- Email: [email protected]
A new apparatus to establish the spontaneous combustion propensity of coals and coal-shales
International Journal of Mining Science and Technology 28 (2018) 649–655
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
A new apparatus to establish the spontaneous combustion propensity of coals and coal-shales M. Onifade, B. Genc ⇑, A. Carpede School of Mining Engineering, University of the Witwatersrand, Johannesburg 2050, South Africa
a r t i c l e
i n f o
Article history: Received 17 November 2017 Received in revised form 7 March 2018 Accepted 15 May 2018 Available online 17 May 2018 Keywords: Coal Coal-shale Spontaneous combustion Wits-Ehac index Wits-CT index
a b s t r a c t Coal and coal-shale both tend to undergo spontaneous combustion under favourable atmospheric conditions. The Wits-Ehac index has been developed in South Africa since the late 1980’s to test the spontaneous combustion liability of coal. However, in some cases, the Wits-Ehac index fails to produce tangible results when testing coal-shales. To overcome this problem, a new apparatus has been developed to test carbonaceous materials such as coal and coal-shale under chemical reactions with oxygen and an index has been obtained. This index is called the Wits-CT index. The equipment emulates the influence of oxygen adsorption on carbonaceous material for a period of 24 h without a heating system. The Wits-CT index uses the total carbon content of the sample and the temperature variations obtained from the samples during reaction with oxygen to predict the spontaneous combustion liability. Eighteen samples have been analyzed using both indices and the results are in-line. It was found that coals and coal-shales with higher values of the Wits-CT index are more liable to spontaneous combustion. Further research on different coal-shales is underway in order to establish an extensive database for coal and coal-shales, together with known incidences of self-heating. Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction The occurrence of spontaneous combustion is one of the major challenges faced by the South African coal mining sector. The physical and chemical reactions between oxygen and external active structures of coal particles which releases heat are behind the event of spontaneous combustion. These incidents occur naturally due to the chemical reactions and oxidation of coal and coal-shales. The spontaneous combustion characteristics of coal may be influenced by the natural convection within a coal seam, spoil heaps and other mining piles [1]. Sasaki and Sugai studied the effects of chemical and oxidation reactions to measure the characteristics of temperature distribution within a stockpile [2]. Premature combustion temperatures at different points in a coal reaction vessel were reported by Gray and Lee [3]. The spontaneous heating of coal arises under definite atmospheric conditions. Coal oxidation occurs in an environment where there is sufficient oxygen. The oxidation rate of coal cannot be easily detected due to the reaction being very slow. In an open cast
⇑ Corresponding author. E-mail addresses: [email protected] (M. Onifade), Bekir.Genc@wits. ac.za (B. Genc), [email protected] (A. Carpede).
mining, the ingress of oxygen in the air into discontinuities and exposure of the highwall for long periods are among the factors affecting spontaneous heating. In underground coal mines, the combination of an insufficient ventilation system and selfheating areas of coal can cause spontaneous combustion. It is known that coal, roof shales, mining piles and other carbonaceous materials become self-heated and liberate heat naturally when subjected to atmospheric conditions for a long time as shown in Figs. 1 and 2. The heat generated may become faster than the heat dissipated to the surrounding. The experience of spontaneous combustion has approached into a phase that makes practical illustrations feasible as a result of the re-established awareness of self-heating of coal and coal-shales in coal mines. Past researcher’s efforts to measure the liability of coal stockpiles were primarily based on laboratory tests of one or more of the coal characteristics associated with the self-heating behavior. A limited number of medium-large scale tests on coal have been carried out in the past. However, such tests have not been conducted frequently due to the high cost and long period of time required and the difficulties in interpretation of the results. Experimental investigations on spontaneous combustion of bulk coal samples have been carried out under a medium-large scale test with a heating system used to initiate the self-heating process [4–10]. Studies on spontaneous combustion of coal
https://doi.org/10.1016/j.ijmst.2018.05.012 2095-2686/Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
650
M. Onifade et al. / International Journal of Mining Science and Technology 28 (2018) 649–655
of spontaneous combustion motivates this new apparatus. This current work presents the results of self-heating tests carried out on highly reactive coal and coal-shales in South African coal mines using the Wits-Ehac test and also the new apparatus (Wits-CT). Eighteen carbonaceous materials were tested with the new apparatus. The method is reliable, accurate and provides the required information on the effects of oxygen on self-heating of coals and coal-shales under field conditions.
Fig. 1. Self-heating of inseam shale and burning coal seam in Witbank, South Africa.
2. Materials and methods 2.1. Sample collection The coals and coal-shales used for this research were collected from two different coal mines in the eMalahleni area of South Africa and kept in airtight bags to avoid oxidation. Twenty representative in situ coal and coal-shales samples were obtained from the affected areas (highwalls and overburden shales). The samples subjected to the Wits-Ehac tests and tests in the newly developed apparatus.
Fig. 2. Burning spoil heaps at Tweefointein Mine, Witbank, South Africa.
stockpiles under the influence of atmospheric conditions have been reported [11–15]. However, no previous studies reported experimental methodologies to emulate the effects of atmospheric conditions on coal spontaneous combustion in the laboratory without an applied heating system. An acceptable standard widely used in South Africa to predict the propensity of coal to spontaneous combustion, known as the Wits-Ehac index has been in existence and used for more than 30 years [16–18]. The test involved the use of a relatively small amount of pulverized, dry samples to determine liability of coal to spontaneous combustion. The performed laboratory small-scale studies carried out in degrees and the influence of limiting factors can only be evaluated only under the available experimental conditions, unlike the medium-large scale test that considered the self-heating of a substantial coal mass under atmospheric conditions. Spontaneous combustion is dependent on the composition of the material, air temperature, sample temperature and mine environment. It has been indicated that the ignition temperature tests are at high temperatures where an external source of heat has been applied at the surface of the coal particles [16,17]. The effects of oxygen, moisture and temperature variations within a coal mass under sluggish airflow conditions, i.e., the most likely scenario at a mine, cannot be integrated into the small-scale test. The self-heating characteristics of some coal-shales in this study could not be determined by the Wits-Ehac index because of their low reactivity under the available experimental conditions. This motivated the need to develop a device that can measure the self-heating of coal, coal-shales and other carbonaceous materials under the influence of oxygen, which can produce similar results compared to the existing Wits-Ehac index. The use of realistic medium experimental scale tests that imitate the influence of atmospheric conditions on the various coal, coal-shale types to predict self-heating is important. The principal argument against laboratory testing in predicting the tendency of coal to spontaneous heating is that the process is unlike the natural conditions under which self-heat takes place. On the basis of these criteria, a medium scale laboratory apparatus has been developed to model and predict the occurrence of spontaneous combustion in coal mines. The model considers number of parameters such as airflow rate and temperature variation. The need for a reliable method to capture the large volume of data used to predict the occurrence
2.2. Sample preparation and characterisation The samples were reduced using a crusher and ball mill to suitable specify sizes as required for each test. The determinations of the moisture, ash, volatile matter and fixed carbon contents were carried out according to the American Society for Testing and Material (ASTM) standards [19–21]. Fixed carbon was obtained by subtracting the sum of the percentage of volatile matter, ash and moisture from 100. The carbon content were determined using a LECO TruSpec CHNS analyser (Fig. 3) after calibration with sulfamethazine based on the International Standard Organisation Standards [22]. The results were given in weight percent of airdried (%, by weight, ad). The results for proximate and total carbon content and spontaneous combustion tests carried out on the coal and coal-shale samples are given in Tables 1 and 2, respectively. 2.3. Wits-Ehac test The Wits-Ehac tests at the School of Mining, University of the Witwatersrand is a small-scale test in which freshly pulverized (<212 mm) and dried coal samples of 20–25 g in weight is used. The Wits-Ehac index has been developed in South Africa in the late
Fig. 3. Leco TruSpec CHNS analyser.
