Research Note pubs.acs.org/IECR
Effects of Cooling Conditions on Surface Properties of Heated Coals Wencheng Xia,* Guangyuan Xie, Dong Pan, and Jianguo Yang School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, Jiangsu China ABSTRACT: Cooling coal mines to prevent spontaneous combustion is usually considered to be one important method to prevent coal combustion. Cooling methods may be divided into two main types, a water cooling method and an isolate cooling method (under a vacuum condition). This investigation is to find out the effects of cooling conditions on surface properties of heated coals. Heated coals were cooled in a vacuum chamber and cooled in water. X-ray photoelectron spectroscopy, scanning electron microscope, and contact angle measurements were used to indicate surface properties of heated coals under different cooling conditions. The results showed that heated coal cooled in water (Coal2) was more hydrophilic than heated coal cooled in a vacuum chamber (Coal1). Coal2 could be oxidized more than Coal1. This paper gives an opinion that cooling heated coals in spontaneous combustion using an isolate cooling method (under a vacuum condition) is useful for the flotation upgrade of fine coals, because the flotation upgrade of fine coals is based on the hydrophobicity of the coal surface. However, cooling heated coals in water reduces this hydrophobicity because an oxidation process can occur on a heated coal surface in water.
1. INTRODUCTION Taixi coal was heated and oxidized heavily during the spontaneous combustion of a coal mine. Water was usually used to cool the coal mine and prevent the coal combustion. As a result, the hydrophobicity of Taixi coal was reduced, and Taixi oxidized coal was difficult to float with common oily collectors.1−6 However, there is another method under a near vacuum condition to cool and prevent coal combustion which is called the isolate cooling method.7−9 Heated coal could also be cooled under the vacuum condition. This paper investigates the effects of cooling conditions on surface properties of heated coals. Throughout the paper, a reasonable suggestion on cooling heated coal is proposed. Furthermore, the results of this paper might be used to explain why unburned carbon in wet-stored fly ash was more difficult to float than unburned carbon in dry-stored fly ash.10−14 It might be that unburned carbon in wet-stored fly ash is oxidized to a further extent by the weathering process during the wet-stored period. In this investigation, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), and contact angle measurements were used to indicate surface properties of heated coals under different cooling conditions.
(Coal2) was dried in air. Both of them were stored in air for 48 h. 2.2. XPS, SEM and Contact Angle Measurements. For the indication of surface properties of heated coals, Coal1 and Coal2 were pressed into the pellets first. The XPS experiments were carried out at room temperature in an ultrahigh vacuum (UHV) system with a surface analysis system (ESCALAB 250Xi, America). The data processing (peak fitting) was performed with XPS Peak fit software, using a Smart type background subtraction and Gaussian/Lorentzian peak shapes. The binding energies were corrected by setting the C 1s hydrocarbon (−CH2−CH2-bonds) peak at 284.6 eV. The FEI Quanta 250 SEM was used to analyze the surface morphology of coal surface. The magnification times were fixed at 2000 and 4000. Coal1 and Coal2 were prepared by surface cleaning using absolute ethyl alcohol. After surface cleaning, Coal1 and Coal2 were dried in air. Before SEM, Coal1 and Coal2 were sputter-coated with a layer of gold. Coal1 and Coal2 were first pressed to the plates. The plates of coal samples were measured using a water contact angle analyzer (JC2000D), measuring a water droplet on the surface of the coal plate in air. The contact angles are obtained at the exact moment when the water droplet contacts with the coal plate.
