961 Bulli 450 2 2 CH & CO4 2Ž .Corrimal Cordeaux 1967–1995 Bulli 400 4 50 CH & CO4 2

Kemira 1980 Bulli 220 2 100 CO2

Metropolitan 1895–1995 Bulli 400–450 40 300 CO minor CH2 4

North Bulli 1911 Bulli 370 1 1 CH 4

South Bulli 1991–1995 Bulli 400 7 120 CO2

Tahmoor 1981–1985 Bulli 410 88 400 CO2

Tower 1981–1995 Bulli 480 19 80 CH 4

West Cliff 1977–1995 Bulli 480 250 320 CH 4

Bowen BasinCollinsville State Mine 1954–1961 Bowen 215–235 13 500 CO2

Collinsville No.3 Mine 1972 Bowen 230 2 1 CO2

Collinsville No.2 Mine 1978–1981 Bowen 250–265 7 35 CO2

Leichhardt 1975–1982 Gemini 370 200q 500 CH 4

Moura No.4 Mine 1980q C upper 130 2q 20 CH 4

Ž .Colliery points out quoted in Roberts, 1995 : ‘‘Seams in Queensland are becomingdeeper and are known to be as gassy and have similar structures to those commonlyassociated with the Bulli seam outbursts. Outbursts have been recorded at LeichhardtColliery and at Collinsville and as other mines in other areas become deeper they shouldalso consider the risk’’. This would appear sound advice given the history of instanta-neous outbursts in Australia, and the temptation to become complacent once theincidence of events diminishes.

Table 4Ž .Summary of Australian fatal outbursts in part from Harvey, 1995

Ž .Colliery Year Fatalities Size t Gas

Sydney BasinMetropolitan 1896 3 unknown CH4

Metropolitan 1925 2 140 CO2

Metropolitan 1954 2 90 CO2

Tahmoor 1985 1 400 CO and CH2 4

South Bulli 1991 3 120 CO2

West Cliff 1994 1 300 CO2

Bowen BasinCollinsville State Mine 1954 7 500 CO2

Leichhardt 1978 2 350 CH4


4. Instantaneous outburst mechanisms

Many models exist to explain the occurrence of instantaneous outbursts from coal.Generally, consideration is given to components of gas content and flow, stress, and coal

Ž .failure. Kidybinski 1980 proposed the presence of three zones in the coal seam ahead

Ž .Fig. 1. Coal face conditions in an outburst zone from Williams and Weissmann, 1995 .


Ž . Ž .of the mining operations starting at the coal face: 1 protectionrdegassed zone, 2 highŽ .gas pressureractive zone and 3 abutment pressure zone.

Within this model, three fundamental conditions are assumed to be met for anŽ . Ž .outburst to occur: 1 failure of the coal in compression within the active zone, 2

Ž .penetration of a hole through the protection zone and 3 fluidized bed outflow of theproducts from outburst cavern.

Ž .Gray 1980 considered two gas-initiated-failure mechanisms to exist, either tensileŽ .failure of unconfined coal or piping of sheared material. Paterson 1986 took the

general view that when gas is released from coal there are body forces on the coal equalto the pressure gradients of the flowing gas. His models therefore were based on thefundamental assumption that an outburst is the structural failure of coal due to excessstress resulting from these body forces.

Ž .A model proposed by Litwiniszyn 1985 was based on the gas existing in acondensed state within the coal. When a shock wave passes through the coal a phasetransformation occurs of the liquid substance into a gaseous state. This sudden creationof gas causes the skeleton of the medium to be destroyed and an outburst is initiated.

Ž .Support for this model is found in the following observations: 1 sometimes ‘bumps’and instantaneous outbursts occur together, and some ‘bumps’ are regarded as initiation

Ž .of instantaneous outbursts, and 2 in hand-working, especially without noise of machin-ery, successive knocks in the coal were often precursors to an instantaneous outburstŽ . Ž .Hargraves pers. comm., 1997 . However, Paterson 1986 identified several flaws inthis model, in particular cause and effect; where do the shock waves originate?

ŽThermodynamic descriptions have also been proposed for outburst modelling Jagiełło.et al., 1992 . In this case the premise is liberation of gas, upon entering a coal seam, is

connected to a decrease in temperature of the system. As a result of the work performedby the gas, the internal energy of the system decreases. The gas contained in the coal hassome ‘store’ of work, which can be used in an outburst process.

Ž .Williams and Weissmann 1995 used a schematic of an outburst in frequentlyŽ .encountered Australian conditions Fig. 1 to discuss gas content thresholds for out-

bursts. They placed emphasis on a gas pressure gradient existing ahead of the workingface. However, they also believed ‘‘the most important parameter is gas desorption rate,in conjunction with the gas pressure gradient ahead of the face’’.

