Environ Monit Assess (2010) 166:529–541DOI 10.1007/s10661-009-1021-7

Greenhouse gas emissions from Australian open-cut coalmines: contribution from spontaneous combustionand low-temperature oxidation

Stuart J. Day · John N. Carras ·Robyn Fry · David J. Williams

Received: 1 March 2009 / Accepted: 26 May 2009 / Published online: 27 June 2009© Springer Science + Business Media B.V. 2009

Abstract Spontaneous combustion and low-temperature oxidation of waste coal and othercarbonaceous material at open-cut coal minesare potentially significant sources of greenhousegas emissions. However, the magnitude of theseemissions is largely unknown. In this study,emissions from spontaneous combustion andlow-temperature oxidation were estimated for sixAustralian open-cut coal mines with annual coalproduction ranging from 1.7 to more than 16 Mt.Greenhouse emissions from all other sources atthese mines were also estimated and comparedto those from spontaneous combustion andlow-temperature oxidation. In all cases, fugitiveemission of methane was the largest source ofgreenhouse gas; however, in some mines, spon-taneous combustion accounted for almost a thirdof all emissions. For one mine, it was estimatedthat emissions from spontaneous combustionwere around 250,000 t CO2-e per annum. Thecontribution from low-temperature oxidationwas generally less than about 1% of the total forall six mines. Estimating areas of spoil affectedby spontaneous combustion by ground-basedsurveys was prone to under-report the area.

S. J. Day (B) · J. N. Carras · R. Fry · D. J. WilliamsCSIRO Energy Technology, PO Box 330,Newcastle, NSW 2300, Australiae-mail: stuart.day@csiro.au

Airborne infrared imaging appears to be a morereliable method.

Keywords Coal mining · Spontaneouscombustion · Greenhouse gas emissions ·Coal oxidation · Infrared thermography

Introduction

In Australia, coal mining is a significant sourceof greenhouse gases. During 2006, emissions fromthe coal mining industry (both underground andopen-cut) were estimated to contribute around6% of the nation’s total emissions (AustralianDept of Climate Change 2008a). Slightly morethan half of these emissions were attributed tofugitive emissions of methane, which is releasedas the coal is mined, while most of the remainderresulted from energy usage.

Open-cut mining also produces large quantitiesof carbonaceous waste material, which, when ex-posed to air, is subject to low-temperature oxida-tion and, in some cases, spontaneous combustion.These processes produce CO2 and other gases,including methane, and therefore represent a po-tential additional source of greenhouse emissions.Worldwide, the significance of emissions fromcoal fires is well known (Stracher and Taylor 2004;Kuenzer et al. 2007), but quantitative data on thescale of these emissions are rare. In a study of

530 Environ Monit Assess (2010) 166:529–541

greenhouse emissions from the Australian coalindustry, Williams et al. (1998) suggested that upto 25% of the total greenhouse emissions from anopen-cut mine may be derived from spontaneouscombustion and low-temperature oxidation; how-ever, this estimate was based on very limited data.

The Intergovernmental Panel on ClimateChange recognises the potential of low-temperature oxidation and spontaneous com-bustion as a source of greenhouse gases, but atpresent, there is no recognised method for makingreliable estimates. As a consequence, emissionsfrom this source are not presently included inthe Australian National Greenhouse Gas Inven-tory. However, with the likely introduction ofan emissions trading scheme in Australia, it isbecoming increasingly important to be able toaccount for all sources of emissions.

Apart from the Williams et al. (1998) report, weare unaware of any other published estimates ofgreenhouse emissions from spontaneous combus-tion and low-temperature oxidation at the individ-ual mine level. Hence, the scale of these emissionsis largely unknown. Accordingly, research into de-veloping a tractable methodology for estimatingemissions has been conducted in Australia overthe past decade. Carras et al. (2009), for instance,recently reported results of a study conducted at11 Australian open-cut coal mines where emis-sions of CO2 and CH4 from low-temperature ox-idation and spontaneous combustion in spoil andother waste were measured using flux chambers.In that study, emissions were categorised accord-ing to the intensity of spontaneous combustion,and despite considerable scatter in the data, threebroad categories of spontaneous combustion wereidentified. Each category was assigned an emis-sion factor ranging from about 12 to 8,200 kgCO2-e year−1 m−2. With appropriate classifica-tion of the affected ground, these emission factorsprovide the basis for estimating emissions fromspontaneous combustion and low-temperatureoxidation of coal wastes.

Other work has investigated the use of airborneinfrared thermography to measure the surfacetemperature of spoil piles (Carras et al. 2002).This, combined with the finding that the emissionflux is roughly proportional to the surface temper-ature (Carras et al. 2009), also provides a potential

method for estimating emissions over relativelylarge areas. To date, however, these methods havenot been applied to quantify emissions from indi-vidual mines.