Table 1 Risk rating classification for coal, coal-shales and other carbonaceous materials to spontaneous combustion. Wits-CT Index
Risk rating
<2.5 2.5–5 5–7.5 >7.5
Less reactive Moderately reactive Reactive Highly reactive
651
M. Onifade et al. / International Journal of Mining Science and Technology 28 (2018) 649–655 Table 2 Results of proximate analysis, carbon content obtained from ultimate analysis and spontaneous combustion tests and weight gain for coal. Sample
Mad (%)
Aad (%)
VMad (%)
FC (%)
Cad (%)
Wits-Ehac index
Wits-CT index
Weight gain (g)
CA CC CE CG CH CI CK CL CM CN
2.3 2.2 2.3 2.5 2.4 2.1 1.6 1.6 1.6 1.6
28.0 33.8 28.6 16.8 18.8 48.8 13.7 22.5 17.0 17.0
23.2 24.1 24.3 20.0 26.9 16.7 22.1 26.1 22.0 23.9
46.5 39.9 51.7 60.7 51.9 32.8 62.6 49.8 59.4 57.5
54.4 47.5 53.6 66.0 65.2 36.1 69.7 58.9 66.7 65.8
4.64 4.52 4.76 4.91 4.69 3.82 4.44 4.87 4.76 4.84
6.29 5.31 5.42 7.53 7.51 4.05 9.10 9.59 7.27 7.91
32.9 32.1 32.6 38.7 37.2 30.2 42.5 45.2 36.1 40.7
1980’s to test the spontaneous combustion liability of coal [16–18]. The apparatus consists of an oil bath, six coal and inert material cell assemblies, an oil circular, a heater, a flowmeter, an air supply compressor and a computer. The test apparatus is used to test coals under predefined conditions and a combustibility index is obtained by Wade et al., as shown in Fig. 4 [23]. The system incorporates the determined crossing point temperatures and differential thermal analysis from selected coal samples with periodic measurement of temperatures to obtain a consistent self-heating liability index called the Wits-Ehac index. The temperatures are recorded every 30 s by the computer over an average of 4 h of testing for the oil in the bath to be heated to 200° Celsius. 2.4. New apparatus (Wits-CT) 2.4.1. Wits-CT experimental setup An apparatus to predict the spontaneous combustion liability of carbonaceous materials such as coal and coal-shale under the influence of airflow without any heating system was recently developed in the School of Mining Engineering, University of the Witwatersrand, referred to as the Wits-CT test. Most coals undergo spontaneous combustion under atmospheric conditions, i.e., when
Fig. 4. Wits-Ehac apparatus setup [23]
exposed to oxygen in the air. The equipment developed is based on this fact and it emulates the influence of oxygen adsorption on carbonaceous material in a closed system. Therefore, the equipment does not involve the heating of carbonaceous material to a high temperature like the adiabatic method (such as the R70, WitsEhac test and so on). The equipment prevents heat loss that may occur under testing condition by ensuring the lid is fixed and fastened, and we also placed a glass mineral wool insulator inside the equipment in order to arrest any heat loss. The insulating material was selected due to the temperature that may be generated during testing and the mode of heat transfer that may be involved. However, as no heating system is applied to the equipment to increase the temperature of the sample, the equipment is not expected to generate a high temperature. The equipment determines the temperature variation within and on the surface of the coal particles with six temperature sensors under sluggish airflow conditions (oxygen) for 24 h. This scenario is related to the atmospheric condition at the mines. The influence of oxygen absorption on the sample leading to the temperature variations during the start and the end of the test is recorded in a computer via a data logger as shown in Fig. 5. The temperatures recorded and the total carbon content of the sample are used to establish a spontaneous combustion liability index, referred to as the Wits-CT index. The apparatus has a capacity to accept a 15 kg sample, depending on the packing density. It oxidizes materials at controlled air pressure and constant flow rate. Three uniformly separated temperature sensors placed along the length of the autoclave are used to check and record the temperatures measurement. The sensor arrangement is typically located at different points within the autoclave (locations A, B, and C within the interior of the container). Each of the three thermocouples has two temperature sensors which measure temperatures at different levels at the locations A, B, and C in order to keep a recording of the temperature distribution during the reaction of a carbonaceous material and oxygen. At location A, sensors are located at levels 13 and 23 cm
Fig. 5. Wits-CT experimental setup and apparatus.
652
M. Onifade et al. / International Journal of Mining Science and Technology 28 (2018) 649–655
within the container, location B, 53 and 63 cm, and location C, 33 and 43 cm. The device comprises an upright mounted cylindrical tube of 0.75 m long and with an internal diameter of 160 mm. It is made of 5 mm thick mild steel. An illustration is given in Fig. 5a while Fig. 5b shows the new apparatus setup. Oxygen is supplied and controlled by a fixed flow rate by means of a flowmeter before being fed into a manifold attached below the lid of the autoclave. The connection between the manifold and the autoclave is achieved by means of a spiral copper tube. Oxygen gas is supplied from the lid through the manifold. The equipment is placed on a digitalize scale (Richterscale, Model W113) to measure any changes in weight during testing due to oxygen absorption. The experimental process simulates a well-insulated system relative to actual conditions in situ. The results obtained from this apparatus are in-line with the studies reported by Sensogut and Ozdeniz and Ozdeniz et al., in coal stockpiles [13,15].
during testing. Samples with high Wits-CT index have higher oxygen absorption than those with lower Wits-CT index. The temperature increase for each sample depends on the oxygen consumed. The results obtained from the Wits-CT liability index are in -line with the Wits-Ehac index. The Wits-CT index was developed to measure the spontaneous combustion liability of coals, coal-shales and other carbonaceous materials under condition that emulate atmospheric condition. The apparatus monitors and determines the temperature distributions within a mass of carbonaceous material with the use of thermocouples and imitates the ingress of oxygen in the air. As this was based on tests carried out on only 18 coal and coal-shale samples, to expand the database of the samples investigated, this new liability index was compared with the Wits-Ehac index [16,17]. The formulation of this new index based on the data obtained is discussed below.
2.4.2. Wits-CT testing procedure In order to determine spontaneous combustion liability via selfheating, a representative sample of particle size (<6.39 mm) was weighed and loaded into an autoclave. The particle size used is based on the study reported by Kunni and Levenspiel to determine the surface-volume area from the size distribution [24]. The system is sealed at the base and loaded with a sample from the top. The lid of the system was detached during the loading of the sample. The process was repeated till the autoclave was filled to the marked point. The lid was fixed and fastened as soon as the autoclave was filled with the sample. This technique of testing gives a closely sized sample that would be preferable to measure the rate of the oxidation process. Each temperature probe was inserted into the column as the correct sample level was reached. The position of each temperature probe to the column was placed vertically and spaced uniformly to the center as soon as the sample level was reached. Oxygen was supplied and controlled at a fixed flow rate (20 mL/min) by means of a flowmeter before being fed into a manifold attached below the lid of the autoclave. The logging began and the variation in temperatures distribution of the sensors were stored to a computer every minute. The test period lasts for 24 h. At the end of each experiment, samples were discarded and the autoclave is cleaned. The conditions in which the experimental tests were carried out closely resembled the situation in the mine environment.