5. Instantaneous outburst assessment

5.1. Emission rates and gas contents

The majority of gas emission research work has been applied to longwall mining withmethane as the dominant coal seam gas, although in recent times the presence of carbondioxide in Australian coal mines, particularly the Sydney Basin, has been an increasingarea of research interest. Gas make in longwall systems appears to be closely linked with

Žcoal face production and face advance rate Airey, 1971; Klebanov, 1971; Myszor,.1974; Cybulski and Myszor, 1974; Noack, 1976 . In bord and pillar workings, increase

in rate of advance also appears to correlate with increased gassiness in the working face


Ž .Hargraves et al., 1964 and conversely any period of face delay results in a decrease ofŽ .gassiness in the working face Beamish, 1990 . The presence of any form of coal seam

Ž .gas predrainage similarly impacts to reduce the seam gassiness Beamish, 1990 .However, caution is required when assessing the effects of these gas reduction measuresas unsurveyed boreholes can deviate markedly from their intended path, leaving areas ofundrained coal. In-seam gas drainage may not be uniform through the coal seam profileor in lateral extent.

General gas assessment work in the Sydney and Bowen Basins of Australia isroutinely performed by measuring gas contents of all coal seams as input to underground

Žmine planning of ventilation. An Australian gas content measurement standard Anon,. Ž .1991 identifies three components to evaluate the total desorbable gas content Q :TD

Q sQ qQ qQ , where Q is the lost gas content, Q the measurable gas contentTD 1 2 3 1 2Ž .and Q the residual gas content. Desorbable gas content Q is the sum of lost gas3 D

Ž . Ž .content Q and measurable gas content Q .1 2

Regional assessment of outburst-prone areas has been conducted in Australia, butprojections have been confined to deep mines with high rank bituminous coal. The mostcommon parameter used on a regional level is the gas content of the coal. Originally,threshold values were determined as 9 m3rt for methane and 5 m3rt for carbon dioxide,

Žbased on empirical experience from operations in Australia and Germany Beamish,.1984 . However, these limits are constantly under review, particularly for mixed gas

Ž .situations, and may vary from one coal mine to another Williams and Slater, 1995 .Ž .Truong and Williams 1989 highlighted the variations that occur in gas content and

composition of Australian coals. They concluded the processes controlling the eventualin situ gas content of coal are highly complex and cannot be explained by simplegeologic factors.

The New South Wales Department of Mineral Resources has implemented a set ofstandard practices for dealing with underground coal mining, which require gas contents

Ž .to be known in advance of any mining Harvey, 1995 . At the forefront of theseŽprocedures is the quick crush gas content method Beamish and Vance, 1990; Williams

. Žand Weissmann, 1995 that is a modified version of the Australian Standard Anon,.1991 . In this procedure a fast and reliable result is obtained that is supplied to mining

operators to decide on a safe mining procedure for the conditions established.

5.2. Seam gas composition

Generally, coal adsorbs about twice as much pure carbon dioxide as it does puremethane. A range of intermediate isotherms are obtained for mixed gas composition,with a regular increase in total gas content occurring as the percentage of carbon dioxideincreases. Consequently, for a given coal seam gas pressure, larger volumes of carbondioxide can exist and emission problems therefore become more acute. Surface areameasurements also indicate that vitrinite-rich coals have a larger carbon dioxide sorption

Ž .capacity than their inertinite-rich equivalents Beamish and O’Donnell, 1992 . SimilarŽ .results have been obtained for methane in recent studies Lamberson and Bustin, 1993 .

Ž .Greaves et al. 1993 reported on a mixed gas sorption experiment using an initial gascomposition of 75% CH r25% CO , which resulted in a final adsorbed gas composition4 2


Fig. 2. Methane sorption history for an initial adsorption gas composition of 75% CH r25% CO . Adsorbed4 2Ž .gas composition at the final pressure of 7.1 MPa was 28% CH r72% CO from Greaves et al., 1993 .4 2

at a pressure of 7.1 MPa of 28% CH r72% CO . Subsequent desorption resulted in the4 2Ž .following: 1 methane was released relatively quickly as pressure falls and the methane

Ž . Ž .content of the coal drops rapidly Fig. 2 and 2 significant desorption of carbon dioxideŽ .did not occur until pressures were reduced to less than 0.7 MPa Fig. 3 . These results

support the observed strong affinity of coal for CO compared to CH . The delay in2 4

release of the carbon dioxide until low pressures are achieved has implications for gasemissions and outbursts in coal seams containing carbon dioxide.

Fig. 3. Carbon dioxide sorption history for an initial adsorption gas composition of 75% CH r25% CO .4 2ŽAdsorbed gas composition at the final pressure of 7.1 MPa was 28% CH r72% CO from Greaves et al.,4 2

.1993 .


