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The rate of temperature rise of a subbituminous coal during spontaneous combustion in an adiabatic device: The effect of moisture content and drying methods
The Rate of Temperature Rise of a Subbituminous Coal during Spontaneous Combustion in an Adiabatic Device: The Effect of Moisture Content and Drying Methods W. E. VANCE, X. D. CHEN* and S. C. SCOT1TM Department of Civil and Resource Engineering (formerlyDepartment of Mining Engineering) (W.E. V., S.C.S.), Department of Chemical and Materials Engineering (X.D.C.), The Universityof Auckland, Auckland City, New Zealand This work investigates the effect of the moisture content of coal on its spontaneous ignition in oxygen (40°C-140°C). It has been found that the highest heating rate is achieved at a m e d i u m moisture content of ~ 7 wt% for an initial inherent moisture content of the coal before drying (in dry nitrogen at 65°C) of - 20 wt%. This is particularly noticeable at temperatures below 80°C and tends to support previous studies showing that a m a x i m u m oxidation rate occurs at such a moisture content in the same temperature range. Two drying m e t h o d s have been adopted in the current work and the effects of their operating conditions on the heating rates are described.
INTRODUCTION The important role of moisture on the development of spontaneous combustion of coal is now well recognized. However, the effect of moisture has not yet been fully comprehended and quantified [1, 2]. This paper is an attempt to investigate further the effect of moisture content on the spontaneous combustion of a subbituminous coal. Role of Moisture in Spontaneous Combustion
In general, one can divide the overall effect of moisture content (and its transfer inside a coal stockpile pile or during mining of coal underground) into two aspects [2, 3], namely (i) the effect of moisture transfer (evaporation, condensation, diffusion and convection) on the overall heat balance; (ii) the effect of moisture (at equilibrium or during drying or wetting) on the rate of coal oxidation. The complex nature of the above effects hindered a comprehensive mathematical modeling of spontaneous cornbustion, as well as a comprehensive practical application of the information generated over some 70 years since Winmill [4] and Graham [5]. The effect of moisture transfer i.e., (i) was initially emphasized by Berkowitz [6] and Stott and Baker [7]. They showed how the heat of
* Corresponding attthor,
COMBUST1ON AND FLAME 106:261-270 (1996) Copyright © 1996 by The Combustion Institute Published by Elsevier Science Inc.
absorption and desorption of water vapor in coal is a very strong controlling factor in the spontaneous heating of coal. Bannerjee [8], summarized the major conclusions from these earlier studies. Others [9-18] have published further related experimental and theoretical work. With respect to the effect of moisture content on the oxidation rate, i.e., (ii), it has been found that a coal reacts with oxygen more rapidly when wet than when dry, at room ternperature [19]. Other studies [20, 21], however, suggested that stripping moisture from coal exposes more fresh active sites on the coal's surface for contacting oxygen and thereby accelerates oxidation. The work carried out by Mukherjee and Lahiri [22], later summarized by Beier [19], showed that, at temperatures above 70°C, the oxidation of coals occurs more readily in dry than in moist air. This could be explained by noting that Berkowitz [23] published evidence to show that the mechanism of coal oxidation was not the same above 70°C as at lower temperatures. Below 70°C the oxidation rate is low enough to be unaffected by restrictions on the supply at the coal surface. Only acid functions and peroxides are generated [23] and a higher moisture content promotes these reactions. Above 70°C but lower than 150°C, however, moisture plays a different role as peroxides form only transiently or not at all. 0010-2180/96/$15.00 SSDI 0010-2180(95)00276-6
262
W. E. VANCE ET AL.
Sondreal and Ellman [27] showed conclusively that the oxidation rate of a lignite increased as the moisture was being removed until a moisture content of ~ 20 wt% was reached, when it fell as the moisture was further driven off. However, in their experiments, this trend becomes less conspicuous when the temperature increases to 70°C. Chen and Stott [3] studied the effect of changes in the moisture content of a subbituminous coal with "near-equilibrium" drying and wetting at 50°C. Similar behavior as that found by Sondreal and Ellman [24] was obtained, although the mechanism described was slightly different from the work by Chen and Stott [3]. It has also been found by Unal et al. [25] that different drying methods can affect the oxidation rate, Chen and Stott [3] used a nearequilibrium drying procedure (very low rate of drying) with nitrogen; this gave different trends for how the oxidation rate was influenced by moisture content from that investigated using a vacuum drying procedure [26]. Vacuum drying was believed [26] to have damaged the solid structure of coal, thereby causing the variation, The Adiabatic Methods
In practice, one is usually concerned about the overall effect of moisture on the spontaneous combustion of coal, i.e., the effect over the whole process of ignition. The adiabatic system to be described later is well-suited for such a purpose. In order to label the coal's propensity to fire spontaneously, many devices have been built to try to enable a thorough evaluation, e.g., the adiabatic method [27], the crossing point method [28, 29], the isothermal method [10] and the ignition temperature method [30]. Of these methods, the adiabatic one appears to be used more often, due to the small sample size and simplicity of the system. However, this does not mean that the test can represent comprehensively spontaneous combustion in
