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Thermodynamic characteristics of coal reaction under low oxygen concentration conditions
Journal of the Energy Institute xxx (2016) 1e12
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Thermodynamic characteristics of coal reaction under low oxygen concentration conditions Xuyao Qi a, b, *, Qizhong Li a, Huijun Zhang a, b, Haihui Xin a, b a b
School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China Key Laboratory of Gas and Fire Control for Coal Mines, China University of Mining and Technology, Xuzhou 221116, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2016 Received in revised form 12 May 2016 Accepted 20 May 2016 Available online xxx
In order to further understand the characteristics of coal reaction under low oxygen concentration atmosphere, this study tested the thermodynamic characteristics during reaction processes under low oxygen concentrations. The kinetic factors were also analyzed based on testing results. The results show that there are some characteristic temperatures during coal spontaneous combustion and the influences of oxygen concentration on these temperatures are different. The influences of oxygen concentration on the mechanism function and kinetic parameters depend on the coal ranks and the reaction stages. They are different at different reaction stages for the same coal, and at the same reaction stage for different coals. This phenomenon is essentially resulted from the different influences of oxygen concentration on the micro reaction sequences during coal spontaneous combustion. The study will be helpful for further understanding of the actual development process of coal spontaneous combustion. © 2016 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Coal Spontaneous combustion Characteristic temperature Activation energy Mechanism function
1. Introduction The spontaneous combustion of coal often occurs during the processes of coal mining, storage or applications. Almost all the coal mining countries are facing the danger of coal spontaneous combustion [1e4]. In addition to burning of coal resource, different types of harmful gases are generated during the process of coal spontaneous combustion, which will seriously threaten people's safety and environment. Under some special conditions, coal spontaneous combustion may result in serious gas or dust explosions. The spontaneous combustion of coal is resulted from the accumulation of heat released from coal reaction. So the thermodynamic characteristics in different stages of coal spontaneous combustion are important for further understanding of this disaster. Many investigators have done a lot of studies on the heat production and dynamic parameters during coal spontaneous combustion. Most of these previous investigations focused on the coal reaction under oxygenic or dry-air conditions. In contrast, few literatures have been published on the topic of thermodynamic characteristics under low oxygen concentration conditions. By the method of TG, some researchers investigated the thermogravimetric changes, exothermic properties and thermodynamic parameters during coal spontaneous combustion process under the conditions of dry-air, high concentration oxygen or pure oxygen [5e14]. Based on the TG testing results, some investigators analyzed the effects of different oxygen concentration on the reaction degree, ignition time and characteristic temperatures of coal [15e18]. However, most of these conclusions are qualitative and cannot provide enough data for further understanding coal reaction under low oxygen concentration conditions. By the method of DSC, some researchers investigated the heat release characteristics of coal reaction under the conditions of dry-air or pure oxygen atmosphere, and proposed some determining methods for the propensity of coal to spontaneous combustion [19e22]. By the method of DTA, Pis et al. [23] tested the thermal characteristics of coal samples oxidized for different times under air atmosphere at 200 C and also proposed determining methods for the propensity of coal to spontaneous combustion. Haykiri-Acma et al. [24] applied DTA and DTG techniques investigated combustion characteristics of bituminous coal samples, found that volatile matter and fixed carbon contents of the investigated samples on a dry-ash free basis were seen to have a strong effect on the cumulative mass loss values. Marinov et al. [25] used TGA/DTA to study the changes in the combustion behavior of microbial treated
* Corresponding author. School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China. E-mail address: [email protected] (X. Qi). http://dx.doi.org/10.1016/j.joei.2016.05.007 1743-9671/© 2016 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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coals. In recent years, some new thermal analysis instruments began to be used in the field of coal spontaneous combustion. Using C80 Micro-calorimeter, Wang et al. [26,27] and Qi et al. [28] researched the exothermic characteristics of coal oxidation at temperatures ranging from room temperature to 300 C, found that the oxygen consumption was proportional to the amount of heat release. Using Pulse Calorimeter, Li et al. [29,30] analyzed the thermodynamic properties of the adsorption heat, condensation heat and evaporation heat of coal during coal oxidation process at low temperatures. In addition, some investigators studied the thermal characteristics of coal oxidation through the analysis based on chemical theory [31e34]. Xu [31,32] inferred the distribution range of exothermic intensity of coal oxidation at low temperatures according to oxygen consumption, heat production rate and bond energy change, and also analyzed the influence of oxygen concentration on the exothermic intensity of coal oxidation [33]. Cheng et al. [34] deduced the thermal effect during coal oxidation at low temperatures under dry-air atmosphere through the analysis based on chemical theory. In summary, the previous investigations on thermal effects of coal reaction were usually carried out under the atmospheres of pure oxygen, dry-air or other sufficient oxygen supply conditions, while the investigation under low oxygen atmosphere is few. And these existing few investigations taken under low oxygen concentration conditions cannot provide enough data for further understanding coal reaction under low oxygen concentration conditions. Actually, most of the coal spontaneous combustion zones, such as goaf, loose coal bulk, sealed fire zone, coal pile, coalfield fire zone, etc., usually occur under the conditions of low oxygen concentration below 15%. Therefore, the existing research results of coal reaction obtained under the experimental conditions of sufficient oxygen supply cannot reflect the actual process of coal spontaneous combustion, and cannot provide an efficient guidance for the prevention of this hazard. This study analyzed the thermodynamic characteristics during the reaction processes of three different ranks of coal samples under different oxygen atmospheres by the thermogravimetric analyzing method. The results will be helpful to further understand of the actual development process of coal spontaneous combustion occurring during the processes of coal mining, storage or applications.
2. Experimental 2.1. Coal samples Three ranks of raw coal were collected from different coalfields in China, i.e. Changcun jet coal, Jiaxiang gas coal and Zhaogezhuang fat coal. They have different propensity to spontaneous combustion. Based on the statistics, their shortest times to spontaneous combustion are 7 days, 25 days and 182 days. The coal samples were processed to remove the surface layers and the material was then crushed in an oxygenfree glove box. This resulted in the coal particles ranging between 0.18 mm and 0.38 mm in size which were sieved and used as the experimental coal samples. The coal particles were kept under an inert atmosphere before they were tested. Considering on the effects of storage on coal self-heating [35,36], the coal samples should be tested no longer than one week. The proximate analyses of the samples are listed in Table 1.
2.2. Testing facility The testing system includes SDT-Q600 synchronous thermal analyzer, MF-4 gas mixture, pure nitrogen bottle, pure oxygen bottle and computer (see Fig. 1). The synchronous thermal analyzer can continuously test TG curves during coal reaction process. The pure nitrogen bottle and pure oxygen bottle are connected with gas mixture through gas tube, which form a high precision gas mixture system. This gas mixture system can provide mixed N2eO2 gas with different oxygen concentrations by setting the gas pressure, gas flow rate and mixing ratio of pure nitrogen and pure oxygen. So the coal reaction processes under the atmospheres at different oxygen concentrations can be simulated accurately. During the testing process, the coal sample will be put into the coal reaction vessel. Then we turn on the instruments and set their running parameters, such as initial temperature, final temperature and temperature rising rate. At the same time, let the mixed gas with different oxygen concentrations to flow into the coal reaction vessel. The thermal analysis data during coal reaction under different oxygen atmosphere will be collected on-line until reaching the final temperature.
2.3. Testing procedure First, 8 mg of coal particles were packed into the coal reaction vessel. Then the programmed temperature program was set to run at a programmed heating rate of 5 C/min while mixture gas of oxygen and nitrogen with different oxygen concentration was permitted to flow through the coal reaction vessel at a rate of 100 mL/min. The temperature was set rising from room temperature to 1000 C. The oxygen concentration used in this study include 5%, 7%, 10%, 13%, 16%, 19% and 21%. During the temperature rising process, the instrument continuously monitored the thermodynamic data and show with changing curves.
