Experimental Verification of the Mechanism of Free Radicals Reactions
in Spontaneous Combustion of Coal
WEI Aizhu, LI Zenghua & YANG Yongliang
(School of Mining & Safety Engineering, China University of Mining & Engineering, Xuzhou 221008, Jiangsu, China)
Abstract: In order to verify the correctness of free radicals reactions in spontaneous combustion of coal, electron spin resonance (ESR) spectroscopy was used
to measure the parameters including Lande factors (g) and concentrations of paramagnetic centers (Ng) as well as linewidth (ΔH) of coal free radicals in the
processes of fragmentation and oxygenation. The variations of these parameters under different particle size and different oxidative conditions were analyzed.
The results show that g-values are mainly related to the types of coal. Smaller particle size, higher oxidation temperature and longer oxidation time can lead to
higher concentration of paramagnetic center. It can also be seen that the variation trend of ΔH is complicated. Finally, according to the experimental data, the
reaction steps of free radicals are deduced and the relation between reaction of free radicals and spontaneous combustion of coal is proved.
Keywords: coal; spontaneous combustion; free radicals; ESR; experimental verification
1 Introduction
Spontaneous combustion has imposed a serious problem in coal related industries. Researching on the mechanism of
spontaneous combustion is significant for predicting and preventing fire disasters due to spontaneous combustion of coal.
Currently, most of researchers approve the theory of coal and oxygen reactions. But the detailed processes of spontaneous
combustion are argued drastically. The mechanism of free radicals reactions is a chemical explanation for the theory of coal and
oxygen reactions. The mechanism can be stated as follows[1]:
Coal mass can be fragmentized by mechanical forces (e.g. ground stress and shear stress resulted by excavator) and then lots of
rips can be generated. During such fragmentizing process, the covalent bands of coal molecule are ruptured and numbers of free
radicals are formed which can react easily. These radicals can be oxidized by O2 with heat releasing. The spontaneous combustion of
coal will occur if the conditions (full of oxygen and heat) are suitable.
Some researches[2, 3] show that there must be some relations between the spontaneous combustion of coal and the free radical
reactions in theory. Based on the mechanism of free radicals reactions, Yang et al[4] have studied on rubber antioxidant which can
catch active free radicals and invented a new inhibitor which possesses high efficient property for inhibiting the oxidation of coal. In
addition, when the coal is mixed with N-phenyl-naphthylamin (an inhibitor of free radicals), the concentrations of CO will decrease
during temperature rising. However, up to now there is no direct experimental evidence to support this mechanism. Therefore, the
authors design an experimental method to measure the free radicals directly during the process of coal fragmentizing and oxidizing
and then deduce the chemical reaction steps while coal is oxidizing. Accordingly, the correctness of the mechanism of free radicals
reactions can be proved directly by experiment.
2 Experimental Details
Two different ranks of coal (including fat coal of Zhangji Mine (Z) and anthracite of Baijigou Mine (B)) are investigated in this
research by JES-FA200 ESR spectrometer using X-band (9 GHz) spectrometer with magnetic modulation 100 kHz. The ESR spectra
are obtained with high attenuation of microwave power 1 mW to avoid signal saturation. The parameters (Lande factor (g),
concentrations of paramagnetic centers (Ng) and linewidth (ΔH)) of free radicals of coal are checked by Tampol and Mn2+ as
standard samples.
Relevant experimental contents and methods are as follows.
1) Measuring ESR spectra parameters during coal fragmentizing
Firstly, take a center part of big fresh coal mass and fragmentize in agate bowl manually. Then these coal particles are disparted
by different griddles and loaded into a special cuvette for ESR spectrometer and measured immediately at normal temperature.
2) Measuring ESR spectra parameters during coal oxidizing
The method of samples preparation is the same as above-mentioned. And the samples are heated in the resonance cavity of ESR
spectrometer directly by additional equipments which can control temperature and aerate air at the same time. ESR spectra can be
obtained at prearranged temperature or time.
3 Results and Discussion
3.1 Comparison of Results of Fragmentizing Experiment
The samples used for this experiment are from Z and B with 5 mg. The g-values, concentrations of paramagnetic centers and
linewidth measured by ESR spectrometer are summarized in Table 1.
The g-values of the two samples are: 2.0061-2.0063 (Z) and 2.0049-2.0054 (B). There are few differences in g-values among
same types of coal, but obvious differences among different types of coal (Table 1). Spin-orbit coupling constants and g-values for
unpaired electrons localized on N, O and S atoms are higher than those for unpaired electrons localized at C atoms[5].The higher
g-values of samples of Z indicate existence of nitrogen, oxygen or sulphur free radicals in the simplest aromatic structures of coal,
correspondingly the lower g-values indicate that those mainly paramagnetic centers with unpaired electrons located at carbon atoms
in B.
