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Structural order and dielectric properties of coal chars
Fuel 137 (2014) 164–171
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Structural order and dielectric properties of coal chars Liang Xu, Haiyu Liu ⇑, Yan Jin, Baoguo Fan, Xiaolei Qiao, Bo Jing College of Electrical and Power Engineering, Taiyuan University of Technology, Taiyuan 030024, China
h i g h l i g h t s This work investigated structure changes and dielectric properties of coal chars. The crystallite array of coal chars was more ordered with increasing temperature. The complex permittivity of coal chars depended on treatment temperature. The crystallite structure was responsible for dielectric properties of coal chars.
a r t i c l e
i n f o
Article history: Received 10 April 2014 Received in revised form 25 July 2014 Accepted 2 August 2014 Available online 13 August 2014 Keywords: Coal char Crystallite structure Dielectric properties Microwave
a b s t r a c t For studying the influence of the structural evolution on the dielectric properties, X-ray diffraction and Raman spectroscopy were used to examine the phase and the degree of crystallite structure of coal chars prepared in the temperature range of 850–1600 °C. Additionally, structural changes were visualized by high-resolution transmission electron microscopy. The dielectric properties were measured in the 2–18 GHz frequency range using the transmission/reflection method. Generally, the fraction of carbon ordering of coal chars increased, while more parallel-aligned aromatic layers were presented, confirming that the crystallite structure of coal chars became ordered with increasing temperature. The dielectric properties of chars were found strongly dependent on heat-treatment temperature. Increasing temperature could produce higher values of the real part and imaginary part of the complex permittivity. Although mineral matters in coal chars were also observed to transform during heat treatment, the conversion of inorganic matters might have a marginal effect on the complex permittivity. As a result, it is reasonable to conclude that the dielectric properties primarily result from the crystallite structure of coal chars. Furthermore, the dielectric loss may contribute much more than the magnetic loss to the further application of microwave technology. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Carbon content in fly ash is a major concern for combustion efficiency of a coal-fired boiler in the power plant. High unburned carbon levels mean poor combustion and cause substantial loss of energy. Real-time monitoring enables carbon content of fly ash to be kept at a reasonable level to improve economic efficiency. Based on much stronger microwave absorption of carbon than other oxides (e.g., SiO2, Al2O3, Fe2O3, CaO, MgO) in fly ash, microwave technique [1–5] has great application potentials for on-line determination of unburned carbon in fly ash. Since unburned carbon is produced from coal particles of incomplete combustion, coal char undergoes a complex succession of physicochemical evolution during the combustion process [6–10], which governs the degree
⇑ Corresponding author. Tel.: +86 13466871287. E-mail address: [email protected] (H. Liu). http://dx.doi.org/10.1016/j.fuel.2014.08.002 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
of char burnout and carbon structure [11–17]. In circulating fluidized bed boilers the temperature of coal combustion is controlled at 850–950 °C for deep desulfurization, while coal combustion is performed in the temperature range of approximately 1200– 1600 °C in most pulverized coal utility boilers. The formation of coal chars with a variety of crystallite is dominated by heat conditions in boilers. It is well known that coal consists of crystallite phases that are randomly arranged, called turbostratic structure. The evolution of coal char structure properties has been extensively studied not only in combustion but also in pyrolysis and gasification processes [10,18–23]. Hurt and co-workers [12,24] previously compared the evolution of char structure between laboratory-prepared chars and residual carbon from pulverized-coal combustion. Residual unburned chars had been found a high degree of crystallinity than laboratory-prepared chars. Although the growth of regions of turbostratic order developed even in such a short combustion time
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of 100 ms, true graphitization was not observed under peak temperature of 1527 °C in their experiments. Feng et al. [25] found that the increase of heat treatment time and temperature improved the fraction of the organized carbon of Australian coal chars but nearly did not change the carbon crystallite size at 850–1150 °C, in agreement with the study of Davis et al. [24]. Meanwhile, the organized carbon structure of ash-free coal chars kept unchanged, which was the result of the loss of catalytic action from the inorganic matter in the coal chars. Emmerich [26] found that the coalescence increase of the crystallites along the a-axis and the c-axis was depend on the temperature range and the carbon used. The coalescence increase dominated after 1350 °C for the graphitizable chars compared with non-graphitizable chars until 2500 °C. Through microscopic structure of chars pyrolyzed at 400–1500 °C by Yoshizawa et al. [27], the average number of stacking layers per stack had been found maximize around 800 °C, which exhibited