Table 1 Proximate analyses (air-dry basis) of coal samples. Coal samples
M (wt%)
A (wt%)
V (wt%)
FC (wt%)
Qnet (MJ/kg)
H (wt%)
St (wt%)
Jet coal Gas coal Fat coal
9.86 2.82 1.20
20.65 7.85 17.48
30.01 34.82 26.71
34.08 54.51 55.10
21.81 31.89 28.48
4.49 3.21 4.59
0.37 0.21 1.16
Notes: M e moisture content; A e ash content; V e volatile matter; FC e fixed carbon; Qnet e net calorific value; H e hydrogen content; St e total sulfur content.
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3. Results and discussion 3.1. General trend and subsection characteristics of thermogravimetric changes According to the testing results, thermo gravimetry (TG) curves during reaction processes of three ranks of coal samples under different oxygen concentration conditions were obtained, as shown in Fig. 2. The testing results show that the weight changing trends with the rise of temperature are similar for different coal reaction process. With the decrease of oxygen concentration, the combustion process was delayed and the combustion intensity was weakened. This phenomenon indicates that the coal oxidation and combustion process is inhibited by low oxygen concentration. The reaction rate of coal is slowed down in varying degrees with the decrease of oxygen concentration. Consequently, the whole reaction process of coal oxidation and combustion is delayed, which results in the differences in TG curves under different oxygen concentrations. Based on the previous literatures and a series of TG curves obtained in this study, there is piecewise characteristics in the TG curves during coal reaction process. There are several characteristic temperature points existing on the TG curves, as shown in Fig. 3 [37e39]. (1) The critical temperature T1 in coal reaction process. That is the temperature corresponding to the first maximum peak point of weight loss rate. After coal temperature reaching this temperature, the reaction rate between oxygen and active groups existing in coals will accelerate obviously, which result in the increase of oxygen consumption rate and released gas production amount (CxHy, H2, CO and CO2 etc.). In addition, due to the rise of coal temperature, more and more initial absorbed gas in coal pore structure begin to desorb and release with the gas produced by coal reaction. The consumption of coal reaction and desorption of initial absorbed gas result in the increase of coal weight loss rate and formation of the first maximum peak point of weight loss rate on the curve. (2) The dry temperature T2. At this temperature, the losing of coal mass would be the minimum in the range before reaching ignition temperature. When coal temperature reaches T2, some functional groups existing in coal structure, such as high molecular aromatic nucleus, hydrocarbon alkyl side chains, oxygen-containing functional groups, bridge bonds and some small molecules, begin to crack or depolymerize. The reaction of these groups will release different types of gases, which consume coal sample. On the other hand, the compound intensity between coal and oxygen reactions will be enhanced in this stage. The coaleoxygen complex produced will not decompose until reaching a higher temperature, which increase the coal mass. The coal consumption by depolymerization and the mass increase by coaleoxygen compound become a dynamic equilibrium state at the dry temperature T2. As a result, the coal sample weight will not decrease from this point. (3) The maximum weight temperature T3, is the temperature when coal's weight by absorbing oxygen reaches the maximum value. Due to the increase of temperature, the number of surface active structures of coal is increased, and the aromatic ring structures which are not involved in the oxidation process at low temperature are also participating in the oxidation reaction, the amount of active structures in coal and the adsorption oxygen quantity reach the maximum value, results in coal weight increasing to the maximum. Meanwhile, coal weight loses quickly after T3. (4) The speed-up temperature T4, namely the temperature corresponding to the maximum weight increasing rate. At this temperature point, many cyclic large molecules existing in coal decompose rapidly. These decomposition reactions form a large amount of active groups, which obviously enhance the compound intensity between coal and oxygen. So the mass increase by coaleoxygen compound will higher than the coal consumption by depolymerization and desorption of initial gas in coal. As a result, the coal weight increasing rate reaches a peak point value. (5) The ignition temperature T5, namely the temperature at which coal samples begin to burn. This value reflects the propensity of coal to spontaneous combustion. (6) The maximum peak temperature T6. It is the point that the chemical reaction intensity of coal reaches to the largest. The corresponding decreasing rate of coal weight reaches to the largest. It shows as the valley point on the DTG curve. (7) The burnout temperature T7. The combustible components of coal have burnt out at all. Its value can reflect the length of the time that the coal oxidation and burning lasting.
3.2. Characteristic temperatures under different oxygen concentration 3.2.1. The critical temperature (T1) Based on the testing results, the changing trends of T1 under different oxygen concentration conditions were obtained, as shown in Fig. 4. The critical temperature (T1) is the starting point for automatic acceleration of coaleoxygen complex reaction. With the decrease of oxygen concentration, the testing results show that the T1 demonstrate a “W” type changing trend, i.e. decrease below 8%, increase in the
Fig. 1. The testing system: (a) Gas bottles; (b) MF-4 dynamic stability distribution device; (c) SDT-Q600 synchronous thermal analyzer; (d) Computer.
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Fig. 2. TG curves under different oxygen concentration conditions: a. jet coal, b. gas coal, c. fat coal.
Fig. 3. Diagram of characteristic temperature points.
range of 8e13%, decrease in the range of 13e18% and increase again above 18%. Under low O2 concentration conditions (O2% < 8%), the changing ranges of T1 are greater than that under the conditions of oxygen concentrations bigger than 8%. The changing trends of T1 with the changes of oxygen concentration are essentially caused by the different reaction intensity of coaleoxygen compound under different oxygen supply conditions. The low oxygen concentration conditions inhibit the reaction intensity between coal and oxygen, which delay the formation of the balance of coal consumption and mass increase by coaleoxygen complex. In contrast, with the rise of oxygen concentration, the active groups existing in coal gain more chances to contact and react with oxygen, which enhance the intensity of coal oxidation. The formation of the balance will need shorter time. As a result, under the oxygen concentration Please cite this article in press as: X. Qi, et al., Thermodynamic characteristics of coal reaction under low oxygen concentration conditions, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.05.007
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Fig. 4. The T1 values under different oxygen concentration conditions.
below 8%, the T1 rapidly decrease with the increase of oxygen concentrations. Under the oxygen concentrations ranging in 8e13%, the coaleoxygen compound reaction rate accelerate again with a large amount of gas release. These released gases limit the diffusion of oxygen molecules into coal pore structure and inhibit the chemical reaction rate of coal oxidation. So the corresponding balance need more time to form, which show as the rise of T1. Under the oxygen concentrations ranging in 13%e18%, an appropriate gradient of oxygen concentration is formed around coal microstructure and enhance the oxygen diffusion in pore structure of coal. So the time for the formation of corresponding equilibrium becomes short, which shows as the second decrease of T1. If oxygen concentration increases to a value higher than 18%, an over large oxygen concentration gradient forms, which inhibits oxygen to further diffuse into pore structure of coal and the desorption of initial gas absorbed in coal. So the reaction rate of coal is inhibited again, which leads to the second rise phenomenon of the critical temperature. 3.2.2. The dry temperature (T2) Based on the testing results, the changing trends of the dry temperature (T2) under different oxygen concentration conditions were obtained, as shown in Fig. 5. For the reaction process of jet coal under 5% oxygen concentration, the oxygen consumption rate is very large at low temperatures, the great shortage of oxygen inhibits the coaleoxygen compound rate obviously. The coaleoxygen compound oxygen uptake is far less than the volume of gas desorption from coal, which means that the coal weight is always in a decreasing state at low temperatures. So the dry temperature T2, the maximum weight temperature T3 and the speed-up temperature T4 for jet coal reacted in 5% oxygen concentration atmosphere cannot be calculated. The testing results show that the dry temperatures generally increase in varying degrees with the decrease of oxygen concentration under low oxygen concentration. In contrast with the dry temperatures tested under the 21% oxygen concentration atmosphere, the increase ranges of dry temperatures tested under 5% oxygen concentration atmosphere obviously differ from each other for different ranks of
Fig. 5. The T2 values under different oxygen concentration conditions.
