Energy Fuels 2009, 23, 4871–4876 Published on Web 09/01/2009
: DOI:10.1021/ef900372w
Spontaneous Combustion Prediction of Coal by C80 and ARC Techniques Qingsong Wang,* Song Guo, and Jinhua Sun State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, P.R. China Received April 26, 2009. Revised Manuscript Received August 12, 2009
Many coal fires were caused by spontaneous combustion in coal mines or coal storehouses, which resulted in a great loss and energy wastage. To identify and evaluate the hazardous degree of coal stockpile, a C80 microcalorimeter and accelerating rate calorimeter (ARC) were employed in this work. The coal samples undergo an exothermal process start at 80 °C with heat generation of -75.1 J g-1(mean value) detected by C80 experiment. The activation energies of the first exothermal process were calculated for the three experiments, and the mean value is 80.76 kJ mol-1, which is lower than that of obtained from the ARC result, 127.0 kJ mol-1. For a 300 tons coal stockpile, the self-heating oxidation temperatures (SHOT) were calculated as 164, 60, 90, and 68 °C based on the ARC experiment and three C80 experiments, respectively. Further research on the mass effect on SHOT shows that if the coal mass is less than 12 tons, the danger of thermal spontaneous combustion is less. However, if the mass amount is more than 12 000 tons, the danger of thermal spontaneous combustion is difficult to avoid even at ambient temperature if no special measures are taken.
chemical kinetics of coal oxidation.7-9 Fourier transform infrared spectroscopy (FTIR),10,11 gas-chromatography massspectrometry (GC-MS)12-14 were used to analysis the decomposition products. TGA is performed on coal to determine changes in weight in relation to change in an imposed temperature range. Differential thermogravimetric (DTG) plots can be derived from the thermogravimetric (TG) plot, to illustrate the variation in the rate of mass loss with temperature or time. The DTG and TG information can be applied to develop the kinetic models of coal oxidation.4,5 In the DTA technique, the temperature difference between a coal sample and a reference material is measured as a function of temperature, while the coal sample and reference material are subjected to a controlled temperature program. The output thermogram, a record of temperature difference against the temperature in the inert medium, is used in the analysis of the heat evolution and chemical reactions occurring during coal oxidation.3 In isothermal calorimeters, the vessel is placed in a large bath held at a constant temperature, and the heat released by chemical reactions occurring in the sample is determined by measuring the heat dissipated to the environment. In adiabatic calorimeters, the heat liberated from coal oxidation is not allowed to transfer to the environment, and then, any heat generated by the coal sample causes the coal to increase in temperature, thus fuelling the reaction. The heat released is then calculated from measuring the temperature rise within the vessel. For this reason, adiabatic calorimeters also serve to measure the maximum temperature rise during the self-heating of a sample. The minimum self-heating temperature
1. Introduction Coal reacts with atmospheric oxygen even at ambient temperature and the reactions are exothermic. If the heat liberated from the reactions is unable to escape, the temperature of the coal will rise up. The temperature of the coal rises over its ignition point, combustion begins if sufficient oxygen is present. This phenomenon is described as spontaneous combustion. For the coal with high sulfur content and where the heat retention is high the coal may start burning at temperatures as low as 30-40 °C. Some parameters were introduced to discuss the oxidation process of coal, including rate of oxidation, mass change, heat evolution, oxygen consumption, nature of the gaseous products, as well as concentration of bound oxygen and oxygenated complexes at coal surfaces.1 The relevant measurements were used