5.3. Mining and geological factors

Mining methods are important considerations in assessing the risk of instantaneousoutbursts. These methods may be categorized in order of decreasing outburst-pronenessŽ . Ž . Ž .Hargraves, 1993 : 1 cross measuring into the seam, 2 development heading in the

Ž . Ž . Ž .seam, 3 longwall advancing, 4 longwall retreating and 5 pillar extraction.Cross measuring into a coal seam is given the highest rating as mining is advancing

from a more competent material to a less competent material. Upon intersecting the coalseam, the confining pressure of the surrounding rock is suddenly removed, and if gas ispresent in the seam in sufficient quantity it can be released very rapidly.

Heading advance creates a situation of atmospheric conditions at the working facewith much higher virgin gas pressures only a short distance ahead. Encountering anycoal seam weakness or disruption therefore can be catastrophic, as again confinement ofthe coal seam is seriously diminished. Longwall advance suffers in a similar manner todevelopment headings, although the longwall rate of advance is usually less and moregas is able to drain away naturally.

Mining by longwall retreat is often performed through an area already drained bynatural drainage into the roadways used to delineate the longwall block and perhapsin-seam drainage boreholes, thus reducing the risk of outbursts. Nevertheless, gasemission into the coal face area can still pose a problem in longwall retreat due to thehigh coal outputs achieved with this technique. Again, this can be adequately dealt withby well planned in-seam drilling and drainage. Gas problems with pillar extraction arenonexistent as the pillars have drained naturally into the surrounding roadways afterdevelopment.

ŽSeam disruptions are a common feature associated with outburst phenomena Shepherd.et al., 1981 , however, faulting is not a prerequisite for instantaneous outbursts

Ž .Hargraves, 1993 . Fault types may be categorized in order of proneness as strike-slip)Ž .reverse)normal Shepherd et al., 1981 , although this concept has been refuted

Ž .Hargraves, 1993 . Other features such as palaeochannels and floaters in the coal seamŽ .also cause disrupted sheared coal to be present due to differential compaction. Dykes

cutting a seam may provide conditions resembling cross measuring according toŽ .Hargraves 1993 .

Only the high rank bituminous coals have been considered outburst-prone fromŽ .Australian experience high volatile bituminous A–low volatile bituminous . However,

instantaneous outbursts are not impossible in low rank coals, with occurrences recordedŽ .at Valenje Colliery, Yugoslavia Hargraves, 1983 . ‘Gas blowers’ have been reported for

Ž .New Zealand subbituminous to high volatile bituminous B coals Patterson et al., 1967 .

5.4. Summary of sorption properties of coal

Adsorption isotherms for two coals are used to illustrate the effects of rank, moistureŽ .content, and coal lithotype Fig. 4 . Sample ERDC7 is a medium volatile bituminousŽ .coal from the Bowen Basin Australia and sample 52r085 is a subbituminous coal from

Ž .the Huntly Coalfield New Zealand . On a raw coal basis, ERDC7 bright coal has thehighest sorption capacity in the dry state. Based on this result, a higher gas content couldbe expected for bright coals in situ than dull coals.


Ž .Fig. 4. Methane isotherms for a Bowen Basin medium volatile bituminous coal ERDC7 , and a HuntlyŽ .Coalfield subbituminous coal 52r085 .

The most significant effect on sorption capacity is the level of moisture present in thecoal. For an increase in moisture from 0.3% to 1.1%, the ERDC7 dull coal sample hasits sorption capacity reduced from 25.1 ccrg to 19.5 ccrg at 5.1 MPa. Sample 52r085has its sorption capacity reduced from 22.6 ccrg to 6.5 ccrg for an increase in moisturefrom dry state to 11.6% at a similar pressure. Adsorption capacity is normally said toincrease with rank, but it is also strongly influenced by the moisture content. High rankcoals have low moisture and therefore there is less competition between gas andmoisture for sorption sites.

Desorption rates of bright and dull coal types in the same seam differ, with theŽprimary cause attributable to the maceral composition of each type Beamish and

.Crosdale, 1995; Fig. 5 . Bright coal bands rich in vitrinite are predominantly micro-porous and their slow desorption rate is generally described by a unipore diffusion

Ž .model Smith and Williams, 1984 . An example of this is ERDC5 bright coal in Fig. 5.Dull coal bands rich in fusinite and semifusinite, with minor amounts of vitrinite have a

Fig. 5. Typical desorption data for Bowen Basin coal lithotypes with diffusion model curves superimposed.


large degree of macroporosity and their fast desorption rate is generally described by aŽ .bidisperse pore diffusion model Ruckenstien et al., 1971 . An example of this is

ERDC1 dull coal in Fig. 5. Dull coal bands containing the inertinite maceral inertodetri-nite behave similarly to bright coals. An example of this is ERDC5 dull coal in Fig. 5.The different desorption rates displayed by dull and bright coal bands have seriousimplications for face emissions and gas contents, in-seam gas drainage, and outburst-proneness of a coal seam.