In the study reported in this paper, estimatesof emissions from spontaneous combustion andlow-temperature oxidation of waste material weremade for a number of operating open-cut coalmines in Australia using the methods developedby Carras et al. (2002, 2009). To place theseestimates into context, emission data from allother sources at each mine were also consid-ered and compared with the estimates madefor spontaneous combustion and low-temperatureoxidation.

Methodology

Greenhouse gas emission data were analysed fromsix open-cut coal mines located in two of Aus-tralia’s major coal producing regions: four in theHunter Valley in New South Wales (NSW) andtwo in the Bowen Basin in Queensland. Annualproduction of run-of-mine (ROM) coal from thesemines ranged from approximately 1.7 Mt to morethan 16 Mt. Mines were selected to include thosewith low, medium and severe occurrences of spon-taneous combustion.

Data on electricity, fuel and explosives con-sumption, land clearing, fugitive emissions andother sources of greenhouse gas sources were ob-tained from each mine. Information on the extentand severity of spontaneous combustion was alsocollected. For two of the mines selected, infrareddata from the Carras et al. (2002) study wereavailable, which were also used to estimate spon-taneous combustion derived emissions from thesemines.

The data were collated to show the relativecontribution from each source compared to thetotal greenhouse emissions from individual mines.Details on the data collection and analysis arediscussed in the following sections.

Electricity, fuel and explosives

Open-cut coal mines operate a range of elec-trically powered equipment, which may include

Environ Monit Assess (2010) 166:529–541 531

Table 1 Emission factors used for estimating greenhousegas emissions from electricity, diesel, waste and explosives

Source Emission factor

ElectricityNSW 1.068 t CO2-e MWh−1

Queensland 1.046 t CO2-e MWh−1

Diesel 3.0 t CO2-e kL−1

Waste to landfill 0.9 t CO2-e t−1

ExplosivesANFO 0.17 CO2-e t−1

Heavy ANFO 0.18 t CO2-e t−1

Emulsion 0.17 t CO2-e t−1

draglines, electric shovels and conveyors. Manymines also have coal preparation plants on site,which are large consumers of electricity. In addi-tion to the electrical equipment, substantial fleetsof diesel-powered machinery, such as haul trucks,bulldozers and drilling rigs, are maintained. Emis-sions from electricity and diesel consumption wereestimated by multiplying the activity (i.e. themegawatt hour of electricity or kiloliter of dieselused) by an appropriate emission factor. Theemission factors used for electricity and diesel areshown in Table 1 (Australian Dept of ClimateChange 2008b).

Explosives based on ammonium nitrate andfuel oil (ANFO) mixtures are used universallythroughout the Australian coal mining industry.These explosives contain approximately 6% fueloil and produce about 170 kg of CO2 per tonneof explosive, depending on the formulation. Theemission factors for the most commonly used ex-plosives are shown in Table 1 (Australian Dept ofClimate Change 2008b).

Land clearing

Land clearing is a necessary consequence of min-ing and therefore contributes to the greenhousefootprint of mines. To estimate emissions due toland clearing, it is necessary to know both the areaof land cleared and an emission factor appropriatefor the type of vegetation removed. The emissionfactor must take into account the amount of car-bon in the biomass and the fate of the carbonreleased. The simplest case is to assume that allof the carbon is released as CO2 during the yearof clearing.

Most mines were able to provide the area ofland disturbed each year but usually did not havespecific information relating to the type or amountof vegetation removed. Three of the mines sur-veyed (two in NSW and one in Queensland) did,however, make their own estimates of emissionsdue to land clearing using an emission factor sup-plied by their respective parent company. Theseemission factors were 165 t CO2 ha−1 for NSWand 99 t CO2 ha−1 for Queensland.

There have been studies in central Queens-land where the amount of biomass in eucalyptwoodlands, typical of the Bowen Basin where thecoal mines are located, has been measured, andit is interesting to compare the results with theemission factors used by the three mines.

Burrows et al. (2000) found that the averagebiomass density of eucalypt woodland across 33central Queensland sites was 5.86 t m−2 of treebasal area. They also found that the average basalarea over these sites was 12.0 m2 ha−1. Assuminga carbon fraction of 0.5 for the biomass (Burrowset al. 2002), this equates to an emission factor ofabout 129 t CO2 ha−1, which compares reasonablywell with the factors used by the three mines.Thus, in the absence of mine-specific data, weapplied the emission factors used by the mines tocalculate the emissions due to land clearing fromthe other mines included in this study.

General waste

Most of the mines examined generate a certainamount of general waste material that is sent tolandfill. An emission factor of 0.9 t CO2-e pertonne of waste was applied to estimate emissionsfrom this source (Australian Dept of ClimateChange 2008b).

