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M.E.T.T.S. - Consulting Engineers > R&D Projects > The Estimation of the Propensity For Spontaneous Combustion in Colliery Wastes
The Estimation of the Propensity For Spontaneous Combustion in Colliery Wastes
Michael C. Clarke, Sydney Technical College; Joseph A. Shonhardt, Univ. of New South Wales; David F. Bagster, The University of Sydney.
Abstract
This paper firstly examines the pollution potential of burning colliery wastes. A comparison of two means of estimating the propensity for coal to spontaneously combust is next presented. The use of microwaves, as a new procedure, is discussed in detail, while a comparison of the results is made with a modified adiabatic technique. A number of coals that are known to spontaneously combust were tested, as were some coals that are not considered to have self heating potential.
Introduction
The propensity of colliery wastes to combust spontaneously, is related to the specific ability of seams or splits of seams to self heat during or after mining. The instances of burning coal wastes are increasing with the increase in the percentage of coal mined by open cut methods. Wastes created in open cut mining often contain coal from seams and splits that for either reasons of quality and/or thickness are not reclaimed. This coal is often blended with the overburden by the heavy machinery used in mining, and if liable to spontaneous combustion results in numerous pockets of heating across and through the wastes. This process of spreading the source of heatings through the overburden makes reclamation of mined out areas very awkward and in two cases known to authors reclamation has failed over large areas due to spontaneous combustion.
Spontaneous combustion in washery rejects has also been a problem in with coal from certain seams. Washery rejects can be seen burning after many years in a number of locations in New South Wales. The extent of environmental impact of such reject fires however is less in potential than that from burning overburden, in that the rejects are normally more concentrated and not as extensive(and therefore more easily disposed of by deep burial) as overburden. Colliery rejects are also often able to be re-washed to obtain otherwise lost coal values1, while at the same time reducing the propensity for spontaneous combustion.
Other sources of environmental damage from coal spontaneous combustion are burning coal stockpiles and insitu coal seams. These sources of pollution are normally short lived due to the economic cost of losing mined or minable coal.
If a major source of pollution is from burning spot fires in overburden and reclaimed mined areas, a means of reducing this environmental hazard is to identify coal that is liable to spontaneous combustion but not to be taken in the mining process(but is to be disposed of with the overburden). If this coal was identified during the exploration or mine planning it could be selectively removed during mining and safely disposed of.
No simple method of estimating the propensity for coal to spontaneously combust exists. The taking of samples of coal and subjecting them to calorimetry testing takes time and resources2. There are five methods that use calorimetry in some form, to test the propensity of coals to spontaneously combust. These methods are:
1) Direct Observation. This involves the direct observation of the behaviour of coal in situ, ie. the measurement of temperatures and gas concentrations,
2) Constant Temperature or Isothermal Procedures. Laboratory studies that observe the absorption of oxygen by coal,
3) Adiabatic Testing3. The measurement of heat given off by the reaction of coal and oxygen, in a thermally isolated surrounding,
4) Ignition Temperature Method(Crossing Point Procedure). These methods were pioneered by Wheeler4 and Kreulen5, and consist of heating coal in a container at a constant rate. The temperatures of both the coal and the container are recorded, with the crossing point being taken as the temperature at which the instantaneous coal temperature exceeds that of the container for the first time, and
5) Chemical Methods. Here coal is oxidised with strong oxidising agents, with the temperature of the coal being recorded.
The number of samples that could be examined using any of the above calorimetry techniques would be limited to the number of testing units that were available; subsequently if multiple seams were to be tested from multiple core samples, a large investment in testing units would need to be made. Since coal seams that are liable to spontaneous combustion, only self heat comparatively rarely, any test designed to observe the propensity for spontaneous combustion would need to able to handle many samples from across whole seam sections, and from numerous examples of those sections. The need is for a simple test that is capable of process numerous samples.