5.5. Moisture effects on gas content in the working face

Coal in the vicinity of an instantaneous outburst is often reported as being dryŽ .Hargraves, 1993 . Isotherm data show, for a given pressure, coal adsorbs more gaswhen in a low moisture state and consequently, gas migrating to the exposed, dry coalface may become readsorbed in the immediate face zone. Gas contents may be therebyelevated, helping to explain the larger than expected volumes of gas released in anoutburst. The existence of such a mechanism would be most important in coals of lowrank, which have high in situ moisture contents and which are known to dry out veryreadily. A pressure lag effect exists along similar lines when draining the coal. Low ranksubbituminous coals with their high moisture content have high pressure gradients for agiven gas content. Consequently, gas content threshold limits for outbursts in high rankcoals do not directly transfer to low rank coals.

5.6. Regional effect of Õitrinite content on coal seam gassiness

Distinct relationships exist between the coal rank and instantaneous outbursts. At No.Ž .2 Mine, Collinsville Bowen Basin , outbursts only occurred in coal with greater than

Ž .1.2% vitrinite reflectance Williams and Rogis, 1980 . Here it was noted that theŽ .vitrinite content of the mining section the top 2.5 m to 2.8 m of the coal seam

Ž .increased from 28%, 200 m east of the Western Panels Fault Fig. 6 to 48% near thefault. Towards the fault, vitrinite content increased to 63% in the lower half of theworking coal section but remained a constant 26% in the upper part.

Ž .Comparison of Hargraves emission value EV; Hargraves, 1962 measurements alongŽa profile of the Western Panels area with vitrinite reflectance and vitrinite content Fig.

.7 , show a clear relationship between vitrinite content and gassiness of the coal. Such acorrelation would be expected since laboratory data suggest vitrinite-rich coal has a

Ž .greater propensity to retain its gas. Face cut-out cycle FCC emission profiles forŽ .roadways approaching the Western Panels Fault Beamish, 1990 , show a response

similar to the EV readings. High gas emissions were recorded during face cutting ofvitrinite-rich coal sections. Additionally, the vitrinite-rich coal was often highly sheared,allowing a much finer particle size to be produced during cutting and resulting in highemissions as the vitrinite content increased approaching the fault. During prolongedperiods of face stoppages and slow advancement of roadways, the sheared coal drainednaturally, because of its fine particle size and significant interparticle porosity andpermeability.


Ž .Fig. 6. Mining layout for Western Panels area, No. 2 Mine, Collinsville from Beamish, 1990 .

5.7. Coal lithotype effect on emission Õalues across a working face

EV readings from a full face exposure in the Western Panels area of No. 2 Mine,Ž . Ž .Collinsville Biggam et al., 1980 have been redrawn and contoured Fig. 8 to

Fig. 7. Profiles of vitrinite reflectance, vitrinite content and EV readings approaching the Western Panels Fault,No. 2 Mine, Collinsville.


Ž .Fig. 8. Hargraves EV readings taken across a working coal face modified from Biggam et al., 1980 .

emphasize the significance of the results in relation to coal lithotype variations in theface. EV values ranged from 0.43 to 1.18 ccrg, and displayed a preferential increasefrom the top of the working coal section to the bottom of the working coal section. Coallithotypes in the upper half are dominated by dull coal and the lower half by bright coal,

Ž .which is reflected in their relative vitrinite contents Williams and Rogis, 1980 . Theupper and lower coal sections are separated by a bedding plane fault.

In a free face environment, according to the desorption results presented in Fig. 5,rapid initial desorption would have occurred in the dull, inertinite-rich coal seam sectionprior to retrieving the EV sample. Gas release after retrieving the EV sample would beon the slow part of the desorption curve. In contrast, the bright, vitrinite-rich part of thecoal seam would have retained most of its gas and would rapidly release it on reducingthe particle size as is the case with the EV reading procedure. This same differentialdesorption phenomenon was well recognized by the face crews at Collinsville, and itwas common knowledge that taking an EV reading in the dull coal produced a lower EVthan in the bright part of the seam. The significance of this ‘home truth’ was not fullyunderstood at the time and was often mistaken for implying the dull coal was

Ž .impermeable in comparison to the bright coal. Hargraves 1962 , acknowledged thatcoal lithotype may make a difference to EV readings. He recommended that EVreadings be taken from duro-clarain or in a consistent coal band for direct comparison.


A second trend is also observable in Fig. 8. The EV readings progressively decreaseaway from the virgin rib side, which is again more noticeable in the upper part of the

Žworking coal section. High bedding-plane permeability Bartosiewicz and Hargraves,.1985 is reflected by the EV readings.

High EV readings are also observed when nearing a hole-through to complete pillarŽ .formation Biggam et al., 1980 . High stress in this region reduces the coal’s permeabil-

Ž .ity and retards gas drainage. Hargraves 1969 showed increase of gas pressure in thisregion due to increase of stress. As the hole-through is completed, removal of theconfining stress and particle size reduction result in larger than expected gas release.