Fugitive emissions from seam gas

Fugitive emissions of methane and, to a lesserextent, CO2 represent a large proportion of green-house gas emissions from the Australian coal min-ing industry. For open-cut mining, however, theseemissions are not readily measured. Saghafi et al.(2008) recently proposed a method for estimat-ing fugitive emissions from individual mines;however, at present, there are insufficient data

532 Environ Monit Assess (2010) 166:529–541

available for most Australian mines to allow re-liable estimates to be made. Consequently, esti-mates of fugitive emissions were made by mul-tiplying the annual ROM coal production by anemission factor. This is the method currently usedfor compiling the Australian National Green-house Gas Inventory.

For NSW, the emission factor is 2.17 kg t−1 CH4

(45.5 kg t−1 CO2-e), while for Queensland, it is0.81 kg t−1 CH4 (17.1 kg t−1 CO2-e; AustralianDept of Climate Change 2008b). Although theseemission factors have high uncertainties, we usedthis methodology to ensure consistency for all themines studied.

Emissions from spontaneous combustionand low-temperature oxidation

Spontaneous combustion—ground measurements

Emissions from spontaneous combustion were ini-tially estimated by determining the area of groundaffected by spontaneous combustion and apply-ing an emission factor based on the work of Carraset al. (2009). Those authors observed a correlationbetween the surface temperature of a spoil pileand surface emission rate. While acknowledgingthat the surface temperature can at best be acrude measure of spontaneous combustion activ-ity within a spoil pile and that the emissions arealso governed by the nature of the topmost layersof spoil, the authors nevertheless found a usefulrelationship. This is shown in Fig. 1 (from Carraset al. 2009).

Methane is often formed and emitted duringspontaneous combustion, and the emission factorsdeveloped by Carras et al. (2009) took this intoaccount.

Linear regression of the data presented inFig. 1 yielded the empirical expression shown inEq. 1, where the emission rate, ER, in units ofkg CO2-e year−1 m−2 is related to the surfacetemperature, T (◦C), by the expression:

ER = 59.6T − 496.4 (1)

Equation 1 is based on measurements made whenthe ambient temperature of unaffected groundwas about 9◦C (i.e., threshold temperature below

Temparture (oC)0 10 20 30 40 50 60

CO

2-e

Em

issi

ons

(kg

y-1 m

-2)

0

500

1000

1500

2000

2500

3000

3500

4000

Fig. 1 Average emission rates from spoil affected by spon-taneous combustion (as CO2-equivalent) as a function ofaverage surface temperature (from Carras et al. 2009)

which there are no emissions). Applying Eq. 1to datasets measured when the threshold tem-perature is significantly different, such as duringsummer months, may substantially over- or un-derestimate emissions. Hence, care was taken toensure that ground temperature measurementswere made in the early morning during wintermonths.

Estimates of the area of ground (in squaremeter) affected by spontaneous combustion ineach mine were made by visual inspection and theintensity categorised as either “minor” or “major”according to the following criteria:

• Minor—active spontaneous combustion withless obvious signs

◦ Surface discolouration◦ No large cracks◦ Little smoke or steam

• Major—active spontaneous combustion withmarked surface signs

◦ Large cracks with obvious signs of gasventing

◦ Smoke and steam◦ Hot gases◦ Surface discolouration

This information was gathered during site visits tomines and detailed discussions with mine person-nel. Because of statutory environmental licensingobligations, a number of the mines already collect

Environ Monit Assess (2010) 166:529–541 533

this information through regular ground surveysand prepare maps showing the area and intensityof heating within the mine. Where available, thesedata were used for estimating the area and sever-ity of spontaneous combustion.

During site visits, surface temperatures weremeasured at various locations showing signs ofspontaneous combustion. In areas classified as mi-nor, the ground temperature was less than about10◦C above unaffected ground so, from Fig. 1, anemission factor of 500 kg year−1 m−2 was applied.Major outbreaks typically had temperatures of60◦C or more, and this category was assigned anemission factor of 3,000 kg year−1 m−2. There is,of course, a continuum between the categories,and it is recognised that this approach is only anapproximation at best.

Spontaneous combustion—airbornemeasurements

An alternative, potentially more accurate ap-proach to determining the extent and intensityof heating across mines in the Hunter Valley hasbeen investigated previously using airborne in-frared thermography (Carras et al. 2002). Datafrom that study, which were collected during Sep-tember 2000, were available for two of the minesincluded in the current study. These data wereused in the present study to determine both thesurface temperature and area of the ground un-dergoing heating at each of these mines.

The infrared data were obtained with aDaedalus 1268 airborne thematic mapper systemfitted to a Cessna 414A aircraft. Measurementswere made using bands 11 and 12 of the instru-ment covering a wavelength range of 8–14 μm. Tominimise the effects of solar radiation and max-imise the thermal contrast, data were collectedbefore dawn during winter months in cloudless,windless conditions. The ground temperature inareas unaffected by spontaneous combustion wasbelow 10◦C (i.e., comparable to the threshold tem-perature inferred by Eq. 1).

