Coal spontaneous combustion: Examples of the self-heating incubation process

Coal spontaneous combustion: Examples of the self-heating incubation process

International Journal of Coal Geology 215 (2019) 103297 Contents lists available at ScienceDirect International Journal of Coal Geology journal home...

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International Journal of Coal Geology 215 (2019) 103297

Contents lists available at ScienceDirect

International Journal of Coal Geology journal homepage: www.elsevier.com/locate/coal

Coal spontaneous combustion: Examples of the self-heating incubation process

T



B. Basil Beamisha,b, , Jan Theilera a b

B3 Mining Services Pty Ltd, Brisbane, Australia School of Minerals and Energy Resources Engineering, University of New South Wales, Sydney, Australia

A R T I C LE I N FO

A B S T R A C T

Keywords: Self-heating incubation Adiabatic oven Moisture moderation Ambient start temperature Seam gas Reactive pyrite

Coal spontaneous combustion continues to pose a significant hazard to mining operations. It is a complex process that ranges from low-temperature oxidation in the normal mine environment to thermal runaway once temperatures exceed 120 °C. At the thermal runaway stage, the coal becomes dry locally after moisture liberation and evaporation and a well-defined hot spot forms. This process takes place over a period of time, which can be referred to as the incubation period. Assessing the spontaneous combustion hazard likelihood has normally relied on the use of laboratory testing to produce index parameters that give a propensity rating. These are often a single value on a relative rating scale, which gives no indication of the nature of the coal self-heating with respect to time as it would occur under various mine site conditions. To overcome this deficiency it is necessary to consider the incubation behaviour of the spontaneous combustion process, particularly as it relates to coal self-heating at initial mine ambient temperatures. This has recently been achieved using adiabatic oven testing, which shows the modifying influences of moisture content, initial start temperature, seam gas content and reactive pyrite content on the low temperature coal self-heating rate. The incubation testing procedure is able to determine whether self-heating can reach thermal runaway and if so in what timeframe this can take place for the environmental conditions present at the mine site.

1. Introduction Spontaneous combustion is considered a Principal Mining Hazard in Australian coal mines and as such an appropriate management plan is essential to mitigate and control the hazard. A key part of this management plan is hazard assessment, which requires the input of testing results for spontaneous combustion propensity rating to evaluate the likelihood of a heating event developing in the mine environment. There are numerous spontaneous combustion propensity tests that produce index parameters based on a relative rating scheme (Nelson and Chen, 2007). These index parameters are reported as a single value that provides no information on the self-heating behaviour of the coal over time. Also, some index parameters, such as Crossing Point Temperature (CPT) and Relative Ignition Temperature (RIT) are obtained at high temperatures where the coal is essentially in thermal runaway (Banerjee, 1985). In both of these tests the coal is subjected to forced heating at a fixed oven temperature ramping rate (usually 2 °C/min) until the rise in coal temperature exceeds the oven temperature. These high temperature index parameters are therefore not indicative of the coal self-heating behaviour at initial ambient mine conditions, but are



more indicative of the likely ignition behaviour of the coal once thermal runaway is reached (Beamish and Theiler, 2017a). This same deficiency of obtaining an index parameter at high temperature is inherent in the UN Wire Basket test for shipping certification of coal, which is performed at an oven temperature of 140 °C. The spontaneous combustion process is described as the heat from coal oxidation exceeding the heat loss to the surroundings resulting in an increase in coal temperature that creates a feedback mechanism for increased self-heating rate and consequently a rise in temperature (Green et al., 2012). This description gives the impression that the spontaneous combustion process is a one way street similar to ignition (high temperature combustion) theory. However, this is not always the case and this paper contains examples that demonstrate the complex interaction of intrinsic coal reactivity with modifying factors such as moisture, initial start temperature, presence of seam gas and presence of reactive pyrite. These examples also illustrate that spontaneous combustion is best described as an incubation process where time can be a critical component that is often overlooked.

Corresponding author at: B3 Mining Services Pty Ltd, Brisbane, Australia. E-mail address: [email protected] (B.B. Beamish).

https://doi.org/10.1016/j.coal.2019.103297 Received 19 April 2019; Received in revised form 18 September 2019; Accepted 18 September 2019 Available online 17 October 2019 0166-5162/ © 2019 Published by Elsevier B.V.