ŽDesorption measurements of lump samples taken from coal faces Creedy, 1986;.Lama, 1986; Ashton, 1990 also display a spread of values, which has been previously

Ž .unexplained. Creedy 1986 has formulated a statistical method for analysing this data toobtain a representative in situ gas content and he recommends using the bright coalsamples from a working face.

Gas emission rate into the ventilating system will also be affected by differentialŽ .desorption and is reflected in face monitoring Beamish, 1990 . Emissions are more

erratic throughout a gas-drained panel compared to an undrained panel, which isexpected if differential desorption occurs in response to the in-seam drainage boreholes.Localized changes in specific gas emissions may also result from a change in the coalseam composition with respect to banding. The effect may be accentuated by thediffering storage capacities of the coal lithotypes. A predominantly dull coal seam richin fusinite and semifusinite with large open macropores would be expected to producehigh general body emissions of gas. A predominantly bright coal seam rich in vitrinitewould produce the opposite effect. However, the bright coal seam would produce highemissions of gas during periods of face cutting as the particle size of the coal is reducedenabling the retained gas to be liberated.

ŽFig. 9. Relationship between uniaxial compressive strength as determined from NCB cone indenter measure-.ments and vitrinite reflectance of Bowen Basin coals.


5.8. Coal strength

A number of strength tests on small scale coal specimens from the Bowen BasinŽ .using a uniaxial compression testing machine and a National Coal Board NCB cone

Ž .indenter were reported by Beamish et al. 1991 . Coal strengths of dull and bright coalbands and the deformation behaviour they display under loading parallel and perpendic-ular to the banding were investigated. Microstructures developed during uniaxial com-

Ž .pression testing were examined by scanning electron microscopy SEM .NCB cone indenter numbers have been converted to uniaxial compressive strength

Ž .and plotted against coal rank Fig. 9 . Limited bright coal data is due to the tendency forthe samples to break under point load in the tester, presumably along the finer

Ž .microcleats observed by SEM Gamson and Beamish, 1991 . Uniaxial compressive

Fig. 10. Axial deformation in dull and bright coals from the Bowen Basin.


strength decreases substantially as rank increases from high volatile bituminous A to lowŽ .volatile bituminous Fig. 9 . The bright coal sample stressed perpendicular to banding

appears to have a lower strength than its dull rank equivalent, whereas the opposite istrue when the sample is stressed parallel to banding. Of the high volatile bituminouscoals tested, the lowest strength is displayed by the sample rich in inertodetriniteŽ .ERDC5 dull .

Ž .Uniaxial compression testing results Fig. 10 show dull coals loaded parallel and atright angles to the banding failed at around 27 MPa. Similarly, bright coals loaded

Ž .parallel to the banding right angles to the cleat network failed at 29 MPa. InŽcomparison, load applied to bright coals at right angles to the banding parallel to the

.cleat network created failure at significantly low pressures of 11 MPa. Typically, themicrofractures that have developed under stress in the bright coal bands do notpropagate into the surrounding dull coals or clays. This competency contrast hasinfluenced the way in which microfractures have propagated through coal bands inresponse to an applied stress.

When the coal strength information is combined with the sorption behaviour of coalŽ .macerals Crosdale and Beamish, 1995 , it can be seen that the outburst-proneness of

coals rich in vitrinite and inertodetrinite is greatly increased. Basically, these coallithotypes will retain their stored gas for long periods of time in the face environmentand when failure of the coal occurs, which is more likely in these coal lithotypes due totheir low strength, the small block sizes produced will release large quantities of gasvery rapidly.

6. Prediction and alleviation

6.1. Gas emission indices for outburst-proneness assessment

Ž .The D P index Ettinger et al., 1958 , based on the initial rate of gas desorption fromcoal, has been widely adopted in Europe and elsewhere. Coals with high initial

Ž .desorption rates are considered prone to instantaneous outbursts Lidin et al., 1954 . TheAustralian experience indicates conflicting results, with mines having a history ofinstantaneous outbursts giving low D P values. Striking differences were also foundbetween D P values for coals of the Sydney and Bowen Basins, which were otherwise

Ž .analytically identical Bartosiewicz and Hargraves, 1985 . This index makes no al-lowance for differences in emission rates of inertinite- and vitrinite-rich coals, whichcould account for the differences between Bowen and Sydney Basin coals. Therefore,D P does not appear to be a reliable index of proneness to instantaneous outbursts for

Ž .Australian coals Bartosiewicz and Hargraves, 1985 .Ž .The L2 index Lama, 1980 is based on the gradient of the desorption curve between

the interval 30 and 600 s, using a log–log plot. This value is analogous to effectiveŽ .diffusivity as defined by Smith and Williams 1984 . Solid dull coal gives the best

Ž .response, with little differentiation between crushed y0.5q0.25 mm bright and dullcoals. It was inferred that the dull coal could be used in solid form to predict

Ž .outburst-prone conditions. However, Beamish 1996 suggests that the desorption ob-


Ž .served by Lama 1980 is more the norm for the dull coal, which contains the inertinitemacerals fusinite and semifusinite. Therefore, caution must be used when trying toimplement desorption indices for outburst prediction in Gondwanan coals. The role ofmacerals should not be overlooked and the blanket use of fast desorption being moreoutburst-prone needs to be taken into context with the size fraction of the sample used toobtain the result.