the influence of swelling on the char structure. Thus, it can be seen from the above discussion that the growth of regions of carbon structure appears to occur during heat treatment. But the inconformity between different studies also presents the evidence that the evolution of crystallite structure is depend on the specific process of heat treatment, as well as the behavior of inorganic matter in coal chars [25,28,29]. Anyway, the crystallite structure of coal chars varies during combustion and pyrolysis, and its changes may have notable effects on the electromagnetic parameters of resultant carbon, which further leads to the performance of reliability of microwave measurement. The changes in the dielectric properties are essential and highly necessary for the microwave application. However, very few researches have been reported involved in the influence of structural ordering of coal chars on the dielectric properties. Herein, in the present work, coal chars were prepared at different temperatures, covering the main heat-treatment range of pulverized coal combustion. For investigating the dielectric properties of coal chars, electromagnetic parameters including the complex permittivity and the complex permeability, were measured by the transmission/reflection method. The crystallite structure of coal chars was characterized by X-ray diffraction (XRD) technique. Raman spectroscopy (RS) analysis was additionally performed to reveal the degree of carbon ordering. The technique of high-resolution transmission electron microscopy (HRTEM) was also employed to provide supporting evidence through visualization of structural changes.
freely cooled to room temperature and char samples were collected for the following tests. The entire process was strictly performed under a nitrogen atmosphere. 2.2. XRD analysis A shimadzu diffractometer, model XRD-6000, was applied to record X-ray intensity scattered from char samples and to determine the phase and crystal structure of coal chars. Cu Ka radiation (40 kV, 30 mA) was used as the X-ray source. Char samples were scanned in a velocity of 8° per minute and a step size of 0.2°, over the 2theta angular range of 5–65°. 2.3. RS technique The Raman spectrum of char samples was acquired by Renishaw confocal microprobe Raman spectroscopy (inVia). A highpower pulsed argon ion laser was used as the excitation light source with a laser beam of 514 nm exciting line, corresponding to the grating of 1800 line/mm. The laser power at the sample surface was controlled at about 2 mW. The spectrum was recorded in the range of 800–2000 cm1. 2.4. HRTEM observation High-resolution transmission electron microscopy (HRTEM) images of char samples were captured using a JEM-2100F (JEOL) field emission transmission electron microscopy operated at an accelerating voltage of 200 kV. Before examination, the dispersions and dilution of sample were carried out with ethanol, treated with ultrasonic waves at ambient temperature for 15 min, and then dried onto copper grids for preparation. 2.5. Dielectric properties
2. Experimental procedure
The dielectric properties of coal chars were probed on the basis of the coaxial transmission/reflection line theory. Char samples were uniformly mixed with dissolved paraffin at a weight ratio of 1:1 and die-pressed into a toroidal shape with 7.0 mm outer diameter, 3.04 mm inner diameter and approximately 2.0 mm thickness. The test ring was fixed to the input section of reflection access of an Agilent N5244A vector network analyzer, and electromagnetic parameters, i.e., the complex permittivity and the complex permeability were determined in the range of 2–18 GHz. The test apparatus is clearly shown in Fig. 1.
2.1. Raw coal and preparation of coal char samples
3. Results and discussions
Pulverized coal originating from a utility boiler was selected for this investigation. Proximate analysis and ultimate analysis of the raw material are presented in Table 1. Table 2 summarizes the chemical composition of coal ash. Coal chars were prepared at various temperatures by a horizontal tube furnace. The furnace reactor was electrically preheated at a heating rate of 10 °C/min to the temperature range of 850–1600 °C. About 2 g coal powder was spread uniformly in an alundum copple and then pushed into the center of the heated tube. After held at the desired temperature for 60 min, the coal in the copple was
3.1. Phase and crystal structure of coal chars by XRD Phase qualitative analysis can be made from XRD patterns of chars prepared at different temperature. As shown in Fig. 2, there are two major aspects of changes observed during heat treatment. The diffraction intensity, of the main peak corresponding to the (0 0 2) reflection of graphite, is very low but becomes higher with increasing temperature, indicating coal chars composed of turbostratic structure carbon, especially for those generated at relatively low temperature, while increasing temperature encourages
Table 1 Proximate and ultimate analysis of raw coal. Sample
Raw coal
Proximate analysis (%, mass, ad)
Ultimate analysis (%, mass, daf)
Volatile
Ash
Fixed carbon
C
H
N
S
O
34.49
7.43
53.28
78.63
3.79
2.28
0.25
10.66
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Table 2 Analysis data of coal ash. Sample
Raw coal
Ash component (%) SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
TiO2
K2O
Na2O
P2O5
43.6
15.58
10.36
17.82
2.5
4.53
0.62
1.12
0.51
0.46
Fig. 1. Test apparatus of the dielectric properties of coal char sample.
