Oxidation kinetics regularity in spontaneous combustion of gas coal

Oxidation kinetics regularity in spontaneous combustion of gas coal

M INING SCIENCE AND TECHNOLOGY Mining Science and Technology 20 (2010) 0059–0063 Oxidation kinetics regularity in spontaneous combustion of gas coal ...

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M INING SCIENCE AND TECHNOLOGY Mining Science and Technology 20 (2010) 0059–0063

Oxidation kinetics regularity in spontaneous combustion of gas coal WANG Lanyun1,2,*, JIANG Shuguang1,2, WU Zhengyan2, SHAO Hao1,2, ZHANG Weiqing1,2, CHEN Yueqin1,2, ZOU Lili1,2 1

State Key Laboratory of Coal Resources and Mine Safety, China University of Mining & Technology, Xuzhou 221008, China 2 School of Safety Engineering, China University of Mining & Technology, Xuzhou 221008, China

Abstract: In order to investigate the oxidation kinetics of gas coal at low temperatures, we derived a rate equation of oxygen consumption during low-temperature oxidation of gas coal and deduced an E-c equation, expressing the relation between active energy E and oxygen concentration c. The reaction order n and active energy E were calculated with this equation based on experiments of static oxygen consumption tests. In addition, we proved the rationality of the E-c equation using a kinetic compensation effect and obtained the isokinetic temperature Tc. The results show that: 1) the gas coal oxidizes easily with increasing temperature and the oxidation tends to be spontaneous at higher temperatures; 2) the oxygen concentration c affects oxygen consumption very much at lower temperatures but has only a small effect at higher temperatures; 3) the isokinetic temperature Tc was 127 °C which has been experimentally validated as the key turning point during low-temperature spontaneous combustion of gas coal. Keywords: static oxygen consumption test system; reaction order; active energy; kinetic compensation effect; isokinetic temperature

1

Introduction

Low-temperature oxidation of coal is accompanied by oxygen consumption and emission of gaseous products which suggest that spontaneous coal combustion is approximately divided into several stages. Previous work by professor Xu Jingcai et al. on coal oxidation has indicated that oxygen consumption is an important macroscopic property reflecting the oxidation ability of coal[1-7]. Moreover, this consumption characteristic can also reveal the rules of chemical kinetics in low-temperature oxidation of coal, but it has been never investigated. Thermogravimetric Analysis (TGA) and adiabatic oxidation have been used to obtain the chemical active energy E of coal oxidation. As well, many other useful attributes have been explored with these two methods[1,8-12]. However, TGA is usually used to investigate the pyrolysis kinetics of coal and to analyze different oxidation mechanisms[5-7]. It is more applicable for Received 05 March 2009; accepted 04 June 2009 *Corresponding author. Tel: 86 15952181013 E-mail address: [email protected] doi: 10.1016/S1674-5264(09)60161-7

pyrolysis at a high temperature stage. Additionally, TGA is usually affected by flotage, convection, condensation of volatiles, tested errors of temperature and other factors. In the TG experiment, the weight in coal loss may be affected by vaporizing water which exacerbates the test errors. Adiabatic oxidation experiments, providing an insulated reaction surrounding for coal oxidation to investigate the oxidization self-potential of coal, provides data for building a numerical relation between reaction temperature and heating rate[1]. The active energy E is educed from curves of rising temperature. This method is usually applied to predict the susceptibility of spontaneous coal combustion. In addition, some researchers obtained the average reaction order n in the oxidation of coal on the basis of isothermal experiments and concluded results which show that the reaction order n and reaction velocity v are related to temperature T. It should be noted that n is seldom equal to 1. The value of n increases and the consumption rate v decreases with decreasing temperature[2]. A Static Oxygen Consumption Test (SOCT) experiment can provide the variation of oxygen concentrations as well as the corresponding temperatures, two easily detected physical parameters. We applied the SOCT experiment results to analyze the oxidation

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kinetics of gas coal. This is the first time that kinetic regularity of spontaneous coal combustion in lowtemperature stages has been demonstrated on the basis of oxygen consumption performance.