Wits-CT index ¼ ðTM =24 þ T R Þ %C ad
3. Results and discussions 3.1. Test results The testing process included complex reactions between coal and coal-shale and oxygen, heat transfer and oxygen transmission in the autoclave. The extent to which the temperature distribution of the samples increases is assumed to be a measure of the sample to spontaneous combustion liability. The oxygen adsorbed by each sample causes an increase in weight and temperature variations
ð1Þ
where TM is the difference between the sum of maximum temperatures of each thermocouple in the autoclave and room temperature (22 °C); TR the difference between the peak temperature and initial temperature during testing in degree Celsius;%Cad the airdried percentage of carbon content of the sample; and 24 the test duration and is constant. Eq. (1) is the formula for obtaining the Wits-CT index of coal, coal-shales and other carbonaceous materials. The index increases with increasing carbon content as shown in Tables 1 and 2. The new Wits-CT index for the tested samples is based on the experience observed in the mines on spontaneous combustion and comparison with the Wits-Ehac index. A new classification of spontaneous combustion liability index for carbonaceous materials according to the new apparatus is shown in Table 3. Coal and coalshales having a higher spontaneous combustion liability are identified as having a higher Wits-CT liability index value and vice versa. The risk rating classification in Table 3 compares the Wits-CT index to what is happening on the mines in terms of the spontaneous combustion liability. This classification provides mine operators a better understanding of the spontaneous combustion liability index of various carbonaceous materials. This index is suitable in rating the liability of a material to spontaneous combustion like the Wits-Ehac index. The fact that different carbonaceous materials have dissimilar spontaneous combustion characteristics makes it essential to classify the materials according to their liability as shown in Table 3. The classification is dependent on two principles: Determination of the carbon content of the material; and Temperature variation due to oxygen absorbed by the material in the equipment. With an improved knowledge and the accumulation of large volume of test data obtained during testing, this study has established a liability index in-line with the experience of spontaneous combustion in the mines.
Table 3 Results of proximate analysis, carbon content obtained from ultimate analysis and spontaneous combustion tests and weight gain for coal-shales. Sample
Mad (%)
Aad (%)
Vad (%)
FC (%)
Cad (%)
Wits-Ehac index
Wits-CT index
Weight gain (g)
SA SE SH SI SK SL SM SN
1.4 1.7 0.8 1.0 1.0 1.0 0.8 1.5
78.5 68.4 88.7 79.6 79.1 74.0 76.9 51.5
11.2 15.9 8.5 11.9 11.7 16.0 11.7 16.6
8.9 14.0 2.0 7.5 8.2 9.0 10.6 30.4
11.5 15.8 2.66 9.12 9.75 10.5 12.5 33.7
3.09 3.73
1.33 1.60 0.27 0.95 1.18 1.34 1.44 3.99
27.5 31.5 15.0 20.1 28.1 28.9 30.5 32.3
2.98 2.99 3.77
M. Onifade et al. / International Journal of Mining Science and Technology 28 (2018) 649–655
3.2. Discussion The new liability index developed used the carbon content of the sample as one of the parameters to establish a formula to measure the spontaneous combustion liability. The carbon content varies between 36.1% to 69.7% for coals and 2.66% to 33.7% for coalshales as seen in Tables 1 and 2. Coal CI has the lowest carbon content and the lowest liability index, while coals with a high carbon content has higher liability index (see Table 3). Coal-shales SN and SE contained the highest carbon content and have the highest liability index compared to other coal-shales as shown in Table 1. It was found that an increase in carbon content indicates coals and coal-shales more liable to spontaneous combustion. Coal CG and coal-shale SN showed the highest liability index based on the Wits-Ehac value, while the Wits-CT indicates that coal CL and coal-shale SN has the highest spontaneous combustion liability index. The two spontaneous combustion liability indices shows a high liability for samples CG, CL and SN respectively. The records of the mine revealed that an incident of spontaneous combustion has been experienced on these samples in the recent time. This indicates that the new liability index can be used to predict the spontaneous combustion liability of coal mines having records of a high spontaneous combustion incidents. It is shown that most of the coal samples displayed high liability index due to their high carbon content compared to the coal-shales with considerably low carbon content. Both samples CI and SJ, with the lowest carbon contents, indicate the lowest liability index. The samples analyzed show that the capacity of a material to undergo spontaneous combustion appears to be directly related to the amount of carbon content. Coals with high carbon content indicate a high liability index while coal-shales which contained more than 15% carbon content show a high liability to spontaneous combustion as shown in Tables 1 and 2. It was found that coal-shales with high carbon and low carbon contents show similar characteristics to coal containing a high and low carbon content with respect to spontaneous combustion. This implies that both coals and coal-shales have similar characteristics in terms of the carbon content. It was found that coals contain a very high amount of carbon than the coal-shales in this study. It might be expected that the rise in temperature would basically increase the reaction rate if the spontaneous combustion liability of carbonaceous materials is associated with the heat generated and heat dissipated from a single process. The carbon content found in the coals and coal-shales varied from one sample to the other; hence, this might indicate that different reactions with different activation energies might take place in the spontaneous combustion process. When the carbon content for the coal-shales was compared with each other, the samples SN and SE are more liable to spontaneous combustion both under WitsEhac tests and the Wits-CT tests. Coal-shales SN and SE have approximately the same liability indices based on the results obtained from the Wits-Ehac and Wit-CT index. For other coal-
653
shales, the relative liability was different under the spontaneous combustion tests conducted. While the Wits-Ehac values for coal-shales SH, SI, and SM could not be determined due to their low reactivity, Wits-CT index successfully obtained liability indices for the same samples. The newly established Wits-CT index values varies between 4.05–9.59 for coals and 0.27–3.99 for coal-shales. Coal CL has the highest Wits-CT values, while coal CI has the lowest Wits-CT values. Coal-shale SH has the lowest Wits-CT value and coal-shale SN has the highest Wits-CT value among the coal-shales. The weight gained due to oxygen adsorbed by each sample increases with increasing Wits-CT index for the coal and coal-shale samples. The coals with a higher Wits-CT liability index have a higher weight gain than those with low weight gain. This is the same with the coal-shales. 3.3. Relationship between weight gain and spontaneous combustion liability The weight increase recorded during the experimental test can be related directly to the spontaneous combustion liability. The weight gained by each sample during the reaction with oxygen increases with the increasing Wits-CT index for the coals and coal-shales as shown in Fig. 6. Samples with a high Wits-CT index have a high oxygen absorption than those with a lower Wits-CT index. The study shows that the temperature variation for the samples depends on their characteristics to absorb oxygen. The purpose of the measured of weight enables the determination of how much oxygen has been absorbed by coals or coal shales under experimental conditions and their relationship with spontaneous combustion lability. The influence of oxygen absorption on the samples indicated that samples with a high oxygen absorption being confirmed by the scale are more liable to spontaneous combustion. Findings show that the samples increase in weight after part of it absorbed oxygen. This may likely be the same when freshly mined coal is exposed to oxygen in air. Figs. 6–10 show that coal and coal-shales having high weight gain, high carbon content, high Wits-CT index values and high Wits-Ehac index are more liable to spontaneous combustion. This shows that the results of the Wits-CT index are in-line with
Fig. 7. Relationship between coal Cad content and Wits-CT index.