During a visit to a major New South Wales Coal Mine, an observation of the inflagration of some coal particles that were being dried in a microwave oven was made. An inquiry was made of the mine chemist as to whether the seam from which that particular coal was taken was liable to spontaneous combustion. The chemist answered in the affirmative.
The drying trays being used by the coal mine contained several hundred individual coal pieces. Four to ten pieces of coal were noted to well alight in those trays that contained burning coal, while a number of other pieces were noticed to be smouldering. The burning pieces were picked out and disposed of.
Process Development (Microwave tests)
Initially some grab samples were obtained from several Hunter Valley Coal Mines to test the theory that a relationship existed between spontaneous combustion potential and the inflagration of certain coal specimens in a microwave oven. The samples were roughly crushed, and spread on a large dinner plate. The plate was subsequently placed in a microwave oven at maximum energy for several minutes( a similar procedure to that observed at the coal mine where the phenomenon was first observed).
Samples were obtained from one coal mine(mining the Liddell Seam) that had a current problem with spontaneous combustion. The other coal was from two coal mines that had no current problem with spontaneous combustion(the Big Ben and the Wambo seams). The samples were obtained by junior mining officials, from close to the working face, during their normal shifts. No attempt was made to protect the samples from oxidation or moisture.
The result of this simple investigation was that the Wambo and Big Ben coals did not inflagrate or even become hot. Some of the Liddell specimens did burst into flame, others smouldered, some became hot to touch while others were no hotter than the plate after the test. The test was repeated after several days, over which the specimens were allowed to air dry. Similar results to the first test were obtained for the second test.
Analytical Procedure(Microwave tests)
It was desired to standardise the above simple test to allow comparison of future results. The areas of standardisation that were identified as being desirable were;
1) The energy delivered to the specimens,
2) The size of the specimens,
3) The number of specimens tested per sample and the
4) Layout of specimens on the test plate.
The energy delivered to the system, is dependant on three factors. The first is the microwave frequencies that are used in microwave ovens. The second is the irradiation time of specimens, while the third is the amount of energy delivered to the oven. It is assumed that the modern microwave oven gives a good spread of energy around the interior of the oven. The frequency used is 2.45 GHz6 which is designed to excite water and oil molecules. The radiation times were chosen along with the level of radiation('cooking setting') to give two levels of energy delivered to the specimens.
(Relative energy levels in the oven were measured by adding 50mL of cold water (temperature recorded) to a 100mL beaker and placing the beaker in the centre of the microwave oven at a chosen energy level for a predetermined time. The temperature of water in the beaker was recorded at the end of the test. Another similar beaker was later placed on the periphery of cooking area in the oven and tested. In the case of microwave oven used(Toshiba E.R. 662) for the tests reported in this paper, temperature increases of 53°C and 54°C were found for an energy level of 3 for three minutes, for the central and peripheral positions respectively, while the temperature increases were 68°C and 72°C for an energy level of 9 for one minute, for the same positions.)
The size of specimens was chosen to be material passing three mesh but retained on eight mesh. This size range was chosen because particles of this range are easy to pick up with tweezers, while allowing ample specimens-25- to be placed on a plate for a single irradiation. The plates used for testing were ruled up with a 25mm grid(125mm x125mm = 25 specimen places). The number of specimens usually used per test were 100 (four plate loadings). (One long series of tests was run with only 50 specimens per test, due to time restrictions.)
Recording of Results(Microwave tests)
Specimens were firstly tested at energy level 3 for three minutes. Any inflagrations were recorded on score sheets. The same specimens were re-tested at energy level 9 for one minute, with any inflagrations being recorded. If no inflagrations were observed, the plate was next 'smelt' for the odour of burning coal, and if this was apparent each specimen was felt to see if it had an increased temperature. Lastly the surface of the plate was observed to see if any coal distillates were adhering. If any of the above occurrences were noticed, they were recorded as heatings in the specimens.