practice, e.g., this test would not provide a full picture of heat and mass transfer during heating. This test can, at least, be used to study the effects of oxidation rate, reabsorption of water vapour, degree of dryness on spontaneous combustion of coal. The adiabatic systern used in the current study is a modified, robust version of that developed by Humphrey et al. [31]. The work focused on the effect of moisture content and the effects of the two drying methods. EXPERIMENTAL PROCEDURES Sample Preparation A fresh sample of Renown Seam coal was obtained from the Huntly East mine in New Zealand. It was stored in an airtight container for transportation to our laboratory. To simulate the behaviour of freshly crushed coal partitles, the large coal lumps were reduced in size to about 210 /zm in a three-stage operation involving: Jaw and cone crusher and hammer mill. The coal particle sizes were also limited by the small volume of the test vessel and the size of the adiabatic oven. The crushed coal was then sealed in airtight bags at room temperature until required. To prevent excessive preoxidation before subsequent experiments, once the coal was reduced to the final size, the coal was bagged separately in approximately 150 g lots. The coal is of subbituminous rank and the proximate analysis is given in Table 1. In order to study the effect of different drying methods and the effects of moisture content on spontaneous heating rate, several samples with different moisture content had to be obtained by following two different drying methods: (D1) Drying tube method. A sample of ~ 150 g of coal each time was placed in a drying tube. The drying tube was placed in a standard laboratory oven which previously had
TABLE 1 Proximate Analysis of Renown Seam Coal, Huntly East (Provided by Coal Research Association of New Zealand Ltd.) Moisture 20.5 wt%
Ash 3.9 wt%
Fixed carbon 39.5 wt%
Volatile Matter 36.1 wt%
Calorific Value 22.71 M J / k g
Sulphur 0.2 wt%
EFFECT OF MOISTURE ON COAL COMBUSTION been heated to 110°C. A nitrogen flow of about 600 m l / m i n at 20°C and 1 atm was passed through the sample, with the oven maintained at 110°C. Upon completion of each drying trial of typically 8-12 h, the tube was removed from the oven and the coal was allowed to cool in an inert atmosphere of dry nitrogen, (D2) Minimum free space drying method. The oven was designed so that coal could either be dried in a vacuum or an atmosphere of nitrogen. The oven consists of a cast aluminium chamber with a front lifting vacuum seal door. Inside the oven is a rack that can take up to 6 trays. A coal sample is placed on these trays for drying. Heating is provided by elements at the bottom of the oven. Preheating chambers are provided in the side and top of the oven for nitrogen flow. In the current study, nitrogen ( ~ 3000 m l / m i n at 20°C and I a t m ) was passed through these chambers via a flow meter,
263
The temperature is controlled by an insulated thermocouple (at the top of the oven) connected to a PID controller. Two temperatures were used, i.e. for two separate drying runs; they were 65 ° and 110°C. The first one was expected to give minimum damage to the coal sample prepared. Samples of different moisture contents were prepared using the two drying methods for various purposes of the current study to be described later. Figure 1 shows the typical drying curves for the different methods of drying. It can be seen that the drying tube method gives a much slower drying than the minimum free space oven at either 110° or 65°C. The high temperature of 110°C used above in drying was intended to see if there was any difference in the behaviour of coals dried at the low temperature. The results from this high-temperature drying may be closely related to commercial coal drying, which employs elevated temperatures.
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264
The Adiabatic System for Spontaneous Combustion Test In this study, an adiabatic oven was designed like that by H u m p h r e y et al. [31]. The oven was built by the Electric Furnace Company Auckland and consists of three essential parts: the reaction vessel, the oven, and the controller. A coal sample is filled in the reaction vessel. This is illustrated in Fig. 2 and is a 500-ml vacuum thermal flask. The top of the flask is fitted with a Teflon plug. The Teflon plug is held in place by two springs. These springs, in conjunction with a rubber washer, ensure an airtight seal when the apparatus is in use. The vessel is fitted on the inner side wall of the oven door by a supporting bracket to allow easy access to the vessel and its attachments. Passing through the Teflon plug are two glass tubes, one of these tubes is 150 mm long with an internal diameter of 3 mm. The end of this tube is pointed to allow easy access through the coal. A type K thermocouple monitors the temperature rise. The second tube of the same diameter is used as an exhaust to allow reaction product gases to be released, To investigate a spontaneous heating, the controller mode was initially set to "manual" and the oven was preheated to 40°C. The reaction vessel was clamped in place. Nitrogen (50 m L / m i n at 20°C and 1 atm) was passed to allow stabilization of the coal at 40°C. The oven was then set at the "automatic" mode and the data acquisition system was activated to follow the heating (recording temperature rise) once every minute. This mode of operation allowed the oven temperature to follow, as closely as possible, the temperature of the coal (measured by a thermocouple located in the reaction vessel, as shown in Fig. 2) as heating progressed. The gas supply was then switched to oxygen flowing at similar rate to initiate oxidation. The gas stream was preheated in a copper coil of ~ 16 m long, also located on the inner wall of the oven door. A heat balance calculation on the oven an oxygen flow inside the copper tube (assuming a low heat transfer coefficient of 2 W / m 2 K between the oven and the tube) indicates that the length of the copper tube ( ~ 16 m) is sufficient to allow the oxygen (50 m L / m i n at 20°C and 1 atm) to be
W . E . V A N C E E T AL. heated to the oven temperature, very quickly on entry to the reaction vessel. The oven safety shut-off was activated at 150°C; this stopped the flow of oxygen and hence allowed the oven and coal sample to cool. The absolute error in heating time has been found to be + 0.3 h for a test that completes the heating up to 140°C within 5 h, but the error deteriorates to +_.1.5 h if the process lasts more than 15 h. However, the reproducibility of each experiment between the start and the temperature of 100°C was found to be better than + 0.2 h, if the whole experiment stops in 5 h.
