1612 by stariya

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									     Relation Between Spontaneous Combustion Risk and Microstructure of Coal
                                          GUO Liwen1, CHANG Wenjie 2 & WANG Fusheng1
                 (1 College of Resource and Environment of Hebei Polytechnic University, Tangshan 063009, Hebei, China;
                                    2 Kailuan Group Cooperation Ltd., Tanshan 063018, Hebei, China)

Abstract: The risk classify of coal to spontaneous combustion was made and the liability of that was appraised based on the coal adsorptive oxygen capacity in
room temperature. It outlined the methods and system of microstructure analysis experiment. According to the result of microstructure analysis experiment of
coal samples, the relation between spontaneous combustion risk and microstructure of coal was analyzed about the different coal samples. From the analysis
result, the abundant fissures and micro holes will make the coal seam easy to spontaneous combustion because the fissures and micro holes make the oxygen
easily come into the coal seam and speed up spontaneous combustion.
Keywords: spontaneous combustion; oxidation; microstructure analysis; spontaneous combustion risk


1 Introduction
      Spontaneous combustion of coal seam is one of the most serious natural disasters in colliery. There are many methods to assess
the risk of liability to coal of spontaneous combustion. For example, the spontaneous combustion risk was forecasted by establishing
large-scale laboratory bench to simulate coal spontaneous combustion process in many countries [1]. However, the experimental
technology factor was unitary that not be adapted the complex condition of coal mine. Singh R N proposed a method of appraise
spontaneous combustion risk considerating internal factor and external factor in the summarizing predecessor's foundation. The
method already have been used generally in England and Australia[2-5]. Moreover, the fuzzy clustering analysis[6] and artificial neural
network[7]have been applying to assess the spontaneous combustion risk of coal seam. Every method has its special character.
      Factors affecting liability of coal to spontaneous combustion depend on intrinsic factors as well as external factors promoting
the self-heating[8]. The intrinsic factors are mainly oxidizability of coal in room temperature and physical characteristics. It includes
carbonization degree of coal, microstructure of coal, component of coal petrology, sulphur content of coal, etc. It is necessary that
analyze the microstructure of coal. By analyzing the relative about maceral of coal, initial temperature of spontaneous combustion
and grain size of coal, we can know the macro crake of coal and the macro break are the important entryway of oxygen when coal
speed-up oxidation that is the decisive effect to spontaneous combustion of coal. Analyzing the microstructure of coal can provide the
auxiliary parameters to estimate the risk of liability of spontaneous combustion.
      In this paper, we analyzed sectional photos of coal to microstructure in the electric microscope. The spontaneous combustion
was analyzed from micro geography.
2 Experiment
2.1    Experiment of Liability of Coal to Spontaneous Combustion
      The experiment mainly tests the adsorptive capacity of coal to oxygen in the low temperature and normal atmosphere, and
divides the gradation of liability of coal to spontaneous combustion. The laboratory instrument of main block uses ZRJ-1
determinator of liability of coal to spontaneous combustion. The experiment system includes standard column calorstat, detector and
its calorstat, gas circuit control system and electric control system etc [9].
      When sampling in the mining work face, stripping oxidizing layer of coal bed surface first, along the vertical direction scoring
two lines which width is 100 to 150 mm, picking 50 mm thickness coal sample. The coal sample was pounded to less than 20 to 30
mm, and then the coal sample was smashed in crashing engine. Finally, the less than 0.15 mm coal powder was screened from
smashing coal sample and sealed into wild-mouth bottle for experiment.
      The coal samples were collected from Jinggezhuang Colliery in China. Its fundamental data was in Table 1.
      According to the classification criteria of liability of coal to spontaneous combustion and different ranks of coal, total sulfur and
adsorptive oxygen capacity of coal in 30 degree Centigrade and normal pressure, the risk of spontaneous combustion can be divided
into three ranks: Ⅰ, Ⅱ, Ⅲ.
      Table 1 shows the adsorption oxygen capacity and the rank of liability of coal to spontaneous combustion. From Table 1, we can
know that there is liability of spontaneous combustion in the all seams of Jinggezhuang colliery. There is different rank in different
place.
2.2 Microstructure analysis Experiment of Coal Samples
      The microstructure analysis experiment can analyze surface characteristics of coal samples and auxiliary study risk of coal to
spontaneous combustion by observing micro-electronic photo of coal section under scanning electron microscope. The experimental
system of scanning electron microscope includes electron-optical system; scanning system; signal collecting, processing and display
system; power supply and control system; cooling system etc six parts.
      When observe the samples under the scanning electron microscope, first cutting the samples along the cross section into nubs
that the diameter is 10-15 millimeter and the thickness is 5 millimeter. The new sections of observing don’t be damaged. After
affixing it onto the copper specimen holder using conducting resin, a film conducting layer about 100 angstrom is jetted plating in
vacuum plating device.
      After the scanning electron microscope is adjusted, setting up the sample and observing the picture. The high pressure is 20-kilo