2

Model of kinetic mechanism of lowtemperature oxidation of coal

Low-temperature oxidation of coal is a complex process involving oxygen consumption and emission of gaseous products; as well some solid products cling to the inner surface of coal. The rate of oxygen consumption v is considered as the rate of the entire chemical reaction if we ignore the small changes on the inner surface area of coal during the surface reaction. The relationship between v (mol/(m3·s)) and oxygen concentration c (mol/m3) is written as v = −dc / dt = kc n , where n is the reaction order and t (s) represents the reaction time. According to the Arrhenius formula, the chemical reaction rate constant k is expressed as Ae − E / RT , where E (J/mol) is the active energy, A is the pre-factor and R denotes the gas constant with a value of 8.314 J/(mol·K), where K is the symbol used for the Kelvin unit of temperature. Hence the oxygen consumption equation of the n-order reaction can be written as: dc − = Ae− E / RT ⋅ c n (1) dt If we define the program heating rate as w = dT / dt and assume that the environmental heat can be quickly transferred into coal, then a rearrangement of Eq.(1) yields dc A − E / RT n c − = e (2) dT w Integrate Eq.(2) as[8]: d c A T − E / RT dT = ³ e c0 c n w T0

−³

c

(3)

The left part of Eq.(3) is defined as f(c) and can be arranged as ­ln c0 − ln c dc ° −³ n = f (c) = ® c01− n − c1− n c0 c ° ¯ 1− n c

(n = 1) (n ≠ 1)

(4)

Define the integration of the right part of Eq.(3) as , then T = E and g(T) and let λ = E RT Rλ

A T − E / RT A T § E · e dT = ³ e − λ d ¨ ¸ ³ T T w 0 w 0 © λR ¹ AE λ − λ −2 AE e λ dλ = =− ⋅ P (λ ) wR ³λ0 wR

g (T ) =

where λ

P ( λ ) = − ³ e − λ λ −2 dλ = ∞

e − λ § 2! 3! 4! · 1− + − + ... ¸ λ 2 ¨© λ λ 2 λ 3 ¹

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E e− λ ⋅ R λ2

§ 2! 3! 4! · ¨1 − + 2 − 3 + ... ¸ λ λ λ © ¹ Previous studies have shown that RT <<1≈ 0 . E Eq.(5) is obtained based on the Agrawal approximation method with higher accuracy[13].

Hence:

³

T

T0

³

T

T0

e − E / RT dT =

e − E / RT dT =

2

RT − E / RT e E

ª 2 RT º « 1− » E « » 2 « § RT · » «1 − 5 ¨ ¸ » © E ¹ ¼ ¬

(5)

The combination of Eqs.(3), (4) and (5) leads to ª AR 1 − 2 RT / E º E § f (c ) · ln ¨ 2 ¸ = ln « − 2 » © T ¹ ¬ wE 1 − 5( RT / E ) ¼ RT

(6)

If we consider the right first part of Eq.(6) as a § f (c ) · 1 constant, then ln ¨ 2 ¸ ∝ , otherwise © T ¹ T f (c ) § · ¨ ª ¸ 2 RT º ¸ ¨ « AR E »¸ 1− ln ¨ 2 « = ln − E » T wE RT ¨ « 2 ¸ » ¨ «1 − 5 § RT · » ¸ ¨ ¸ ¨ © E ¹ ¼ ¹¸ © ¬

(7)

As RT E ≈ 0 , the first part of the right side becomes the constant ln( AR / wE ) , so that Eq.(7) can be written as E (8) ln( f (c) / T 2 ) = ln( AR / wE ) − RT and ln( f (c) / T 2 ) ∝ 1/ T which expresses a linear

relationship between ln(f (c) / T 2 ) and 1/T. Hence, the slope of the fiited line is − E / R and the intercept is ln( AR wE ) , from which the active energy E and pre-factor A can be obtained. By substituting f (c ) into Eq.(8), the expressions of the relations between oxygen concentration c and temperature T can be calculated by Eqs.(9) and (10) with different reaction mechanisms: a one-order (n=1) reaction: AR E § ln c0 − ln c · ln ¨ = ln − ¸ 2 T wE RT © ¹

(9)

and a n-order (n ≠ 1) reaction: § c 1− n − c1− n ln ¨ 0 2 © (1 − n)T

· AR E − ¸ = ln wE RT ¹

(10)