Fig. 3. Variation of structural parameters of coal chars at various temperature. Fig. 2. XRD diffraction patterns for chars prepared at different temperature.
structure conversion from amorphous carbon to more microcrystalline carbon. Another noticeable transformation is the performance of silicon and its derivative. The SiO2 substance is from the raw coal (Table 2), and still remains in coal chars after heat treatment at 850–1150 °C, through its corresponding peak in XRD spectra. Thereafter the disappearance of the SiO2 peak at 1300–1450 °C implies its transformation of silicon compound glass phase. The produced phase of SiC identified from the relevant peak is attributed to the reaction by silicon with carbon at 1600 °C. By means of Bragg’s law and conventional Scherrer equation, the obtained structure parameters from the XRD (0 0 2) profiles reveal the dependence on treatment temperature, as shown in Fig. 3. The interlayer spacing (d002) remains almost unchanged
from 850 °C to 1000 °C and then decreases significantly from 3.83 Å to 3.55 Å at temperatures above 1000 °C. The stacking height of the crystallite (Lc) increases from 8.0 Å to 16.2 Å. In contrast to the d value (3.354 Å) of the single crystal of graphite, the large interlayer spacing, in conjunction with the low stacking height indicates that carbon structure of coal chars is poor crystallinity. Furthermore, the average layer number (Nc) was calculated from d002 and Lc by the equation N c ¼ Lc =d002 and shown in Fig. 3. The N c value is 2.09 for chars at 850 °C, just increasing to 4.55 for chars at 1600 °C. The little change of the stacking layers implies the graphitization is difficulty. Although calculated from the XRD (10) carbon bands, the lateral size of the crystallite (La) is not shown here. Based on possible large errors consideration from the diffuse peak and weak intensity of (10) bands at 44°,
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La may be unsuitable for analyzing the crystallite dimension in the a-axis direction. However, the crystallite structure is regularly developed after higher temperature treatment. The presence of mineral materials may have a non-negligible affect on the evolution of coal char structure, since a variety of metal compounds promote or restrain carbon structural ordering [28,30]. The increasing degree of graphitization in carbonaceous materials has been mainly attributed to the occurrence of catalytic ordering from metals including boron, calcium, aluminum, iron, silicon, etc., especially their carbides [31]. Among them, silicon and its derivative are considered as the main contribution to catalytic graphitization here, instead of iron, which, although was reported to act as effective catalyst to graphitic structure in previous literatures [25]. Two reasons for this can be identified. First, silicon catalytically forms only graphitic carbon, while iron causes formation of both graphitic and turbostratic carbon. Silicon is thought to be more effective than iron for acceleration of graphitization. Second but not least, it can be seen that silicon is the major constituent of inorganic matter in coal chars, whose content is much higher than that of any other metal. Phenomena of catalytic graphitization results from the carbide formation–decomposition mechanism [30,32], and the formation of silicon carbide detected through XRD patterns also suggests that it is no more than silicon which exists in carbide forms, thus the transition of mineral matters is believed to provide the motivation for the development of carbon crystallite structure. The XRD quantitative analysis reveals the overall change of coal chars during heat treatment, similar with other studies of char structure [18,23,25]. Moreover, silicon or its carbide accelerating the improvement of coal char structure is proposed in this paper, which has never been explicitly emphasized before. The change of the degree of ordering or graphitization during thermal process, as well as catalysis from mineral matters would be confirmed and elucidated in the subsequent sections.