Data were recorded by the scanner as pixelsand assigned an integer value ranging from 0 to255. The numerical value of each pixel was relatedto ground temperature according to a calibra-tion function established by a series of “ground-

truthing” temperature measurements made at thetime of the flight. The resolution of the thermalimages was such that each pixel corresponded toan area on the ground of about 7 × 7 m. Data fromthe airborne scanner were analysed with Terras-can software (Resource Imaging Australia).

Using the ground temperatures derived fromthe infrared data and Eq. 1, emissions from thetwo NSW mines covered by the airborne in-frared survey were calculated and compared tothe ground-based method described above.

Low-temperature oxidation

Low-temperature oxidation of waste coal in spoilpiles was considered in detail by Carras et al.(2009). Using the results of laboratory measure-ments of oxidation rates of coal and carbonaceousmaterial as well as numerical modelling, they cal-culated greenhouse gas emission rates from min-ing waste for a range of carbon contents andtemperatures. They found that the emission ratesfrom a variety of sources, including bare spoil,revegetated spoil, uncovered tailings, uncoveredreject, natural forest floor and suburban lawn,were all similar. From this, they suggested thatgreenhouse emissions of CO2 from rehabilitatedspoil, with no spontaneous combustion, could notbe distinguished from other natural land surfaces,although they also indicated that further workwould be required to disentangle the biogenic andcoal-based contributions.

For the purpose of the current study, we as-sumed that the average temperature and car-bon content of the spoil were 27◦C and 5%,respectively. According to the results of Carraset al. (2009), this yields an emission factor of2.2 kg year−1 m−2, which was applied to low-temperature oxidation from uncovered spoil, tail-ings or reject with no spontaneous combustion.

To estimate the contribution from low-temperature oxidation, it is also necessary toknow the surface area of exposed spoil. Aerialphotographs of mines A, B, C, and D indicatedthat the spoil areas were roughly 4, 4, 0.5, and2 km2, respectively. Photographs for mines E andF were not available so the exposed spoil area wasassumed to be 3 km2 in each case since they wereabout three quarters the size of mines A and B.

534 Environ Monit Assess (2010) 166:529–541

Tab

le2

Sum

mar

yof

the

gree

nhou

sega

sem

issi

onda

tafo

rth

esi

xop

en-c

utco

alm

ines

RO

MM

ine

Pro

duct

ion

AB

C(M

tpa)

16.8

16.8

1.7

Em

issi

ons

Spec

ific

emis

sion

sP

erce

ntE

mis

sion

sSp

ecifi

cem

issi

ons

Per

cent

Em

issi

ons

Spec

ific

emis

sion

sP

erce

nt(t

CO

2-e

year

−1)

(kg

CO

2-e

year

−1t−

1)

ofto

tal

(tC

O2-e

year

−1)

(kg

CO

2-e

year

−1t−

1)

ofto

tal

(tC

O2-e

year

−1)

(kg

CO

2-e

year

−1t−

1)

ofto

tal

Ele

ctri

city

171,

820

10.2

16.0

130,

910

7.8

11.9

3,88

62.

32.

8F

uel

111,

568

6.7

10.4

164,

412

9.8

15.0

16,1

429.

611

.5E

xplo

sive

s68

570.

40.

66,

390

0.4

0.6

584

0.3

0.4

Fug

itiv

es76

3,24

045

.571

.276

3,66

345

.569

.676

,518

45.5

54.3

Was

te40

10.

00.

066

10.

00.

1N

otas

sess

edL

and

clea

ring

9,40

50.

60.

922

,935

1.4

2.1

Not

asse

ssed

Spon

Com

568

0.0

0.1

00.

00.

042

,723

25.4

30.3

Low

tem

p8,

830

0.5

0.8

8,83

00.

50.

81,

104

0.7

0.8

oxid

atio

nT

otal

1,07

2,68

963

.910

0.0

1,09

7,80

265

.410

0.0

129,

459

83.9

100.

0

RO

MM

ine

Pro

duct

ion

DE

F(M

tpa)

7.3

12.2

13.1

Em

issi

ons

Spec

ific

emis

sion

sP

erce

ntE

mis

sion

sSp

ecifi

cem

issi

ons

Per

cent

Em

issi

ons

Spec

ific

emis

sion

sP

erce

nt(t

CO

2-e

year

−1)

(kg

CO

2-e

year

−1t−

1)

ofto

tal

(tC

O2-e

year

−1)

(kg

CO

2-e

year

−1t−

1)

ofto

tal

(tC

O2-e

year

−1)

(kg

CO

2-e

year

−1t−

1)

ofto

tal

Ele

ctri

city

56,2

227.

77.

047

,271

3.9

12.4

205,

083

15.7

33.5

Fue

l11

8,49

016

.214

.830

,115

2.5

7.9

140,

773

10.8

23.0

Exp

losi

ves

2,88

20.

40.

41,

790

0.1

0.5

6,36

80.