to investigate the characteristics of these parameters during coal oxidation experiments. Thermogravimetric analysis (TGA), differential thermal analysis (DTA),2-5 and isothermal and adiabatic calorimeters were used to analysis the kinetics. The method of basket heating was developed based on the Frank-Kamenetskii model for thermal ignition of packed solids.6 Crossing-point temperature and Chen’s method have been used for the assessment of apparent *To whom correspondence should be addressed. Telephone: þ86551-360-6431. Fax: þ86-551-360-1669. E-mail address: pinew@ustc. edu.cn. (1) Wang, H. H.; Dlugogorski, B. Z.; Kennedy, E. M. Prog. Energy Combust. 2003, 29, 487–513. (2) Chen, Y.; Mori, S.; Pan, W. P. Energy Fuel 1995, 9, 71–74. (3) Kok, M. V.; Keskin, C. J. Therm. Anal. Calorim. 2001, 64, 1265– 1270. (4) Straka, P.; Nahunkova, J. J. Therm. Anal. Calorim. 2004, 76, 49– 53. (5) Kok, M. V. J. Therm. Anal. Calorim. 2007, 88, 663–668. (6) Pehlivan, D.; Gurkahraman, M.; Pamuk, V. Fuel Sci. Techn. Int. 1996, 14, 1097–1110. (7) Chen, X. D. Process. Saf. Environ. 1999, 77, 187–192. (8) Ogunsola, O. L. Fuel 1991, 70, 258–261. (9) Chen, X. D.; Chong, L. V. Process. Saf. Environ. 1995, 73, 101– 107. r 2009 American Chemical Society
(10) Rui, M.; Sun, X. G. Spectrosc. Spect. Anal. 2008, 28, 61–66. (11) Ibarra, J. V.; Moliner, R. J. Anal. Appl. Pyrol. 1991, 20, 171–184. (12) Gryglewicz, G.; Gryglewicz, S. Fresen. J. Anal. Chem. 2001, 370, 60–63. (13) Ryan, N. J.; Boon, J. J.; Given, P. H. Abstr. Pap. Am. Chem. Soc. 1984, 188, 6-Geoc. (14) Greenwood, P. F.; Zhang, E.; Vastola, F. J.; Hatcher, P. G. Anal. Chem. 1993, 65, 1937–1946.
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(SHT) of coal is defined as the lowest temperature of thermal runaway of a sample in the reactor under controlled experimental conditions, and it has been recognized as an index for ranking the propensity of coal to self-heating and spontaneous combustion.1 Some mathematical models were proposed based on the thermal analysis.15-18 Guruz et al.17 used different kinetic models to simulate the decomposition of coal in the variable and constant temperature regions of pyrolysis. Sensogut19 and Ozdeniz20 researched the temperature distribution of coal stockpiles and introduced the temperature model. The particle size, moisture and diffusion of oxygen effects were also explored.21-23 Such researches have drawn the common results that the coal can be ignited by itself, and it should to be regarded as hazardous goods, especially for large stockpiles. For the storage of hazardous material, the self-accelerating decomposition temperature (SADT) is one of the important parameters to evaluate the hazard degree.24,25 It is defined as the lowest ambient temperature at which the temperature increase of a chemical substance is at least 6 K in a specified commercial package during a period of seven days or less.26,27 For the coal spontaneous combustion, the temperature rise is caused by the oxidation of coal, rather than a decomposition process. However, the SADT is focus more on the temperature increase of the materials, rather than on its reaction types. Therefore, SADT can be employed and changes to self-heating oxidation temperature (SHOT) to evaluate the hazard degree of coal stockpile. However, until now, there have been few reports on the SADT or SHOT of coal stockpiles, as one of the important parameters to character the hazard degree. Microcalorimeter tests can be a possible candidate to assess the risk of self-ignition of coal and the corresponding efforts have been done.28 The C80 calorimeter (Setaram, France) is a heat flux calorimeter that has merits of high sensitivity with a quite wide testing temperature range from ambient temperature to 300 °C. Herein, it was used to identify the thermal behavior of coal at elevated temperature, and to calculate the kinetics and to predict the SHOT in this study. Furthermore, the accelerated rate calorimeter (ARC) also was used to research the coal thermal behavior as an assistant method.