    Supported by National Natural Science Foundation of China: 50474080
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volt and the observing scope is 100 micron, partly 10 micron or 1 millimeter. Then, the micro-electronic pictures are taken.
             Table 1 Coal sample fundamental data and result of perfection to liability of spontaneous combustion
   Serial           Coal                                                    Total    Real specific   Adsorptive oxygen
                                    Location           Moisture/%                                                              rank
  number            seam                                                  sulfur/%     gravity       capacity/(cm3·g-1)
     1                9           Eastern wing              4.26            0.38         1.48               0.71                Ⅰ
     2                9           Eastern wing              5.22            0.32         1.55               0.79                Ⅰ
     3                9          Western wing               4.19            0.50         1.48               0.66                Ⅱ
     4                9          Western wing               5.82            0.49         1.43               0.52                Ⅱ
     5                9          Southern wing              4.38            0.36         1.46               0.63                Ⅱ
     6                9          Western wing               4.52            0.49         1.43               0.68                Ⅱ
      7               9          Southern wing              6.32           0.25          1.43                1.16               Ⅰ
      8               9           Western wing              4.35           0.40          1.42                1.43               Ⅰ
Ⅰ is easy spontaneous combustion; Ⅱ is spontaneous combustion; Ⅲ is non-spontaneous combustion.

3 Results and Discussion
     Table 1 shows the liability of coal to spontaneous combustion of seam 9 eastern wing is highest in these seams. It belongs to
type Ⅰ, i.e. easy self combustion. The type of western wing coal seam is type Ⅱ that is spontaneous combustion coal seam. The type
of southern wing is same to western wing coal seam.
     The 8th coal sample was fetched from western wing. Its adsorptive oxygen capacity is higher than other coal samples of western
wing. The coal sample is hard, massive, non-broken and is heavier by hand feeling. But it is easy to spontaneous combustion during
the practical production. Figs.1 and 2 are the sectional microstructure of the 8th coal sample under the electronic microscope. These
photos show that there are abundant fissures and some samdwich in the sample. The samdwich look like more breaking and spongy
than the solid mass. From the surface of the solid mass there are some micro holes. It is different from the other samples of western
wing. That forms the different character and makes the coal sample easy to spontaneous combustion.




          Fig.1     8th sample of magnification 400 times                     Fig.2 8th sample of magnification 500 times

     The adsorptive oxygen capacity of 7th coal sample is higher than others coal samples of southern wing obviously. Though this
coal sample was fetched from the same working face with 5th coal sample, the 7th coal sample presents ribbon in the all seam. It is
obviously different to others coal samples. Figs.3 and 4 show the photo of 7th coal sample section in microscope. Magnification
times in Fig.3 is 50, and in Fig.4 is 180. From these two photos it can be found that the bedding and hole of the sample is more
abundant than other samples. There are more abundant transcrystalline cracking in the sample. This is not different from 5th sample.
This makes the 7th sample special character, i.e. the oxygen easily come into the sample, and easily supplies the condition of
oxidation. So the 7th sample is easily to spontaneous combustion.