International Journal of Coal Geology 215 (2019) 103297

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Table 1 Coal quality data and test parameters for incubation test coal samples. Sample

Location

R70 (°C/h)

Volatile matter (%, dmmf)

Calorific value (Btu/ lb., mmmf)

ASTM rank

Ash content (%, db)

Sulphur content (%, db)

Moisture content (%, ar)

Incubation test start temperature (°C)

Rank suite Coal A New Zealand Coal B Indonesia Coal C Indonesia Coal D Australia Coal E Colombia Coal F Australia Coal G New Zealand Coal H United States Coal I Australia Coal J Australia Coal K Australia Coal L Australia Coal M Australia Coal N Australia

99.67 74.72 28.57 14.61 11.43 6.34 5.87 4.19 4.10 3.94 3.18 1.95 0.37 0.22

62.2 55.4 51.6 30.2a 42.5 36.7 41.3 41.2 46.0 31.8 45.8 28.5 23.4 20.6

7464 7846 9755 10,540 11,678 12,858 13,749 13,787 13,668 13,134 14,664 14,347 na na

ligA ligA subB subA hvCb hvCb hvBb hvBb hvBb hvBb hvAb hvAb mvb lvb

6.8 3.5 2.8 11.5 1.7 4.2 3.1 5.8 6.2 6.9 4.6 13.7 8.4 5.5

0.34 0.19 0.10 0.12 0.40 0.57 0.30 0.49 1.18 0.39 0.45 0.74 0.52 0.49

47.2 34.6 24.0 16.7 17.7 12.0 11.7 5.0 9.2 11.0 3.0 4.7 3.0 3.0

25.9 27.4 24.4 27.2 39.5 39.9 27.0 27.9 35.4/20.2 27.2/35.3 27.5 24.3 37.5 37.7

Gassy coal samples Coal O Australia Coal P Australia

5.95 3.17

37.7 33.8

13,293 14,192

hvBb hvAb

14.8 4.8

0.53 0.36

7.3 5.3

27.3 35.8

Pyritic coal samples Coal Q Australia Coal R Australia Coal S Australia Coal T Australia

12.33 7.20 7.11 5.88

40.6 47.7 47.9 44.8

11,658 12,061 12,115 11,309

hvCb hvCb hvCb hvCb

4.8 7.9 15.7 30.4

0.71 4.93 8.10 15.48

16.6 12.7 11.7 12.3

26.9 27.3 27.2 27.4

na – not applicable. a Inertinite-rich coal with suppressed volatile matter. 80

70

0

Volatile Matter (%, dmmsf)

1

2 3

60

A

4 6

7

8

9

5

B

10

50

C

11 12

40

I

K

E

G

H

13

Q

O P

14

F

J

M

L

5

15

20

16

10

N 14

17

10

0 16500

0

D

30

18 19 20

25

16000

15500

15000

14500

14000

13500

13000

12500

12000

11500

11000

10500

10000

Calorific Value (Btu/lb, dmmsf) Fig. 1. Suggate rank plot of coal samples.

2. Methodology

Australian coal industry. Since 1979 this technique has been used to produce a propensity rating index parameter known as R70 self-heating rate (Humphreys et al., 1981; Beamish and Beamish, 2011). It is the only test method where the index parameter is obtained in the low temperature self-heating region of the coal spontaneous combustion process where non-Arrhenius kinetics dominates (Arisoy and Beamish,

2.1. Adiabatic oven testing methods Of the many spontaneous combustion testing methods available, adiabatic oven self-heating testing has been the preferred option for the 2

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• Starting the test at a temperature equivalent to the mine ambient condition; • Using a larger sample mass of approximately 200 g; and • Using a low oxygen flow rate to create a high mass to flow ratio.