The rationality of these mechanism formulas cannot be confirmed even if good linearity were to exist. Sometimes there are different reaction mechanisms corresponding to the same set of kinetic data, which

WANG Lanyun et al

Oxidation kinetics regularity in spontaneous combustion of gas coal

are inevitable even under the strictest experimental conditions. Therefore, we introduce the kinetic compensation function ln A = a + bE as a principle for checking, where, a and b ((moO·K)/J) denote the compensation parameters. The mechanism formulas are rational if all calculated dots are distributed close to the fitted line[13-14]. There are different reactions in spontaneous coal combustion at different temperature stages. Investigations by Wang et al. have indicated that the oxidation of coal is complicated and indeterminate[15]. They proposed that the chemical reactions include two parallel sequences consuming oxygen and two thermal decomposition pathways producing carbon oxides. Due to the complicated structures of coal and variable chemical reactions, there is no clear dividing point symbolizing the low-temperature oxidation and uncontrollable combustion. However, the active energy E and reaction order can represent the entire range of oxidation characteristics of coal. The following equation, i.e., Eq.(11), indicating the relation of E and reaction velocity v, is deduced by substituting the compensation function ln A = a + bE into the logarithmic format of the Arrhenius function E ln v = ln A − . RT 1 ln v = a + (b − )E (11) RT According to this equation, there is also an isokinetic temperature Tc with a value1/(bR) , leading to the constant v. Tc is the crossing point formed by the Arrhenius curves (lnv~1/T) for all reactions. It should be noted that Tc is just an approximate isokinetic temperature, because not all data dots are perfectly placed on the fitted line[14].

3

Static test of oxygen consumption experiment

The structure of the static test of the oxygen consumption system with a surrounding adiabatic layer is shown in Fig. 1. A 1000 mL distillation flask filled with air is used to display the gas coal sample. A bag connected to a branch of the flask is used to collect gases and to suck the heated inflation gas for pressure balance. This bag could stand temperature as high as Table 1 Parameters

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200 °C and more. A fan is used to accelerate heat exchange to form an isothermal field. The gas analysis is carried out by a chromatography analyzer [1] .

Fig. 1

Composition of static oxygen consumption test system[1]

We selected the Chaili gas coal for our test and placed the coal sample (mass of 40 g, particle size of 80~120 mesh) into the closed distillation flask. The initial oxygen concentration in the flask is 20.9%. We increased the temperature in steps of ten degree Celsius from 40 to 190 °C and kept each temperature constant for one hour. We detected the oxygen concentration for each temperature after one hour isothermal heating. The results of oxygen consumption per gram coal are shown in Fig. 2.        





      7 ƒ&

Fig. 2

4

Oxygen consumption at different temperatures[1]

Results and discussion

Five reaction stages, 40~75 °C, 75~100 °C, 100~130 °C, 130~160 °C and 160~190 °C were divided based on the trend in oxygen consumption. Reaction orders, corresponding slopes and intercepts were obtained using a minimum variance programmed calculation. The average active energy E at every stage was calculated (Table 1).

Chemical kinetic parameters of low-temperature oxidation of Chaili gas coal Temperature stage (°C) 40~75

75~100

100~130

130~160

160~190

Reaction order n

3

0.83

0.06

0.05

0.15

Slope

–8414.5

–3425.2

–695.5779

–2.9345

594.7401

Intercept

5

–1.5

–8.9145

–9.7541

–11.1344

Frequency factor A

3609.7

2.2

0.00025994

4.7362×10-7

2.4141×10-5

Active energy E (kJ/mol)

69.9959

28.477

5.783

0.024

–4.945

Note: Rate of temperature increase: w = 10/3600 = 0.00278 K/s .