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coal samples. For jet coal, gas coal and fat coal, their corresponding temperature increasing ranges are 24.3%, 17.0% and 21.5% respectively (corresponding increased values: 46.3 C, 26.8 C and 43.2 C). The decrease of oxygen concentration can slow down the fracture of side chains existing in coal structure and inhibit the reaction of the active groups, pyrolysis and thermal decomposition process. The dry temperature change trends of jet coal and fat coal are similar, while the gas coal's dry temperature is significantly smaller. This phenomenon is formed due to the special structure characteristics of gas coal. The alkyl side chains, oxygen-containing groups, bridge bonds of big molecular aromatic nucleus and some small molecules existing in gas coal structure are easier to crack or depolymerize, and release different types of productions. So the dynamic balance may achieve earlier and the corresponding dry temperature of gas coal is lower than the other two rank ones. 3.2.3. The maximum weight temperature (T3) Based on the testing results, the changing trends of the maximum weight temperature (T3) under different oxygen concentration conditions were obtained, as shown in Fig. 6. The testing results show that the maximum weight temperatures increase in varying degrees with the decrease of oxygen concentration under low oxygen concentration. It indicates that the generation rate of active structures is inhibited by the low oxygen concentration. Under low oxygen atmosphere, it will take longer time to adsorb oxygen and generate a large number of active groups for the following decomposition and combustion process. Therefore, the lower the oxygen concentration is, the longer the coal oxygen-absorption process will continue. So the T3 change to a larger value. In addition, it shows that the oxygen concentration has a greater influence on jet coal than gas coal and fat coal. If oxygen concentration decreases from 21% to 7%, the rising range is 14.75% for jet coal, while they are only 8.6% for fat coal. 3.2.4. The speed-up temperature (T4) Based on the testing results, the changing trends of the speed-up temperature (T4) under different oxygen concentration conditions were obtained, as shown in Fig. 7. The testing results show that the changing ranges of speed-up temperatures with the changes of oxygen concentration are smaller than other characteristic temperatures. It indicates that the speed-up temperature is not affected by O2 concentration very great. In addition, the changing rule of speed-up temperature with the changes of oxygen concentration conditions is also affected by coal ranks. Among the coal samples tested, the speed-up temperatures of gas coal and fat coal rise with the decrease of oxygen concentration. In contrast with the speed-up temperatures tested under 21% oxygen concentration atmosphere, the temperature rising ranges of gas coal and fat coal are only 5.1% and 6.2% respectively (corresponding increased values: 13.3 C and 18.0 C). On the other hand, the speed-up temperature of jet coal shows a certain fluctuation with the changes of oxygen concentration, the relative minimum (212.2 C) and relative maximum (217.5 C) were found at 13% and 16% O2 concentrations respectively. This phenomenon is due to the different types and amount of active groups existing in different ranks of coal samples. The lower rank coal samples usually have more types and larger amount of active groups than higher rank coal samples. So there will be more reaction sequences occurring during the reaction process of low rank coal samples. The interaction among different reaction sequences and their different affection degree by oxygen supply conditions are the main reasons for the fluctuation on the changing curve. 3.2.5. The ignition temperature (T5) Based on the testing results, the changing trends of the ignition temperature (T5) under different oxygen concentration conditions were obtained, as shown in Fig. 8. With the decrease of O2 concentration, the ignition temperatures similarly keep as a constant at first and then transfer to continuously rising trend. Under the oxygen concentrations ranging in 19e21%, the ignition temperatures almost keep as a constant. On the other hand, if under the oxygen concentrations below 19%, the ignition temperatures present a nearly linearly increasing trend with decrease of oxygen concentration. In contrast with the ignition temperatures under 19% O2 concentration atmosphere, the temperature
Fig. 6. The T3 values under different oxygen concentration conditions.
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Fig. 7. The T4 values under different oxygen concentration conditions.
Fig. 8. The T5 values under different oxygen concentration conditions.
increasing ranges of ignition temperatures under 5% O2 concentration conditions of jet coal, gas coal and fat coal are 5.1%, 4.2% and 6.1% (corresponding increased values: 21.8 C, 18.7 C, 28.8 C). The coal spontaneous combustion is essentially the result of acceleration of thermal released in the contact and reaction between oxygen and active groups existing in coal. Under an oxygen concentration above 19%, the oxygen supplied is enough and the reaction intensity is strong at first. So the remained oxygen and gas production will form a gas bearing ring, which limit the release of gas production and the diffusion of oxygen. As a result, the reaction intensity will slow down later and lead to the decrease of ignition temperature. However, this closure effect of gas bearing ring do not produce a substantial and continuous effect on the coal spontaneous combustion process. The corresponding affecting range on ignition temperature is not very great. 3.2.6. The maximum peak temperature (T6) Based on the testing results, the changing trends of the maximum peak temperature (T6) under different oxygen concentration conditions were obtained, as shown in Fig. 9. The results show that the maximum peak temperatures present an increasing trend with the decrease of O2 concentration. In contrast with the ignition temperatures under 21% O2 concentration atmosphere, the temperature increasing ranges of ignition temperatures under 5% O2 concentration conditions of jet coal, gas coal and fat coal are 6.8%, 7.6% and 9% respectively (corresponding increased values: 33.3 C, 38.9 C, 49.2 C). Coal spontaneous combustion is essentially due to the heat release during the reaction processes of initial and secondary active groups. The decrease of oxygen concentration weakens the generation of active groups and cannot provide enough new active centers for further reaction. It leads to the decrease of oxidation reaction rate and heat release rate, which result in the delay for reaching the maximum weight and the rise of the corresponding maximum peak temperatures.
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Fig. 9. The T6 values under different oxygen concentration conditions.
3.2.7. The burnout temperature (T7) Based on the testing results, the changing trends of the burnout temperature (T7) under different oxygen concentration conditions were obtained, as shown in Fig. 10. The testing results show that the burnout temperatures increase with the decrease of O2 concentration. In contrast with the ignition temperatures under 21% O2 concentration atmosphere, the temperature increasing ranges of burnout temperatures under 5% O2 concentration conditions of jet coal, gas coal and fat coal are 8.9%, 12.9% and 13.2% (corresponding increased values: 48.1 C, 76.7 C and 80.8 C). It shows that we can prevent a fire zone throughout keeping a low oxygen concentration condition in this area. During coal spontaneous combustion process, the oxygen amount needed for the reaction of active groups is a determined value. Under low oxygen concentration conditions, the coal reaction intensity and gas diffusion are small. The corresponding reaction time will be longer.
3.3. Kinetic parameters at different reaction stages 3.3.1. Calculation theory Coal oxidation and combustion is a typical gasesolid reaction process. The kinetic parameters in this process, such as reaction mechanism function f(a), activation energy E and pre-exponential factor A, can be determined by thermogravimetric analysis method. Under the condition of single scan rate, there are many integral methods and differential methods for the calculation of kinetic parameters [40]. In this study, the CoatseRedfern integral method and AchareBrindleyeSharpeWendworth differential method are used to solve the thermo kinetic factors of coal oxidation at low-temperatures and combustion stage at high-temperatures under low oxygen concentration conditions. (1) CoatseRedfern integral method According to thermodynamics theory, the CoatseRedfern equation is expressed as follows [41,42].