Table 2 Forms of sulphur (%, air-dried basis) present in pyritic coal samples. Sample

Pyritic Sulphur

Sulphate Sulphur

Organic Sulphur

Coal Coal Coal Coal

0.06 1.70 4.61 13.23

0.09 0.13 0.07 0.20

0.47 2.55 2.67 0.67

Q R S T

The results for all the coal samples in this study have been obtained using this methodology over a period of almost 10 years since the development of this new test procedure. Wang et al. (2018) have also recently used this new method to study rewetting effects on the spontaneous combustion behaviour of low rank coals. It should be noted that several refinements to the test procedure have been developed commercially since 2012 to produce an acceptable benchmark scale for the Coal Mining Industry and provide direct comparison with mine site coal self-heating performance. This includes allowing the coal to reach higher temperatures to assess any variations that occur once the thermal runaway region is reached. A large database of examples now exists for coals from all Australian coal mining regions and many countries from around the world. The new test procedure has been adopted as part of interactive spontaneous combustion hazard assessment and management at Meandu Mine (Beamish et al., 2018) and since then it has been routinely used across the Australian Coal Mining Industry. It is now referred to as the Incubation test method.

2015a). The R70 test has been used to study the effects of rank (Beamish et al., 2001; Beamish, 2005; Beamish and Beamish, 2011), mineral matter (Beamish and Blazak, 2005; Beamish and Arisoy, 2008), moisture (Beamish and Hamilton, 2005) and ageing (Beamish et al., 2000; Devlin et al., 2009; Beamish et al., 2019) on coal self-heating rates. The R70 test is conducted on a dry basis as described by Beamish et al. (2000). Drying the sample at high temperature under nitrogen (110 °C) has been perceived as an issue in terms of the possibility of leading to irreversible alteration of the coal properties (Kaminsky et al., 2017). Since all samples are prepared for testing in the same standard manner, the relative self-heating rate index is still applicable for hazard assessment. Nevertheless, to overcome this issue, and obtain a better understanding of the self-heating incubation behaviour of the coal with time, a modification to the R70 test procedure was developed by Beamish and Beamish (2010) and described as a Moist Adiabatic Benchmark (MAB) test method. This effectively consisted of:

2.2. Coal samples

• Testing the coal with its as-received moisture content;

`Analytical data and key test parameters for all of the samples studied are contained in Table 1. They cover the rank spectrum from

Fig. 2. Images of Coal T: a) polished block under oil immersion showing finely layered pyrite parallel to coal banding; b) scanning electron microscope image of micron thick pyrite layers; c) photograph of pyrite reaction products after exposure to air leading to coal layer separation and disintegration as a result of volumetric expansion. 3

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100.0

ligA

subB subA hvCb

R70 (oC/h, db)

10.0 hvBb

Increase in mineral matter of pyritic coal

hvAb

1.0 A

B

C

D

Q

R

S

T

E

F

O

G

H

I

J

K

P

L

M

N

mvb

lvb

0.1

Coals Fig. 3. R70 self-heating rate values for coals ranging in rank from lignite to low volatile bituminous.

160

R70 = 28.57C/h, M = 24.0%

R70 = 74.72C/h, M = 34.6%

Temperature (oC)

140

120 100 80

R70 = 99.67C/h, M = 47.2%

60 40 20 0 0

6

12

18

24

30

36

42

48

54

60

66

72

Time (hours) Coal A

Coal B

Coal C

Fig. 4. Adiabatic self-heating incubation test results for Coals A, B and C.

2.3. Adiabatic incubation test results and discussion

lignite to low volatile bituminous, which can be seen in the Suggate rank diagram (Suggate, 2000) shown in Fig. 1. Several of the coals from Australia plot within the low-medium vitrinite coal band. This is indicative of the inertinite-rich nature of these coals (Suggate, 1998). Three of the coals plot above the high vitrinite coal band. Coal A is liptinite-rich, whereas Coals I and K contain per-hydrous vitrinite associated with an elevated volatile matter content (Wilkins et al., 1992). Coals Q, R, S and T are from the same coal deposit and show a range of sulphur contents (Table 2) that is related to the presence of pyrite. The pyrite occurs as bands parallel to the coal banding that are only a few microns thick (Fig. 2a,b). This pyrite form is very reactive to oxygen and the coal samples disintegrate rapidly on exposure to air due to the pyrite oxidation reaction forming hydrated iron sulphate minerals that create substantial volumetric expansion (Fig. 2c). Coals O and P contain significant amounts of seam gas, which is predominantly carbon dioxide for the coals tested. The retained seam gas content of these samples prior to incubation testing is in excess of 5 m3/t.