meter. Then, a simple reaction mechanism is assumed to be dependent on the Arrhenius law. The rate expression for the consumption of reaction is defined as eq 1,25 dx E ¼ A exp ð1 -xÞn ð1Þ dt RT where x=(M0 - M)/M0 is the conversion rate, M is the mass of reactant at time t (g), M0 is the initial mass of reactant (g), t is the time (s), A is the frequency factor (s-1), E is activation energy (J mol-1), R is the universal gas constant (J mol-1 K-1), T is the temperature of system (K), and n is reaction order (dimensionless). Substituting x into eq 1, the following equation can be easily obtained. n dM E M ¼ A exp ð2Þ M0 dt RT M0 At the initial stage the reactant consumption can be negligible. Therefore, M is approximately equal to M0. By multiplying eq 2 by the heat of reaction ΔH (J g-1), the heat flow of the reaction is obtained as: dH=dt E ð3Þ ¼ A exp ΔHM0 RT Taking the natural logarithm of eq 3, dH=dt E1 ln ¼þln A ΔHM0 RT
ð4Þ
By plotting the curve of ln[(dH/dt)/(ΔHM0)] versus inverse temperature (T -1), the activation energy (E) and frequency factor (A) can be easily calculated.25 2.2. Derivation of Kinetics by ARC Method. The reaction kinetics describes the speed of the reaction, how it accelerates with temperature and how it decreases in rate as concentration is depleted. The basic starting point is always the Arrhenius equation.29 dC ¼ Ae -E=RT C n ð5Þ dt Where A, E, n, T, t, and R are same with eq 1, and C is the concentration (g mol m-3). Only when a fully adiabatic experiment is performed can the assumption be made that:29 dC dT ð6Þ dt dt
2. Principle Theory of Calculation 2.1. Derivation of Kinetics by C80 Method. The SADT and SHOT calculation are based on the reactant kinetics para-
As the concentration is depleted so heat is produced and the rates are equivalent. Thus, dT µAe -E=RT C n ð7Þ dt
(15) Edwards, J. C. Jom.- J. Min. Met. Mat. Soc. 1982, 35, A84–A84. (16) Brooks, K.; Glasser, D. Soc. Afr. J. Sci. 1988, 84, 874–875. (17) Guruz, G. A.; Uctepe, U.; Durusoy, T. J. Anal. Appl. Pyrol. 2004, 71, 537–551. (18) Rafsanjani, H. H.; Jamshidi, E. Chem. Eng. J. 2008, 140, 1–5. (19) Sensogut, C.; Ozdeniz, A. H.; Gundogdu, I. B. Energy Source, Part A 2008, 30, 339–348. (20) Ozdeniz, A. H.; Corumluoglu, O.; Kalayci, I.; Sensogut, C. Energy Source, Part A 2008, 30, 1085–1097. (21) Akgun, F.; Arisoy, A. Combust. Flame 1994, 99, 137–146. (22) Hull, A. S.; Lanthier, J. L.; Chen, Z. M.; Agarwal, P. K. Combust. Flame 1997, 110, 479–493. (23) Arisoy, A.; Akgun, F. Fuel 1994, 73, 281–286. (24) Wang, Q. S.; Sun, J. H.; Guo, S. Ind. Crop. Prod. 2008, 28, 268– 272. (25) Sun, J. H.; Li, Y. F.; Hasegawa, K. J. Loss Prevent. Proc. 2001, 14, 331–336. (26) United Nations Recommendations on the Transport of Dangerous Goods, Manual of Tests and Criteria. In ST/SG/AC.10/11/Rev.4, Fourth revised ed.; United Nations: New York, 2003. (27) Kossoy, A. A.; Sheinman, I. Y. J. Hazard. Mater. 2007, 142, 626– 638. (28) Jones, J. C. Fuel 1999, 78, 89–91.
Or for a zero order reaction, dT µAe -E=RT dt Taking the natural logarithm of eq 8, dT E1 ln þ lnðAÞ ¼dt RT
ð8Þ
ð9Þ
Plotting the reaction data for reactions obeying Arrhenius kinetics should be curves. The slopes of the initial part of the self-heat rate curve (or all the curves for a zero order (29) THT Analysis of Accelerating Rate Calorimetry Data: Theory; Thermal Hazard Technology: Bletchley, 2004.
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Table 1. Components of the Coal Powder moisture ash VM C H N S heat value (%) (%) (%, daf) (%, db) (%, db) (%, db) (%, db) (MJ kg-1) 2.96
9.66
45.77
70.64
3.38
1.12
2.84
25.88
reaction) are E/R and thus the activation energy is readily obtained. The curvature of the self-heat rate plot is indicative of the order of reaction. 2.3. Frank-Kamenetskii Model. Frank-Kamenetskii formulated one of the earliest mathematical models to study self-heating in reactive systems. The model, formulated for a one-dimensional slab, assumed that heat transfer was primarily due to conduction; the oxidation reaction, with the Arrhenius law temperature dependence, was assumed to be zero order in oxygen concentration. It has been made a common sense, in the study of thermal explosion of liquids, to stir liquid samples or circulate the air around the containers in an attempt to satisfy the prerequisite of Semenov.30 For solid, it has been already well confirmed that the values can be determined by applying the Frank-Kamenetskii critical condition for the thermal explosion as eq 10.28,31 δc ¼
r20 ΔHEFA expð -E=RT0 Þ KRT02
Figure 1. Heat flow plot of coal in closed vessel at a 0.2 °C min-1 heating rate (M = 0.5 g).