GðaÞ AR 2RT E 1 ¼ ln ln E RT bE T2
(1)
where GðaÞ is the integral form of coal oxidation mechanism function; T the coal temperature, K; E the activation energy, J/mol; A the preexponential factor, K/s; b the heating rate, C/min, which is set as 5 C/min in this testing; R the gas constant, 8.314 J/(mol K); a the weight loss rate, which can be obtained by the equation as follows.
a¼
m0 mt m0 m∞
(2)
where m0 is the initial mass of coal, g; mt the coal mass at the time of t, g; m∞ the remaining mass after reaction, g. For general temperature ranges and most activation energy, the value of E/RT in Equation (1) is far greater than 1, and the value of ð1 2RT=EÞ is approximately equal to 1. Therefore, the CoatseRedfern equation can be approximately expressed as,
GðaÞ AR E ¼ ln ln 2 bE RT T
(3)
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Fig. 10. The T7 values under different oxygen concentration conditions.
(2) AchareBrindleyeSharpeWendworth differential method [43,44] According to thermodynamics theory, the AchareBrindleyeSharpeWendworth equation is expressed as follows.
ln
da A E ¼ ln f ðaÞdT b RT
(4)
where f ðaÞ is differential form of coal oxidation reaction mechanism function. For coal spontaneous combustion process, its mechanism function demonstrates a relationship functional between coaleoxygen reaction rate and weight losing rate a. The characteristics of thermal analysis curves are depended on the mechanism function. There are many gasesolid reaction mechanism functions proposed in previous literatures [45]. Considering on the piecewise characteristics of coal spontaneous combustion, we selected the Bagchi method for determining the mechanism function model, the activation energy E and the preexponential factor A. There are some steps in the application of the Bagchi method. Firstly, substitute the thermogravimetric testing results of different reaction stages in different gasesolid reaction mechanism models [45]. Secondly, the obtained G(a) and f(a) values are substituted into equations (3) and (4) respectively, and then analyze the linear correlation coefficients between the two functions. Based on the comprehensive comparison, the mechanism function model having the largest linear correlation coefficient was selected as the mechanism functions, as shown in Table 2. Finally, the trendlines were fitted to the calculation data, with ln½GðaÞ=T 2 as Y axis and 1/T as X axis. Then the activation energy E and pre-exponential factor A were obtained based on the slope and intercept of fitting equations. 3.3.2. Discussion on the calculation results The coal spontaneous combustion process presents an obvious piecewise characteristic. The TG changing characteristics of coal at different reaction stages are also different. From the TG changing curves, coal spontaneous combustion process can be generally divided into four stages: water evaporation and desorption weightlessness stage (onset temperature ~ T2), oxygen-absorption mass-gain stage (T2 ~ T3), decomposition and combustion weight loss stage (T3 ~ T7) and burnout stage (T7 ~ end temperature). Besides of coal oxidation, the water evaporation and desorption of gases absorbed in raw coal (i.e. CH4, CO2 etc.) also occur during water evaporation and desorption weightlessness stage. Thus, the kinetic parameters calculated at this stage cannot reflect the actual oxidation process of coal. Therefore, the second stage and third stage are respectively selected for the calculation of kinetic parameters during coal oxidation at low-temperatures and coal combustion at high-temperatures. Based on the calculation methods described in Section 3.3.1, the activation energy and the preexponential factor at different reaction stages can be determined respectively, as shown in Figs. 11 and 12. Fig. 11 shows that the activation energy E and pre-exponential factor A decrease with the increase of O2 concentration during coal oxidation at low temperatures. It indicates that decreasing of O2 concentration can inhibit the types and quantity of active groups taking part in the coal oxidation at this stage. As a result, the corresponding heat generation during coal oxidation would decrease, and the activation of some micro structures in coal need to absorb more heat, which cannot provide enough active centers for further reaction.
Table 2 The mechanism functions of different coal samples in different reaction stages. Mechanism functions
Jet coal
Gas coal
Fat coal
Oxidation at low-temperatures
G(a) ¼ ln(1a) f(a)¼(1a) G(a) ¼ 1(1a)1/3 f(a)¼3(1a)2/3
G(a) ¼ [(1a)1/31]2 f(a) ¼ 3/2[(1a)1/31]1$(1a)4/3 G(a) ¼ 1(1a)1/2 f(a)¼2(1a)1/2
G(a) ¼ [(1a)1/31]2 f(a) ¼ 3/2[(1a)1/31]1$(1a)4/3 G(a) ¼ 1(1a)1/3 f(a)¼3(1a)2/3
Combustion at high-temperatures
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Fig. 11. Relationship between kinetic parameters and oxygen concentration during coal combustion at low temperatures. (a) activation energy E versus oxygen concentration; (b) pre-exponential factor A versus oxygen concentration.
Fig. 12. Relationship between kinetic parameters and oxygen concentration during coal combustion at high temperatures. (a) activation energy E versus oxygen concentration; (b) pre-exponential factor A versus oxygen concentration.
Fig. 12 shows that the activation energy E and pre-exponential factor A increase with the increase of O2 concentration during coal combustion at high temperatures. It is completely opposite comparing with the changing trend during coal oxidation at low temperatures. And the influence degree is smaller than low temperature stage. As known, the coal spontaneous combustion is a piecewise process. The main reaction sequence and influence factors differ from each other for different reaction stages. At high temperatures, the main reaction occurred include the oxidation, pyrolysis and pyrocondensation of aromatic hydrocarbon molecules, which need high activation energy. So the effects of O2 concentration on coal reaction at different stages are also different. With the decrease of oxygen concentration, the quantity of large molecules participated in reaction decreases. So the activation energy required is reduced.
