Procedia Earth and Planetary Science
Procedia Earth and Planetary Science 1 (2009) 718–723
www.elsevier.com/locate/procedia
The 6th International Conference on Mining Science & Technology
Research on low-temperature oxidation and pyrolysis of coal by thermal analysis experiment Lu Changa,b, Zheng Yan-mina,b, Yu Ming-gaoa,b,* a
Henan Province Key Laboratory of Prevention and Cure of Mine Methane & Fires, Jiaozuo, China,454003 b School of Safety Science and Engineering,Henan Polytechnic University, Jiaozuo, China,454003
Abstract: In order to understand the law of low-temperature oxidation and pyrogenation well, thermal analyzer is used to study the oxidation and pyrogenation of coal in Yangquan below 150 . Through the TG-DSC curves, the heat release of two kinds of experiments is compared. Combining with the curve, the polynomial form of the mechanism is set up to analyze the function of its dynamic characteristics. It shows that the process of low-temperature oxidation is a clear exothermic oxidation process contrasted with the pyrogenation process and the oxidation of coal needs smaller activation energy than pyrogenation in nitrogen atmosphere. The mechanism is different from the oxidation at ordinary low-temperatures and the process of pyrogenation is the function of polynomial. The mechanism function is applied to the TG and DSC curves. Keywords: coal safety; oxidation; pyrogenation; kinetic analysis
1. Introduction Spontaneous combustion of coal leads to resource and enormous economic loss. It also gives a great security risk for production. In our country, more than half of key coal mines have serious spontaneous combustion[1]. Inr the past years, scholars have put forward many hypothesis, theories or mechanisms, however because of its complexity of physical and chemical structure of coal, as well as the factors that affect spontaneous combustion of coal involved in many aspects, there are still many ambiguities on the mechanism of oxidation reaction, especially on the kinetic low-temperature oxidation of coal. The thermal analysis is a method to research the physical or chemical reaction rate of material, which could be used in isothermal and non-isothermal condition. This method, which has greatly been developed in the past half century[2], is considered to have the rapid and simple advantages and so on. This article employs this method to research the process of spontaneous combustion, and aims at understanding the oxidation mechanism of coal and obtaining the mechanism function. It could provide theoretical basis for simulating the process of coal spontaneous combustion. 2. Experimental 2.1. Experimental method and results analysis
* Corresponding author. Tel.: +86-391-3987456. E-mail address:
[email protected].
1878-5220 © 2009 Published by Elsevier B.V. Open access under CC BY-NC-ND license. doi:10.1016/j.proeps.2009.09.113
L. Chang et al. / Procedia Earth and Planetary Science 1 (2009) 718–723
719
The experiment is performed by using STA449c thermal analyzer under oxidation atmosphere. Though carefully sieving and mortaring, the particles are reduced to less than 0.15mm. The experiments are conducted under two atmospheres, oxygen and nitrogen atmosphere respectively. A certain amount of coal samples are added to experiments, and the nitrogen gas is used to protect the instrument. The flow rate for reaction environments is set at 20ml/min, with temperature rising at 1.5 /min from room temperature to 150 . The slow heating rates ensure that the heat transfer limitations can be ignored. In this way, TG-DSC curves under two reaction environments are obtained. All the tests and experiments are conducted once again and deemed reproducible when the results are within ±5%. The TG-DSC curves of two atmospheres are shown in Fig.1.
Fig. 1. TG-DSC curves of different atmosphere
(1) From the fFig.1, it is known that there are similar curve shapes under two atmospheres, while the degree of curves and steep slope have a slight difference. Compared to nitrogen atmosphere, the shape of the curve is larger under oxygen atmosphere. This may be explained that the low temperature oxygen reaction with coal occurred more intensely than the process of coal pyrogenation. In addition, at the beginning of both reaction environments, evaporation phenomenon has been accompanied, which results in weightlessness. At the same time, endothermic phenomenon have emerged on the DSC curves of two atmospheres, as is that water evaporation required to absorb heat. And then with the coal and oxygen adsorption, desorption and chemical reaction, the heat is gradually released. (2) It is evident from Fig.1 that DSC curves change more greatly in the oxygen atmosphere at the initial stage of the experiment, which is compared to nitrogen atmosphere. In the early experiments of the oxygen atmosphere, it not only has moisture evaporation, but also accompanies by the occurrence of physical adsorption and heat release. It shows that low-temperature oxidation of coal is a multi-step and mutual contact reaction process. Compared the DSC curves of different atmospheres, if the process of pyrogenation does not have heat change then oxidation process is exothermic, which may indicate that the coal low-temperature oxidation process was accompanied by heat release. 