in the 1580 cm1 position of Raman spectra, representing E2g symmetry vibration for ideal graphitic lattice. Any disorder, defect and imperfections would cause some additional bands, and bands at different position have been used for special materials and studies [38–41]. From the intensity of the acquired Raman spectra against Raman shift in the first-order region, five bands were assigned and curve-fitted for this study, as shown in Fig. 4. The D1, D2, D3 and D4 bands located approximately 1350, 1620, 1530 and
3.2. Structural order of coal chars by Raman Coal or coal char has a polycrystalline structure between shortrange order and long-range disorder. Although XRD analysis gives straightforward information on the crystallite structure, it may be debatable for characterizing highly disordered crystallite, e.g., chars produced at low temperatures [33–35]. As an effective method for characterizing physicochemical composition of carbonaceous material through vibratory structure, Raman spectroscopy (RS) has also been employed to analyze the evolution of char structure in this work [36,37]. It is well known that only one band exists
Fig. 4. Assignments of curve-fitted bands for a typical Raman spectrum in the firstorder region.
167
Fig. 5. Variation of the band area ratios against temperature.
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1150 cm1, stand for A1g symmetry vibration of disordered graphitic lattice, E2g symmetry vibration of disordered graphitic lattice, amorphous carbon with sp2–sp3 mixed bond forms, and ionic impurities or sp3-rich structures at the periphery of crystallites, respectively [42,43]. Since the identifiable bands from the shape and the height of the Raman peaks are relative to various structures in coal chars, the band area ratios, of the G band relative to the completely integrated area (denoted as IG =IAll ) and the Di bands to the G band (denoted as IDi =IG ), are applied to study the char structural evolution. The impact of treatment temperature on char structure is shown in Fig. 5. Through the temperature dependence of band area ratios, it can be seen that IG =IAll increases significantly from 0.07 to 0.32, while ID1 =IG , ID2 =IG and ID3 =IG decrease gradually with the increase of treatment temperature. Generally, ID4 =IG shows a reduction with a little fluctuation for chars at 1000 °C. The value of IG =IAll is small for chars at low treatment temperature, implying carbon in coal chars is chiefly amorphous, but these organic carbons with disordered structure would transform into periodically organized aromatic layers with temperature. Several major types of defect are also eliminated under severer heat treatment. The results signify well an agreement with the above analysis by
XRD, which are also consistent with the literature studies of char structural characteristics [36,43]. Besides, some researchers had deduced a relationship between La and ID =IG , and the parameters from different methods revealed linear [33,44]. But the relationship was not inferred here, in view of the disordered carbon structure and the possible calculated errors of La . Nevertheless, the XRD technique is still valid to estimate char crystallite parameters for studying the structural changes of crystallites during heat treatment, which has been confirmed via the consistency of the results from XRD and RS analysis. 3.3. Morphological evolution and phase transition by HRTEM HRTEM photographs, for their abilities in visualizing structural changes, are also significant for structural properties of coal chars. Small selected regions from HRTEM images of coal chars at different temperatures are shown in Fig. 6. The variety of the lattice fringes in HRTEM can be used to characterize the structural evolution [22,23,45]. With the help of element detection by EDS module of HRTEM apparatus, the 850 °C chars exhibit silicon-rich components, as well as abundant amorphous carbon from the observation of distorted layers (see Fig. 6a). The lattice crystals of carbon
Fig. 6. HRTEM images of chars at the temperature of (a) 850 °C, (b) 1000 °C, (c) 1150 °C, (d) 1300 °C, (e) 1450 °C, and (f) 1600 °C.