2.3.1. Moisture moderation and intrinsic reactivity heat balance The R70 self-heating rate value for all of the coals is shown in Fig. 3. There is a non-linear relationship with coal rank. It can be seen that the lignite samples have R70 values two orders of magnitude higher than medium and low volatile bituminous coals and up to one order of magnitude higher than the high volatile bituminous coals. Hence, low rank coals are considered to have a higher spontaneous combustion propensity than high rank coals based on this index parameter rating. However, the R70 test is conducted on a dry basis and is therefore a measure of the intrinsic reactivity of the coal to oxygen in the absence of moisture. Lignite generally has moisture contents in the range of 30–50%, which is an order of magnitude higher than the high rank coals and as such the high moisture content of the coal could be expected to create a moderating effect on the self-heating rate. This feature of coal self-heating is shown in Fig. 4. Coal A is classified as lignite A and has the highest R70 value of all the samples at 99.67 °C/h, and has a moisture content of 47.2%. Initially, the lignite self-heats at a rapid rate consistent with its high 4

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160

R70 = 11.43C/h, M = 17.7%

R70 = 14.61C/h, M = 16.7%

Temperature (oC)

140

120 100 80

R70 = 6.34C/h, M = 12.0%

60 40 20 0 0

10

20

30

40

50

60

70

80

90

100

100

110

Time (hours) Coal D

Coal E

Coal F

Fig. 5. Adiabatic self-heating incubation test results for Coals D, E and F.

160 R70 = 6.34 oC/h

Temperature (oC)

140

120 100 80 60 40 20 0 0

10

20

30

40

50

60

70

80

90

Time (hours) 12.0% moisture

9.3% moisture

0% moisture

Fig. 6. Adiabatic self-heating incubation test results for Coal F in varying moisture states.

brief period self-heating acceleration resumes and the coal rapidly approaches thermal runaway. Consequently from a hazard likelihood perspective it is the sub-bituminous coal that poses the greater risk in terms of hazard management. As the coal rank increases the moisture content of the coal decreases. Fig. 5 shows the incubation behaviour of Coal D (sub-bituminous A) and Coals E and F (high volatile bituminous C). Coal E develops a minor moisture shoulder at 90 °C but reaches thermal runaway before the lower rank Coal D. This is because the mine start temperature condition for Coal E is higher than Coal D and consequently the initial self-heating rate due to the coal oxidation reaction is higher. Coal F selfheats to a maximum temperature of 64 °C before the heat loss from moisture liberation and evaporation dominates the self-heating process and subsequently the coal temperature decreases. Coal F was retested with 2.7% moisture removed from the sample under nitrogen at low temperature. This was sufficient to tip the moisture moderation/intrinsic reactivity balance in favour of heat gain and the coal was able to progress to thermal runway in a short period of time (Fig. 6). Arisoy et al. (2017) provide a numerical modelling explanation of this moisture moderation effect.

intrinsic reactivity, but once the temperature reaches 93 °C (Fig. 4), the sample begins to lose heat due to moisture liberation and evaporative cooling from the moisture removal process. The sample temperature continues to decrease over time due to this heat loss mechanism and no thermal runway is achieved. Further examples of this effect are presented by Beamish et al. (2019). They also show that as the lignite ages on exposure to air, the moisture moderation/intrinsic reactivity heat balance changes, such that thermal runaway is possible if the lignite is rehandled to create newly exposed coal particle surfaces. Coal B, which is also a lignite has a similar initial self-heating rate behaviour to Coal A, but it also develops a moisture shoulder and begins to lose heat for a significant period of time after reaching 90 °C (Fig. 4). However, a point is reached where the coal dries out sufficiently for fresh reactive sites to become available for oxidation to take place. The heat balance then tips in favour of accelerated self-heating to thermal runaway. Coal C is a sub-bituminous B coal with a high intrinsic reactivity and high moisture content. The initial self-heating of this coal is quite rapid, but once the coal reaches a temperature of 80 °C there is a decrease in the self-heating rate as moisture is liberated and evaporated. After a 5

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160

R70 = 4.10C/h, M = 9.2%

R70 = 4.19C/h, M = 5.0%

R70 = 5.87C/h, M = 11.7%

Temperature (oC)

140

120 100 80 60 R70 = 3.94C/h, M = 11.0%

40 20 0 0

10

20

30

40

50

60

70

80

90

100

Time (hours) Coal G

Coal H

Coal I

Coal J (27.2C)

Coal J (35.3C)

Fig. 7. Adiabatic self-heating incubation test results for Coals G, H, I and J.