Hargraves EV readings are a raw coal measurement, which greatly exaggerates thedifference between dull and bright coals sorption capacities. Even if the dull and brightcoals have not partially desorbed prior to measurement, bright coal would give a highresult for Bowen Basin coals. The fact that dull coal desorbs faster than bright coalincreases this difference. Nevertheless, a high Hargraves EV reading works in terms ofoutburst-proneness as it indicates the gas content of the coal is still high, and that thecoal also desorbs rapidly when particle size is reduced. Assessment of bright-coalfractions would appear to be the best indicator for this index, a fact well known to theexperienced coal miners at No. 2 Mine, Collinsville.

Ž .The FCC emission of Beamish 1990 works as it is a direct reading of the coal in themining face, as long as driveage is not interrupted. In gas drainage areas care must betaken to establish if the coal seam has segregated intervals of bright and dull coal. Lowreadings may be a false sense of security as they may be a result of little gas in the dullcoal, but a large proportion of the original gas content still remaining in the bright coal.Therefore, on a tonnage mined basis, the overall FCC value may appear low, whenspecific intervals of the coal face may still be prone to outbursts.

6.2. In-seam gas drainage

Incremental flow differences in a horizontal borehole will exist as a result of theborehole intersecting different coal bands. Most documented histories of gas flows fromin-seam boreholes refer to high gas flow zones associated with cleat direction andstructural disturbances. However, a more simple explanation of these regions can beattributed to the changing coal seam lithology, which may be occurring both through theseam profile or laterally. Consequently, a uniform drainage pattern will not exist awayfrom the borehole due to differential desorption of the coal bands.

When using in-seam drilling for gas drainage it is therefore necessary to ensure goodcoal seam coverage and to assess the borehole location with respect to the coallithotypes. This means surveying the boreholes and preferably using directional drillingwith in-hole motors to ensure the coal seam is criss-crossed and no ‘shadow zones’exist.

6.3. Water infusion for outburst-proneness reduction

Water infusion has been used as a method of controlling instantaneous outbursts inŽ .some countries Jackson, 1984 . It is believed that as humidity or moisture increases, the

capability of the coal to accumulate elastic strain energy decreases and the permanentŽ .non-recoverable strain energy increases. In this way Peng 1978 proposes that the

Ž .energy index of liability to outburst W decreases. Hargraves 1983 points out thatet


Fig. 11. Effect of water infusion on sorption capacity.

when the infusion is stopped, the process reverses and so the effects of infusion are notlong lasting.

The influence of water infusion could equally be explained using sorption isothermresults. With an increase in the coal moisture state at high pressure, the water moleculeswould begin to compete with the methane molecules for sorption sites and subsequently

Ž . Ž .displace them, hence lowering the gas content of the coal Fig. 11 . Hargraves 1983reported on longwall faces in the Commonwealth of Independent States, where highpressure infusion had the effect of driving the gas from the coal with a whistling sound.Once the water infusion pressure is removed, natural drainage would recommence

Ž .including loss of moisture. As seen from the isotherm Fig. 11 , the gas content wouldagain increase for a given pressure.

7. Case history examples showing coal lithotype effects on instantaneous outbursts

7.1. Outburst potential of coal from Leichhardt Colliery, Blackwater

Ž .Gray 1980 used material from Leichhardt Colliery in his paper on energy releaseassociated with instantaneous outbursts. His desorption tests were conducted on three

Ž . Ž .types of coal: 1 normal outburst coal, D heading east, straddling the 3 m parting, 2Ž .brecciated material from the ribs of the December 1st, 1978 outburst and 3 mylonitized

Ž .sheared material from the December 1st, 1978 outburst. These samples were subjectedto methane repressurisation followed by rapid gas pressure release, and the amount ofgas liberated was monitored. The mylonitized coal showed the largest volume of initial

Ž .gas released. This is not surprising as the details of the coal samples Table 5 clearly


Table 5Ž .Leichhardt Colliery coal samples tested by Gray 1980

Normal coal Brecciated coal Mylonitized coal

Ž .Vitrinite reflectance % 1.24 1.23 1.28

Ž .Maceral composition %Vitrinite 35 51 56Exinite 1 0 1Inertinite 59 43 38Mineral matter 5 6 5