2.2. Activation energy of oxidation kinetics Oxidation and decomposition of coal is a complex process, which may contain multi-step reactions. In general, for the convenience of calculating and analyzing, we often consider it as a relatively simple form of gas-solid reactions to solve the kinetic parameters. However, this does not comply with the reality of coal oxidation reactions, and even some scholars have different assumptions which make opposite results. The low-temperature oxidation of coal may be a multi-step reaction which is carried out simultaneously, so we should consider the low-temperature oxidation of coal comprehensively. According to Taylor formula, its oxidation process is applied as a function of the model f (a ) = cx 2 + dx + e , and its integral function is g (a ) = c x 3 + d x 2 + ex + f . 3 2 2.2.1. Oxidation and pyrogenation kinetics of TG The rate of heterogeneous solid-state reactions can be generally described by:
L. Chang et al. / Procedia Earth and Planetary Science 1 (2009) 718–723
720
E − da = Ae RT f (a ) dt
The conversion rate
(1)
a of the samples can be get from TG curves: a=
(w0 − w) (w − w∞ )
(2)
Where a is the conversion degree or fraction of combusted material, t is time, w0 is initial mass and w∞ is final mass, w is the residual mass of the coal on t times, T is temperature, A is Arrhenius parameter, E is activation energy and f (a ) a function called the reaction model, which describes the reaction rate of dependence on the extent dT of reaction. As temperature changes at a constant positive rate β = dt , so, Eq. (1) can changes to Eq. (3):
da A − RTE = e f (a ) dT β Integrated conversion of
(3)
a , Eq. (3) gives:
∫
a
0
E
da A T − = ∫ e RT dT f (a ) β T0
(4)
A problem with Eq. (4) is that it is not analytically solvable. The function g (a ) , however, can be expressed by some approximate equations. Many approximations have been derived and discussed in [4-5].Here the numerical analysis method is used to get the solution, and adapt to the Coats-Redfen formula:
AR 2 RT E g (a ) ln 2 = ln 1 − − E RT T βE
(5)
g (a ) is integral function of TG curves, for different reaction mechanism, the dynamics of integral function g (a ) can be written as: From the Eq.(5):
g (a ) = − ln(1 − a ) , n = 1
1 − (1 − a ) g (a ) = 1− n
1− n
,
n ≠ 1.
a and T are selected from different temperature ranges[3-6]. For each conversion value a , the E g (a ) term of ln 2 varies linearly with 1 as slope of the line − which is plotted in Fig.2. Subsequently, values T R T A E of and may be respectively estimated from the interception and slope of a plot of ln g (a ) versus 1 . From the Eq.(5),
T 2
T
2.2.2. Oxidation and pyrogenation kinetics of DSC
The major premise of DSC kinetics is that the extent of reaction responses to the release or absorption of thermal effect in direct proportion with the area under the DSC curve. Therefore, the conversion rate of DSC curves can be defined as[6-9]:
L. Chang et al. / Procedia Earth and Planetary Science 1 (2009) 718–723
Where
721
da 1 dH = dt H T dt
(6)
dH = ∇W dt
(7)
H T is the total enthalpy, which is the total heat release after the reaction completes; Eq. (7) is the flux
rate of material; and then makes points of Eq. (7): H
t
H0
t0
∫ dH = ∫ ∇Wdt (8)
Combining Eq.(6) and Eq.(1) leads to Eq.(9):
ln
H −H E dH 1 − n ln T =− + ln A = ln K dt H T HT RT
(9)
a and T are selected from two different atmospheres, with the correct E order of reaction, the plot of ln K versus 1 gives a linear relationship with slope − , so the E is obtained as a R T Hence for different temperature ranges,
function of the conversion. 2.2.3. Kinetic calculate and analysis
From the TG curves of the two atmospheres, the dates of a and T are selected respectively and the precise instruments are used to analyze and calculate dates, then integrate the calculation results into the mechanism of function model of the integral function which is set to 2.2, here c = d = e = 1 is set. The value f which changes , with time (in this case
g (a ) f = 0, 1, 2, 1.5 are selected), is discussed to get the plot of ln 2 versus 1 of TG T T
curves, and obtain the order of reaction through the good linear correlation curves. Similarly from the DSC curves of two atmospheres, the dates of a and T are selected respectively, with the correct order of reaction on the DSC curves, the plot of ln K versus 1 are obtained which are shown as follows: T t he TG cur ves under
2
-8
R
oxygen at m ospher e
t he TG cur ves of
f =0
= 0. 9903
f =1
] 2 - 10 T / ) a (- 12 g [ n l- 14
f =0. 5 f =2 l i near ( f =0)
2
-8
R
ni t r ogen at m ospher e
= 0. 9885
-9
] 2 T - 10 / ) - 11 a ( g - 12 [ n - 13 l
f =0 f =1 f =0. 5 f =2 l i near ( f =0)