160

R70 = 3.18C/h, M = 3.0%

Temperature (oC)

140

120 100 80 60 R70 = 0.22C/h, M = 3.0%

40 R70 = 1.95C/h, M = 4.7%

R70 = 0.37C/h, M = 3.0%

20 0 0

10

20

30

40

50

60

70

80

90

100

110

Time (hours) Coal K

Coal L

Coal M

Coal N

Fig. 8. Adiabatic self-heating incubation test results for Coals K, L, M and N.

27 °C. Retesting the sample from a start temperature of 35.3 °C, which replicates the ambient conditions in summer for the surface stockpiles of this coal, shows that the moisture moderation/intrinsic reactivity balance still favours heat loss despite the elevated temperature condition. Fig. 8 shows the incubation behaviour of four high rank coals. The two high volatile A bituminous coals show contrasting behaviours. Coal K reaches thermal runaway after a considerable period of time. There is no moisture shoulder present due to the low moisture content of the coal. Coal L initially self-heats to a maximum temperature of 36 °C, but due to the high moisture content of the coal it begins to lose heat due to the moisture liberation and evaporation effect. The low intrinsic values of Coals M and N are insufficient to overcome the moderating effects of the moisture content present in these coals. Consequently, no thermal runaway is recorded. Spontaneous combustion events have been recorded for low intrinsic reactivity coals (Cliff et al., 2014). In each case the coal temperature was raised artificially above ambient temperature by an external heat source. For example, curing compounds such as polyurethane resin can produce an exothermic reaction, which if in

Fig. 7 shows the incubation behaviour of four high volatile B bituminous coals. Coal G develops a substantial moisture shoulder once the coal temperature reaches 80 °C. After approximately another 24 h testing the self-heating rate begins to accelerate and thermal runaway soon follows. The shape of this self-heating incubation curve is very similar to that obtained from numerical models (Arisoy and Akgun, 1994; Monazam et al., 1998), which show prolonged incubation until the coal becomes dry locally before access to new reactive sites produces additional heat to continue the self-heating process to thermal runaway. Coal H does not develop a moisture shoulder due to the low moisture content of 5% and consequently it gradually progresses to thermal runaway. In contrast, Coal I has a very similar intrinsic reactivity to Coal H, but it has a moisture content of 9.2% and therefore produces a small moisture shoulder at approximately 100 °C before progressing to thermal runaway. Coal J has a lower intrinsic reactivity than the other three coals, but it also has a moisture content of 11.0%, which in this case is sufficient to tip the moisture moderation/intrinsic reactivity balance in favour of heat loss similar to Coal F shown in Fig. 5. The underground mine ambient temperature is approximately 6

International Journal of Coal Geology 215 (2019) 103297

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R70 (oC/h, db)

100

TR

10

NTR

1

0 0

5

10

15

20

25

30

35

40

45

50

Moisture content (%, ar) ligA

subB/A

hvCb

hvBb

hvAb

mvb

lvb

Fig. 9. Relationship between coal moisture content and R70 self-heating rate showing the boundary between thermal runaway (TR) and no thermal runaway (NTR).

160

Temperature (oC)

140

120 100 80 60 40 20 0 0

10

20

30

40

50

60

70

80

90

100

110

120

130

Time (hours) 20C

35C

Fig. 10. Adiabatic self-heating incubation test results for Coal I at different start temperatures representative of the mine conditions in summer and winter.