4. Conclusions Considering on few investigation of coal spontaneous combustion under low oxygen concentration, this study tested the thermodynamic characteristics during reaction processes of three different coal samples under low oxygen concentrations. The experiments were carried out under oxygen concentrations of 5%, 7%, 10%, 13%, 16%, 19% and 21%. The mechanism function, activation energy E and pre-exponential factor A under different conditions were analyzed. The results show that there are some characteristic temperatures during coal spontaneous combustion. With the decrease of oxygen concentration, the critical temperatures (T1) show a “W” type changing trend; the dry temperatures (T2), the maximum weight temperatures (T3), the maximum peak temperatures (T6) and the burnout temperatures (T7) show increasing trend; the changing ranges of the speed-up temperatures (T4) are small, which indicates that the influence of oxygen concentration on T4 is not very great; the ignition temperatures (T5) similarly keep as a constant at first and then transfer to continuously rising trend. The influences of oxygen concentration on the mechanism function and kinetic parameters are affected by coal ranks and the reaction stages. Both of the changing trends and changing ranges are different at different reaction stages for the same coal sample, and are also different at the same reaction stage for different coal samples. For coal oxidation at low temperatures, the activation energy (E) and preexponential factor (A) increase with the decrease of O2 concentration. In contrast, their changing trends are completely opposite during Please cite this article in press as: X. Qi, et al., Thermodynamic characteristics of coal reaction under low oxygen concentration conditions, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.05.007
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the coal combustion at high temperatures. This phenomenon is essentially resulted from the difference in micro reaction sequences during coal spontaneous combustion. The coal spontaneous combustion process is essentially the gradual strengthening of reactions between active groups and oxygen. The decrease of oxygen concentration inhibits the intensity of many reaction sequences and even leads to the nonoccurrence of some reaction sequences. These changes of micro reactions will result in the difference of characteristic temperatures and kinetic parameters. Acknowledgments The authors deeply appreciate the financial support provided by “the Fundamental Research Funds for the Central Universities (No. 2014QNA06)”. References [1] M.A. Trevits, L. Yuan, A.C. Smith, E.D. Thimons, G.V. Goodman, The status of mine fire research in the United States, in: Proceedings of the 21st World Mining Congress, Taylor & Francis Group, Krakow, Poland: London, 2008. [2] B. 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Thermodynamic characteristics of coal reaction under low oxygen concentration conditions Xuyao Qi a, b, *, Qizhong Li a, Huijun Zhang a, b, Haihui Xin a, b a b
School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China Key Laboratory of Gas and Fire Control for Coal Mines, China University of Mining and Technology, Xuzhou 221116, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2016 Received in revised form 12 May 2016 Accepted 20 May 2016 Available online xxx
In order to further understand the characteristics of coal reaction under low oxygen concentration atmosphere, this study tested the thermodynamic characteristics during reaction processes under low oxygen concentrations. The kinetic factors were also analyzed based on testing results. The results show that there are some characteristic temperatures during coal spontaneous combustion and the influences of oxygen concentration on these temperatures are different. The influences of oxygen concentration on the mechanism function and kinetic parameters depend on the coal ranks and the reaction stages. They are different at different reaction stages for the same coal, and at the same reaction stage for different coals. This phenomenon is essentially resulted from the different influences of oxygen concentration on the micro reaction sequences during coal spontaneous combustion. The study will be helpful for further understanding of the actual development process of coal spontaneous combustion. © 2016 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Coal Spontaneous combustion Characteristic temperature Activation energy Mechanism function
1. Introduction The spontaneous combustion of coal often occurs during the processes of coal mining, storage or applications. Almost all the coal mining countries are facing the danger of coal spontaneous combustion [1e4]. In addition to burning of coal resource, different types of harmful gases are generated during the process of coal spontaneous combustion, which will seriously threaten people's safety and environment. Under some special conditions, coal spontaneous combustion may result in serious gas or dust explosions. The spontaneous combustion of coal is resulted from the accumulation of heat released from coal reaction. So the thermodynamic characteristics in different stages of coal spontaneous combustion are important for further understanding of this disaster. Many investigators have done a lot of studies on the heat production and dynamic parameters during coal spontaneous combustion. Most of these previous investigations focused on the coal reaction under oxygenic or dry-air conditions. In contrast, few literatures have been published on the topic of thermodynamic characteristics under low oxygen concentration conditions. By the method of TG, some researchers investigated the thermogravimetric changes, exothermic properties and thermodynamic parameters during coal spontaneous combustion process under the conditions of dry-air, high concentration oxygen or pure oxygen [5e14]. Based on the TG testing results, some investigators analyzed the effects of different oxygen concentration on the reaction degree, ignition time and characteristic temperatures of coal [15e18]. However, most of these conclusions are qualitative and cannot provide enough data for further understanding coal reaction under low oxygen concentration conditions. By the method of DSC, some researchers investigated the heat release characteristics of coal reaction under the conditions of dry-air or pure oxygen atmosphere, and proposed some determining methods for the propensity of coal to spontaneous combustion [19e22]. By the method of DTA, Pis et al. [23] tested the thermal characteristics of coal samples oxidized for different times under air atmosphere at 200 C and also proposed determining methods for the propensity of coal to spontaneous combustion. Haykiri-Acma et al. [24] applied DTA and DTG techniques investigated combustion characteristics of bituminous coal samples, found that volatile matter and fixed carbon contents of the investigated samples on a dry-ash free basis were seen to have a strong effect on the cumulative mass loss values. Marinov et al. [25] used TGA/DTA to study the changes in the combustion behavior of microbial treated
* Corresponding author. School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China. E-mail address: [email protected] (X. Qi). http://dx.doi.org/10.1016/j.joei.2016.05.007 1743-9671/© 2016 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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coals. In recent years, some new thermal analysis instruments began to be used in the field of coal spontaneous combustion. Using C80 Micro-calorimeter, Wang et al. [26,27] and Qi et al. [28] researched the exothermic characteristics of coal oxidation at temperatures ranging from room temperature to 300 C, found that the oxygen consumption was proportional to the amount of heat release. Using Pulse Calorimeter, Li et al. [29,30] analyzed the thermodynamic properties of the adsorption heat, condensation heat and evaporation heat of coal during coal oxidation process at low temperatures. In addition, some investigators studied the thermal characteristics of coal oxidation through the analysis based on chemical theory [31e34]. Xu [31,32] inferred the distribution range of exothermic intensity of coal oxidation at low temperatures according to oxygen consumption, heat production rate and bond energy change, and also analyzed the influence of oxygen concentration on the exothermic intensity of coal oxidation [33]. Cheng et al. [34] deduced the thermal effect during coal oxidation at low temperatures under dry-air atmosphere through the analysis based on chemical theory. In summary, the previous investigations on thermal effects of coal reaction were usually carried out under the atmospheres of pure oxygen, dry-air or other sufficient oxygen supply conditions, while the investigation under low oxygen atmosphere is few. And these existing few investigations taken under low oxygen concentration conditions cannot provide enough data for further understanding coal reaction under low oxygen concentration conditions. Actually, most of the coal spontaneous combustion zones, such as goaf, loose coal bulk, sealed fire zone, coal pile, coalfield fire zone, etc., usually occur under the conditions of low oxygen concentration below 15%. Therefore, the existing research results of coal reaction obtained under the experimental conditions of sufficient oxygen supply cannot reflect the actual process of coal spontaneous combustion, and cannot provide an efficient guidance for the prevention of this hazard. This study analyzed the thermodynamic characteristics during the reaction processes of three different ranks of coal samples under different oxygen atmospheres by the thermogravimetric analyzing method. The results will be helpful to further understand of the actual development process of coal spontaneous combustion occurring during the processes of coal mining, storage or applications.
2. Experimental 2.1. Coal samples Three ranks of raw coal were collected from different coalfields in China, i.e. Changcun jet coal, Jiaxiang gas coal and Zhaogezhuang fat coal. They have different propensity to spontaneous combustion. Based on the statistics, their shortest times to spontaneous combustion are 7 days, 25 days and 182 days. The coal samples were processed to remove the surface layers and the material was then crushed in an oxygenfree glove box. This resulted in the coal particles ranging between 0.18 mm and 0.38 mm in size which were sieved and used as the experimental coal samples. The coal particles were kept under an inert atmosphere before they were tested. Considering on the effects of storage on coal self-heating [35,36], the coal samples should be tested no longer than one week. The proximate analyses of the samples are listed in Table 1.
2.2. Testing facility The testing system includes SDT-Q600 synchronous thermal analyzer, MF-4 gas mixture, pure nitrogen bottle, pure oxygen bottle and computer (see Fig. 1). The synchronous thermal analyzer can continuously test TG curves during coal reaction process. The pure nitrogen bottle and pure oxygen bottle are connected with gas mixture through gas tube, which form a high precision gas mixture system. This gas mixture system can provide mixed N2eO2 gas with different oxygen concentrations by setting the gas pressure, gas flow rate and mixing ratio of pure nitrogen and pure oxygen. So the coal reaction processes under the atmospheres at different oxygen concentrations can be simulated accurately. During the testing process, the coal sample will be put into the coal reaction vessel. Then we turn on the instruments and set their running parameters, such as initial temperature, final temperature and temperature rising rate. At the same time, let the mixed gas with different oxygen concentrations to flow into the coal reaction vessel. The thermal analysis data during coal reaction under different oxygen atmosphere will be collected on-line until reaching the final temperature.