Supported by the National Natural Science Foundation of China (No. 50474067).
1616
Table 1 ESR parameters of coal in different particle size at normal temperature
Z fat coal B anthracite
Particle diameter
Concentration Linewidth Concentration Linewidth
/mm g 17 -1
g 17 -1
Ng/10 spin·g ΔH/mT Ng/10 spin·g ΔH/mT
0.42-0.84 2.00263 1.4936 653.9 2.00254 3.6198 437.8
0.18-0.42 2.00261 1.6596 653.4 2.00252 4.2644 474.0
0.125-0.18 2.00262 1.9775 643.1 2.00249 5.1764 480.9
0.1-0.125 2.00261 2.0815 653.4 2.00254 5.2687 487.1
The concentrations of paramagnetic centers increase as particle diameter decreases (Table 1). The smaller particle size is, the
higher concentrations of paramagnetic centers are. As all of the reactions occurred in air can be ignored due to the fast operation of
the experiment, the number of free radicals inevitably increases as molecule bond breaks when the outside mechanical force acts.
Exactly, fragmentizing is an approach to produce free radicals and induce reactions. It is also shown from Table 1 that the
concentrations of B are obviously higher than that of Z, which relates to the inactive inherent free radicals in different types of coal[6].
It is difficult to discuss on linewidth. Spin-lattice and spin-spin of electron are responsible for the variation of linewidth. On the
one hand, spin-lattice of electron lengthens relaxation time due to more aromatic rings in high rank coal, which make linewidth
narrow; on the other hand, spin-spin of electron strengthens due to more free radicals, which make linewidth broad. In Table 1, the
linewidth in Z is broader than that in B. It can be concluded that spin-lattice relaxation is responsible for the coal samples of B and
mainly spin-spin relaxation is responsible for the linewidth of Z. In addition, paramagnetic centers of multi-ring aromatic structures
of B, mainly delocalizedπ electrons with strong exchange interactions, are also responsible for narrow lines.
3.2 Comparison of Results of Oxidizing Experiment
5 mg of samples of Z in the particle size 0.125-0.18 mm are measured. The velocity of air-flow is 10 mL/s. Three oxidizing
ways are used: heating directly from 50 ℃ to 200 ℃; heating for 40min at the same temperature; heating to 110 ℃ then cooling.
The experimental results show that g-values are 2.00262-2.00271 in oxidizing, which is little higher than that at normal
temperature. This is the reason why more oxygen-containing free radicals could occur (e.g. hydroxyl, carbonyl and carboxyl). The
concentrations of paramagnetic centers and linewidth are mainly discussed in this section (Figs 1, 2 and 3).
It can be seen in Fig.1 that concentrations of paramagnetic centers increase quickly with oxidizing temperature increasing.
Oxidizing temperature is the main factor of free radicals increasing and spontaneous combustion of coal. This viewpoint is
consentaneous with most of scholars’. The increasing rate of concentrations is higher after 125 ℃ than that before 125 ℃ (Fig 1).
The possible reasons are: firstly, with temperature increasing, there are some new free radicals produced by thermal decomposition;
secondly, with temperature increasing, the reaction rate of free radicals is accelerated and some inactive inherent free radicals
participate in reactions, resulting in more free radicals formed.
10 700
8
Ng/ 1017 spin/g
600
ΔH /mT
6
500
4
400
2
0 300
50 75 100 125 150 175 200
Temperature/℃
concentrations linewidth
Fig.1 Influence of temperature on free radical of coal
At the same oxidizing temperature, the concentrations of paramagnetic centers are increased with oxidizing time (Fig 2). The
small increment indicates that gather of free radicals requires quite long time, that is to say, enough time is necessary to gather
high-concentrations of new free radicals for reactions, which can explain the period of coal spontaneous combustion. We also can see
in Figure 2 that the increasing rate of concentrations at 90 ℃ is remarkably higher than that at 70 ℃, which indicates that the critical
temperature of coal spontaneous combustion is 70 ℃. The experimental results are well fit with the experiential values.
1617
10 700
8
600
Ng /1017 spin/g
ΔH /mT
6
500
4
400
2
0 300
0 10 20 30 40
Time/min
concentrations in 70℃ concentrations in 90℃
linewidth in 70℃ linewidth in 90℃
Fig.2 Influence of oxidative time on free radicals of coal
The ESR spectra of coal samples during heating and cooling process are compared. The concentrations of paramagnetic centers
increase while coal sample is heated to 110 ℃ and cooled to 90 ℃, 70 ℃ and 50 ℃ (Fig 3). It can be speculated that numbers of new
free radicals are produced due to the cleavage of covalent bonds of high polymer caused by thermal stresses expanding and shrinking
on coal. The phenomenon[7] that the ignition point of coal can be lowered and then it is more prone to spontaneous combustion can be
explained according to this experiment. If coal is oxidized at low-temperature, even though lower environment temperature, the
concentrations of paramagnetic centers are still increased because the reactions of free radicals can be promoted.