51.

0F

ugit

ives

332,

661

45.5

41.7

209,

133

17.1

54.8

223,

350

17.1

36.4

Was

teN

otas

sess

ed55

00.

00.

137

10.

00.

1L

and

clea

ring

32,1

754.

44.

07,

227

0.6

1.9

27,4

232.

14.

5Sp

onC

om25

1,50

434

.531

.578

,750

6.4

20.6

3,00

00.

20.

5L

owte

mp

4,41

50.

60.

66,

623

0.5

1.7

6,62

30.

51.

1ox

idat

ion

Tot

al79

8,35

010

9.4

100.

038

1,45

831

.210

0.0

612,

990

46.9

100.

0

Environ Monit Assess (2010) 166:529–541 535

Results

The results of the greenhouse gas emissions es-timates for each of the six mines are shown inTable 2. Emissions from each source are ex-pressed as t CO2-e per year and also as a “spe-cific emission” where the emissions have beennormalised to the annual ROM coal production(expressed as kg CO2-e year−1 t−1). The specificemissions for each mine are plotted in Fig. 2 (emis-sions from waste to land fill have been omittedsince, in all cases, it contributed less than 0.1%of the total). The percentage contribution fromthe various sources relative to each mine’s totalannual emissions are also listed in Table 2 andplotted in Fig. 3.

The results for individual mines are discussedin more detail below.

Mine A is a large Hunter Valley mine withan annual production of around 16 Mt of ROMcoal. Spontaneous combustion is comparativelyrare at this mine, with only a few small outbreaksoccurring from time to time in reject dumps anddisused tailings emplacements. The total area thatwas thought to have had occurrences of heatingin the past was estimated to be about 20,000 m2.

However, because there was no sign of activespontaneous combustion anywhere on the mine,an emission factor of 500 kg CO2-e year−1 m−2

was considered to be too large for the intensityof heating present. Instead, an estimate of emis-sions due to spontaneous combustion was madeby assuming that all of this area correspondedto the lowest category for rejects and tailings asdefined by Carras et al. (2009), i.e. 28.4 kg CO2-eyear−1 m−2.

Fugitive emissions from mine A representnearly three quarters of the mine’s total green-house gas emissions, but, as noted previously, thisestimate is subject to a large uncertainty. Most ofthe remaining emissions originate from electricityand fuel usage, which together comprise about27% of the total. Not surprisingly, spontaneouscombustion contributes only about 0.1% to thetotal. Low-temperature oxidation of waste coalin the spoil contributes about 1% of the totalgreenhouse emissions from the mine.

Similar results were obtained for mine B, whichis also in the Hunter Valley and has aroundthe same annual output as mine A. There areno known outbreaks of spontaneous combustionat mine B. Again, fugitive emissions comprise

Fig. 2 Specificgreenhouse gas emissions(i.e., emissionsnormalised to the annualROM coal production)attributed to the mainsources for the six mines.Note that waste to landfillhas been omitted for thesake of clarity

Electri

city

Fuel

Explos

ives

Fugitiv

es

Land

Clea

ring

Spon

Com

Low T

emp

Oxidat

ion

Spe

cific

Em

issi

on (

kg C

O2-

e y-1

t-1)

0

10

20

30

40

50

Mine AMine BMine CMine DMine EMine F

536 Environ Monit Assess (2010) 166:529–541

Fig. 3 Greenhouse gasemissions expressed as apercentage of the totalemissions from individualmines

Mine

Mine A Mine B Mine C Mine D Mine E Mine F

Gre

enho

use

Em

issi

ons

(% o

f Min

e's

Tot

al)

0

20

40

60

80

100

ElectrictyFuel Explosives

FugitivesLand Clearing

Spon ComLow Temp Ox

the bulk of greenhouse emissions, while energy-related emissions make up slightly more than aquarter of the total.

Compared to the other mines studied, mine C,which is located in the Hunter Valley, is relativelysmall and produces around 1.7 Mt of ROM coalannually. The contribution from land clearing wasnot assessed for mine C, but since this mine dis-turbs little fresh ground, it is unlikely to contributesignificantly to the mine’s total greenhouse emis-sions. Mine C has, for many years, been affectedby moderate spontaneous combustion throughoutthe site.

It is apparent from Figs. 2 and 3 that the pro-portion of energy-related emissions derived fromelectricity is relatively low in mine C comparedto other mines. This is because mine C usesmostly diesel powered equipment in their miningoperations; the other mines use large electricallypowered draglines and shovels. Fugitive emissionsaccounted about 76,000 t CO2-e, about 55% of thetotal greenhouse gas emission from the mine.

Ground-based assessments of the spontaneouscombustion at Mine C yielded areas of 9,950 m2

for the minor and 8,750 m2 for the major cate-gories. On this basis, emissions from spontaneous

combustion were calculated to be approximately31,000 t CO2-e.