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present a high degree of turbostratic arrangements at lower temperature. As temperature increases, turbostratic crystallites in coal chars become recognizable and oriented gradually (see Fig. 6b–e). In the 1300 °C chars, carbon crystallite is found to stack in groups of several short graphene layers with orientation and arrangement. Furthermore, the 1600 °C chars display a better degree of crystallization, for long parallel-aligned layers perform a close-grained and unified stacking (see Fig. 6f). The monitored lattice fringes indicate that carbon crystallite structure has became more ordered during heat treatment. On the other hand, the crystallite phases of silicon compound were also identified constantly, which could support the XRD findings. Although revealing a relatively scattered distribution in lower temperature chars (e.g., Fig. 6a), carbon and silicon would migrate for structural transformation. Silicon compounds tend to concentrate with the compacted carbon layers stacking tighter and tighter, leading to an increasing rate of nucleation graphite. Meanwhile, silicon carbide is produced during the diffusion of carbon atoms within silicon-rich components, especially in liquid form above the ash fusion temperature. It is formation and decomposition of silicon carbide that is responsible for the mechanism of the acceleration of ordered carbon arrays. Since the crystallite structure cannot be fully mirrored only through some selected regions, no effort was made to obtain the crystallite size from HRTEM. However, HRTEM lattice fringe images show directly visual evidence of the structural evolution at a crystallite level, strongly proving the quantitative results from XRD and Raman. 3.4. Dielectric properties of coal chars When materials are exposed to microwave radiation, the interaction between materials and the electromagnetic field can be characterized with the complex permittivity ðe ¼ e0 je00 Þ and the complex permeability ðl ¼ l0 jl00 Þ, of which the real part is relative to energy storage and the imaginary part is associated to the dielectric loss [46,47]. Fig. 7 is the real part (e0 ) and imaginary part ðe00 Þ of the complex permittivity plotted as a function of frequency. One can observe that both the e0 and e00 curves display a gradual decrease with increasing frequency. However, the dependence of the measuring frequency differs for chars at various temperatures. Take, for example, Fig. 7b. The e00 values of chars at 850 °C vary from 6.5 to 2.8, while the e00 values of 1600 °C chars decrease sharply from 62.9 to 15.1 with frequency. The results show that lower temperature chars have a slight reduction, while higher temperature chars decrease significantly in the frequency range. On the other hand, the complex permittivity is essentially dependent on heat temperature. From Fig. 7a and b, it is observed that both the e0 and e00 values of chars increase significantly as heat temperature increases from 850 °C to 1600 °C. The complex permittivity represents macroscopic measurement of interaction between microscopic particle and electromagnetic wave, and the response of atomic or molecular structures to an alternating electric field commonly relied on polarization degree [48,49]. For a given dielectric material there are several basic mechanisms of polarization, such as space-charge, relaxation, dipolar orientation and electron-ion shift polarization. The sum of contributions for every mechanism determines the net polarizability.
P ¼ Ps þ Pr þ Pd þ Pe
ð1Þ
The temperature dependence of the complex permittivity can be explained as follows. The structural transformation of coal chars is schematically illustrated in Fig. 8, according to the former subsection. In general, coal chars treated at low temperature exhibit disordered structure (Fig. 8a), providing little space-charge or
Fig. 7. The (a) real part, (b) imaginary part of the complex permittivity, and (c) the dielectric loss tangent plotted against frequency for coal chars.
orientation because of the canceling influences from different atoms and molecules of turbostratic structure. The low values of the complex permittivity are the overall result of weak polarization. Elevated temperatures increase the crystal growth, and crystallites with a group of several carbon layers could be induced to orient by microwave radiation (Fig. 8b). Electron-ion shift polarization is established quickly without energy loss, but its contribution to the complex permittivity is small in comparison to other polarization at measuring microwave frequency in this paper. Therefore, the strengthened orientation polarization, losing energy during the slow formation of relaxation process, is regarded as the dominant mechanism to the complex permittivity. In addition, structural order enhances sp2-hybridization carbon in a single plane and generates delocalized electrons in the graphite interplanar bonding. Electron migration along the layer plane direction and possible hopping between crystallites further contribute to dielectric loss (Fig. 8c). By analysis above, it is concluded that the degree of orientation polarization and electron conductivity could be proposed to the main contribution to the increase of the complex permittivity, and the behavior of structure ordering is eventually responsible for dielectric properties of coal chars.
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Fig. 8. Schematic illustration showing the structural order of coal chars according to these analyses by XRD, Raman and HRTEM (a) char disordered structure, (b) crystallites with several aligned layers, and (c) graphitic carbon planes.