            Fig.3    7th sample of magnification 50 times           Fig.4 7th sample of magnification 180 times

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      The gradation of liability of coal to spontaneous combustion in western wing is lower than eastern wings. But actually the
spontaneous combustion is easier in western wing than eastern wing. This is because of the external factors. In the western wing
seam the geological condition is bad, there are many faults, the density of fault is big, and the coal near the fault is very cracked. That
gives the condition for heating. The coal near the fault is dry. It made the spontaneous combustion more graveness. Fig.5, fig.6 and
fig.7 are the photos of electrical microscope in western wing.
      Fig.5, fig.6 and fig.7 show that there are many fissures and micro fissures in the coal sample of western wing, and the coal is
very soft and cracked. The many fissures and the cracked or floppy coal increase the adsorptive oxygen capacity and thermal storage
capacity. So the western wing coal seam is easy for spontaneous combustion.




  Fig.5      Western wing sample of magnification 100 times                 Fig.6   Western wing sample of magnification 1000 times




                                Fig.7   Fissure of western wing sample (magnify 100 times)

     From Table 1 we can know the risk of liability of spontaneous combustion is type Ⅰ, i.e. the eastern wing coal seam easy to
spontaneous combustion. This is the same to practical situation. Fig.8 shows that there are many different fissures in the section of
the eastern wing coal sample. The coal is soft and there are some cracked coal filled into the density part. These characters make the
eastern wing coal seam easy for thermal storage and self-combustion. When magnifying the coal sample to 3 000 times, you can find
that there are many microholes in the surface of coal sample. If there is some water in the micro holes, the risk of spontaneous
combustion will reduce. If the water in the micro holes dries out, the area of coal contact air will increase, and the risk of spontaneous
combustion will increase too.




     Fig.8     Fissures of eastern wing sample ( 250 times)         Fig.9    Pore space of eastern wing coal sample ( 3 000 times)


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     The gradation of liability of coal to spontaneous combustion of southern wing coal seam is type Ⅱ. Figs.10 and 11 are the
sectional photos of microstructure in the electric microscope. The Figs. show that the surface structure of the southern wing coal
sample is flake, the particle is fine and small and loosening. When it is amplified to 1 000 times, we can find there are some fissures
in the surface, but the micro fissure is not development. This structure makes it containing some crevice water. In the same time
because of the artesian aquifer of seam 7, seam 9 and above seam 5, the coal is moist. It is hardly to accumulation of heat. So the risk
of coal to spontaneous combustion is reduced.




      Fig.10   southern wing sample of magnification 100 times                        Fig.11   the fissure of southern wing sample (1 000 times)

4 Conclusions
     The abundant fissures and micro holes will make the coal seam easy to spontaneous combustion. The more abundant
transcrystalline cracking makes the oxygen easily come into the coal seam and speed up spontaneous combustion. If the density of
faults is big, the coal near the faults will become very cracking, and which increase the adsorptive oxygen capacity and thermal
storage capacity, and which make the coal seam easy for spontaneous combustion.

References
[1]   Singh R N, Denby B. A Knowledge-based system for assessing spontaneous combustion risk in longwall mining. Australia Mining Science and
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[2]   Wu Jun. Study of coal to spontaneous combustion in coal petrology analysis. The Journal of Coal Engineer, 1987(3): 9-13
[3]   Chandra D, Prasad Y V S. Effect of coalification on spontaneous combustion of coal. International Journal of Coal Geology, 1990(6): 225-229
[4]   Demirbilek S, Sc B, Sc M. The development of a spontaneous combustion risk classification system for coal seam. Thesis submitted to the University of
      Nottingham for the degree of Doctor of Philosophy, 1987: 19-22
[5]   Singh R N, Shonhardt J A. Spontaneous Combustion Risk Managementing Longwall Mining in New South Wales. Mining Risk Management Conference,
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[7]   Wang Hongde, Di Yingquan, Xiao Jiaxin. Using BP's algorithm to Realize the forecast of spontaneous combustion risk of the Coal Seam to Be Mined.
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[8]   Singh R N, Demirbilek S, Turney M. Application of spontaneous combustion risk index to mine planning, safe storage and shipment of coal. Journal of
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[9]   Guo Liwen, Wang Haiyan, Zhang Fusheng. Text and study of the law governing spontaneous combustion of coal seam in Ginggezhuang coal mine.
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[10] Wang Fusheng. The study on indicator gases of spontaneous combustion in Zhaogezhuang coal mine. Ms D Thesis. Tangshan: Hebei Polytechnic
      University, 2002(in Chinese)




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