2. EXPERIMENTAL METHOD AND PROCEDURE 2.1. Materials and Experiment Design. Fresh coals were selected from Ningxia province in China. The proximate analysis of fresh coal is given in Table 1. Where Mad is the moisture content; Vad is the volatile matter content; FCad is the fixed carbon content and Aad is the ash content. The contents of elements (C, H, O, N, and S) based on dry ash free are also shown in Table 1. The heating process was done in a crucible in an electric muffle furnace. The heating temperature was 800 °C and heating time was 120 min. Heated coals were cooled in a vacuum chamber and cooled in water, respectively. After the cooling processes, heated coal cooled in a vacuum chamber (Coal1) was stored in air, while heated coal cooled in water © 2014 American Chemical Society
3. RESULTS AND DISCUSSION Figure 1 shows C 1s peaks for Coal1 and Figure 2 shows C 1s peaks for Coal2. For C 1s peaks, peaks at binding energies of 284.6, 285.6, 286.6, and 289.1 eV correspond to the following groups: C−C or C−H, C−O, CO, and OC−O.15−18 On the basis of XPS peak fit analysis technology, the contents of these four groups could be calculated in Table 2 for Coal1 and Coal2. The groups C−C and C−H are primary hydrophobic Received: Revised: Accepted: Published: 10810
December 20, 2013 April 3, 2014 May 1, 2014 May 1, 2014 dx.doi.org/10.1021/ie404331j | Ind. Eng. Chem. Res. 2014, 53, 10810−10813
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Research Note
Table 1. Proximate analysis of fresh coal samples Mad
FCad
Vad
Aad
Cdaf
Hdaf
Odaf
Ndaf
St,daf
%
%
%
%
%
%
%
%
%
4.20
86.85
7.40
1.55
94.43
3.73
0.91
0.79
0.13
because Coal1 is cooled in a vacuum chamber while Coal2 is cooled in water. Coal2 might be oxidized by the water wetting process. It is well known that high-temperature-heated coal will react with water in a rapid process.21,22 Meanwhile, wetted coal can also be oxidized during the drying process in air.22−24 Therefore, Coal2 should be more heavily oxidized than Coal1. Figure 3 shows SEM images for Coal1, and Figure 4 shows SEM images for Coal2. The surface of Coal2 is rougher than that of Coal1. Meanwhile, the Coal2 surface has more cracks and pores than the Coal1 surface. It indicates that cooling heated coal in water may cause a process in which coal expands when heated and contracts when cooled. Therefore, the Coal2 surface has more cracks than Coal1. Heated coal could be crumbled quickly in cool water and Coal2 should suffer a fast process of temperature decreasing. However, Coal1 should suffer a slow process of temperature decreasing. Besides, the surface of Coal1 and Coal2 is similar to coal char during pyrolysis as there are many pores on their surface.25−31 Figure 5 shows the contact angle for Coal1, and Figure 6 shows the contact angle for Coal2. The contact angle of Coal1 is about 45° while that of Coal2 is about 37°. It indicates that Coal2 should be more heavily oxidized than Coal1. Therefore, the Coal2 surface is more hydrophilic than the Coal1 surface. Meanwhile, there are more cracks and pores on the Coal2 surface than the Coal1 surface. It is thought that the infiltration of water into the cracks and holes can also affect the value of the contact angles.4 Furthermore, the content of hydrophobic functional groups on the Coal1 surface is higher than that on Coal2 surface, while the content of hydrophilic functional groups on the Coal1 surface is lower than that on the Coal2 surface. As a result, the contact angle of Coal2 is less than that of Coal1.
Figure 1. C 1s peaks for Coal1.
Figure 2. C 1s peaks for Coal2.
functional groups while the groups C−O, CO, and OC− O are hydrophilic functional groups in coal.19,20 Table 2. Fraction of C on Coal1 and Coal2 Surface (Relative % of C 1s) coal type (s)
C−C and C−H
C−O
CO
OC−O
Coal1 Coal2
82.49 77.36
9.25 9.90
6.11 5.45
2.15 7.29
4. CONCLUSION A comparison of surface properties of heated coals cooled under different conditions was investigated by XPS, SEM, and contact angle measurements. The content of the hydrophilic functional groups on the Coal2 (heated coal cooled in water) surface is much more than that on the Coal1 (heated coal
As shown in Table 2, it could be seen that the content of C− C and C−H groups on Coal1 surface is higher than that on Coal2 surface. However, the content of C−O, CO, and O C−O groups on Coal1 surface is lower than that on the Coal2 surface. It indicates that Coal2 is oxidized heavier than Coal1
Figure 3. SEM pictures for Coal1. 10811
dx.doi.org/10.1021/ie404331j | Ind. Eng. Chem. Res. 2014, 53, 10810−10813
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Figure 4. SEM pictures for Coal2.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China. We also want to thank the support of Advanced Analysis and Computation Center of China University of Mining and Technology.
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Figure 5. Contact angle for Coal1.
Figure 6. Contact angle for Coal2.
cooled in a vacuum chamber) surface. Coal2 surface has more cracks than Coal1 and the contact angle of Coal2 is less than that of Coal1. The primary finding is that cooling heated coal in water makes the coal surface more hydrophilic than cooling heated coal in a vacuum chamber. This work supports the opinion that cooling heated coals in spontaneous combustion using the isolate cooling method instead of the water cooling method is recommended.
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Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest. 10812
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