Fig. 6. Relationships between weight gained and Wits-CT index for coals and coal-shales.
654
M. Onifade et al. / International Journal of Mining Science and Technology 28 (2018) 649–655
Fig. 8. Relationship between coal Wits-Ehac index and Wits-CT index.
Fig. 9. Relationship between coal-shales Cad content and Wits-CT index.
able to evaluate the liability index of carbonaceous shales that could not be determined by the WITS-Ehac index. A new classification for liability index of carbonaceous materials to spontaneous combustion has been established in this research. It was observed that coals and coal-shales with a high carbon content are more liable to spontaneous combustion. Therefore, the influence of the carbon content present in coals and coal-shales played a significant role in evaluating the incidents of spontaneous combustion. This experimental procedure might allow a high accuracy in predicting spontaneous combustion liability and may be used to establish a database of temperature measurements over an interval of time. Furthermore, this study establishes a relationship between coal and coal-shales in respect to spontaneous combustion liability. This may provide the industry with a better understanding on the awareness of the spontaneous combustion of coal and coal-shales. Simplified comprehensive procedures on the use of the equipment is made available in the School of Mining Engineering Spontaneous Combustion Research Laboratory for both skilled and semi-skilled labour. Acknowledgments This work was conducted in the context of coal-shale spontaneous combustion in the eMalahleni coalfields, South Africa was financially sponsored by Coaltech. The authors wish to express gratitude to Coaltech and the staff of the selected coal mines for their support. The work presented here is part of a PhD research study in the School of Mining Engineering at the University of the Witwatersrand. References
Fig. 10. Relationship between coal-shales Wits-Ehac test and Wits-CT index.
Wits-Ehac index. Most of the fires caused by spontaneous combustion in the affected areas of this study have occurred in the zones that have been identified as having high Wits-Ehac and Wits-CT values. As can be seen from Table 3, coal CG, CL CN and coalshale SN (which were taken from Khwezela at Bokgoni Pit and Impunzi Mines) showed the highest risk value in the two liability indices. The records of the mines revealed that an incident of spontaneous combustion happened in the coal seam. The areas are assigned as being high-risk zone and are the zones from which samples were collected. 4. Conclusions Most of the previous studies measure the ignition temperature at a high temperature where an external source of heat has to be applied at the surface of the coal particles to predict the spontaneous combustion liability. This study has developed and employed a new liability index (Wits-CT index) which emulates the influence of oxygen adsorption on carbonaceous materials. The equipment does not involve the heating of carbonaceous material to a high temperature like the adiabatic method. The equipment determines the temperature variation within the coal particles with six temperature sensors under sluggish airflow conditions (oxygen) for a period of 24 h. The Wits-CT index established in this paper has been used to measure spontaneous combustion liability of coal and coal-shale. This is found to be a reliable tool to predict the experience of self-heating observed in coal mines. The Wits-CT index and the Wits-Ehac index used different methodologies to measure the spontaneous combustion liability of coals and coal-shales and the results are in-line. The Wits-CT index is
[1] Moghtaderi B, Dlugogorski BZ, Kennedy EM. Effects of wind flow on selfheating characteristics of coal stockpiles. Trans IChemE 2000;78(6):445–53. [2] Sasaki K, Sugai Y. Equivalent oxidation exposure - time for low-temperature spontaneous combustion of coal. In: Ahsan A, editor. Heat analysis and thermodynamic effects. InTech; 2011. p. 235–54. [3] Gray P, Lee PR. Thermal explosion theory. In: Tipper CFH, editor. Oxidation and combustion reviews, vol. 2. Amsterdam: Elsevier; 1967. [4] Stott JB. The spontaneous heating of coal and the role of moisture transfer, Final Report, US Bureau of Mines, Contract No. JO; 1980. p. 395–146. [5] Monazam ER, Shaddle LJ, Shamsi A. Spontaneous combustion of char stockpiles. Energy Fuels 1988;12(6):1305–12. [6] Akgun F, Arisoy A. Effect of particle size on the spontaneous heating of a coal stockpile. Combust Flame 1994;99:137–46. [7] Cliff D, Davis R, Bennett A, Galvin G, Clarkson F. Large-scale laboratory testing of the spontaneous combustibility of Australian coals. In: Proceedings of Queensland mining industry health & safety conference. Brisbane: Queensland Mining Council; 1998. p. 175–9. [8] Smith AC, Miron Y, Lazzara CP. Large-scale studies of spontaneous combustion of coal. Report of Investigations 9346. U.S. Bureau of Mines (USBM), Washington, D.C., U.S.A.; 1991. [9] Chen XD. The spontaneous heating of coal-Large-scale laboratory assessment supporting the theory. Ph.D. Thesis, Department of Chemical Engineering, University of Canterbury, Christchurch, New Zealand; 1991. [10] Beamish BB, Lau AG, Moodie AL, Vallance TA. Assessing the self-heating behaviour of Callide coal using a 2-metre column. J Loss Prev Process Ind 2001;15(5):385–90. [11] Fierro V, Miranda JL, Romero C, Andres JM, Arriaga A, Schmal D. Prevention of spontaneous combustion in coal stockpiles: experimental results in coal storage yard. Fuel Process Technol 1999;59:23–34. [12] Fierro V, Miranda JL, Romero C, Andres JM, Arriaga A, Schmal D. Model predictions and experimental results on self-heating prevention of stockpiled coals. Fuel 2001;80:125–34. [13] Sensogut C, Ozdeniz AH. Statistical modelling of stockpile behaviour under different atmospheric conditions, Western Lignite Corporation (WLC) case. Fuel 2005;84:1858–63. [14] Ozdeniz AH, Yilmaz N. Artificial Neural Network modeling of the spontaneous combustion occurring in the industrial scale coal stockpiles with 10-18mm coal grain sizes. Energy Sources 2009;Part A, 31:1425–35. [15] Ozdeniz AH, Sivrikaya O, Kelebek S. Statistical modeling of spontaneous coal combustion due to the effect of the sunrays. Energy Sources Part A 2015;37:2114–22. [16] Gouws MJ, Wade L. The self-heating liability of coal: Predictions based on simple indices. Min Sci Technol 1989;9:75–80.
M. Onifade et al. / International Journal of Mining Science and Technology 28 (2018) 649–655 [17] Gouws MJ, Wade L. The self-heating liability of coal: Predictions based on composite indices. Min Sci Technol 1989;9:81–5. [18] Genc B, Cook A. Spontaneous combustion risk in South African coalfields. J South Afr Inst Min Metall 2015;115:563–8. [19] ASTM, D-3173. Standard Test Methods for moisture in the analysis sample of coal and coke. [20] ASTM, D-3174. Standard test method for ash in the analysis sample of coal and coke from coal and standard classification of coals by rank.
655
[21] ASTM, D-3175. Standard test method for volatile matter in the analysis sample of coal and coke. [22] ISO 12902:2001. Determination of total carbon, hydrogen and nitrogen. [23] Wade L, Gouws MJ, Phillips HR. An apparatus to establish the spontaneous combustion propensity of South African coals, Symposium on safety in coal mines, CSIR, Pretoria; 1987. p. 7.1–7.2. [24] Kunii D, Levenspiel O. Fluidization engineering. New York: Krieger; 1991.