2.3. Testing procedure First, 8 mg of coal particles were packed into the coal reaction vessel. Then the programmed temperature program was set to run at a programmed heating rate of 5 C/min while mixture gas of oxygen and nitrogen with different oxygen concentration was permitted to flow through the coal reaction vessel at a rate of 100 mL/min. The temperature was set rising from room temperature to 1000 C. The oxygen concentration used in this study include 5%, 7%, 10%, 13%, 16%, 19% and 21%. During the temperature rising process, the instrument continuously monitored the thermodynamic data and show with changing curves.
Table 1 Proximate analyses (air-dry basis) of coal samples. Coal samples
M (wt%)
A (wt%)
V (wt%)
FC (wt%)
Qnet (MJ/kg)
H (wt%)
St (wt%)
Jet coal Gas coal Fat coal
9.86 2.82 1.20
20.65 7.85 17.48
30.01 34.82 26.71
34.08 54.51 55.10
21.81 31.89 28.48
4.49 3.21 4.59
0.37 0.21 1.16
Notes: M e moisture content; A e ash content; V e volatile matter; FC e fixed carbon; Qnet e net calorific value; H e hydrogen content; St e total sulfur content.
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3. Results and discussion 3.1. General trend and subsection characteristics of thermogravimetric changes According to the testing results, thermo gravimetry (TG) curves during reaction processes of three ranks of coal samples under different oxygen concentration conditions were obtained, as shown in Fig. 2. The testing results show that the weight changing trends with the rise of temperature are similar for different coal reaction process. With the decrease of oxygen concentration, the combustion process was delayed and the combustion intensity was weakened. This phenomenon indicates that the coal oxidation and combustion process is inhibited by low oxygen concentration. The reaction rate of coal is slowed down in varying degrees with the decrease of oxygen concentration. Consequently, the whole reaction process of coal oxidation and combustion is delayed, which results in the differences in TG curves under different oxygen concentrations. Based on the previous literatures and a series of TG curves obtained in this study, there is piecewise characteristics in the TG curves during coal reaction process. There are several characteristic temperature points existing on the TG curves, as shown in Fig. 3 [37e39]. (1) The critical temperature T1 in coal reaction process. That is the temperature corresponding to the first maximum peak point of weight loss rate. After coal temperature reaching this temperature, the reaction rate between oxygen and active groups existing in coals will accelerate obviously, which result in the increase of oxygen consumption rate and released gas production amount (CxHy, H2, CO and CO2 etc.). In addition, due to the rise of coal temperature, more and more initial absorbed gas in coal pore structure begin to desorb and release with the gas produced by coal reaction. The consumption of coal reaction and desorption of initial absorbed gas result in the increase of coal weight loss rate and formation of the first maximum peak point of weight loss rate on the curve. (2) The dry temperature T2. At this temperature, the losing of coal mass would be the minimum in the range before reaching ignition temperature. When coal temperature reaches T2, some functional groups existing in coal structure, such as high molecular aromatic nucleus, hydrocarbon alkyl side chains, oxygen-containing functional groups, bridge bonds and some small molecules, begin to crack or depolymerize. The reaction of these groups will release different types of gases, which consume coal sample. On the other hand, the compound intensity between coal and oxygen reactions will be enhanced in this stage. The coaleoxygen complex produced will not decompose until reaching a higher temperature, which increase the coal mass. The coal consumption by depolymerization and the mass increase by coaleoxygen compound become a dynamic equilibrium state at the dry temperature T2. As a result, the coal sample weight will not decrease from this point. (3) The maximum weight temperature T3, is the temperature when coal's weight by absorbing oxygen reaches the maximum value. Due to the increase of temperature, the number of surface active structures of coal is increased, and the aromatic ring structures which are not involved in the oxidation process at low temperature are also participating in the oxidation reaction, the amount of active structures in coal and the adsorption oxygen quantity reach the maximum value, results in coal weight increasing to the maximum. Meanwhile, coal weight loses quickly after T3. (4) The speed-up temperature T4, namely the temperature corresponding to the maximum weight increasing rate. At this temperature point, many cyclic large molecules existing in coal decompose rapidly. These decomposition reactions form a large amount of active groups, which obviously enhance the compound intensity between coal and oxygen. So the mass increase by coaleoxygen compound will higher than the coal consumption by depolymerization and desorption of initial gas in coal. As a result, the coal weight increasing rate reaches a peak point value. (5) The ignition temperature T5, namely the temperature at which coal samples begin to burn. This value reflects the propensity of coal to spontaneous combustion. (6) The maximum peak temperature T6. It is the point that the chemical reaction intensity of coal reaches to the largest. The corresponding decreasing rate of coal weight reaches to the largest. It shows as the valley point on the DTG curve. (7) The burnout temperature T7. The combustible components of coal have burnt out at all. Its value can reflect the length of the time that the coal oxidation and burning lasting.
3.2. Characteristic temperatures under different oxygen concentration 3.2.1. The critical temperature (T1) Based on the testing results, the changing trends of T1 under different oxygen concentration conditions were obtained, as shown in Fig. 4. The critical temperature (T1) is the starting point for automatic acceleration of coaleoxygen complex reaction. With the decrease of oxygen concentration, the testing results show that the T1 demonstrate a “W” type changing trend, i.e. decrease below 8%, increase in the
Fig. 1. The testing system: (a) Gas bottles; (b) MF-4 dynamic stability distribution device; (c) SDT-Q600 synchronous thermal analyzer; (d) Computer.
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Fig. 2. TG curves under different oxygen concentration conditions: a. jet coal, b. gas coal, c. fat coal.
Fig. 3. Diagram of characteristic temperature points.
range of 8e13%, decrease in the range of 13e18% and increase again above 18%. Under low O2 concentration conditions (O2% < 8%), the changing ranges of T1 are greater than that under the conditions of oxygen concentrations bigger than 8%. The changing trends of T1 with the changes of oxygen concentration are essentially caused by the different reaction intensity of coaleoxygen compound under different oxygen supply conditions. The low oxygen concentration conditions inhibit the reaction intensity between coal and oxygen, which delay the formation of the balance of coal consumption and mass increase by coaleoxygen complex. In contrast, with the rise of oxygen concentration, the active groups existing in coal gain more chances to contact and react with oxygen, which enhance the intensity of coal oxidation. The formation of the balance will need shorter time. As a result, under the oxygen concentration Please cite this article in press as: X. Qi, et al., Thermodynamic characteristics of coal reaction under low oxygen concentration conditions, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.05.007
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Fig. 4. The T1 values under different oxygen concentration conditions.
below 8%, the T1 rapidly decrease with the increase of oxygen concentrations. Under the oxygen concentrations ranging in 8e13%, the coaleoxygen compound reaction rate accelerate again with a large amount of gas release. These released gases limit the diffusion of oxygen molecules into coal pore structure and inhibit the chemical reaction rate of coal oxidation. So the corresponding balance need more time to form, which show as the rise of T1. Under the oxygen concentrations ranging in 13%e18%, an appropriate gradient of oxygen concentration is formed around coal microstructure and enhance the oxygen diffusion in pore structure of coal. So the time for the formation of corresponding equilibrium becomes short, which shows as the second decrease of T1. If oxygen concentration increases to a value higher than 18%, an over large oxygen concentration gradient forms, which inhibits oxygen to further diffuse into pore structure of coal and the desorption of initial gas absorbed in coal. So the reaction rate of coal is inhibited again, which leads to the second rise phenomenon of the critical temperature. 3.2.2. The dry temperature (T2) Based on the testing results, the changing trends of the dry temperature (T2) under different oxygen concentration conditions were obtained, as shown in Fig. 5. For the reaction process of jet coal under 5% oxygen concentration, the oxygen consumption rate is very large at low temperatures, the great shortage of oxygen inhibits the coaleoxygen compound rate obviously. The coaleoxygen compound oxygen uptake is far less than the volume of gas desorption from coal, which means that the coal weight is always in a decreasing state at low temperatures. So the dry temperature T2, the maximum weight temperature T3 and the speed-up temperature T4 for jet coal reacted in 5% oxygen concentration atmosphere cannot be calculated. The testing results show that the dry temperatures generally increase in varying degrees with the decrease of oxygen concentration under low oxygen concentration. In contrast with the dry temperatures tested under the 21% oxygen concentration atmosphere, the increase ranges of dry temperatures tested under 5% oxygen concentration atmosphere obviously differ from each other for different ranks of
Fig. 5. The T2 values under different oxygen concentration conditions.