RESULTS AND DISCUSSION The Effect of Moisture Content on Spontaneous Ignition of Coal To study this effect, several coal samples of different moisture contents were prepared by the drying method D2 at 65°C (see Table 2). Drying at this fairly low temperature was expected to cause minimum structural damage to the coal, compared with vacuum drying or the same type of drying at a much higher temperature, e.g., l l 0 ° C [3]. In all the tests performed, as soon as the temperature went beyond 140°C, thermal run away became evident. The temp e r a t u r e - t i m e profiles as influenced by the initial moisture contents are shown in Fig. 3. One can see that the time required to reach 140°C varied with initial moisture contents of the samples. There is a medium moisture content of ~ 7 wt%, beyond or below which the rate of temperature rise decreases. The times to reach 80°C and 140°C can also be found in Table 2 for each sample (Instead of 40°C, 42°C was used in the calculations as the initial ternperature; this is to avoid error caused by the initial operation that switched the gas flow from nitrogen to moist oxygen.) The rates of temperature rise for all the samples in Table 2 were calculated between temperature intervals 45°-60°C, 60°-80°C and 80°-100°C are plotted in Fig. 4. The data provided in Table 2 as well as the heating rates shown in Fig. 4 illustrate that the considerable differences in rate of temperature rise started to occur right from the beginning of the experiments, which have
EFFECT OF MOISTURE ON COAL COMBUSTION Gas InletPipe ~----x ThermocoupleLeadsTo Controller/DataLogger. ~
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shown the great importance of the oxidation rates at these low temperatures. It can be seen that the maximum rate appears to shift, as temperature increases, towards a higher moisture removal (i.e. small amount of residual moisture). More drastic changes in heating
rates with increasing moisture removal occurred at high residual moisture contents. If this trend was due to a change in the specific heat (Cp) and the density of coal ( p ) with moisture, it would require that their product (pCp) in the energy balance of self-heating
266
W. E. V A N C E E T AL. TABLE2
Coal Sampleswith Different Moisture Contents Obtained Using the Minimum Free Space Oven Method Operated Under Nitrogen Flow at 65°C; Time to 80~Cand Time to 140°C. Moisture Content (wt%)a
Time to reach 80~C(h)
Time to reach 140~C(h)
15.7 14.3 11.7 9.5 8.3 5.0 4.5
10.1 8.1 6.0 4.5 3.2 4.0 4.2
16.2 12.4 8.6 6.2 4.7 4.9 5.0
Retained moisture content based on inherent moisture content of 20.5 wt% of the crushed coal before drying; the crushing did not appear to alter this value to more than +0.5%. [14] possessed a minimum value at the corresponding moisture content. A coal sample with zero moisture content has the lowest density (due to dry pores) and with any increase in moisture content without swelling (due to interaction with water) this density must increase. Similarly, the specific heat of coal should increase from that of the dry coal with addition of moisture inside the pores. As a result, ( p C p ) should increase monotonically with increasing moisture content. This, therefore could not have caused the maximum temperature rise at a medium moisture content, Previous studies [3, 24] have demonstrated the effect of moisture content on oxidation rate under isothermal conditions at or below 70°C. A maximum oxidation rate occurs at a medium moisture content, for a very similar sub-hituminous coal investigated by Chen and Stott [3]. These previous findings are therefore supported by the current study. The current study is more significant in the sense that it gives, for the first time, the integrated effect of moisture on the whole ignition process (from about 40 ° to 140°C).
The Effect of Drying Methods on Spontaneous Ignition As shown in Table 3, several samples were prepared to show a comparison between the performance of various drying methods adopted in the current study. Table 3 shows that the drying method D2 at 110°C did not give reproducible results; this may be caused by the nonuniform temperature distribution in
the minimum free space oven as detected. However, a lower temperature, e.g., 65°C, gave fairly consistent results. The drying method D1 at l l 0 ° C also produced reproducible results. It is interesting to note that the heating rate obtained using D1 at l l 0 ° C was at least twice as much as that when using D2 at 65°C while only a difference in moisture content of ~ 2 wt% between the two. To explain this observation, it would have been desirable to have full chemical analyses before and after drying. Procedure D2 at l l 0 ° C consistently gave the lower rates of heating compared with D1 at ll0°C. The explanation is not dear. One apparent reason causing a slower rate for D2 at l l 0 ° C was that the dried coal samples reabsorbed moisture from the ambient air while cooling in open air. In one case recorded, a sample picked up 1% moisture in 40 min. During this period, the coal started at a temperature of ~ 100°C, which should lead to a fast rate of preoxidation. Normal stockpiling is unlikely to produce a drying process as severe as that described for D1 and D2 at 110°C. Laboratory studies carried out using such a high temperature may not be relevant to practice. On the other hand, drying at low temperatures would not give information related to the handling of commercial coals dried at elevated temperatures. Observations on the Effects of Weathering and Gas Humidity With its inherent moisture content, the coal could not self-heat and a maximum temperature rise of about 3°C was observed. A fresh
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TABLE 3
Coal Sampleswith Different Moisture Contents Obtained using the Two Drying Methods (DI and D2); Time
to 140°c. Moisture Removed
(wt%) 18.6 18.9 18.8 18.4
Drying Method D1, ll0°C D1, ll0°C D1, ll0°C D2, ll0°C
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sample with inherent moisture content and an old sample with similar moisture content, after 4 months of exposure to air at room temperature, were tested for their tendency to self-heat, The t e m p e r a t u r e - t i m e profiles are shown in Fig. 5, the old coal sample did not show any
sign of spontaneous heating. Two dried samples obtained using D2 at l l 0 ° C with similar moisture removal ( ~ 18.5 wt%) were tested separately. One was exposed to saturated oxygen and another reacted with dry oxygen (in both cases, the dry oxygen flow rates remained the same). The former gave a faster rate of heating as expected, since the reabsorption of water vapour released the latent heat of water vaporization (as shown in Fig. 6). In this moist case, a saturated oxygen flow was required. All these observations reconfirm previous findings, e.g., those by Guney and Hodges [27] and Stott [29] etc. CONCLUSIONS For the first time, the integrated effect of moisture of a subbituminous coal on its selfignition process (40" to 140°C) has been studied. It has been found that, at a medium moisture content of ~ 7 wt%, the self-heating rate