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
A new apparatus to establish the spontaneous combustion propensity of coals and coal-shales M. Onifade, B. Genc ⇑, A. Carpede School of Mining Engineering, University of the Witwatersrand, Johannesburg 2050, South Africa
a r t i c l e
i n f o
Article history: Received 17 November 2017 Received in revised form 7 March 2018 Accepted 15 May 2018 Available online 17 May 2018 Keywords: Coal Coal-shale Spontaneous combustion Wits-Ehac index Wits-CT index
a b s t r a c t Coal and coal-shale both tend to undergo spontaneous combustion under favourable atmospheric conditions. The Wits-Ehac index has been developed in South Africa since the late 1980’s to test the spontaneous combustion liability of coal. However, in some cases, the Wits-Ehac index fails to produce tangible results when testing coal-shales. To overcome this problem, a new apparatus has been developed to test carbonaceous materials such as coal and coal-shale under chemical reactions with oxygen and an index has been obtained. This index is called the Wits-CT index. The equipment emulates the influence of oxygen adsorption on carbonaceous material for a period of 24 h without a heating system. The Wits-CT index uses the total carbon content of the sample and the temperature variations obtained from the samples during reaction with oxygen to predict the spontaneous combustion liability. Eighteen samples have been analyzed using both indices and the results are in-line. It was found that coals and coal-shales with higher values of the Wits-CT index are more liable to spontaneous combustion. Further research on different coal-shales is underway in order to establish an extensive database for coal and coal-shales, together with known incidences of self-heating. Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction The occurrence of spontaneous combustion is one of the major challenges faced by the South African coal mining sector. The physical and chemical reactions between oxygen and external active structures of coal particles which releases heat are behind the event of spontaneous combustion. These incidents occur naturally due to the chemical reactions and oxidation of coal and coal-shales. The spontaneous combustion characteristics of coal may be influenced by the natural convection within a coal seam, spoil heaps and other mining piles [1]. Sasaki and Sugai studied the effects of chemical and oxidation reactions to measure the characteristics of temperature distribution within a stockpile [2]. Premature combustion temperatures at different points in a coal reaction vessel were reported by Gray and Lee [3]. The spontaneous heating of coal arises under definite atmospheric conditions. Coal oxidation occurs in an environment where there is sufficient oxygen. The oxidation rate of coal cannot be easily detected due to the reaction being very slow. In an open cast
⇑ Corresponding author. E-mail addresses: [email protected] (M. Onifade), Bekir.Genc@wits. ac.za (B. Genc), [email protected] (A. Carpede).
mining, the ingress of oxygen in the air into discontinuities and exposure of the highwall for long periods are among the factors affecting spontaneous heating. In underground coal mines, the combination of an insufficient ventilation system and selfheating areas of coal can cause spontaneous combustion. It is known that coal, roof shales, mining piles and other carbonaceous materials become self-heated and liberate heat naturally when subjected to atmospheric conditions for a long time as shown in Figs. 1 and 2. The heat generated may become faster than the heat dissipated to the surrounding. The experience of spontaneous combustion has approached into a phase that makes practical illustrations feasible as a result of the re-established awareness of self-heating of coal and coal-shales in coal mines. Past researcher’s efforts to measure the liability of coal stockpiles were primarily based on laboratory tests of one or more of the coal characteristics associated with the self-heating behavior. A limited number of medium-large scale tests on coal have been carried out in the past. However, such tests have not been conducted frequently due to the high cost and long period of time required and the difficulties in interpretation of the results. Experimental investigations on spontaneous combustion of bulk coal samples have been carried out under a medium-large scale test with a heating system used to initiate the self-heating process [4–10]. Studies on spontaneous combustion of coal
https://doi.org/10.1016/j.ijmst.2018.05.012 2095-2686/Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
650
M. Onifade et al. / International Journal of Mining Science and Technology 28 (2018) 649–655
of spontaneous combustion motivates this new apparatus. This current work presents the results of self-heating tests carried out on highly reactive coal and coal-shales in South African coal mines using the Wits-Ehac test and also the new apparatus (Wits-CT). Eighteen carbonaceous materials were tested with the new apparatus. The method is reliable, accurate and provides the required information on the effects of oxygen on self-heating of coals and coal-shales under field conditions.
Fig. 1. Self-heating of inseam shale and burning coal seam in Witbank, South Africa.
2. Materials and methods 2.1. Sample collection The coals and coal-shales used for this research were collected from two different coal mines in the eMalahleni area of South Africa and kept in airtight bags to avoid oxidation. Twenty representative in situ coal and coal-shales samples were obtained from the affected areas (highwalls and overburden shales). The samples subjected to the Wits-Ehac tests and tests in the newly developed apparatus.
Fig. 2. Burning spoil heaps at Tweefointein Mine, Witbank, South Africa.
stockpiles under the influence of atmospheric conditions have been reported [11–15]. However, no previous studies reported experimental methodologies to emulate the effects of atmospheric conditions on coal spontaneous combustion in the laboratory without an applied heating system. An acceptable standard widely used in South Africa to predict the propensity of coal to spontaneous combustion, known as the Wits-Ehac index has been in existence and used for more than 30 years [16–18]. The test involved the use of a relatively small amount of pulverized, dry samples to determine liability of coal to spontaneous combustion. The performed laboratory small-scale studies carried out in degrees and the influence of limiting factors can only be evaluated only under the available experimental conditions, unlike the medium-large scale test that considered the self-heating of a substantial coal mass under atmospheric conditions. Spontaneous combustion is dependent on the composition of the material, air temperature, sample temperature and mine environment. It has been indicated that the ignition temperature tests are at high temperatures where an external source of heat has been applied at the surface of the coal particles [16,17]. The effects of oxygen, moisture and temperature variations within a coal mass under sluggish airflow conditions, i.e., the most likely scenario at a mine, cannot be integrated into the small-scale test. The self-heating characteristics of some coal-shales in this study could not be determined by the Wits-Ehac index because of their low reactivity under the available experimental conditions. This motivated the need to develop a device that can measure the self-heating of coal, coal-shales and other carbonaceous materials under the influence of oxygen, which can produce similar results compared to the existing Wits-Ehac index. The use of realistic medium experimental scale tests that imitate the influence of atmospheric conditions on the various coal, coal-shale types to predict self-heating is important. The principal argument against laboratory testing in predicting the tendency of coal to spontaneous heating is that the process is unlike the natural conditions under which self-heat takes place. On the basis of these criteria, a medium scale laboratory apparatus has been developed to model and predict the occurrence of spontaneous combustion in coal mines. The model considers number of parameters such as airflow rate and temperature variation. The need for a reliable method to capture the large volume of data used to predict the occurrence
2.2. Sample preparation and characterisation The samples were reduced using a crusher and ball mill to suitable specify sizes as required for each test. The determinations of the moisture, ash, volatile matter and fixed carbon contents were carried out according to the American Society for Testing and Material (ASTM) standards [19–21]. Fixed carbon was obtained by subtracting the sum of the percentage of volatile matter, ash and moisture from 100. The carbon content were determined using a LECO TruSpec CHNS analyser (Fig. 3) after calibration with sulfamethazine based on the International Standard Organisation Standards [22]. The results were given in weight percent of airdried (%, by weight, ad). The results for proximate and total carbon content and spontaneous combustion tests carried out on the coal and coal-shale samples are given in Tables 1 and 2, respectively. 2.3. Wits-Ehac test The Wits-Ehac tests at the School of Mining, University of the Witwatersrand is a small-scale test in which freshly pulverized (<212 mm) and dried coal samples of 20–25 g in weight is used. The Wits-Ehac index has been developed in South Africa in the late
Fig. 3. Leco TruSpec CHNS analyser.