As well as the ground-based survey of sponta-neous combustion, an infrared image of mine C(Carras et al. 2002) was used to estimate emis-sions. Figure 4 shows the infrared image of mineC; areas with elevated surface temperatures areshown in lighter shades.

Data from the infrared image were used tomeasure the area and temperature of groundshowing signs of heating, from which CO2-e emis-sions for the mine were calculated (Table 3).

The area of heat-affected ground estimatedfrom the thermography data was approximately41,000 m2, more than twice that estimated by aground survey. Spontaneous combustion-relatedgreenhouse gas emissions based on this value andEq. 1 were around 43,000 t CO2-e y−1, about50% higher than the value determined from theground survey data. This is equivalent to a specificemission of 25.4 kg CO2 t−1 year−1 or around 30%of the total greenhouse gas emission.

The difference between the two methods isthe result of ground surveys tending to underes-timate the area because they rely on some formof surface expression to identify that spontaneous

Environ Monit Assess (2010) 166:529–541 537

Fig. 4 Infrared image of mine C. The surface resolution ofthe image is about 7 m

combustion is present. Thermography, on theother hand, is more sensitive to detecting evensmall changes in surface temperature that may notbe associated with visible manifestations.

Mine D regularly surveys the area of the minethat is affected by spontaneous combustion. Dur-ing 2003, the area of spontaneous combustion wasmonitored each month and found to range fromabout 650 to 2,500 m2, with an average value forthe year of 1,326 m2. Information on the severitywas not available so a 50:50 minor/major split wasassumed. On this basis, the emissions attributedto spontaneous combustion were very low, rep-resenting only about 0.4% of the total from themine.

Infrared data from the study of Carras et al.(2002) were also available for mine D, so emis-sions were estimated using the same technique ap-plied to mine C. The area affected by spontaneouscombustion and associated emissions determinedby airborne thermography for mine D are listed inTable 3.

Using the infrared data yielded an area of about22 ha of heat affected ground, which is more thana factor of 160 higher than that determined by theground survey. Accordingly, the greenhouse gasemissions determined by this method were muchhigher, totalling more than 250,000 t CO2-e. Thisrepresents approximately 30% of the mine’s totalannual greenhouse emissions.

Although the infrared survey of mine D wasmade several years before the ground-based es-timate, it is unlikely that the large discrepancybetween the two methods can be accounted for byimproved spontaneous combustion managementat the mine (such as covering active areas withinert material). Results of ground measurementsmade at the time of the airborne survey indicatedthat the area of affected ground was even less thanthe 1,326 m2 used in these calculations. It seemsthat most of the spoil undergoing heating at mineD is not readily detected by visible inspection.

The estimate for Mine D based on the in-frared data compares well to measurements madeduring the late 1990s by CSIRO (unpublisheddata). In that instance, plumes of CO2 originat-ing from mine D (and an adjacent mine whichalso has spontaneous combustion) were traversedin an instrumented vehicle, which measured theCO2 concentration in the plumes. Local mete-orological data and a plume model were thenused to infer the flux of CO2 emitted from the

Table 3 Surfacetemperature and area ofground affected byspontaneous combustionand associated emissionsin mines C and Ddetermined by airbornethermography

Average Mine C Mine Dtemperature Area Emissions Area Emissions(◦C) (m2) (t CO2-e year−1) (m2) (t CO2-e year−1)

17.5 16,660 9,103 73,500 40,16022.5 8,869 7,488 45,080 38,06227.5 3,969 4,534 27,293 31,17532.5 2,793 4,022 21,266 30,62737.5 1,617 2,811 14,259 24,78442.5 7,252 14,765 42,581 86,696Total 41,160 42,723 223,979 251,504

538 Environ Monit Assess (2010) 166:529–541

mine site. This technique yielded an emissionestimate of up to 260,000 t CO2-e year−1. Thisvalue, however, may be an overestimation sinceit also included emissions from vehicles and otherequipment operating on the mine site at the timeof the measurements.

Overall specific emissions for mine D were thehighest of the six mines at 109.4 kg CO2-e t−1 y−1.

Mine E, located in the Bowen Basin, is a largemine producing about 12 Mt of thermal coal perannum. The coal is subject to spontaneous com-bustion, and there are numerous outbreaks inwaste material throughout the mine. There arealso old underground workings on the mine lease,which are prone to spontaneously combust as theopen-cut operations uncover the old shafts.

Table 2 shows that the electricity and dieselusage in this mine were the lowest of all six mines;the combined electricity and diesel contributionto greenhouse gas emissions amounted to 6.4 kgCO2-e t−1 year−1. This compares to the next low-est mine (mine C) with nearly twice that emissionand the highest, mine F, at 26.5 kg CO2-e t−1

year−1. The reason for the large differences is thatthe amount of material moved to produce the coalvaries considerably from mine to mine. In the caseof mine E, which has a very low stripping ratio(i.e. the ratio of the volume of overburden tothe volume of coal), the total amount of materialmoved is only about a third of that at mines A andB. The amount of energy required to extract thecoal is therefore correspondingly lower.