Sample Collection
To test the propensity of coal wastes to spontaneously combust, it was decided to test some coals, from which such wastes could be generated in the mining process, for spontaneous combustion potential. The collection of coal samples from seams that are 'known'* to be liable to spontaneous combustion and from seams that are 'known never'* to spontaneously combust was next undertaken.
(*Information supplied by competent mining officials.)
The coals that were considered to be liable to spontaneously combust were:
1a) Ravensworth 4
1b) Ravensworth 3
2) Liddell(Sampled from two mines)
3) Greta(Two samples tested)
4) Collie(Only an old sample was available)
The coals that were not considered to be liable to spontaneously combust were:
5) Bulli
6) Wongawilli
7) Buller(New Zealand)
8) Gundawarra
9) Muswellbrook(Occasional heatings have been reported)
10) Hallett( Occasional heatings have been reported)
Descriptions of the above coals are given in an appendix.
Samples were tested as soon as possible after collection. In the case of samples 1, 2, 5, 6, 9 & 10, testing was carried out within forty eight hours of collection. Samples 3, 7 & 8 were tested within four weeks of collection, while the Collie coal was over six months old.
Results
The inflagrations or heatings in samples are recorded as percentages.
| 1a) Ravensworth | No heatings. |
| 1b) Ravensworth 3 | 3% (level 9) |
| 2a) Liddell | No heatings * [1] |
| 2b) Liddell | No heatings [2] |
| 3a) Greta | 5% (2 @ level 9, 3@9) |
| 3b) Greta | No heatings [3] |
| 4) Collie | No heatings |
| 5) - 8) | No heatings |
| 9) Muswellbrook | 2% (level 3) |
| 10) Hallett | 2% (level 9) |
* Faint smell of burning coal.
[1] The manager stated that this seam had been free of heatings for some time.
[2] Heatings were reported in the adjacent panel, however no heatings were apparent in the panel tested.
[3] The manager stated that the incidents of heatings were decreasing.
In sample 3a) pieces of coal showing obvious pyrites were placed in the microwave. No sign of heating was apparent.
Comparison of Microwave Tests with Modified Adiabatic Tests.
To relate the newly developed method of testing for spontaneous combustion, a series of modified adiabatic tests were run on some of the coals. The test procedure was pioneered by Mr. J. Shonhardt2, and consists of placing coal in a specially built calorimeter, that contains a heating coil (See Figure 1). The procedure could be considered to be a hybrid of adiabatic and crossing point techniques. The criteria by which the coals propensity to self heat is judged is the temperature at which the coal generates sufficient heat to achieve a heating rate detectibly greater than that generated by the system itself.
Test Results (Calorimetry)
This table gives the temperatures at which self heating for a number of the coals became apparent.:
|
Seam/Split.
|
Temperature of the
onset of self heating. |
| Ravensworth 3 | 58.0°C |
| Ravensworth 4 | 61.8°C |
| Muswellbrook | 69.3°C |
| Hallett | 73.8°C |
| Liddell | 76.8°C |
| Buller N.Z. | 97.8°C |
| Wongawilli | 114.5°C |
Plots of temperature v's time are given for each coals mentioned in Table1. (Figures 2 - 8). The shape of the time temperature curve produced by heating a coal that is not liable to spontaneous combustion is typical of the form shown for the Buller material. The linear section is due solely to the effects of the one hundred and fifty miliwatt heat source incorporated in the sample chamber.
In general the shape of the heating curve is controlled by two factors superimposed on the linear heating curve due to the resistance; heat generation in the sample due to low temperature oxidation and heat loss due to the evaporation of water. These can compete to give a curve of the shape shown for the Muswellbrook sample, with a decrease in the slope from 65°C to 75°C where both factors are significant, or may result in the straight line plot obtained for the Buller, where the sample produced little low temperature oxidation and had negligible water loss. The Wongawilli sample showed no significant heat generation, but did show some cooling due to water loss.