ð10Þ
where δc is the Frank-Kamenetskii critical parameter (dimensionless), r0 is the characteristic dimension (m), F is the bulk density (kg m-3), κ is the thermal conductivity (W m-1K-1), and T0 is the ambient temperature (K). The Frank-Kamenetskii theory allows for the temperature gradient to be taken into account. There could be a considerable resistance to heat transfer in the reacting system, or the system has reactants with a low thermal conductivity and the system having highly conducting walls. So the thermal conductivity κ and characteristic size r0 are considered as the main influence factors. The critical value of δc, above which there is ignition and below which there is failure to ignite, which is determined by the style of the package. Then, the SHOT, that is T0, can be calculated by eq 10.
Figure 2. Heat flow plot of coal in closed vessel at a 0.2 °C min-1 heating rate (M=0.19 g).
ARC is a bench-scale apparatus, which is designed to operate under almost true adiabatic condition. ARC can be used to characterize a runaway chemical reaction in a closed cell. An ARC (ARC 2000 model, Arthur D. Little) was independently used to evaluate the thermal stability of the coal. The ARC sample was placed in the titanium bomb, sealed in air atmosphere, initially heated to 40 °C, and then equilibrated for 5 min, followed by a 10 min seek for an exothermic signal (self-heating rate >0.02 °C min-1). If no exothermic signal was detected, then the temperature was increased by 5 °C at a rate of 2.0 °C min-1 with the subsequent repetition of the heat-wait-seek periods. This heat-wait-seek mode continued until to an exothermic signal was detected below 500 °C.
3. Experimental Section The coal sample was bought from China coal research institute, and the main components of the coal powder are listed in Table 1. The average particle diameter is 54 μm, which is measured by a Malvern Mastersizer 2000 particle analyzer. The coal sample was stored in a drier set at 50 °C for 10 h until constant weight was reached. The coal samples were exposed to atmospheric oxygen during their preparation and storage. The C80 microcalorimeter is a new microcalorimeter made by Setaram. The C80 is used in isotherm or scanning mode from ambient temperature to 300 °C; the C80 detects low power thermal phenomena. It is especially intended to provide measurements of heat of mixtures and reactions that will be useful in the chemical, specialty chemical, and petrochemical industries, among others.32 In this study, the prepared samples (0.5 or 0.19 g) were sealed in the C80 high-pressure stainless vessel (volume: 8.5 mL) in air atmosphere and then heated from ambient temperature to 300 °C at a 0.2 or 0.05 °C min-1 heating rate.
4. Result and Discussions 4.1. C80 Result. Figure 1 shows the heat flow plot of 0.5 g coal at a 0.2 °C min-1 heating rate from ambient temperature to 300 °C. The most-related exothermic peak for spontaneous combustion is the first exothermic peak, as the first reaction heat provides the necessary energy for the following reactions. Here, one exothermic process and another uncompleted exothermic process were detected in the C80 experiment of coal powder. The pure coal powder starts to release heat at 80 °C, and reaches a peak at 123 °C with the heat generation of -48.69 J g-1. After 200 °C, it starts to release heat again and drops sharply at 256 °C. After that, it continues to release heat. As the reactivity of coal rapidly decreases on exposure to oxygen, even at ambient temperatures, the thermal behaviors here detected are the thermal characteristics with surface oxidized. The onset temperature and peak temperature are put off because of the reactivity decreased at the coal surface.
(30) Semenov, N. N., Some Problems in Chemical Kinetics and Reactivity; Princeton University Press: Princeton, 1959. (31) Fisher, H. G.; Goetz, D. D. J. Loss Prevent. Proc. 1993, 6, 183– 194. (32) Sun, J. H.; Li, X. R.; Hasegawa, K.; Liao, G. X. J. Therm. Anal. Calorim. 2004, 76, 883–893.
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Figure 3. Heat flow plot of coal in closed vessel at a 0.05 °C min-1 heating rate (M = 0.5 g).
Figure 5. ln[(dH/dt)/ ΔHM0] vs T-1 for the coal C80 test in Figure 2.