10 700
8
Ng /1017 spin/g
600
ΔH /mT
6
500
4
400
2
0 300
50 70 90 110 90 70 50
Temperature/℃
concentrations linewidth
Fig.3 Comparison of parameters in the process of heating and cooling
It can be seen from the above three figures that the regularity of variations in linewidth during oxidizing processes of coal is
different. In Figures 1 and 2, the values of linewidth are lower than those in Table 1, but in Figure 3, the linewidth is higher. In Figure
1, linewidth shows decreasing trend with temperature, but in Figure 3, linewidth increases. The results of the experiment have
indicate the complex system of paramagnetic centers in coal. It can be concluded that broad linewidth due to more oxygen-center free
radicals and narrow linewidth due to some carbon-center free radicals are both existed during oxidizing process of coal.
3.3 Deducing Reaction Steps of Free Radicals in Oxidation of Coal
It’s well known that gases including CO, CO2, alkane, alkene, methanol and aldehyde etc. can be produced during coal
oxidizing. According to above discussion and relevant researches[1, 2], the reaction steps are deduced as follows:
When coal mass is broken into pieces by mechanical forces, the active free radicals will be produced:
R R′ R· + R′· (1)
Where R and R′ denote segments of the aromatic or aliphatic structure of coal. If O2 exists, oxidizing reaction will occur and peroxid
radical will be produced:
R·+ O2 R O O· (2)
Reaction (2) is an exothermic reaction. The temperature of coal will rise and then further reaction occurs:
R O O· + R′H R O O H+ R′· (3)
Different peroxids can decompose and form different products. The reactions are described by the following steps:
1618
RCHO + H2O
(4a)
R CH2OOH
RCH2O· + ·OH
R· + HCHO
O
R CHOOH R C CH3 + H2O (4b)
CH3
CH3 O
R C OOH R C CH3 + CH3OH (4c)
CH3
(4d)
R CH CH OOH R CH CHO· + ·OH
O
R CH2 C· R· +CO
Simultaneously, CO2 can be generated by the following reaction:
O
R C O· R· +CO2 (5)
After those reactions, the gases such as CO, CO2, H2O, methanol and formaldehyde and new free radicals are produced. The
new free radicals react with O2 again, then those reactions repeat again and again, and further more, the temperature rises gradually.
When the temperature rises to a higher degree, coal will be pyrolyzed and the gases of alkane and alkene series are generated. When
the conditions (enough oxygen and heat) are suitable, the spontaneous combustion of coal will occur.
4 Conclusions
It can be drawn from the experiment that the factors such as the degree of fragmentation, the oxidation temperature and the
oxidation time are responsible for the concentration of paramagnetic center, g-factor and linewidth of free radicals of coal. Smaller
particle size, higher oxidation temperature and longer oxidation time can lead to higher concentration of paramagnetic center. The
g-factors and linewidth are relative to the amount of carbon-center or oxygen-center free radicals. The deduced reactions steps of free
radicals can explain the way of gases generating (including CO, CO2, H2O, methanol and formaldehyde) during coal oxidizing.
This work has directly verified that the mechanism of free radicals reactions can explain spontaneous combustion of coal. Due
to the complex system of paramagnetic centers in coal, more works need to be conducted in the future.
Acknowledgment
The authors are very grateful to all the members of Physics Laboratory of Chinese University of Mining & Technology for
providing experiment apparatus.
References
[1] Li Z H. Mechanism of free radical reactions in spontaneous combustion of coal. Journal of China University of Mining & Technology, 1996, 25(3):
111-114
[2] Wang H, Dlugogorski B Z, Kennedy E M. Analysis of the mechanism of the low-temperature oxidation of coal. Combustion and Flame, 2003, 134(2):
107-117
[3] Yang S Q, Zhang R W, Di Z Q, et al. Mechanism analysis of coal spontaneous combustion and flame retard of conventional fire preventing and
extinguishing measures. Journal of China Coal Society, 1998, 23(6): 620-623
[4] Yang Y L, Yu S J, Zhang R Y, et al. Study on new stopping agent of preventing coal spontaneous combustion. Journal of China Coal Society, 1999, 24(2):
163-166
[5] Weickowski A B. Thermally excited multiplet states in flame coal. Fuel, 2001(80): 451-453
[6] Zhang P Z, Wang Z F. The use study on free radicals in Chinese coal. Journal of Fuel Chemistry and Technology, 1992, 20(3): 307-312
[7] Wang X S. Prevention and Cure in Mine Fire. Xuzhou: China University of Mining & Technology Press, 1990
1619