- 14
- 16 0. 0026
- 15
0. 00265
0. 0027 1/ T
0. 00275
0. 0028
0. 00245
0. 0025
0. 00255 1/ T
0. 0026
0. 00265
L. Chang et al. / Procedia Earth and Planetary Science 1 (2009) 718–723
722
t he DSC cur ve of
oxygen at m ospher e
t he DSC cur ve of
- 10. 4
ni t r ogen at m ospher e
2
- 10. 2
2
- 10. 6
R
R
= 0. 9999
- 10. 4
= 0. 9984
- 10. 8 K n l
- 10. K n l
- 11
- 11. 2
- 11
- 11. 4 - 11. 6 0. 002
6
- 10. 8
- 11. 2
0. 0022
0. 0024
0. 0026
0. 0028
0. 003
0. 0032
0. 002
0. 0022
0. 0024
1/ T
0. 0026
0. 0028
0. 003
1/ T
Fig. 2. The kinetic analysis of TG-DSC curves under two atmospheres
From Fig.2, it shows that: kinetic mechanism has very good linear correlation under different atmospheres of the TG-DSC curves in the application of polynomial function, and corresponding slopes are combined, then the kinetic parameters can be obtained respectively, as are reported in Table 1. Table 1. Kinetic parameters of two atmospheres
Activation energy
Correlation parameter
TG
39.202
0.9903
DSC
44.477
0.9984
TG
51.797
0.9885
DSC
51.928
0.9999
Atmospheres/curves
oxygen
nitrogen
From the above analysis, it is known that: the low-temperature reaction of coal is a relatively complex process. From the kinetic mechanism of low-temperature oxidation and pyrogenation, the polynomial form is used to solve the kinetic parameters, which is different from what the previous scholars proposed. And the results show that it has very good linear correlation in the application of polynomial, and the correlation coefficient is greater than 0.98 in the TG and DSC curves. From the kinetic point of view, the mechanism of the polynomial function is applied to two reaction environments, which both obtain better correlation and may indicate that the process of coal-oxygen reaction is simultaneous and interrelated. From the view point of activation energy, the oxidation of coal needs smaller activation energy than pyrogenation in nitrogen atmosphere, which shows that the coal at low temperature reaction occurs more easily with small apparent energy, and coal is more susceptible to oxidation at low temperatures with exothermic reaction under certain conditions. 3. Conclusions (1) From the TG curves, it is known that there are similar curve shapes under two atmospheres, while the degree of curves and steep slope have a slight difference which indicates the size changes differently under low temperature. (2)Comparing the two DSC curves of different atmospheres, if the process of pyrogenation does not have heat change in this case, then oxidation process is exothermic, which is shown that the low-temperature oxidation of coal is accompanied by heat release. (3) The polynomial form is used to solve the kinetic parameters and the results show that the application of polynomial has a very good linear correlation. From the TG and DSC curves of the kinetic point, the mechanism of the polynomial function, applied to two reaction environments which have better correlations, indicates that the
L. Chang et al. / Procedia Earth and Planetary Science 1 (2009) 718–723
723
process of coal-oxygen reaction is simultaneous and interrelated. (4) From the view point of activation energy, the coal reaction occurs more easily with small apparent energy at low temperature. (5) Because the experimental samples and the experimental process are affected by a lot of factors, it is only researched and analyzed from the theoretical and experimental aspects , so many problems have to be solved in the future research work.
Acknowledgements Financial support for this work, provided by Natural Science Foundation of China (50274061), by Basic and Advanced Technology Research Program of Henan Province (072300420180, 082300463205), and by Doctoral Foundation of Henan Polytechnic University(648183), is gratefully acknowledged.
References [1] X. Wang, G. Zhang, Mine Fire Prevention. Xuzhou: China University of Mining and Technology Press, 1990. [2] Q. He, D. Wang, Research on coal oxidation and pyrolysis reaction kinetics. Journal of Beijing University of Science and Technology, 28 (2006) 1~5. [3] E. Kathy, B. Benfell, B. Bcamish, Thermogravimetric analytical procedures for characterizing New Zealand and Eastern Australian coals. Thermochim Acta, 286 (1996) 67. [4] R.Hu, Thermal Analysis Kinetics. Beijing: Science Press, 2001. [5] L. Jian, Indicators based on the activation energy of coal spontaneous combustion stage research. Journal of Liaoning Technical University, 25 (2006) 162~163. [6] T. L. Zhang, R.Z. Hu, F. P. Li, Oxidative pyrolysis and char combustion. Thermochim Acta, 1994. [7] N. Liu, The kinetics of materials biomass pyrolysis and its analysis method. China University of Science and Technology, 5 (2000) 1. [8] H. Wang, Experimental Study on Low-Temperature Oxidation of an Australian Coal.Energy & Fuels, 13 (1999) 1174~1177. [9] D. Liang, Research on the relationship between heating rate and coal oxidation characteristics. Coal Science and Technology, 35 (2007) 72~74.