Figure 6. ln[(dH/dt)/ ΔHM0] vs T-1 for the coal C80 test in Figure 3. Figure 4. ln[(dH/dt)/(ΔH M0)] vs T-1 for the coal C80 test in Figure 1.
reaction is out of the detectable range in Figure 2. The above tests show that the mass and heating rate do not play much effect on the thermal behavior trend of coal samples. However, the more rapid heating rate, the lower onset temperature and exothermic peak temperatures were detected. The more mass was tested, the lower onset temperature and exothermic peak temperature were detected. By using the method motioned in Section 2.1, the plots of ln[(dH/dt)/(ΔH M0)] versus T -1 were drawn and shown in Figures 4-7. The thermal decomposition activation energy and frequency factor of coal are obtained. For the first experiment shown in Figure 1, the activation energy and frequency factor are E=99.26 kJ mol-1 and A=1.22 1010 s-1, respectively. For the second and third experiments shown in Figures 2 and 3, the activation energies and frequency factors are E = 74.75 kJ mol-1 and A = 1.21 105 s-1 and E = 68.26 kJ mol-1 and A = 1.01 105 s-1, respectively. The difference in activation energy maybe contributed by the occasional factor, and mean value can be taken as 80.75 kJ mol-1. Furthermore, the fitting degrees, which means the correlation coefficient calculated from linear regression of the results, are close to 1, that is, they keep good linear relation. 4.2. ARC Result. Figure 7 shows the self-heating rate versus temperature plot of the coal sample tested using ARC (sample mass is 0.50 g, thermal inertia φ = 6.8). The heat release was detected when the temperature over 329 °C. The first stage obtained in C80 experiment does not appear in this record. This is because the ARC sensitivity of selfheating rate (0.02 °C min-1) is a low and the thermal inertia
A little thermal behavior difference was found in Figure 2 with less coal sample. Here, 0.19 g of coal was tested using C80 with the same heating rate, 0.2 °C min-1, from ambient temperature to 300 °C. Similar with the 0.5 g sample, it starts to release heat at 84 °C, and reaches to the peak temperature at 168 °C with the heat generation of -111.06 J g-1. After 268 °C, it releases heat again. The coal thermal response to heating rate was also investigated, and Figure 3 shows the thermal behavior of 0.5 g coal at a 0.05 °C min-1 heating rate from ambient temperature to 300 °C. Similar with above tests, the sample undergoes a small exothermic process and an uncompleted exothermic process, but its onset temperature and peak temperatures are 56 and 104 °C, respectively, which are lower than the above tests results with 0.2 °C min-1 heating rate. The heat generation is -65.56 J g-1, which is close to the result with the same mass sample test in Figure 1. After 200 °C, it starts to release heat again. In Figure 1, the first exothermic peak temperature is 123 °C, the endothermic reaction occurs at the rising stage of exothermic progress, 256 °C. In Figure 3, a small heat flow decreasing peak is also detected at 254 °C. In Figure 2, the first exothermic peak temperature is 168 °C, for less mass, the peak is put off. Comparing with Figure 1, it is supposed that the endothermic peak also appears after being increased to 133 °C; and then in Figure 3, the endothermic peak should appear at 301 °C, which is out of the C80 working range (from ambient temperature to 300 °C). Therefore, it is speculated that the endothermic 4874
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: DOI:10.1021/ef900372w
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Figure 7. The self-heating rate versus temperature plot of the coal in the ARC (M = 0.5 g, φ = 6.8).
Figure 8. ln(dT/dt) vs T -1 for the coal ARC test in Figure 7.