heating incubation behaviour replicate samples of the coal were tested at two different start temperatures. In summer the normal mine temperature is approximately 35 °C, whereas in winter the temperature is approximately 20 °C. This has a major impact on the incubation period to thermal runaway as shown in Fig. 10. It takes almost three times longer in winter for the coal to reach thermal runaway, which has been the experience at this mine site. These laboratory results are consistent with numerical model results obtained by Schmal et al. (1985) using a moist coal model. They are also similar to experimental data obtained by Li and Skinner (1986). Meandu Mine in Queensland takes this temperature difference into consideration as part of their spontaneous combustion management planning. The mine implements additional trigger levels in summer and also uses an interactive spontaneous combustion hazard assessment and management practice (Beamish et al., 2018) to ensure no spontaneous combustion events occur at the mine. Leading into and during the summer months, strip samples are systematically collected from each new strip as it is exposed and these are supplied to the laboratory for incubation testing the same day. The results obtained are benchmarked against the mine site performance of a previous spontaneous

contact with the coal produces an increase in the reaction rate to the point of altering the moisture moderation/intrinsic reactivity balance in favour of heat gain (Cliff et al., 2009). Under these circumstances, depending on the temperature achieved by the curing compound, the coal can then incubate to thermal runaway in a short timeframe. A summary of the moisture moderation/intrinsic reactivity balance that controls whether there is thermal runaway (TR) or no thermal runaway (NTR) is contained in Fig. 9 for the coals ranging in rank from lignite to low volatile bituminous. Boundary lines have been drawn where the difference between the two outcomes is distinct; however, there is clearly a transition zone where the outcome can only be determined from testing of the coal. This zone may be a function of both chemical and physical properties of the samples and highlights how sensitive the heat balance is for coals that fit within this region. On a site basis this boundary may be more clearly defined. 2.4. Initial start temperature effect on incubation period The mine temperature conditions for Coal I can vary significantly between winter and summer. To show the impact this has on the self7

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160 140

Temperature (oC)

R70 = 3.17C/h, M = 5.3%

R70 = 5.95C/h, M = 7.3%

120 100 80 60 40 20 0 0

50

100

150

200

250

300

350

400

450

500

Time (hours) Coal O

Coal P

Fig. 11. Adiabatic self-heating incubation test results for Coals O and P containing carbon dioxide seam gas.

160

R70 = 5.88C/h, M = 12.3%

R70 = 7.11C/h, M = 11.7%

Temperature (oC)

140

R70 = 12.33C/h, M = 16.6%

R70 = 7.20C/h, M = 12.7%

120 100 80 60 40 20 0 0

5

10

15

20

25

30

35

40

Time (hours) 13.23% Spy

4.61% Spy

1.70% Spy

0.06% Spy

Fig. 12. Adiabatic self-heating incubation test results for Coals Q, R, S and T containing varying amounts of reactive pyrite.

combustion event and the new information is relayed to all relevant mining personnel so that the coal is handled according to its incubation behaviour.

spontaneous combustion as reactive sites become more readily available and the moderating heat loss created by the presence of moisture in the coal decreases.

2.5. Seam gas inhibition of coal self-heating rate

2.6. Mutual effects of reactive pyrite and moisture on self-heating acceleration

Many underground coal mines have gassy coals present. The seam gas in the coal acts as a natural inhibitor to the coal oxidation process as access to reactive sites is controlled by the gas desorbing from the coal to free up the sites. In addition, the gas desorption process is endothermic. This can lead to prolonged incubation as shown by the two examples in Fig. 11. Both Coals O and P show an initial rapid selfheating rate, but then the coal temperature gradually decreases as the seam gas desorption process takes place. It should be noted that moisture is also removed from the coal as the seam gas desorbs from coal pore internal surfaces. Eventually, the heat from coal oxidation at freshly exposed reactive sites begins to dominate the heat balance and the coal finally self-heats to thermal runaway. These results also imply that the use of seam gas drainage, which removes both water and gas from the coal, can contribute to increasing the likelihood of

The US Bureau of Mines conducted an investigation into the cause of floor self-heatings in an underground coal mine operating in a high rank low volatile bituminous coal seam and found that pyrite oxidation was the primary cause of the heatings (Miron et al., 1992). One of the main conclusions of the investigation was that there was a need to amend current spontaneous combustion testing methods to fully assess the effect of pyrite on the self-heating process. Correcting this deficiency in testing methods has not been possible until the development of the adiabatic incubation test method. More recently, Beamish and Theiler (2016) have shown that this modified test method can also be used to assess the self-heating behaviour of reactive pyrite in black shale waste rock from a metalliferous mine. Arisoy and Beamish (2015b) showed that the presence of reactive 8