The total area of the mine affected by sponta-neous combustion was estimated to be betweenabout 30,000 and 40,000 m2 with about 30%classed as minor (500 kg CO2-e y−1 m−2) and70% major (3,000 kg CO2-e year−1 m−2). Infraredimagery of this mine was not available, but basedon the experience of mines C and D, it is possi-ble that this is less than the actual area of spoilwith elevated surface temperatures. Assuming anaverage area of 35,000 m2 of active spontaneouscombustion, the total annual emission equates toaround 79,000 t CO2-e (6.4 kg CO2-e year−1 t−1).This represents around 20% of the mine’s totalemissions.

Due to the relatively low energy requirementsand the lower emission factor for fugitive emis-sions of seam gas from Queensland mines, specific

emissions (from all sources) at mine E were thelowest (∼31 kg CO2-e year−1 t−1) of all the minessurveyed.

Mine F is located in the Bowen Basin and pro-duces about 13 Mt of ROM coal annually. In thismine, the amount of material moved to producea tonne of coal is high, and this is reflected in thehigher energy usage compared to the other mines.The combined CO2 emissions for electricity andfuel of 26.5 kg CO2-e year−1 t−1 was the highest ofthe six mines and accounted for more than half ofthe mine’s total emissions.

Mine F has relatively few occurrences of spon-taneous combustion, all of which can be classi-fied as minor. The total area of affected groundwas estimated to be around 6,000 m2. Using thisinformation (but bearing in mind the problems as-sociated with estimating the area of spoil undergo-ing spontaneous combustion by ground surveys),emissions from spontaneous combustion at mineF were estimated to be 3,000 t CO2-e per yearor about 0.5% of the mine’s total greenhouse gasemissions.

Discussion

The eight sources of greenhouse gases examinedin this study account for the bulk of the green-house emissions from open-cut mining; however,before considering the magnitude of these esti-mates, it is worthwhile discussing the associateduncertainties.

The most reliable estimates are those for elec-tricity and fuel usage since the activities aremeasured accurately for billing purposes and theemission factors are well established. Emissionsfrom explosives are also reasonably accurate be-cause consumption is closely monitored and ro-bust emission factors, determined by assumingthat the fuel oil in the ANFO mixture is burntto produce CO2, are used. Although other gasesmay be produced during blasting, most of thecarbon in the fuel oil is converted to CO2 withonly relatively small quantities of CO and hydro-carbons formed under some conditions (Attallaet al. 2008). The uncertainty for energy-derivedemission estimates (which includes explosives) isprobably less than 5% (Leung 2002).

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Emissions from waste sent to landfill are poorlydefined, but since they comprise only a very smallproportion of each mine’s total emission, evenlarge errors in the waste component have verylittle effect on the overall estimate. Similarly, landclearing is subject to moderate uncertainty, butthe overall effect of these errors is low because thecontribution accounts for no more than a few percent of the total.

Fugitive emissions of seam gas, on the otherhand, represent the largest source of greenhousegas emissions from open-cut coal mining, but theuncertainty associated with fugitive emissions es-timates is very high. Leung (2002) suggested thatthe uncertainty on open-cut fugitives was ±50%,while Williams et al. (1998) considered that itmay be as high as a factor of two. Consequently,estimates of total greenhouse gas emissions fromindividual mines have a relatively high uncertaintybecause of the strong contribution from fugitiveemissions. Recent work by Saghafi et al. (2008)has resulted in the development of a method todirectly measure fugitive emissions from individ-ual open-cut coal mines and with sufficient data,this methodology may substantially reduce theuncertainty of these estimates.

Spontaneous combustion emission estimatesare also subject to high uncertainty. The emis-sion factors and the emission/temperature rela-tionship developed by Carras et al. (2009) have,in themselves, significant uncertainty as evidencedby the scatter in measurements on which they arebased (Fig. 1). In addition, there are a numberof practical problems that compound this uncer-tainty. In the methodology used in this study, it isnecessary to know the area of the spoil affectedby spontaneous combustion, which is difficult tomeasure accurately, especially at ground level.The results of this study suggest that ground-basedsurveys underestimate the area, in some cases

by very large margins. This is probably becauseground surveys rely on detecting visible surfaceexpressions of spontaneous combustion; less ob-vious signs may be overlooked. As well, physicalaccess to parts of spoil piles is often very diffi-cult or impossible due to operational and safetyfactors.