calculated (heat transfer coefficient κ = 0.143 W m-1K-1, characteristic dimension r0=d/2=4 m, F=1500 kg m-3).28,33,34 For the C80 experiments, the SHOTs based on the three experiments were calculated as 60, 90, and 68 °C, respectively. The SHOT values from the experiments with 0.5 g of coal are very close, and they are 60 and 68 °C, respectively. However, the SHOT calculated from the experiment result with 0.19 g of coal is higher. It is known that the SHOT calculated from the thermodynamics and kinetics, for the less mass sample, because it releases too little heat to be detected by the C80, yielded a lower SHOT value. On another aspect, it also maybe caused by the incomplete oxidation of coal at low temperature with the limited oxygen in the sealed vessel for the larger mass sample. Therefore, the larger sample mass will result in a lower SHOT. For the ARC experiment, the SHOT based on the experiment was calculated is 164 °C, which is higher than the C80 results. This difference of SHOT may be mainly caused by the calorimeters. In these experiments, the heating rates and scanning modes are different. For the C80 test, the heating rate was set as 0.2 °C min-1 or 0.05 °C min-1, which is quite slower than the ARC heating rate, 2.0 °C min-1, in its heating stage. At the lower heating rate, more heat effect can be detected by the instruments, which can discover smaller reactions. Another aspect is the sensitivity of the calorimeters. The resolution of C80 is 0.10 μW, and that for the ARC detection of exotherm is 0.02 °C min-1, as the bomb weight is 25.34 g with capacity of 0.518 J g-1 K-1, using ΔH = mcpbΔT, then the heat absorption by bomb is 4.38 mW; the ARC minimum detectable accuracy is bigger than 4.38 mW in value. The heating rate and accuracy of C80 are better than that of ARC, and the bomb thermal inertia of ARC also decrease its accuracy, therefore, the C80 result should be more reliable and closer to the reality, which is in agreement with Yu and Hasegawa’s results.35 4.4. Mass effect on the SHOT. For the cylinder storehouse, here the diameter is specified to the twice length of highness, that is d=2l, then, from eq 10 and 13, the following eq 14 is obtained.
is high in this experiment. Here the thermal inertia φ is determined by eq 11,29 Mb cpb φ ¼ 1þ ð11Þ M s cps where Mb is the mass of bomb, cpb is the specific heat of the bomb, Ms is the mass of sample, and cps is the specific heat of the sample. In this experiment, the φ value is 6.8, calculated by eq 11. The sensitivity of the ARC of 0.02 °C min-1 is unchangeable, whereas the value of the φ depends on sample mass and bomb mass as shown in eq 11. So, in order to measure the heat generation at lower temperatures, it is favorable to increase the mass of the sample. However, it should be pointed out that if the mass of the sample is too large, the danger of rapid reaction at higher temperatures could exceed the insulating controlling ability of the ARC and even lead to the bomb exploding. For test safety, the sample mass in this experiment is small when the φ value is high, so the result should be modified based on the φ value as: -
ΔH ¼ φ cv ðΔTad Þsys
ð12Þ
where ΔH is the heat of reaction, cv is the average specific heat of the sample at constant volume over the course of the reaction, and ΔTad is the adiabatic temperature rise (K). By this method, the heat of reaction is calculated as ΔH=485.5 J g-1. By using the ARC method, the plot of ln(dT/dt) versus T -1 for the coal ARC test in Figure 7 was shown in Figure 8. The activation energy and frequency factor are E=127.0 kJ mol-1 and A=6.55 108 s-1, respectively. Comparing it with the C80 results, the activation energy is larger than the former results, and the fitting degree is 0.938, which is lower than the C80 results. 4.3. Evaluation of SHOT. A cylinder storehouse with 300 tons of coal was selected as the study object. The height (l) and diameter (d) of the storehouse are 4 and 8 m, respectively. For such a finite cylinder (l < d), the Frank-Kamenetskii critical parameter δc can be calculated by: δc ¼ 2:0þ0:195ðd=lÞ2
ð13Þ
δc ¼
Then, the critical parameter is calculated as δc=2.78. Substituting the relevant parameters into eq 10, the SHOTs can be (33) Sun, J. H.; Wang, Q. S.; Sun, Z. H., Catalytic Effects of Impurity on the Thermal Stability of Some Typical Hazardous Materials. In New Research on Hazardous Materials, 1st ed.; Warey, P. B., Ed. Nova Science Publishers, Inc.: New York, 2006; pp 111-160.
M 2=3 ΔHEFA expð -E=RTÞ0 πF KRT02
ð14Þ
(34) Tang, Q. J.; Zhang, G. S.; Chen, Q. H. Jiangxi Coal Sci. Technol. 2006, 2006, 24–26. (35) Yu, Y. H.; Hasegawa, K. J. Hazard. Mater. 1996, 45, 193–205.