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coal samples. For jet coal, gas coal and fat coal, their corresponding temperature increasing ranges are 24.3%, 17.0% and 21.5% respectively (corresponding increased values: 46.3 C, 26.8 C and 43.2 C). The decrease of oxygen concentration can slow down the fracture of side chains existing in coal structure and inhibit the reaction of the active groups, pyrolysis and thermal decomposition process. The dry temperature change trends of jet coal and fat coal are similar, while the gas coal's dry temperature is significantly smaller. This phenomenon is formed due to the special structure characteristics of gas coal. The alkyl side chains, oxygen-containing groups, bridge bonds of big molecular aromatic nucleus and some small molecules existing in gas coal structure are easier to crack or depolymerize, and release different types of productions. So the dynamic balance may achieve earlier and the corresponding dry temperature of gas coal is lower than the other two rank ones. 3.2.3. The maximum weight temperature (T3) Based on the testing results, the changing trends of the maximum weight temperature (T3) under different oxygen concentration conditions were obtained, as shown in Fig. 6. The testing results show that the maximum weight temperatures increase in varying degrees with the decrease of oxygen concentration under low oxygen concentration. It indicates that the generation rate of active structures is inhibited by the low oxygen concentration. Under low oxygen atmosphere, it will take longer time to adsorb oxygen and generate a large number of active groups for the following decomposition and combustion process. Therefore, the lower the oxygen concentration is, the longer the coal oxygen-absorption process will continue. So the T3 change to a larger value. In addition, it shows that the oxygen concentration has a greater influence on jet coal than gas coal and fat coal. If oxygen concentration decreases from 21% to 7%, the rising range is 14.75% for jet coal, while they are only 8.6% for fat coal. 3.2.4. The speed-up temperature (T4) Based on the testing results, the changing trends of the speed-up temperature (T4) under different oxygen concentration conditions were obtained, as shown in Fig. 7. The testing results show that the changing ranges of speed-up temperatures with the changes of oxygen concentration are smaller than other characteristic temperatures. It indicates that the speed-up temperature is not affected by O2 concentration very great. In addition, the changing rule of speed-up temperature with the changes of oxygen concentration conditions is also affected by coal ranks. Among the coal samples tested, the speed-up temperatures of gas coal and fat coal rise with the decrease of oxygen concentration. In contrast with the speed-up temperatures tested under 21% oxygen concentration atmosphere, the temperature rising ranges of gas coal and fat coal are only 5.1% and 6.2% respectively (corresponding increased values: 13.3 C and 18.0 C). On the other hand, the speed-up temperature of jet coal shows a certain fluctuation with the changes of oxygen concentration, the relative minimum (212.2 C) and relative maximum (217.5 C) were found at 13% and 16% O2 concentrations respectively. This phenomenon is due to the different types and amount of active groups existing in different ranks of coal samples. The lower rank coal samples usually have more types and larger amount of active groups than higher rank coal samples. So there will be more reaction sequences occurring during the reaction process of low rank coal samples. The interaction among different reaction sequences and their different affection degree by oxygen supply conditions are the main reasons for the fluctuation on the changing curve. 3.2.5. The ignition temperature (T5) Based on the testing results, the changing trends of the ignition temperature (T5) under different oxygen concentration conditions were obtained, as shown in Fig. 8. With the decrease of O2 concentration, the ignition temperatures similarly keep as a constant at first and then transfer to continuously rising trend. Under the oxygen concentrations ranging in 19e21%, the ignition temperatures almost keep as a constant. On the other hand, if under the oxygen concentrations below 19%, the ignition temperatures present a nearly linearly increasing trend with decrease of oxygen concentration. In contrast with the ignition temperatures under 19% O2 concentration atmosphere, the temperature
Fig. 6. The T3 values under different oxygen concentration conditions.
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Fig. 7. The T4 values under different oxygen concentration conditions.
Fig. 8. The T5 values under different oxygen concentration conditions.
increasing ranges of ignition temperatures under 5% O2 concentration conditions of jet coal, gas coal and fat coal are 5.1%, 4.2% and 6.1% (corresponding increased values: 21.8 C, 18.7 C, 28.8 C). The coal spontaneous combustion is essentially the result of acceleration of thermal released in the contact and reaction between oxygen and active groups existing in coal. Under an oxygen concentration above 19%, the oxygen supplied is enough and the reaction intensity is strong at first. So the remained oxygen and gas production will form a gas bearing ring, which limit the release of gas production and the diffusion of oxygen. As a result, the reaction intensity will slow down later and lead to the decrease of ignition temperature. However, this closure effect of gas bearing ring do not produce a substantial and continuous effect on the coal spontaneous combustion process. The corresponding affecting range on ignition temperature is not very great. 3.2.6. The maximum peak temperature (T6) Based on the testing results, the changing trends of the maximum peak temperature (T6) under different oxygen concentration conditions were obtained, as shown in Fig. 9. The results show that the maximum peak temperatures present an increasing trend with the decrease of O2 concentration. In contrast with the ignition temperatures under 21% O2 concentration atmosphere, the temperature increasing ranges of ignition temperatures under 5% O2 concentration conditions of jet coal, gas coal and fat coal are 6.8%, 7.6% and 9% respectively (corresponding increased values: 33.3 C, 38.9 C, 49.2 C). Coal spontaneous combustion is essentially due to the heat release during the reaction processes of initial and secondary active groups. The decrease of oxygen concentration weakens the generation of active groups and cannot provide enough new active centers for further reaction. It leads to the decrease of oxidation reaction rate and heat release rate, which result in the delay for reaching the maximum weight and the rise of the corresponding maximum peak temperatures.
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Fig. 9. The T6 values under different oxygen concentration conditions.
3.2.7. The burnout temperature (T7) Based on the testing results, the changing trends of the burnout temperature (T7) under different oxygen concentration conditions were obtained, as shown in Fig. 10. The testing results show that the burnout temperatures increase with the decrease of O2 concentration. In contrast with the ignition temperatures under 21% O2 concentration atmosphere, the temperature increasing ranges of burnout temperatures under 5% O2 concentration conditions of jet coal, gas coal and fat coal are 8.9%, 12.9% and 13.2% (corresponding increased values: 48.1 C, 76.7 C and 80.8 C). It shows that we can prevent a fire zone throughout keeping a low oxygen concentration condition in this area. During coal spontaneous combustion process, the oxygen amount needed for the reaction of active groups is a determined value. Under low oxygen concentration conditions, the coal reaction intensity and gas diffusion are small. The corresponding reaction time will be longer.