Ž .Size analysis % retainedq12.7 mm 31.0 7.2 12.36.35–12.7 mm 26.6 20.0 14.23.18–6.35 mm 18.1 23.0 16.21.00–3.18 mm 15.0 25.4 21.20.50–1.00 mm 4.8 10.8 11.80.25–0.50 mm 2.6 6.6 10.80.125–0.25 mm 1.0 3.1 6.1y0.125 mm 0.9 3.9 8.0

Ž .show two things: 1 the brecciated and mylonitized coals contain a large percentage offine material, 13.6% and 24.9% less than 500 mm respectively, as opposed to 4.5% from

Ž .the normal coal; therefore, desorption rate is greater and 2 the brecciated andmylonitized coals contain a large percentage of vitrinite, 51% and 56% respectively, asopposed to 35% for the normal coal, therefore, the amount of sorbed gas is greater.

The coal profile in the vicinity of the outburst cavity is shown in Fig. 12. The layersof vitrinite-rich, myolinitized coal are clearly shown and presumably were the main coalcomponent responsible for the outburst material. Another notable feature of this profile

Ž .is the presence of a very dull band of coal. Hunt and Botz 1986 made particularŽ .reference to this band Fig. 12 in their paper on petrographic factors in outbursts in

Australian coal mines. This coal band was subjected to desorption rate testing onŽ .crushed material, using the method of Janas and Winter 1977 . The coal displayed the

highest adsorption and desorption rates of coal lithotypes from the Leichhardt Colliery.The major maceral component of the dull band was inertodetrinite.

Fig. 12. View of eastern rib December 1, 1978, outburst, Leichhardt Colliery, showing mylonite banding andŽ .inertodetrinite-rich dull coal band from Gray, 1980 .


At first, this result would appear to be in conflict with the laboratory results presentedin Fig. 5. However, this apparent difference is simply a result of particle size in thedesorption test method. In fact the two results are entirely consistent with outburst-prone

Ž .coal conditions. Namely at large particle sizes e.g., in situ in the coal face thedesorption rate of coal rich in inertodetrinite is slow, therefore gas is retained at a highlevel. Once the particle size is reduced however, the retained gas is rapidly released.Consequently, the dual presence of vitrinite-rich mylonitized coal and aninertodetrinite-rich coal contributes to a much greater potential for outbursts. Once themylonitized coal breaks from the coal face bringing the inertodetrinite-rich material withit, and particle size reduction occurs, the gas release is almost instantaneous.

7.2. Instantaneous outburst occurrence at No. 2 Mine, CollinsÕille

Ž .At 5.27 p.m. on the 16 April 1981, an instantaneous outburst Fig. 13 , at No. 2Mine, Collinsville released approximately 35 tonnes of coal and 380 m3 of carbon

Ž . Ž .dioxide Beamish, 1981 , in the belt road 29 m inbye of 11A cut-through Fig. 6 . A

Fig. 13. Gas analyzing system chart record for afternoon shift 16 April 1981, showing carbon dioxide outburst,No. 2 Mine, Collinsville.


Fig. 14. Plan view of continuous miner location at the time of outburst No. 22, No. 2 Mine, Collinsville, withface cross-section locations indicated.

plan view of the outburst face is shown in Fig. 14 with corresponding cross-sections inFigs. 15 and 16. The origin of the outburst appeared to be from the floor line in thecentre of the roadway, 4.5 m inbye of the coal face. A rectangular prism-shaped cavity

Ž .resulted. Drillhole data placed the intersection of the Western Panels Fault Fig. 6 withthe mining section floor, approximately 5 m inbye of the outburst face. Consequently,the disruption to the coal seam caused by the fault contributed to the outburst. As in allpreviously recorded cases at Collinsville, the outburst material was extremely shearedwith a sugary texture.

Ž .Analyses of coal samples taken from the outburst location Table 6 reveal that the inŽ . Ž .situ sheared coal and outburst coal have: 1 high crucible swelling numbers and 2


Table 6Coal analyses from outburst locality, 51 Level West Panel, No. 2 Mine, Collinsville

Sample details Inherent Ash Volatile Fixed CrucibleŽmoisture %, matter carbon swell

Ž . . Ž Ž% db %, %, number. .db db

Outburst coal ejected onto miner head 1.9 16.2 23.0 60.8 8Coal from outburst cavity 1.1 16.5 22.0 61.5 8Dull coal from right hand side of the face above slip plane 0.6 16.8 19.4 63.8 2Sheared coal from right hand side of the face below slip plane 1.6 17.9 19.7 62.4 5

relatively high volatile matter content. Both these parameters are consistent withvitrinite-rich coal, which matches the seam profile assessment.