EFFECT OF MOISTURE ON COAL COMBUSTION
269
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215
3
270
W. E. VANCE ET AL.
( r a t e o f t e m p e r a t u r e rise) is a m a x i m u m . T h i s
is most pronounced at temperatures below 80°C. This supports the previous work on oxid a t i o n r a t e as i n f l u e n c e d b y m o i s t u r e c o n t e n t .
The drying method adopted, when using a relatively h i g h t e m p e r a t u r e
o f 110°C, gave less
reproducible results. This may be attributed to "thermal damage" on the solid structure and un-controlled re-absorption of moisture, by a v e r y dry coal. REFERENCES 1. Guney, M., Colliery Guardian 216:137-143 (1968). 2. Chen, X. D., Proceedings Symposium of 4th Coal Research Conference, Wellington, New Zealand 2:383-406 (1991). 3. Chen, X. D., and Stott, J. B., Fuel 72:787-792 (1993). 4. Winmill, T. F., Trans. Inst. Mining Eng. 46:563 (1913-14); 48:503; 48:508 (1914-1915). 5. Graham, J. I., Trans. Inst. Mining Eng. 48:521 (1914-15); 49:35 (1916-17). 6. Berkowitz, N., Fuel 36:355-373 (1957). 7. Stott, J. B., and Baker, O. J., Fuel 32(4):415-427 (1953). 8. Banel~ee, S. C., Spontaneous Combustion of Coal and Mine Fires, A. A. Balkema, Rotterdam, pp. 20-23, 1985. 9. Bhattacharyya, K. K., Fuel 51:214-220 (1972). 10. Stott, J. B., Proceedingsof the Symposium on Nature of Coal, Central Fuel Research Institute, Lealgor, India, 1959, pp. 173-184. 11. Stott, J. B., and Murtagh, B. A., Australian and New Zealand Association for the Advancement of Science, 39th Congress, 1971, Paper 22. 12. Stott, J. B. and Quan, T. N., New Zealand Institute of Mining Seminar, 1974, Paper 1.
13. Schmal, D., Duyzer, J. H., and van Heuven, J. W., Fuel 64:963-972(1985). 14. Chen,X. D., Combust. Flame 90:114-120 (1992). 15. Chen, X. D., Coal Prep. 14:223-236 (1994). 16. Stott, J. B., and Chen, X. D., Colliery Guardian, 9-16 (Jan. 1992). 17. Chen, X. D., and Stott, J. B. J. Fire Sci. 10:352-361
(1992).
18. Chen, X. D., and Wake, G. C., Trans. IChemE Part B: Proc. Safety Environ. Prey. 72(B):135-141 (1994). 19. Beier, E., 'Oxidation of Coal in Air', Mitteilungen der Westfalischen Berggwerk~cha~tskasse,no. 22 (translated from German by Script technica Inc. for US Bureau of Mines, Washington, DC, pp. 15-18, 53-63) (1962). 20. Baner~ee, S. C., Baner~ee, B. D., and Chakravorty, R.N., Fuel 49:324 (1970). 21. Walker, I. K., Fire Res. Abs. Rev. 9(1):10 (1967). 22. Mukherjee, P. N. and Lahiri, A., Brennstoff-Chem. 38:55 (1957). 23. Berkowitz, N., The Chemisay of Coal, Elsevier, Amsterdam, 1995. 24. Sondreal, E. A., and Ellman, R. L., Report oflnvestigation, U.S. Bureau of Mines, No. 7787, (1974). 25. Unal, S., Wood, D. G., and Harris, I. J., Fuel 71:183 (1992). 26. Karsner, G. G., and Pedmutter, D. D., Ind. Eng. Chem. ProcessDes. Dev. 21(2):348 (1982). 27. Guney, M., and Hodges, D. J., Colliery Guardian 217:173-177 (1969). 28. Feng, K. K., Chakravorty, R. N., and Cochrane, T. S., CIM Bull. 66(738):75-84 (1973). 29. Stott, J. B., Nature 188(4744):54-55 (1960). 30. Smith, A. C., and Lazzara, C. P., Spontaneous combustion studies of U.S. coals, Report of Investigation, U.S. Bureau of Mines, No. 9079 (1987). 31. Humphrey, D., Rowland, D. and Cudmore, J. F., Ignitions, Explosions and Fires in Coal Mines Symposium, The AusIMM Illawarra Branch, 5-1-5-19 (1981). Received 3 January 1995; accepted 10 December 1995
INTRODUCTION The important role of moisture on the development of spontaneous combustion of coal is now well recognized. However, the effect of moisture has not yet been fully comprehended and quantified [1, 2]. This paper is an attempt to investigate further the effect of moisture content on the spontaneous combustion of a subbituminous coal. Role of Moisture in Spontaneous Combustion
In general, one can divide the overall effect of moisture content (and its transfer inside a coal stockpile pile or during mining of coal underground) into two aspects [2, 3], namely (i) the effect of moisture transfer (evaporation, condensation, diffusion and convection) on the overall heat balance; (ii) the effect of moisture (at equilibrium or during drying or wetting) on the rate of coal oxidation. The complex nature of the above effects hindered a comprehensive mathematical modeling of spontaneous cornbustion, as well as a comprehensive practical application of the information generated over some 70 years since Winmill [4] and Graham [5]. The effect of moisture transfer i.e., (i) was initially emphasized by Berkowitz [6] and Stott and Baker [7]. They showed how the heat of
* Corresponding attthor,
COMBUST1ON AND FLAME 106:261-270 (1996) Copyright © 1996 by The Combustion Institute Published by Elsevier Science Inc.