Fig. 9 illustrates the frequency dependence of the real part (l0 ) and the imaginary part ðl00 Þ of the complex permeability. It is observed that the complex permeability does not give a constant expression with frequency for all the chars at different temperature, but the values are distributed in the range 0.95 < l0 < 1.15 and 0.1 < l00 < 0.1, fairly small compared to the complex permittivity. Calculated from the equation tan dm ¼ l00 =l0 , the magnetic loss tangent (tan dm) values are found to fluctuate just around 0. Thus, it suggests that the magnetic loss may contribute almost nothing to microwave application, compared with the dielectric loss. 4. Conclusions The most significant result of this paper is to demonstrate the assured influence of the crystallite structure on the dielectric properties of coal chars. Coal chars were produced at 850–1600 °C. The structural variation was inspected by XRD, RS and HRTEM. The following conclusions were drawn from the present investigation: (1) Increasing temperature made the crystallite structure of coal chars ordered, the fraction of organized carbon increased and aromatic layers periodically rearranged. In addition, mineral matters were examined to behave conversion and consumption. The results from different techniques provided concordant evidence of the structural evolution. (2) The complex permittivity of coal chars exhibited significant dependence on treatment temperature. Both the real and imaginary part increased at higher temperature. The dielectric loss tangent behaved similar with heat treatment conditions. The dielectric properties were strongly predominated by char structure resulting in various intensities of polarization, and only marginally influenced by the inorganic matters. (3) The complex permeability and the magnetic loss tangent of coal chars may be insignificant in this work. The dielectric properties of coal chars were mainly responsible for the attenuation properties in microwave application.
Fig. 9. The (a) real part, (b) imaginary part of the complex permeability, and (c) the magnetic loss tangent plotted against frequency for coal chars.
Acknowledgement The authors gratefully acknowledge the support from the plan of the National Natural Science Foundation (No. 51206116).
As a factor representing the fraction of stored energy loss per period of field oscillation, the dielectric loss tangent (tan de) is necessary in microwave application [46,50], which can be calculated by the equation tan de ¼ e00 =e0 and shown in Fig. 7c. It can be seen that tan de is distinctly influenced by heat temperature. From the relationship between the e0 and e00 , the increase of tan de with temperature can be inferred for chars. Moreover, the dielectric loss tangent is directly related to attenuation factor of material in a microwave field, so chars treated at higher temperature imply good attenuating properties for microwave application.
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Structural order and dielectric properties of coal chars Liang Xu, Haiyu Liu ⇑, Yan Jin, Baoguo Fan, Xiaolei Qiao, Bo Jing College of Electrical and Power Engineering, Taiyuan University of Technology, Taiyuan 030024, China
h i g h l i g h t s This work investigated structure changes and dielectric properties of coal chars. The crystallite array of coal chars was more ordered with increasing temperature. The complex permittivity of coal chars depended on treatment temperature. The crystallite structure was responsible for dielectric properties of coal chars.
a r t i c l e
i n f o
Article history: Received 10 April 2014 Received in revised form 25 July 2014 Accepted 2 August 2014 Available online 13 August 2014 Keywords: Coal char Crystallite structure Dielectric properties Microwave
a b s t r a c t For studying the influence of the structural evolution on the dielectric properties, X-ray diffraction and Raman spectroscopy were used to examine the phase and the degree of crystallite structure of coal chars prepared in the temperature range of 850–1600 °C. Additionally, structural changes were visualized by high-resolution transmission electron microscopy. The dielectric properties were measured in the 2–18 GHz frequency range using the transmission/reflection method. Generally, the fraction of carbon ordering of coal chars increased, while more parallel-aligned aromatic layers were presented, confirming that the crystallite structure of coal chars became ordered with increasing temperature. The dielectric properties of chars were found strongly dependent on heat-treatment temperature. Increasing temperature could produce higher values of the real part and imaginary part of the complex permittivity. Although mineral matters in coal chars were also observed to transform during heat treatment, the conversion of inorganic matters might have a marginal effect on the complex permittivity. As a result, it is reasonable to conclude that the dielectric properties primarily result from the crystallite structure of coal chars. Furthermore, the dielectric loss may contribute much more than the magnetic loss to the further application of microwave technology. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Carbon content in fly ash is a major concern for combustion efficiency of a coal-fired boiler in the power plant. High unburned carbon levels mean poor combustion and cause substantial loss of energy. Real-time monitoring enables carbon content of fly ash to be kept at a reasonable level to improve economic efficiency. Based on much stronger microwave absorption of carbon than other oxides (e.g., SiO2, Al2O3, Fe2O3, CaO, MgO) in fly ash, microwave technique [1–5] has great application potentials for on-line determination of unburned carbon in fly ash. Since unburned carbon is produced from coal particles of incomplete combustion, coal char undergoes a complex succession of physicochemical evolution during the combustion process [6–10], which governs the degree