3.3. Kinetic parameters at different reaction stages 3.3.1. Calculation theory Coal oxidation and combustion is a typical gasesolid reaction process. The kinetic parameters in this process, such as reaction mechanism function f(a), activation energy E and pre-exponential factor A, can be determined by thermogravimetric analysis method. Under the condition of single scan rate, there are many integral methods and differential methods for the calculation of kinetic parameters [40]. In this study, the CoatseRedfern integral method and AchareBrindleyeSharpeWendworth differential method are used to solve the thermo kinetic factors of coal oxidation at low-temperatures and combustion stage at high-temperatures under low oxygen concentration conditions. (1) CoatseRedfern integral method According to thermodynamics theory, the CoatseRedfern equation is expressed as follows [41,42].
GðaÞ AR 2RT E 1 ¼ ln ln E RT bE T2
(1)
where GðaÞ is the integral form of coal oxidation mechanism function; T the coal temperature, K; E the activation energy, J/mol; A the preexponential factor, K/s; b the heating rate, C/min, which is set as 5 C/min in this testing; R the gas constant, 8.314 J/(mol K); a the weight loss rate, which can be obtained by the equation as follows.
a¼
m0 mt m0 m∞
(2)
where m0 is the initial mass of coal, g; mt the coal mass at the time of t, g; m∞ the remaining mass after reaction, g. For general temperature ranges and most activation energy, the value of E/RT in Equation (1) is far greater than 1, and the value of ð1 2RT=EÞ is approximately equal to 1. Therefore, the CoatseRedfern equation can be approximately expressed as,
GðaÞ AR E ¼ ln ln 2 bE RT T
(3)
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Fig. 10. The T7 values under different oxygen concentration conditions.
(2) AchareBrindleyeSharpeWendworth differential method [43,44] According to thermodynamics theory, the AchareBrindleyeSharpeWendworth equation is expressed as follows.
ln
da A E ¼ ln f ðaÞdT b RT
(4)
where f ðaÞ is differential form of coal oxidation reaction mechanism function. For coal spontaneous combustion process, its mechanism function demonstrates a relationship functional between coaleoxygen reaction rate and weight losing rate a. The characteristics of thermal analysis curves are depended on the mechanism function. There are many gasesolid reaction mechanism functions proposed in previous literatures [45]. Considering on the piecewise characteristics of coal spontaneous combustion, we selected the Bagchi method for determining the mechanism function model, the activation energy E and the preexponential factor A. There are some steps in the application of the Bagchi method. Firstly, substitute the thermogravimetric testing results of different reaction stages in different gasesolid reaction mechanism models [45]. Secondly, the obtained G(a) and f(a) values are substituted into equations (3) and (4) respectively, and then analyze the linear correlation coefficients between the two functions. Based on the comprehensive comparison, the mechanism function model having the largest linear correlation coefficient was selected as the mechanism functions, as shown in Table 2. Finally, the trendlines were fitted to the calculation data, with ln½GðaÞ=T 2 as Y axis and 1/T as X axis. Then the activation energy E and pre-exponential factor A were obtained based on the slope and intercept of fitting equations. 3.3.2. Discussion on the calculation results The coal spontaneous combustion process presents an obvious piecewise characteristic. The TG changing characteristics of coal at different reaction stages are also different. From the TG changing curves, coal spontaneous combustion process can be generally divided into four stages: water evaporation and desorption weightlessness stage (onset temperature ~ T2), oxygen-absorption mass-gain stage (T2 ~ T3), decomposition and combustion weight loss stage (T3 ~ T7) and burnout stage (T7 ~ end temperature). Besides of coal oxidation, the water evaporation and desorption of gases absorbed in raw coal (i.e. CH4, CO2 etc.) also occur during water evaporation and desorption weightlessness stage. Thus, the kinetic parameters calculated at this stage cannot reflect the actual oxidation process of coal. Therefore, the second stage and third stage are respectively selected for the calculation of kinetic parameters during coal oxidation at low-temperatures and coal combustion at high-temperatures. Based on the calculation methods described in Section 3.3.1, the activation energy and the preexponential factor at different reaction stages can be determined respectively, as shown in Figs. 11 and 12. Fig. 11 shows that the activation energy E and pre-exponential factor A decrease with the increase of O2 concentration during coal oxidation at low temperatures. It indicates that decreasing of O2 concentration can inhibit the types and quantity of active groups taking part in the coal oxidation at this stage. As a result, the corresponding heat generation during coal oxidation would decrease, and the activation of some micro structures in coal need to absorb more heat, which cannot provide enough active centers for further reaction.
Table 2 The mechanism functions of different coal samples in different reaction stages. Mechanism functions
Jet coal
Gas coal
Fat coal
Oxidation at low-temperatures
G(a) ¼ ln(1a) f(a)¼(1a) G(a) ¼ 1(1a)1/3 f(a)¼3(1a)2/3
G(a) ¼ [(1a)1/31]2 f(a) ¼ 3/2[(1a)1/31]1$(1a)4/3 G(a) ¼ 1(1a)1/2 f(a)¼2(1a)1/2
G(a) ¼ [(1a)1/31]2 f(a) ¼ 3/2[(1a)1/31]1$(1a)4/3 G(a) ¼ 1(1a)1/3 f(a)¼3(1a)2/3
Combustion at high-temperatures
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Fig. 11. Relationship between kinetic parameters and oxygen concentration during coal combustion at low temperatures. (a) activation energy E versus oxygen concentration; (b) pre-exponential factor A versus oxygen concentration.
Fig. 12. Relationship between kinetic parameters and oxygen concentration during coal combustion at high temperatures. (a) activation energy E versus oxygen concentration; (b) pre-exponential factor A versus oxygen concentration.
Fig. 12 shows that the activation energy E and pre-exponential factor A increase with the increase of O2 concentration during coal combustion at high temperatures. It is completely opposite comparing with the changing trend during coal oxidation at low temperatures. And the influence degree is smaller than low temperature stage. As known, the coal spontaneous combustion is a piecewise process. The main reaction sequence and influence factors differ from each other for different reaction stages. At high temperatures, the main reaction occurred include the oxidation, pyrolysis and pyrocondensation of aromatic hydrocarbon molecules, which need high activation energy. So the effects of O2 concentration on coal reaction at different stages are also different. With the decrease of oxygen concentration, the quantity of large molecules participated in reaction decreases. So the activation energy required is reduced.
4. Conclusions Considering on few investigation of coal spontaneous combustion under low oxygen concentration, this study tested the thermodynamic characteristics during reaction processes of three different coal samples under low oxygen concentrations. The experiments were carried out under oxygen concentrations of 5%, 7%, 10%, 13%, 16%, 19% and 21%. The mechanism function, activation energy E and pre-exponential factor A under different conditions were analyzed. The results show that there are some characteristic temperatures during coal spontaneous combustion. With the decrease of oxygen concentration, the critical temperatures (T1) show a “W” type changing trend; the dry temperatures (T2), the maximum weight temperatures (T3), the maximum peak temperatures (T6) and the burnout temperatures (T7) show increasing trend; the changing ranges of the speed-up temperatures (T4) are small, which indicates that the influence of oxygen concentration on T4 is not very great; the ignition temperatures (T5) similarly keep as a constant at first and then transfer to continuously rising trend. The influences of oxygen concentration on the mechanism function and kinetic parameters are affected by coal ranks and the reaction stages. Both of the changing trends and changing ranges are different at different reaction stages for the same coal sample, and are also different at the same reaction stage for different coal samples. For coal oxidation at low temperatures, the activation energy (E) and preexponential factor (A) increase with the decrease of O2 concentration. In contrast, their changing trends are completely opposite during Please cite this article in press as: X. Qi, et al., Thermodynamic characteristics of coal reaction under low oxygen concentration conditions, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.05.007
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