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runaway. These examples of coal self-heating incubation behaviour as recorded by adiabatic oven testing help to provide a more informed evaluation of the spontaneous combustion hazard likelihood that can be used to develop an appropriate spontaneous combustion management plan. This same incubation testing approach can be used for managing coal stockpiles, coal overburden spoilpiles and coal shipping certification. The results of this work also highlights spontaneous combustion laboratory tests that produce results in a very short timeframe miss vital information about the coal self-heating behaviour over time, particularly in the low ambient temperature region.

pyrite in coal can create a mutual self-heating acceleration, since the pyrite uses the moisture to react and generate heat, which reduces heat loss from moisture evaporation. This is demonstrated in Fig. 12 for a high volatile bituminous coal seam where successively higher contents of reactive pyrite help to create shorter incubation periods. The R70 selfheating rates for the high pyritic content coals are lower than the nonpyritic coal due to the absence of moisture in the R70 test and hence no pyrite oxidation reaction can occur. Consequently, the pyrite mineral matter only acts as a diluent and heat sink under the R70 test conditions. It should be noted that the pyrite needs to be in an appropriate form (size and morphology) to enhance its oxidation potential (Miron et al., 1992; Beamish and Theiler, 2017b). In this case the micron thick layered pyrite (Fig. 2) is composed of submicron pyrite crystals that provide substantial surface area for the pyrite reaction to take place. There is also ample moisture present in these coal samples to take part in the pyrite oxidation reaction. It can also be seen in Fig. 12 that the moisture shoulder becomes progressively less pronounced as the reactive pyrite concentration increases due to the moisture removal by the pyrite oxidation reaction. Experiments conducted by adding pyrite from ore samples to coal samples, to obtain various pyrite concentration mixes (Deng et al., 2015), are invalid as they do not record the mutual self-heating acceleration that results from the presence of natural reactive pyrite, which has formed with the coal. In addition, often the ore pyrite is unreactive in terms of the rate of reaction since the crystal size is not sufficiently small enough to provide the surface area for reaction to take place at an appreciable rate.