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exothermal process near 80 °C with mean heat generation of -75.1 J g-1 for the C80 experiment. The activation energies for the first exothermal process are calculated for the three experiments, the mean value is 80.76 kJ mol-1. The activation energy value got from the ARC result is 127.0 kJ mol-1, which is larger than that from the C80 results. The SHOT is an important parameter to evaluate the hazardous degree of the materials. The SHOTs based on the ARC experiment and three C80 experiments were calculated are 164, 60, 90, and 68 °C, respectively, for a 300 ton coal stockpile. The SHOT obtained from ARC results is very higher and not of much significance for coal storage, but it can be an assistant method for C80 experiments. Further research on the mass amount effect on SHOT shows that if the coal mass is less than 12 tons, the danger of thermal spontaneous combustion is quite less, however, if the mass amount is over 12 000 tons, the danger of thermal spontaneous combustion is difficult to avoid, even in ambient temperature, and then special measures should be taken to keep the stockpile safe.
Figure 9. SHOT plot varying with the coal mass.
On the basis of eq 14, given a certain coal storage M, the SHOT can be obtained. The SHOT changing plot with mass was shown in Figure 9 for the C80 experiment 3. Figure 9 shows that with the storage increasing the SHOT decreases. In the event that the mass is given as 12 000 tons, the SHOT lowers to 34 °C. Such a lower temperature is very dangerous for storage. In the south of China, the temperature in summer is higher than 34 °C, which means that storehouses of 12 000 tons of coal can be ignited by itself in natural situations. On the other hand, when the mass is given at 12 tons, the SHOT reaches to 104 °C. Comparing it with the experimental result in Figure 3, when the temperature is over 104 °C, the heat generation turns to decreasing, and then the heat accumulation is less than heat loss. In this condition, the coal no longer undergoes the spontaneous combustion process. Therefore, if the coal stockpile is smaller than 12 tons, the dangers from thermal spontaneous combustion is very less. However, one should note that the potential risk for a coal stockpile of 12 tons is less, which does not means it is always safe. The SHOT predicts the seven days temperature rise, if the stockpile is kept in better ventilation and a period longer than seven days, spontaneous combustion may be occurring. If the stockpile is over 12 000 tons, the dangers from thermal spontaneous combustion is difficult to avoid, and special measures should be taken to keep the stockpile cool. It should be note that there are some limitations in the Frank-Kamenetskii model. It does not account for oxygen depletion within the stockpile, the particle size, and size segregation of particles, sample prior oxidation history, wind velocity, and so on. Therefore, comparing with other sophisticated models, as proposed by Carras and Young,36 Krishnaswamy et al.,37 Fierro et al.,38 and so on, it is a simple and easy method to evaluate the self-heating behavior of coal in a confined and sealed space. It can be used as a primary method to predict the stockpile scale, for the prediction of detail self-heating history in the stockpile, the sophisticated models should be employed and numerical simulation work should be done.
Nomenclature A=frequency factor (s-1) C=concentration (g mol m-3) cv= average specific heat(J g-1 K-1) cpb =specific heat of bomb(J g-1 K-1) cps =specific heat of sample (J g-1 K-1) d=reactant diameter (m) dH/dt=overall heat flow (W) dT/dt=self-heat rate (K s-1) E=activation energy (J mol-1) ΔH=heat of reaction (J g-1) l=reactant height (m) M0 =initial mass of reactant (g) M=mass of reactant (g) Mb =mass of bomb(g) Ms =mass of sample(g) n=reaction order r0=reactant characteristic dimension, for cylinder, r0 is its radius R=universal gas constant (J K-1 mol-1) t=time (s) T0 =ambient temperature (K) T=temperature of system (K) ΔTad =adiabatic temperature rise (K) x=conversion rate, x=(M0 - M)/M0 Greek symbols φ=thermal inertia δc =Frank-Kamenetskii critical parameter κ=heat transfer coefficient (J m-1 K-1 s-1) F=reactant density (g m-3)
5. Conclusions By measuring the thermal behavior of coal at elevated temperature by C80 and ARC, the thermal behaviors of coal were examined. It was observed that the coal undergoes an
Subscripts
(36) Carras, J. N.; Young, B. C. Prog. Energy Combust. 1994, 20 1–15. (37) Krishnaswamy, S.; Agarwal, P. K.; Gunn, R. D. Fuel 1996, 75, 353–362. (38) Fierro, V.; Miranda, J. L.; Romero, C.; Andres, J. M.; Arriaga, A.; Schmal, D. Fuel 2001, 80, 125–134.
Acknowledgment. This study was supported by Anhui College Science Research Program (grant No. KJ2007B262). A financial support from China National Natural Science Foundation Key Project (grant No. 50536030) is also appreciated. We also thank Dr. C.J. Wang for his kindly help in the ARC experiment.
sys=system
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