absorption and desorption of water vapor in coal is a very strong controlling factor in the spontaneous heating of coal. Bannerjee [8], summarized the major conclusions from these earlier studies. Others [9-18] have published further related experimental and theoretical work. With respect to the effect of moisture content on the oxidation rate, i.e., (ii), it has been found that a coal reacts with oxygen more rapidly when wet than when dry, at room ternperature [19]. Other studies [20, 21], however, suggested that stripping moisture from coal exposes more fresh active sites on the coal's surface for contacting oxygen and thereby accelerates oxidation. The work carried out by Mukherjee and Lahiri [22], later summarized by Beier [19], showed that, at temperatures above 70°C, the oxidation of coals occurs more readily in dry than in moist air. This could be explained by noting that Berkowitz [23] published evidence to show that the mechanism of coal oxidation was not the same above 70°C as at lower temperatures. Below 70°C the oxidation rate is low enough to be unaffected by restrictions on the supply at the coal surface. Only acid functions and peroxides are generated [23] and a higher moisture content promotes these reactions. Above 70°C but lower than 150°C, however, moisture plays a different role as peroxides form only transiently or not at all. 0010-2180/96/$15.00 SSDI 0010-2180(95)00276-6
262
W. E. VANCE ET AL.
Sondreal and Ellman [27] showed conclusively that the oxidation rate of a lignite increased as the moisture was being removed until a moisture content of ~ 20 wt% was reached, when it fell as the moisture was further driven off. However, in their experiments, this trend becomes less conspicuous when the temperature increases to 70°C. Chen and Stott [3] studied the effect of changes in the moisture content of a subbituminous coal with "near-equilibrium" drying and wetting at 50°C. Similar behavior as that found by Sondreal and Ellman [24] was obtained, although the mechanism described was slightly different from the work by Chen and Stott [3]. It has also been found by Unal et al. [25] that different drying methods can affect the oxidation rate, Chen and Stott [3] used a nearequilibrium drying procedure (very low rate of drying) with nitrogen; this gave different trends for how the oxidation rate was influenced by moisture content from that investigated using a vacuum drying procedure [26]. Vacuum drying was believed [26] to have damaged the solid structure of coal, thereby causing the variation, The Adiabatic Methods
In practice, one is usually concerned about the overall effect of moisture on the spontaneous combustion of coal, i.e., the effect over the whole process of ignition. The adiabatic system to be described later is well-suited for such a purpose. In order to label the coal's propensity to fire spontaneously, many devices have been built to try to enable a thorough evaluation, e.g., the adiabatic method [27], the crossing point method [28, 29], the isothermal method [10] and the ignition temperature method [30]. Of these methods, the adiabatic one appears to be used more often, due to the small sample size and simplicity of the system. However, this does not mean that the test can represent comprehensively spontaneous combustion in
practice, e.g., this test would not provide a full picture of heat and mass transfer during heating. This test can, at least, be used to study the effects of oxidation rate, reabsorption of water vapour, degree of dryness on spontaneous combustion of coal. The adiabatic systern used in the current study is a modified, robust version of that developed by Humphrey et al. [31]. The work focused on the effect of moisture content and the effects of the two drying methods. EXPERIMENTAL PROCEDURES Sample Preparation A fresh sample of Renown Seam coal was obtained from the Huntly East mine in New Zealand. It was stored in an airtight container for transportation to our laboratory. To simulate the behaviour of freshly crushed coal partitles, the large coal lumps were reduced in size to about 210 /zm in a three-stage operation involving: Jaw and cone crusher and hammer mill. The coal particle sizes were also limited by the small volume of the test vessel and the size of the adiabatic oven. The crushed coal was then sealed in airtight bags at room temperature until required. To prevent excessive preoxidation before subsequent experiments, once the coal was reduced to the final size, the coal was bagged separately in approximately 150 g lots. The coal is of subbituminous rank and the proximate analysis is given in Table 1. In order to study the effect of different drying methods and the effects of moisture content on spontaneous heating rate, several samples with different moisture content had to be obtained by following two different drying methods: (D1) Drying tube method. A sample of ~ 150 g of coal each time was placed in a drying tube. The drying tube was placed in a standard laboratory oven which previously had