Table 1 Risk rating classification for coal, coal-shales and other carbonaceous materials to spontaneous combustion. Wits-CT Index
Risk rating
<2.5 2.5–5 5–7.5 >7.5
Less reactive Moderately reactive Reactive Highly reactive
651
M. Onifade et al. / International Journal of Mining Science and Technology 28 (2018) 649–655 Table 2 Results of proximate analysis, carbon content obtained from ultimate analysis and spontaneous combustion tests and weight gain for coal. Sample
Mad (%)
Aad (%)
VMad (%)
FC (%)
Cad (%)
Wits-Ehac index
Wits-CT index
Weight gain (g)
CA CC CE CG CH CI CK CL CM CN
2.3 2.2 2.3 2.5 2.4 2.1 1.6 1.6 1.6 1.6
28.0 33.8 28.6 16.8 18.8 48.8 13.7 22.5 17.0 17.0
23.2 24.1 24.3 20.0 26.9 16.7 22.1 26.1 22.0 23.9
46.5 39.9 51.7 60.7 51.9 32.8 62.6 49.8 59.4 57.5
54.4 47.5 53.6 66.0 65.2 36.1 69.7 58.9 66.7 65.8
4.64 4.52 4.76 4.91 4.69 3.82 4.44 4.87 4.76 4.84
6.29 5.31 5.42 7.53 7.51 4.05 9.10 9.59 7.27 7.91
32.9 32.1 32.6 38.7 37.2 30.2 42.5 45.2 36.1 40.7
1980’s to test the spontaneous combustion liability of coal [16–18]. The apparatus consists of an oil bath, six coal and inert material cell assemblies, an oil circular, a heater, a flowmeter, an air supply compressor and a computer. The test apparatus is used to test coals under predefined conditions and a combustibility index is obtained by Wade et al., as shown in Fig. 4 [23]. The system incorporates the determined crossing point temperatures and differential thermal analysis from selected coal samples with periodic measurement of temperatures to obtain a consistent self-heating liability index called the Wits-Ehac index. The temperatures are recorded every 30 s by the computer over an average of 4 h of testing for the oil in the bath to be heated to 200° Celsius. 2.4. New apparatus (Wits-CT) 2.4.1. Wits-CT experimental setup An apparatus to predict the spontaneous combustion liability of carbonaceous materials such as coal and coal-shale under the influence of airflow without any heating system was recently developed in the School of Mining Engineering, University of the Witwatersrand, referred to as the Wits-CT test. Most coals undergo spontaneous combustion under atmospheric conditions, i.e., when
Fig. 4. Wits-Ehac apparatus setup [23]
exposed to oxygen in the air. The equipment developed is based on this fact and it emulates the influence of oxygen adsorption on carbonaceous material in a closed system. Therefore, the equipment does not involve the heating of carbonaceous material to a high temperature like the adiabatic method (such as the R70, WitsEhac test and so on). The equipment prevents heat loss that may occur under testing condition by ensuring the lid is fixed and fastened, and we also placed a glass mineral wool insulator inside the equipment in order to arrest any heat loss. The insulating material was selected due to the temperature that may be generated during testing and the mode of heat transfer that may be involved. However, as no heating system is applied to the equipment to increase the temperature of the sample, the equipment is not expected to generate a high temperature. The equipment determines the temperature variation within and on the surface of the coal particles with six temperature sensors under sluggish airflow conditions (oxygen) for 24 h. This scenario is related to the atmospheric condition at the mines. The influence of oxygen absorption on the sample leading to the temperature variations during the start and the end of the test is recorded in a computer via a data logger as shown in Fig. 5. The temperatures recorded and the total carbon content of the sample are used to establish a spontaneous combustion liability index, referred to as the Wits-CT index. The apparatus has a capacity to accept a 15 kg sample, depending on the packing density. It oxidizes materials at controlled air pressure and constant flow rate. Three uniformly separated temperature sensors placed along the length of the autoclave are used to check and record the temperatures measurement. The sensor arrangement is typically located at different points within the autoclave (locations A, B, and C within the interior of the container). Each of the three thermocouples has two temperature sensors which measure temperatures at different levels at the locations A, B, and C in order to keep a recording of the temperature distribution during the reaction of a carbonaceous material and oxygen. At location A, sensors are located at levels 13 and 23 cm
Fig. 5. Wits-CT experimental setup and apparatus.
652
M. Onifade et al. / International Journal of Mining Science and Technology 28 (2018) 649–655
within the container, location B, 53 and 63 cm, and location C, 33 and 43 cm. The device comprises an upright mounted cylindrical tube of 0.75 m long and with an internal diameter of 160 mm. It is made of 5 mm thick mild steel. An illustration is given in Fig. 5a while Fig. 5b shows the new apparatus setup. Oxygen is supplied and controlled by a fixed flow rate by means of a flowmeter before being fed into a manifold attached below the lid of the autoclave. The connection between the manifold and the autoclave is achieved by means of a spiral copper tube. Oxygen gas is supplied from the lid through the manifold. The equipment is placed on a digitalize scale (Richterscale, Model W113) to measure any changes in weight during testing due to oxygen absorption. The experimental process simulates a well-insulated system relative to actual conditions in situ. The results obtained from this apparatus are in-line with the studies reported by Sensogut and Ozdeniz and Ozdeniz et al., in coal stockpiles [13,15].
during testing. Samples with high Wits-CT index have higher oxygen absorption than those with lower Wits-CT index. The temperature increase for each sample depends on the oxygen consumed. The results obtained from the Wits-CT liability index are in -line with the Wits-Ehac index. The Wits-CT index was developed to measure the spontaneous combustion liability of coals, coal-shales and other carbonaceous materials under condition that emulate atmospheric condition. The apparatus monitors and determines the temperature distributions within a mass of carbonaceous material with the use of thermocouples and imitates the ingress of oxygen in the air. As this was based on tests carried out on only 18 coal and coal-shale samples, to expand the database of the samples investigated, this new liability index was compared with the Wits-Ehac index [16,17]. The formulation of this new index based on the data obtained is discussed below.
2.4.2. Wits-CT testing procedure In order to determine spontaneous combustion liability via selfheating, a representative sample of particle size (<6.39 mm) was weighed and loaded into an autoclave. The particle size used is based on the study reported by Kunni and Levenspiel to determine the surface-volume area from the size distribution [24]. The system is sealed at the base and loaded with a sample from the top. The lid of the system was detached during the loading of the sample. The process was repeated till the autoclave was filled to the marked point. The lid was fixed and fastened as soon as the autoclave was filled with the sample. This technique of testing gives a closely sized sample that would be preferable to measure the rate of the oxidation process. Each temperature probe was inserted into the column as the correct sample level was reached. The position of each temperature probe to the column was placed vertically and spaced uniformly to the center as soon as the sample level was reached. Oxygen was supplied and controlled at a fixed flow rate (20 mL/min) by means of a flowmeter before being fed into a manifold attached below the lid of the autoclave. The logging began and the variation in temperatures distribution of the sensors were stored to a computer every minute. The test period lasts for 24 h. At the end of each experiment, samples were discarded and the autoclave is cleaned. The conditions in which the experimental tests were carried out closely resembled the situation in the mine environment.