The area being worked in the No. 2 Mine had been undergoing gas drainage toreduce the gas levels, and a borehole was located in the dull, upper part of the coal seamŽ .Figs. 15 and 16 . As a result of the different desorption rates of the dull and bright coallithotypes, the upper part of the coal seam would have drained to a safe level, however,the bright, lower part of the seam would not have. This is reflected in the proximate

Ž .analyses Table 6 . The dull coal had a very low moisture content, which would beexpected after 5 months of draining from a borehole. The vitrinite-rich coal also displays

Fig. 15. Face cross-sections A–A and B–B from Fig. 14.


Fig. 16. Face cross–sections C–C and D–D from Fig. 14.

a high ash content, reflecting the effects of mineral matter infilling cleats. The presenceof this material would also contribute to the slow desorption rate of the bright coal, dueto clogging of the macropore system. A crucible swelling number of 5 from the righthand side of the face below the slip plane, would tend to indicate the presence of bandedcoal. Therefore, the low EV of 0.50 ccrg recorded at the time of face development from

Ž .this part of the coal seam Fig. 13 is consistent with fast desorption in situ from thistype of coal, and a low retained gas content.

7.3. Instantaneous outbursts and gas content gradient

Generally, diagrams of instantaneous outburst mechanisms are presented which placeŽ .emphasis on a gas pressure gradient existing in the coal face environment Fig. 1 .

Ž .Williams and Weissmann 1995 used this same diagram when discussing gas contentthresholds for outbursts. However, they believed the most important parameter is gasdesorption rate, in conjunction with the gas pressure gradient ahead of the face. Thelaboratory results of desorption rates shown in Fig. 5 support this idea. Effectively, in


Fig. 17. Face conditions for outburst No. 22, No. 2 Mine, Collinsville, indicating high gas content gradient inlower part of the working section.

the face environment as pressure is taken off the coal it is the different desorption ratesŽ .of the coal lithotypes which create a large gas content gradient, namely: 1 vitrinite-rich

or inertodetrinite-rich coal bands do not desorb rapidly, retaining their gas and thusŽ .producing a steep gas content gradient and 2 fusinite-rich or semifusinite-rich coal

bands lose their gas more rapidly and thus have a shallow gas content gradient.Consequently, as observed at No. 2 Mine, Collinsville and Leichhardt Colliery,

Blackwater the major thicknesses of vitrinite-rich and inertodetrinite-rich coal plies weresufficient to create a major gas content gradient in the mining face which resulted inoutbursts occurring. Therefore, the outburst face example illustrated in Fig. 16 can be

Ž .explained using a gas content gradient model Fig. 17 . The key features of this modelare the different gas content profiles of the vitrinite- and inertinite-rich parts of theworking coal section. Naturally, the gas content gradient effect is exacerbated if the coalseam gas is carbon dioxide due to its greater sorption capacity at a given pressure and

Ž .the strong retention effect of coal as noted by Greaves et al. 1993 .

8. Conclusions

Instantaneous outbursts in underground coal mines continue to pose a hazard to safe,productive extraction of coal. The problem results from a combination of the effects ofstress, gas content, and physico-mechanical properties of the coal. Research andoperational experiences have provided the opportunity to test theories on the mecha-nisms of an instantaneous outburst as well as to set guidelines for assessing the potentialoutburst-proneness of coal. For example, in the Sydney Basin, gas content thresholdshave been implemented, which take into account changes in gas composition from puremethane to pure carbon dioxide. These thresholds are often modified for individual mineconditions.


Many of the indices used for outburst-proneness assessment of face coal have shownconflicting results for Australian Permian coals. Differences in the gas desorptionbehaviour of banded coal lithotypes, which make up these Australian coals, help explainthese results. Dull coal bands rich in fusinite and semifusinite have fast desorption ratesfrom solid coal. Conversely, bright coal bands rich in vitrinite have slow desorptionrates from solid coal. Dull coal bands rich in inertodetrinite, behave in a similar mannerto bright coal bands.

The significance of this differential desorption behaviour of banded coals, is observ-able in gas emission readings from a mine face. Vitrinite-rich coal sections of a seamretain their gas content due to the slow desorption rate of the solid coal. Once theparticle size of the coal is reduced, either during coal cutting operations or retrieving asample for an emission value reading, the gas is rapidly released. Similarly, the resultingdifferential desorption in the working face can produce a significant gas content gradientahead of the face, sufficient to initiate an outburst. This effect has been documented attwo mines in the Bowen Basin. At Leichhardt Colliery, Blackwater, a combination ofsheared vitrinite-rich coal surrounding an inertodetrinite-rich coal layer was responsiblefor a major methanercoal outburst. At No. 2 Mine, Collinsville, a sheared vitrinite-richlower working section was responsible for a minor carbon dioxidercoal outburst.

Recognition of vitrinite-rich andror inertodetrinite-rich coal seam sections is war-ranted in future delineation of outburst-prone areas of underground operations in theBowen Basin. Assessments based on rapid desorption must be treated with caution inview of particle size and coal lithotype effects.

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Beamish Instantaneous Outbursts in Underground Coal Mines A