TABLE 1 Proximate Analysis of Renown Seam Coal, Huntly East (Provided by Coal Research Association of New Zealand Ltd.) Moisture 20.5 wt%
Ash 3.9 wt%
Fixed carbon 39.5 wt%
Volatile Matter 36.1 wt%
Calorific Value 22.71 M J / k g
Sulphur 0.2 wt%
EFFECT OF MOISTURE ON COAL COMBUSTION been heated to 110°C. A nitrogen flow of about 600 m l / m i n at 20°C and 1 atm was passed through the sample, with the oven maintained at 110°C. Upon completion of each drying trial of typically 8-12 h, the tube was removed from the oven and the coal was allowed to cool in an inert atmosphere of dry nitrogen, (D2) Minimum free space drying method. The oven was designed so that coal could either be dried in a vacuum or an atmosphere of nitrogen. The oven consists of a cast aluminium chamber with a front lifting vacuum seal door. Inside the oven is a rack that can take up to 6 trays. A coal sample is placed on these trays for drying. Heating is provided by elements at the bottom of the oven. Preheating chambers are provided in the side and top of the oven for nitrogen flow. In the current study, nitrogen ( ~ 3000 m l / m i n at 20°C and I a t m ) was passed through these chambers via a flow meter,
263
The temperature is controlled by an insulated thermocouple (at the top of the oven) connected to a PID controller. Two temperatures were used, i.e. for two separate drying runs; they were 65 ° and 110°C. The first one was expected to give minimum damage to the coal sample prepared. Samples of different moisture contents were prepared using the two drying methods for various purposes of the current study to be described later. Figure 1 shows the typical drying curves for the different methods of drying. It can be seen that the drying tube method gives a much slower drying than the minimum free space oven at either 110° or 65°C. The high temperature of 110°C used above in drying was intended to see if there was any difference in the behaviour of coals dried at the low temperature. The results from this high-temperature drying may be closely related to commercial coal drying, which employs elevated temperatures.
20-
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l
i~
102
/
4-
~
/ 20
f
Time (hrs)
Fig. 1. Typical drying curves using different drying methods.
264
The Adiabatic System for Spontaneous Combustion Test In this study, an adiabatic oven was designed like that by H u m p h r e y et al. [31]. The oven was built by the Electric Furnace Company Auckland and consists of three essential parts: the reaction vessel, the oven, and the controller. A coal sample is filled in the reaction vessel. This is illustrated in Fig. 2 and is a 500-ml vacuum thermal flask. The top of the flask is fitted with a Teflon plug. The Teflon plug is held in place by two springs. These springs, in conjunction with a rubber washer, ensure an airtight seal when the apparatus is in use. The vessel is fitted on the inner side wall of the oven door by a supporting bracket to allow easy access to the vessel and its attachments. Passing through the Teflon plug are two glass tubes, one of these tubes is 150 mm long with an internal diameter of 3 mm. The end of this tube is pointed to allow easy access through the coal. A type K thermocouple monitors the temperature rise. The second tube of the same diameter is used as an exhaust to allow reaction product gases to be released, To investigate a spontaneous heating, the controller mode was initially set to "manual" and the oven was preheated to 40°C. The reaction vessel was clamped in place. Nitrogen (50 m L / m i n at 20°C and 1 atm) was passed to allow stabilization of the coal at 40°C. The oven was then set at the "automatic" mode and the data acquisition system was activated to follow the heating (recording temperature rise) once every minute. This mode of operation allowed the oven temperature to follow, as closely as possible, the temperature of the coal (measured by a thermocouple located in the reaction vessel, as shown in Fig. 2) as heating progressed. The gas supply was then switched to oxygen flowing at similar rate to initiate oxidation. The gas stream was preheated in a copper coil of ~ 16 m long, also located on the inner wall of the oven door. A heat balance calculation on the oven an oxygen flow inside the copper tube (assuming a low heat transfer coefficient of 2 W / m 2 K between the oven and the tube) indicates that the length of the copper tube ( ~ 16 m) is sufficient to allow the oxygen (50 m L / m i n at 20°C and 1 atm) to be
W . E . V A N C E E T AL. heated to the oven temperature, very quickly on entry to the reaction vessel. The oven safety shut-off was activated at 150°C; this stopped the flow of oxygen and hence allowed the oven and coal sample to cool. The absolute error in heating time has been found to be + 0.3 h for a test that completes the heating up to 140°C within 5 h, but the error deteriorates to +_.1.5 h if the process lasts more than 15 h. However, the reproducibility of each experiment between the start and the temperature of 100°C was found to be better than + 0.2 h, if the whole experiment stops in 5 h.