Wits-CT index ¼ ðTM =24 þ T R Þ %C ad
3. Results and discussions 3.1. Test results The testing process included complex reactions between coal and coal-shale and oxygen, heat transfer and oxygen transmission in the autoclave. The extent to which the temperature distribution of the samples increases is assumed to be a measure of the sample to spontaneous combustion liability. The oxygen adsorbed by each sample causes an increase in weight and temperature variations
ð1Þ
where TM is the difference between the sum of maximum temperatures of each thermocouple in the autoclave and room temperature (22 °C); TR the difference between the peak temperature and initial temperature during testing in degree Celsius;%Cad the airdried percentage of carbon content of the sample; and 24 the test duration and is constant. Eq. (1) is the formula for obtaining the Wits-CT index of coal, coal-shales and other carbonaceous materials. The index increases with increasing carbon content as shown in Tables 1 and 2. The new Wits-CT index for the tested samples is based on the experience observed in the mines on spontaneous combustion and comparison with the Wits-Ehac index. A new classification of spontaneous combustion liability index for carbonaceous materials according to the new apparatus is shown in Table 3. Coal and coalshales having a higher spontaneous combustion liability are identified as having a higher Wits-CT liability index value and vice versa. The risk rating classification in Table 3 compares the Wits-CT index to what is happening on the mines in terms of the spontaneous combustion liability. This classification provides mine operators a better understanding of the spontaneous combustion liability index of various carbonaceous materials. This index is suitable in rating the liability of a material to spontaneous combustion like the Wits-Ehac index. The fact that different carbonaceous materials have dissimilar spontaneous combustion characteristics makes it essential to classify the materials according to their liability as shown in Table 3. The classification is dependent on two principles: Determination of the carbon content of the material; and Temperature variation due to oxygen absorbed by the material in the equipment. With an improved knowledge and the accumulation of large volume of test data obtained during testing, this study has established a liability index in-line with the experience of spontaneous combustion in the mines.
Table 3 Results of proximate analysis, carbon content obtained from ultimate analysis and spontaneous combustion tests and weight gain for coal-shales. Sample
Mad (%)
Aad (%)
Vad (%)
FC (%)
Cad (%)
Wits-Ehac index
Wits-CT index
Weight gain (g)
SA SE SH SI SK SL SM SN
1.4 1.7 0.8 1.0 1.0 1.0 0.8 1.5
78.5 68.4 88.7 79.6 79.1 74.0 76.9 51.5
11.2 15.9 8.5 11.9 11.7 16.0 11.7 16.6
8.9 14.0 2.0 7.5 8.2 9.0 10.6 30.4
11.5 15.8 2.66 9.12 9.75 10.5 12.5 33.7
3.09 3.73
1.33 1.60 0.27 0.95 1.18 1.34 1.44 3.99
27.5 31.5 15.0 20.1 28.1 28.9 30.5 32.3
2.98 2.99 3.77
M. Onifade et al. / International Journal of Mining Science and Technology 28 (2018) 649–655
3.2. Discussion The new liability index developed used the carbon content of the sample as one of the parameters to establish a formula to measure the spontaneous combustion liability. The carbon content varies between 36.1% to 69.7% for coals and 2.66% to 33.7% for coalshales as seen in Tables 1 and 2. Coal CI has the lowest carbon content and the lowest liability index, while coals with a high carbon content has higher liability index (see Table 3). Coal-shales SN and SE contained the highest carbon content and have the highest liability index compared to other coal-shales as shown in Table 1. It was found that an increase in carbon content indicates coals and coal-shales more liable to spontaneous combustion. Coal CG and coal-shale SN showed the highest liability index based on the Wits-Ehac value, while the Wits-CT indicates that coal CL and coal-shale SN has the highest spontaneous combustion liability index. The two spontaneous combustion liability indices shows a high liability for samples CG, CL and SN respectively. The records of the mine revealed that an incident of spontaneous combustion has been experienced on these samples in the recent time. This indicates that the new liability index can be used to predict the spontaneous combustion liability of coal mines having records of a high spontaneous combustion incidents. It is shown that most of the coal samples displayed high liability index due to their high carbon content compared to the coal-shales with considerably low carbon content. Both samples CI and SJ, with the lowest carbon contents, indicate the lowest liability index. The samples analyzed show that the capacity of a material to undergo spontaneous combustion appears to be directly related to the amount of carbon content. Coals with high carbon content indicate a high liability index while coal-shales which contained more than 15% carbon content show a high liability to spontaneous combustion as shown in Tables 1 and 2. It was found that coal-shales with high carbon and low carbon contents show similar characteristics to coal containing a high and low carbon content with respect to spontaneous combustion. This implies that both coals and coal-shales have similar characteristics in terms of the carbon content. It was found that coals contain a very high amount of carbon than the coal-shales in this study. It might be expected that the rise in temperature would basically increase the reaction rate if the spontaneous combustion liability of carbonaceous materials is associated with the heat generated and heat dissipated from a single process. The carbon content found in the coals and coal-shales varied from one sample to the other; hence, this might indicate that different reactions with different activation energies might take place in the spontaneous combustion process. When the carbon content for the coal-shales was compared with each other, the samples SN and SE are more liable to spontaneous combustion both under WitsEhac tests and the Wits-CT tests. Coal-shales SN and SE have approximately the same liability indices based on the results obtained from the Wits-Ehac and Wit-CT index. For other coal-
653
shales, the relative liability was different under the spontaneous combustion tests conducted. While the Wits-Ehac values for coal-shales SH, SI, and SM could not be determined due to their low reactivity, Wits-CT index successfully obtained liability indices for the same samples. The newly established Wits-CT index values varies between 4.05–9.59 for coals and 0.27–3.99 for coal-shales. Coal CL has the highest Wits-CT values, while coal CI has the lowest Wits-CT values. Coal-shale SH has the lowest Wits-CT value and coal-shale SN has the highest Wits-CT value among the coal-shales. The weight gained due to oxygen adsorbed by each sample increases with increasing Wits-CT index for the coal and coal-shale samples. The coals with a higher Wits-CT liability index have a higher weight gain than those with low weight gain. This is the same with the coal-shales. 3.3. Relationship between weight gain and spontaneous combustion liability The weight increase recorded during the experimental test can be related directly to the spontaneous combustion liability. The weight gained by each sample during the reaction with oxygen increases with the increasing Wits-CT index for the coals and coal-shales as shown in Fig. 6. Samples with a high Wits-CT index have a high oxygen absorption than those with a lower Wits-CT index. The study shows that the temperature variation for the samples depends on their characteristics to absorb oxygen. The purpose of the measured of weight enables the determination of how much oxygen has been absorbed by coals or coal shales under experimental conditions and their relationship with spontaneous combustion lability. The influence of oxygen absorption on the samples indicated that samples with a high oxygen absorption being confirmed by the scale are more liable to spontaneous combustion. Findings show that the samples increase in weight after part of it absorbed oxygen. This may likely be the same when freshly mined coal is exposed to oxygen in air. Figs. 6–10 show that coal and coal-shales having high weight gain, high carbon content, high Wits-CT index values and high Wits-Ehac index are more liable to spontaneous combustion. This shows that the results of the Wits-CT index are in-line with
Fig. 7. Relationship between coal Cad content and Wits-CT index.