Discussion
Microwave tests showed that two of the coals that were considered to have a high spontaneous combustion potential reacted when bombarded with microwaves. Two coals that were considered to be occasionally liable to spontaneously combust also reacted in the microwave oven. All coals that were considered not to be liable to spontaneous combustion, did not react when subjected to microwave bombardment. Over six months had lapsed since the Collie coal had been mined, to the date when it was tested. In the case of the Liddell samples tested during the main experimental period, only a hint of burning coal was detected with one specimen. The Liddell material that was originally grab sampled however had shown a strong propensity to react in a microwave oven. The above results indicate that the performance of some coals when bombarded with microwaves, depends on what samples were taken.
(The difficulties of sampling coals for spontaneous combustion testing, is further indicated in the behaviour of the Liddell samples. The second Liddell sample, tested during the main experimental period, consisted of material sampled across the entire width of the seam. Coal from the adjoining panel had spontaneously combusted on a number of occasions, however the panel from which the sample was taken for testing did not self ignite either before or after sampling. Unfortunately it was not possible to sample the adjoining panel, where spontaneous combustion had occurred, since this had been sealed. Due to the lack of time this later sample of Liddell coal was not tested in the adiabatic system.)
From the adiabatic tests Ravensworth 3 were shown to be the most active. This coal also reacted in the microwave oven. Ravensworth 4 was only marginally less active in the microwave than Ravensworth 3, but did not react in the microwave. The Hallett and Muswellbrook samples were fairly active in the adiabatic tests, and were also active when bombarded with microwaves. Buller and Wongawilli samples did not react in the microwave tests, and were shown to have a low activity in the adiabatic tests.
Conclusion
The above results thus show some correlation among data from adiabatic and microwave testing and reports of incidents. It therefore is important to gather more such data to reach a firmer conclusion. It may then be possible to predict with some confidence if a coal has a propensity to combust spontaneously, and thus deal with a problem which has been causing atmospheric pollution, and destroying property and even life for generations.
Appendix
Coal Seam Descriptions 7.
Ravensworth. H-V Bituminous coals with relatively low specific energy values and medium to high ash. They are used in power generation.
Liddell. H-V, low-ash and low sulphur coking coal.
Greta. Very H-V perhydrous low ash bituminous coal, relatively high in sulphur, used in coking blends.
Collie. A low ash, high moisture, sub-bituminous steaming coal. It is of low sulphur.
Bulli. M-V coking coal, medium to high ash, prime coking coal, sulphur content low.
Wongawilli. M-V coking coal, low sulphur and medium to high ash.
Buller.(8 & 9). Cretaceous, H to M-V Bituminous coals, with very low ash, low sulphur.
Gundawarra. Low ash, H-V steaming coals with low to moderate sulphur content.
Muswellbrook. As for Gundawarra.
Hallett. As for Gundawarra.
All coals are of Permian origin except for the New Zealand Buller Coal.
References
1) Stockton N. D., 1982. Unpublished Ph.D Thesis U.N.S.W.
2) Shonhardt, J. A. Calorimeter design and the assessment of self heating in coal. The Coal Journal, November 1984.
3) Guney, M. and Hodges, D.J., Adiabatic Studies of the Spontaneous Heating in Coal. Part 1. Colliery Guardian, Feb. 1969, pp105 - 108.
4) Wheeler, R. V.,1924. The Ignition of Coal. Fuel in Science and Practice. Vol. III, pp366 - 370.
5) Kreulin, D.J.W., 1948. Elements of Coal Chemistry. Nijgh and Van Ditmar, Rotterdam, pp 138 - 141.
6) Personal communication, Mr. Ken Smith, Technical Officer, Toshiba, Australia.
7) Black Coal In Australia, 1984-85. Joint Coal Board.
8) Australasian Coal Mining Practice., 1986. The Australasian Institute of Mining and Metallurgy.
9) Newman, N. A., 1985. Mineral Matter in West Coast Coals. Draft Report to NZERDC.
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