RESULTS AND DISCUSSION The Effect of Moisture Content on Spontaneous Ignition of Coal To study this effect, several coal samples of different moisture contents were prepared by the drying method D2 at 65°C (see Table 2). Drying at this fairly low temperature was expected to cause minimum structural damage to the coal, compared with vacuum drying or the same type of drying at a much higher temperature, e.g., l l 0 ° C [3]. In all the tests performed, as soon as the temperature went beyond 140°C, thermal run away became evident. The temp e r a t u r e - t i m e profiles as influenced by the initial moisture contents are shown in Fig. 3. One can see that the time required to reach 140°C varied with initial moisture contents of the samples. There is a medium moisture content of ~ 7 wt%, beyond or below which the rate of temperature rise decreases. The times to reach 80°C and 140°C can also be found in Table 2 for each sample (Instead of 40°C, 42°C was used in the calculations as the initial ternperature; this is to avoid error caused by the initial operation that switched the gas flow from nitrogen to moist oxygen.) The rates of temperature rise for all the samples in Table 2 were calculated between temperature intervals 45°-60°C, 60°-80°C and 80°-100°C are plotted in Fig. 4. The data provided in Table 2 as well as the heating rates shown in Fig. 4 illustrate that the considerable differences in rate of temperature rise started to occur right from the beginning of the experiments, which have
EFFECT OF MOISTURE ON COAL COMBUSTION Gas InletPipe ~----x ThermocoupleLeadsTo Controller/DataLogger. ~
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shown the great importance of the oxidation rates at these low temperatures. It can be seen that the maximum rate appears to shift, as temperature increases, towards a higher moisture removal (i.e. small amount of residual moisture). More drastic changes in heating
rates with increasing moisture removal occurred at high residual moisture contents. If this trend was due to a change in the specific heat (Cp) and the density of coal ( p ) with moisture, it would require that their product (pCp) in the energy balance of self-heating
266
W. E. V A N C E E T AL. TABLE2
Coal Sampleswith Different Moisture Contents Obtained Using the Minimum Free Space Oven Method Operated Under Nitrogen Flow at 65°C; Time to 80~Cand Time to 140°C. Moisture Content (wt%)a
Time to reach 80~C(h)
Time to reach 140~C(h)
15.7 14.3 11.7 9.5 8.3 5.0 4.5
10.1 8.1 6.0 4.5 3.2 4.0 4.2
16.2 12.4 8.6 6.2 4.7 4.9 5.0
Retained moisture content based on inherent moisture content of 20.5 wt% of the crushed coal before drying; the crushing did not appear to alter this value to more than +0.5%. [14] possessed a minimum value at the corresponding moisture content. A coal sample with zero moisture content has the lowest density (due to dry pores) and with any increase in moisture content without swelling (due to interaction with water) this density must increase. Similarly, the specific heat of coal should increase from that of the dry coal with addition of moisture inside the pores. As a result, ( p C p ) should increase monotonically with increasing moisture content. This, therefore could not have caused the maximum temperature rise at a medium moisture content, Previous studies [3, 24] have demonstrated the effect of moisture content on oxidation rate under isothermal conditions at or below 70°C. A maximum oxidation rate occurs at a medium moisture content, for a very similar sub-hituminous coal investigated by Chen and Stott [3]. These previous findings are therefore supported by the current study. The current study is more significant in the sense that it gives, for the first time, the integrated effect of moisture on the whole ignition process (from about 40 ° to 140°C).
The Effect of Drying Methods on Spontaneous Ignition As shown in Table 3, several samples were prepared to show a comparison between the performance of various drying methods adopted in the current study. Table 3 shows that the drying method D2 at 110°C did not give reproducible results; this may be caused by the nonuniform temperature distribution in
the minimum free space oven as detected. However, a lower temperature, e.g., 65°C, gave fairly consistent results. The drying method D1 at l l 0 ° C also produced reproducible results. It is interesting to note that the heating rate obtained using D1 at l l 0 ° C was at least twice as much as that when using D2 at 65°C while only a difference in moisture content of ~ 2 wt% between the two. To explain this observation, it would have been desirable to have full chemical analyses before and after drying. Procedure D2 at l l 0 ° C consistently gave the lower rates of heating compared with D1 at ll0°C. The explanation is not dear. One apparent reason causing a slower rate for D2 at l l 0 ° C was that the dried coal samples reabsorbed moisture from the ambient air while cooling in open air. In one case recorded, a sample picked up 1% moisture in 40 min. During this period, the coal started at a temperature of ~ 100°C, which should lead to a fast rate of preoxidation. Normal stockpiling is unlikely to produce a drying process as severe as that described for D1 and D2 at 110°C. Laboratory studies carried out using such a high temperature may not be relevant to practice. On the other hand, drying at low temperatures would not give information related to the handling of commercial coals dried at elevated temperatures. Observations on the Effects of Weathering and Gas Humidity With its inherent moisture content, the coal could not self-heat and a maximum temperature rise of about 3°C was observed. A fresh
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268
W . E . V A N C E E T AL. 40,
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TABLE 3
Coal Sampleswith Different Moisture Contents Obtained using the Two Drying Methods (DI and D2); Time
to 140°c. Moisture Removed
(wt%) 18.6 18.9 18.8 18.4
Drying Method D1, ll0°C D1, ll0°C D1, ll0°C D2, ll0°C
Time to reach 140°C(11) 1.5 1.8 1.8 3.6
18.5 18.5
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D2, 65°C D2, 65°C
2.8 5.2
4.9 4.4
15.5
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5.0
16,0 15.4
sample with inherent moisture content and an old sample with similar moisture content, after 4 months of exposure to air at room temperature, were tested for their tendency to self-heat, The t e m p e r a t u r e - t i m e profiles are shown in Fig. 5, the old coal sample did not show any
sign of spontaneous heating. Two dried samples obtained using D2 at l l 0 ° C with similar moisture removal ( ~ 18.5 wt%) were tested separately. One was exposed to saturated oxygen and another reacted with dry oxygen (in both cases, the dry oxygen flow rates remained the same). The former gave a faster rate of heating as expected, since the reabsorption of water vapour released the latent heat of water vaporization (as shown in Fig. 6). In this moist case, a saturated oxygen flow was required. All these observations reconfirm previous findings, e.g., those by Guney and Hodges [27] and Stott [29] etc. CONCLUSIONS For the first time, the integrated effect of moisture of a subbituminous coal on its selfignition process (40" to 140°C) has been studied. It has been found that, at a medium moisture content of ~ 7 wt%, the self-heating rate