Airborne infrared thermography appears to becapable of providing better area estimates andalso has the advantage of being able to mea-sure the surface temperature of the spoil piles.However, care must be taken when interpret-ing the results of the infrared surveys to inferground temperatures. The radiation measured bythe infrared sensor is dependent on both the ther-modynamic temperature of the surface and itsemissivity. Emissivity is wavelength-dependentbut also depends on the nature of the emitter.Since different substances have different emissiv-ities, for a given thermodynamic temperature, theapparent temperatures inferred from the infrareddata will vary accordingly. Accurate temperaturevalues therefore depend on accurate knowledgeof the emissivities of the materials. Carras et al.(2002) suggested that the uncertainty of the emis-sivity meant that surface temperatures could onlybe determined to about ±3◦C.

This has implications when using ground tem-peratures derived from infrared data to estimategreenhouse gas emissions. Carras et al. (2002)showed that the method is very sensitive to thetemperature below which emissions are assumedto be zero. Since a large proportion of the emis-sions are associated with relatively low temper-atures, variations of even a few degrees in thisthreshold temperature can make a substantial dif-ference to the emission estimate.

Notwithstanding the uncertainties discussedabove, the emission estimates are based on ac-tual mine data and provide some indication of

Table 4 Equivalent amount of ROM coal that would have to be burnt to produce the spontaneous combustion estimatesfor each mine

Mine A Mine B Mine C Mine D Mine E Mine F

Annual spontaneous combustion emission 568 0 31,225 251,504 78,750 3,000(t CO2-e)

ROM coal equivalent (t) 221 0 12,166 97,989 30,682 1,169Per cent of annual production 0 0 0.72 1.34 0.25 0.01

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the relative contribution to greenhouse gas emis-sions from the major sources. Total specific emis-sions from the six mines varied from about 31 kgCO2-e year−1 t−1 up to almost 110 kg CO2-ey−1 t−1. Fugitive emissions of methane from seamgas were the dominant source of greenhousegases, ranging from slightly more than a third toalmost three quarters of individual mines’ totalgreenhouse emissions.

For some mines, emissions from spontaneouscombustion are substantial and are of the sameorder as all of their energy-related emissions, insome cases representing up to about a third of allemissions. For one of the mines (mine D), spon-taneous combustion is estimated to produce morethan 250,000 t CO2 per year. Table 4 summarisesthe emissions from spontaneous combustion andalso shows the amount of ROM coal that wouldneed to be burnt to produce an equivalent amountof CO2 (assuming 70% carbon in the ROM coal).

Table 4 shows that the annual emissions fromspontaneous combustion can be equivalent toburning as much as 100,000 tonnes of coal. Formine D, this represents about 1.3% of its annualproduction. Given that spoil piles may containmillions of tonnes of carbonaceous waste, it isclear that a mine with uncontrolled spontaneouscombustion is likely to release very large quan-tities of greenhouse gases over its lifetime andbeyond.

While the contribution of spontaneous com-bustion is clearly significant for some individualmines, it is, however, unclear at present how im-portant this source is for the industry as a whole,since many of the 60 or so open-cut coal minescurrently operating in Queensland and NSW arenot affected by spontaneous combustion.

Low-temperature oxidation contributed onlyaround 1% of the total greenhouse emissions forall six mines. Estimates were made by assum-ing that the average carbon content of the spoilwas 5%, but this is likely to be highly variableboth between individual mines and also within thespoil. However, even if the emissions from low-temperature oxidation were a factor of two higher,they would still be relatively minor compared tospontaneous combustion and fugitives.

Conclusions

Estimates of greenhouse gas emissions were madeat six Australian open-cut coal mines. Fugitiveemissions of methane from seam gas were in allcases estimated to be the largest source of green-house gas, but for some mines, up to a third of thetotal emissions originated from spontaneous com-bustion. In one mine, annual emissions attributedto spontaneous combustion amounted to about250,000 t CO2-e. There is, however, considerableuncertainty surrounding estimates from sponta-neous combustion.

The emission factors and the emission/temperature relationship established by Carraset al. (2009) are subject to considerable inherentuncertainty, but compounding this are errorsin measuring the area of spoil affected byspontaneous combustion. The area is generallyestimated by ground surveys; however, theseappear to substantially underestimate the truearea because ground without visible signs ofheating may not be included.

Airborne infrared thermography is a more re-liable method for measuring the area of sponta-neous combustion, since it is capable of detectingeven slightly elevated surface temperatures whereother visual signs of heating are not apparent. In-frared data also provides a method for measuringthe surface temperature, which can in turn be usedto infer greenhouse emissions.

Although emissions from spontaneous combus-tion represent a large proportion of the total insome mines, the contribution to greenhouse gasemissions from the coal mining industry as a wholeis probably substantially less since spontaneouscombustion is not present in all mines. Low-temperature oxidation, on the other hand, is likelyto be present at most mines but is a relativelyminor source of greenhouse gas, contributing upto about 1% of a mine’s total emissions.

Acknowledgements We are grateful to the many minepersonnel who provided the data for this project and theirhelpful discussions. This project was partially supported bythe Australian Coal Association Research Program.

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