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The results obtained from Table 1 are as follows: 1) At the first oxidation stage, 40~75 °C, the coaloxygen recombination reaction was of order 3 for the intense chemisorption of oxygen on the inner coal surface. Oxygen diffusion in the coal pores controlled the oxygen consumption at this stage. At temperature above 75 °C, the order of the reactions was always less than 1, which indicates that oxygen diffusion has little effect on the oxidization of coal, but a kinetic reaction controls the oxygen consumption more than before. The oxidation at the higher temperature stage of 100~160 °C, with a reaction order of almost zero, is affected the least by oxygen concentration. At the 160~190 °C temperature stage, the reaction order climbed slightly, possibly due to the thermal decomposition of some solid products on the coal surface. It may also be that it is caused by an increased porosity from thermal distortion of pores, which causes the coal to absorb oxygen easily. In general, a decrease in the reaction order shows a weakening effect of oxygen concentration and an intensifying effect of the chemical reaction kinetics on the oxidation reaction of the gas coal. 2) The active energy E decreased with rising temperature, which shows that coal oxidizes easily at higher temperatures. The active energy at the 40~75 °C stage, with a value of 69.9959 kJ/mol, agrees with the direct burn-off reaction elucidated by Wang et al[15]. A negative E appears at the 160~190 °C temperature stage, indicating that at this point the gas coal oxidizes without an energy barrier. According to Tolman’s definition[16], a negative E is produced usually for the following two reasons: 1) at higher temperatures, accelerating oxidation provides more heat for reactants directly but less heat for complex compounds, leading to the negative E; 2) stable and inactive complex compounds are produced at this stage. The more stable the compound, the less active the energy produced. In addition, inactive compounds on the surface of coal limit direct contact of coal with oxygen. Hence, the oxidation of coal is characterized by self-activation at high temperature stages. Furthermore, a compensation effect was used to check the rationality of our inference (Fig. 3). This figure shows that all calculated dots are located near the fitted line. Given the fitted line, a slope value b=0.00030062 and an isokinetic temperature Tc of 1/(bR ) = 127 ć were obtained. This Tc indicates that the coal oxidizes slowly at temperatures below 127 °C, but violently when temperatures are higher than 127 °C. In order to prove the significance of Tc, we obtained the characteristic peak areas F of the active groups in the low-temperature gas coal oxidation range using a Fourier Transform Infrared (FTIR) spectroscopy. Fig. 4 shows decreases with rising temperatures in such active groups as: aryl hydrocar-

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bon, C=C in aromatic ring with stretch vibration, methyl and methylene with asymmetry flex vibration, methylene with symmetry stretch vibration, methylene with shearing vibration, methylene with plane vibration, which reduced sharply at 120 °C. Carbonyls in aromatic ketone and aldehyde, produced during oxidation, increase with temperature and mount rapidly also at 120 °C. From these experimental results, we conclude that a temperature of 120 °C is the key turning point of coal self-combustion. Since the isokinetic temperature equals 127 °C and is close to 120 °C, we see at this point the transition of the oxidation of gas coal, hence the effect of suppressing spontaneous coal combustion is determined by comparing Tc with the real temperature of coal, i.e., T. The combustion can be suppressed easily when T< Tc, otherwise it will be very difficult to control a violent oxidation.      

Fig. 3







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Kinetic compensation of active energy E and pre-factor A











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Fig. 4 Variation of the active groups during gas coal oxidation[1]

5

Conclusions

1) Based on the theory of chemical kinetics, an oxygen consumption rate equation during low-tem-

WANG Lanyun et al

Oxidation kinetics regularity in spontaneous combustion of gas coal

perature oxidation of coal was derived and an E-c equation was deduced according to the Agrawal approximation. 2) The active energy E decreased with rising temperature and a negative value appears when temperature increases, indicating that with rising temperature, oxidization tends to be easy and becomes spontaneous at stages of higher temperatures. The oxidation order is 3 at temperatures lower than 75 °C but approaches zero at temperatures of 100~160 °C, indicating that the oxygen concentration greatly increases oxygen consumption during the low temperature stage but plays a minor role during higher temperature stages. 3) By introducing the compensation effect, we proved the rationality of the kinetic formulas and calculated an isokinetic temperature Tc which was experimentally validated as the key turning point of the entire oxidation of gas coal.

Acknowledgements We gratefully acknowledge the financial support provided by the National Key Technology R&D Program during the 11th Five-Year Period (No. 2006BAK03B05), the National Natural Science Foundation of China (Nos.50534090, 50674090 and 50804047), the Research Fund of the State Key Laboratory of Coal Resources and Mine Safety, China University of Mining and Technology (Nos.08KF14 and SKLCRSM09X04), and the Scientific Research Foundation of China University of Mining & Technology (No.2007A001).

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