EFFECT OF MOISTURE ON COAL COMBUSTION
269
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215
3
270
W. E. VANCE ET AL.
( r a t e o f t e m p e r a t u r e rise) is a m a x i m u m . T h i s
is most pronounced at temperatures below 80°C. This supports the previous work on oxid a t i o n r a t e as i n f l u e n c e d b y m o i s t u r e c o n t e n t .
The drying method adopted, when using a relatively h i g h t e m p e r a t u r e
o f 110°C, gave less
reproducible results. This may be attributed to "thermal damage" on the solid structure and un-controlled re-absorption of moisture, by a v e r y dry coal. REFERENCES 1. Guney, M., Colliery Guardian 216:137-143 (1968). 2. Chen, X. D., Proceedings Symposium of 4th Coal Research Conference, Wellington, New Zealand 2:383-406 (1991). 3. Chen, X. D., and Stott, J. B., Fuel 72:787-792 (1993). 4. Winmill, T. F., Trans. Inst. Mining Eng. 46:563 (1913-14); 48:503; 48:508 (1914-1915). 5. Graham, J. I., Trans. Inst. Mining Eng. 48:521 (1914-15); 49:35 (1916-17). 6. Berkowitz, N., Fuel 36:355-373 (1957). 7. Stott, J. B., and Baker, O. J., Fuel 32(4):415-427 (1953). 8. Banel~ee, S. C., Spontaneous Combustion of Coal and Mine Fires, A. A. Balkema, Rotterdam, pp. 20-23, 1985. 9. Bhattacharyya, K. K., Fuel 51:214-220 (1972). 10. Stott, J. B., Proceedingsof the Symposium on Nature of Coal, Central Fuel Research Institute, Lealgor, India, 1959, pp. 173-184. 11. Stott, J. B., and Murtagh, B. A., Australian and New Zealand Association for the Advancement of Science, 39th Congress, 1971, Paper 22. 12. Stott, J. B. and Quan, T. N., New Zealand Institute of Mining Seminar, 1974, Paper 1.
13. Schmal, D., Duyzer, J. H., and van Heuven, J. W., Fuel 64:963-972(1985). 14. Chen,X. D., Combust. Flame 90:114-120 (1992). 15. Chen, X. D., Coal Prep. 14:223-236 (1994). 16. Stott, J. B., and Chen, X. D., Colliery Guardian, 9-16 (Jan. 1992). 17. Chen, X. D., and Stott, J. B. J. Fire Sci. 10:352-361
(1992).
18. Chen, X. D., and Wake, G. C., Trans. IChemE Part B: Proc. Safety Environ. Prey. 72(B):135-141 (1994). 19. Beier, E., 'Oxidation of Coal in Air', Mitteilungen der Westfalischen Berggwerk~cha~tskasse,no. 22 (translated from German by Script technica Inc. for US Bureau of Mines, Washington, DC, pp. 15-18, 53-63) (1962). 20. Baner~ee, S. C., Baner~ee, B. D., and Chakravorty, R.N., Fuel 49:324 (1970). 21. Walker, I. K., Fire Res. Abs. Rev. 9(1):10 (1967). 22. Mukherjee, P. N. and Lahiri, A., Brennstoff-Chem. 38:55 (1957). 23. Berkowitz, N., The Chemisay of Coal, Elsevier, Amsterdam, 1995. 24. Sondreal, E. A., and Ellman, R. L., Report oflnvestigation, U.S. Bureau of Mines, No. 7787, (1974). 25. Unal, S., Wood, D. G., and Harris, I. J., Fuel 71:183 (1992). 26. Karsner, G. G., and Pedmutter, D. D., Ind. Eng. Chem. ProcessDes. Dev. 21(2):348 (1982). 27. Guney, M., and Hodges, D. J., Colliery Guardian 217:173-177 (1969). 28. Feng, K. K., Chakravorty, R. N., and Cochrane, T. S., CIM Bull. 66(738):75-84 (1973). 29. Stott, J. B., Nature 188(4744):54-55 (1960). 30. Smith, A. C., and Lazzara, C. P., Spontaneous combustion studies of U.S. coals, Report of Investigation, U.S. Bureau of Mines, No. 9079 (1987). 31. Humphrey, D., Rowland, D. and Cudmore, J. F., Ignitions, Explosions and Fires in Coal Mines Symposium, The AusIMM Illawarra Branch, 5-1-5-19 (1981). Received 3 January 1995; accepted 10 December 1995