Declaration of Competing Interest The authors state that there are no interests to declare in the submission of this research article. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.coal.2019.103297. References Arisoy, A., Akgun, F., 1994. Modelling of spontaneous combustion of coal with moisture content included. Fuel 73, 281–286. Arisoy, A., Beamish, B., 2015a. Reaction kinetics of coal oxidation at low temperatures. Fuel 159, 412–417. Arisoy, A., Beamish, B., 2015b. Mutual effects of pyrite and moisture on coal self-heating rates and reaction rate data for pyrite oxidation. Fuel 139, 107–114. Arisoy, A., Beamish, B., Yoruk, B., 2017. Moisture moderation during coal self-heating. Fuel 210, 352–358. Banerjee, S.C., 1985. Spontaneous Combustion of Coal and Mine Fires. Balkema, Rotterdam, pp. 168. Beamish, B.B., 2005. Comparison of the R70 self-heating rate of New Zealand and Australian coals to Suggate rank parameter. Int. J. Coal Geol. 64, 139–144. Beamish, B.B., Arisoy, A., 2008. Effect of mineral matter on coal self-heating rate. Fuel 87, 125–130. Beamish, B., Beamish, R., 2010. Benchmarking moist coal adiabatic oven testing. In: Proceedings 10th Coal Operators’ Conference University of Wollongong and the Australasian Institute of Mining and Metallurgy, pp. 264–268. Beamish, B., Beamish, R., 2011. Testing and sampling requirements for input to spontaneous combustion risk assessment. In: Proceedings of the Australian Mine Ventilation Conference the Australasian Institute of Mining and Metallurgy Melbourne, pp. 15–21. Beamish, B.B., Blazak, D.G., 2005. Relationship between ash content and R70 self-heating rate of Callide coal. Int. J. Coal Geol. 64, 126–132. Beamish, B.B., Hamilton, G.R., 2005. Effect of moisture content on the R70 self-heating rate of Callide coal. Int. J. Coal Geol. 64, 133–138. Beamish, B., Theiler, J., 2016. Characterising the spontaneous combustion propensity of waste rock. In: Proceedings of Life of Mine Conference 2016 the Australasian Institute of Mining and Metallurgy Melbourne, pp. 93–96. Beamish, B.B., Theiler, J., 2017a. Recognising the deficiencies of current spontaneous combustion propensity index parameters. In: Proceedings of the Australian Mine Ventilation Conference the Australasian Institute of Mining and Metallurgy Melbourne, pp. 113–117. Beamish, B., Theiler, J., 2017b. Assessing the reactivity of pyrite. In: Proceedings 17th Coal Operators’ Conference University of Wollongong, pp. 391–394. Beamish, B.B., Barakat, M.A., St George, J.D., 2000. Adiabatic testing procedures for determining the self-heating propensity of coal and sample ageing effects. Thermochim. Acta 362, 79–87. Beamish, B.B., Barakat, M.A., St George, J.D., 2001. Spontaneous-combustion propensity of New Zealand coals under adiabatic conditions. Int. J. Coal Geol. 45, 217–224. Beamish, B., Edwards, D., Theiler, J., 2018. Implementation of interactive spontaneous combustion hazard assessment and management at Meandu Mine. In: Proceedings 18th Coal Operators’ Conference University of Wollongong, pp. 329–335. Beamish, B., Theiler, J., Garvie, A., 2019. Ageing effects on the self-heating incubation behaviour of lignite. In: Proceedings 2019 Coal Operators’ Conference University of Wollongong, pp. 260–264. Cliff, D., Beamish, B., Cuddihy, P., 2009. Explosions, fires and spontaneous combustion. In: In Monograph. 12. Australasian Coal Mining Practice – Third Edition, The Australasian Institute of Mining and Metallurgy Melbourne, pp. 421–435. Cliff, D., Brady, D., Watkinson, M., 2014. Developments in the management of spontaneous combustion in Australian underground coal mines. In: Proceedings 14th Coal Operators’ Conference University of Wollongong and the Australasian Institute of Mining and Metallurgy, pp. 330–338. Deng, J., Ma, X., Zhang, Y., Li, Y., Zhu, W., 2015. Effects of pyrite on the spontaneous combustion of coal. Int. J. Coal Sci. Technol. https://doi.org/10.1007/s40789-015-

3. Summary and conclusions The majority of test methods for assessing the coal spontaneous combustion hazard do not capture the full self-heating behaviour of the coal under normal mine conditions. Primarily, these laboratory tests provide at best an indication of the intrinsic reactivity of the coal, although many of them are performed at high temperatures in the thermal runaway region and are more indicative of the likely ignition behaviour. Consequently, factors that modify the self-heating rate of the coal at mine ambient temperatures are not taken into consideration and one of the key elements namely time is not measured by the testing. This is quite important for appropriate management of a heating event, as knowing the status of a heating can be crucial to making the correct decisions for adopting control measures (Newham et al., 2016). Coal spontaneous combustion is best described as an incubation process over time, where competing factors can either accelerate or decelerate the self-heating rate of the coal. Moisture present in the coal moderates the self-heating rate due to its liberation and evaporative heat loss. This leads to prolonged incubation until the coal becomes dry locally and access to new reactive sites produces additional heat to continue the self-heating process to thermal runaway. Low rank, high moisture coals show this type of spontaneous combustion behaviour. Conversely, reactive pyrite present in the coal accelerates the selfheating rate due to the additional heat from the pyrite reaction and the mutual effect of reducing the amount of moisture available to produce evaporative heat loss (Arisoy and Beamish, 2015b). This can only occur if the coal contains pyrite in a form that enhances its oxidation potential and that there is sufficient moisture available for the pyrite reaction to take place. The initial start temperature condition of the coal affects the incubation period for spontaneous combustion. The lower the start temperature the longer the coal takes to reach thermal runaway. Presence of seam gas in the coal can also act as a natural inhibitor to self-heating. In this case, oxygen access to reactive sites is governed by the desorption rate of the seam gas from the coal, which can take a considerable amount of time. The desorption process is also endothermic, thus creating a heat loss mechanism. Once fresh reactive sites become available though, self-heating can continue at a faster rate to thermal 9

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