Fig. 6. Relationships between weight gained and Wits-CT index for coals and coal-shales.
654
M. Onifade et al. / International Journal of Mining Science and Technology 28 (2018) 649–655
Fig. 8. Relationship between coal Wits-Ehac index and Wits-CT index.
Fig. 9. Relationship between coal-shales Cad content and Wits-CT index.
able to evaluate the liability index of carbonaceous shales that could not be determined by the WITS-Ehac index. A new classification for liability index of carbonaceous materials to spontaneous combustion has been established in this research. It was observed that coals and coal-shales with a high carbon content are more liable to spontaneous combustion. Therefore, the influence of the carbon content present in coals and coal-shales played a significant role in evaluating the incidents of spontaneous combustion. This experimental procedure might allow a high accuracy in predicting spontaneous combustion liability and may be used to establish a database of temperature measurements over an interval of time. Furthermore, this study establishes a relationship between coal and coal-shales in respect to spontaneous combustion liability. This may provide the industry with a better understanding on the awareness of the spontaneous combustion of coal and coal-shales. Simplified comprehensive procedures on the use of the equipment is made available in the School of Mining Engineering Spontaneous Combustion Research Laboratory for both skilled and semi-skilled labour. Acknowledgments This work was conducted in the context of coal-shale spontaneous combustion in the eMalahleni coalfields, South Africa was financially sponsored by Coaltech. The authors wish to express gratitude to Coaltech and the staff of the selected coal mines for their support. The work presented here is part of a PhD research study in the School of Mining Engineering at the University of the Witwatersrand. References
Fig. 10. Relationship between coal-shales Wits-Ehac test and Wits-CT index.
Wits-Ehac index. Most of the fires caused by spontaneous combustion in the affected areas of this study have occurred in the zones that have been identified as having high Wits-Ehac and Wits-CT values. As can be seen from Table 3, coal CG, CL CN and coalshale SN (which were taken from Khwezela at Bokgoni Pit and Impunzi Mines) showed the highest risk value in the two liability indices. The records of the mines revealed that an incident of spontaneous combustion happened in the coal seam. The areas are assigned as being high-risk zone and are the zones from which samples were collected. 4. Conclusions Most of the previous studies measure the ignition temperature at a high temperature where an external source of heat has to be applied at the surface of the coal particles to predict the spontaneous combustion liability. This study has developed and employed a new liability index (Wits-CT index) which emulates the influence of oxygen adsorption on carbonaceous materials. The equipment does not involve the heating of carbonaceous material to a high temperature like the adiabatic method. The equipment determines the temperature variation within the coal particles with six temperature sensors under sluggish airflow conditions (oxygen) for a period of 24 h. The Wits-CT index established in this paper has been used to measure spontaneous combustion liability of coal and coal-shale. This is found to be a reliable tool to predict the experience of self-heating observed in coal mines. The Wits-CT index and the Wits-Ehac index used different methodologies to measure the spontaneous combustion liability of coals and coal-shales and the results are in-line. The Wits-CT index is
[1] Moghtaderi B, Dlugogorski BZ, Kennedy EM. Effects of wind flow on selfheating characteristics of coal stockpiles. Trans IChemE 2000;78(6):445–53. [2] Sasaki K, Sugai Y. Equivalent oxidation exposure - time for low-temperature spontaneous combustion of coal. In: Ahsan A, editor. Heat analysis and thermodynamic effects. InTech; 2011. p. 235–54. [3] Gray P, Lee PR. Thermal explosion theory. In: Tipper CFH, editor. Oxidation and combustion reviews, vol. 2. Amsterdam: Elsevier; 1967. [4] Stott JB. The spontaneous heating of coal and the role of moisture transfer, Final Report, US Bureau of Mines, Contract No. JO; 1980. p. 395–146. [5] Monazam ER, Shaddle LJ, Shamsi A. Spontaneous combustion of char stockpiles. Energy Fuels 1988;12(6):1305–12. [6] Akgun F, Arisoy A. Effect of particle size on the spontaneous heating of a coal stockpile. Combust Flame 1994;99:137–46. [7] Cliff D, Davis R, Bennett A, Galvin G, Clarkson F. Large-scale laboratory testing of the spontaneous combustibility of Australian coals. In: Proceedings of Queensland mining industry health & safety conference. Brisbane: Queensland Mining Council; 1998. p. 175–9. [8] Smith AC, Miron Y, Lazzara CP. Large-scale studies of spontaneous combustion of coal. Report of Investigations 9346. U.S. Bureau of Mines (USBM), Washington, D.C., U.S.A.; 1991. [9] Chen XD. The spontaneous heating of coal-Large-scale laboratory assessment supporting the theory. Ph.D. Thesis, Department of Chemical Engineering, University of Canterbury, Christchurch, New Zealand; 1991. [10] Beamish BB, Lau AG, Moodie AL, Vallance TA. Assessing the self-heating behaviour of Callide coal using a 2-metre column. J Loss Prev Process Ind 2001;15(5):385–90. [11] Fierro V, Miranda JL, Romero C, Andres JM, Arriaga A, Schmal D. Prevention of spontaneous combustion in coal stockpiles: experimental results in coal storage yard. Fuel Process Technol 1999;59:23–34. [12] Fierro V, Miranda JL, Romero C, Andres JM, Arriaga A, Schmal D. Model predictions and experimental results on self-heating prevention of stockpiled coals. Fuel 2001;80:125–34. [13] Sensogut C, Ozdeniz AH. Statistical modelling of stockpile behaviour under different atmospheric conditions, Western Lignite Corporation (WLC) case. Fuel 2005;84:1858–63. [14] Ozdeniz AH, Yilmaz N. Artificial Neural Network modeling of the spontaneous combustion occurring in the industrial scale coal stockpiles with 10-18mm coal grain sizes. Energy Sources 2009;Part A, 31:1425–35. [15] Ozdeniz AH, Sivrikaya O, Kelebek S. Statistical modeling of spontaneous coal combustion due to the effect of the sunrays. Energy Sources Part A 2015;37:2114–22. [16] Gouws MJ, Wade L. The self-heating liability of coal: Predictions based on simple indices. Min Sci Technol 1989;9:75–80.
M. Onifade et al. / International Journal of Mining Science and Technology 28 (2018) 649–655 [17] Gouws MJ, Wade L. The self-heating liability of coal: Predictions based on composite indices. Min Sci Technol 1989;9:81–5. [18] Genc B, Cook A. Spontaneous combustion risk in South African coalfields. J South Afr Inst Min Metall 2015;115:563–8. [19] ASTM, D-3173. Standard Test Methods for moisture in the analysis sample of coal and coke. [20] ASTM, D-3174. Standard test method for ash in the analysis sample of coal and coke from coal and standard classification of coals by rank.
655
[21] ASTM, D-3175. Standard test method for volatile matter in the analysis sample of coal and coke. [22] ISO 12902:2001. Determination of total carbon, hydrogen and nitrogen. [23] Wade L, Gouws MJ, Phillips HR. An apparatus to establish the spontaneous combustion propensity of South African coals, Symposium on safety in coal mines, CSIR, Pretoria; 1987. p. 7.1–7.2. [24] Kunii D, Levenspiel O. Fluidization engineering. New York: Krieger; 1991.