⇑ Corresponding author. Tel.: +86 13466871287. E-mail address: [email protected] (H. Liu). http://dx.doi.org/10.1016/j.fuel.2014.08.002 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
of char burnout and carbon structure [11–17]. In circulating fluidized bed boilers the temperature of coal combustion is controlled at 850–950 °C for deep desulfurization, while coal combustion is performed in the temperature range of approximately 1200– 1600 °C in most pulverized coal utility boilers. The formation of coal chars with a variety of crystallite is dominated by heat conditions in boilers. It is well known that coal consists of crystallite phases that are randomly arranged, called turbostratic structure. The evolution of coal char structure properties has been extensively studied not only in combustion but also in pyrolysis and gasification processes [10,18–23]. Hurt and co-workers [12,24] previously compared the evolution of char structure between laboratory-prepared chars and residual carbon from pulverized-coal combustion. Residual unburned chars had been found a high degree of crystallinity than laboratory-prepared chars. Although the growth of regions of turbostratic order developed even in such a short combustion time
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of 100 ms, true graphitization was not observed under peak temperature of 1527 °C in their experiments. Feng et al. [25] found that the increase of heat treatment time and temperature improved the fraction of the organized carbon of Australian coal chars but nearly did not change the carbon crystallite size at 850–1150 °C, in agreement with the study of Davis et al. [24]. Meanwhile, the organized carbon structure of ash-free coal chars kept unchanged, which was the result of the loss of catalytic action from the inorganic matter in the coal chars. Emmerich [26] found that the coalescence increase of the crystallites along the a-axis and the c-axis was depend on the temperature range and the carbon used. The coalescence increase dominated after 1350 °C for the graphitizable chars compared with non-graphitizable chars until 2500 °C. Through microscopic structure of chars pyrolyzed at 400–1500 °C by Yoshizawa et al. [27], the average number of stacking layers per stack had been found maximize around 800 °C, which exhibited the influence of swelling on the char structure. Thus, it can be seen from the above discussion that the growth of regions of carbon structure appears to occur during heat treatment. But the inconformity between different studies also presents the evidence that the evolution of crystallite structure is depend on the specific process of heat treatment, as well as the behavior of inorganic matter in coal chars [25,28,29]. Anyway, the crystallite structure of coal chars varies during combustion and pyrolysis, and its changes may have notable effects on the electromagnetic parameters of resultant carbon, which further leads to the performance of reliability of microwave measurement. The changes in the dielectric properties are essential and highly necessary for the microwave application. However, very few researches have been reported involved in the influence of structural ordering of coal chars on the dielectric properties. Herein, in the present work, coal chars were prepared at different temperatures, covering the main heat-treatment range of pulverized coal combustion. For investigating the dielectric properties of coal chars, electromagnetic parameters including the complex permittivity and the complex permeability, were measured by the transmission/reflection method. The crystallite structure of coal chars was characterized by X-ray diffraction (XRD) technique. Raman spectroscopy (RS) analysis was additionally performed to reveal the degree of carbon ordering. The technique of high-resolution transmission electron microscopy (HRTEM) was also employed to provide supporting evidence through visualization of structural changes.
freely cooled to room temperature and char samples were collected for the following tests. The entire process was strictly performed under a nitrogen atmosphere. 2.2. XRD analysis A shimadzu diffractometer, model XRD-6000, was applied to record X-ray intensity scattered from char samples and to determine the phase and crystal structure of coal chars. Cu Ka radiation (40 kV, 30 mA) was used as the X-ray source. Char samples were scanned in a velocity of 8° per minute and a step size of 0.2°, over the 2theta angular range of 5–65°. 2.3. RS technique The Raman spectrum of char samples was acquired by Renishaw confocal microprobe Raman spectroscopy (inVia). A highpower pulsed argon ion laser was used as the excitation light source with a laser beam of 514 nm exciting line, corresponding to the grating of 1800 line/mm. The laser power at the sample surface was controlled at about 2 mW. The spectrum was recorded in the range of 800–2000 cm1. 2.4. HRTEM observation High-resolution transmission electron microscopy (HRTEM) images of char samples were captured using a JEM-2100F (JEOL) field emission transmission electron microscopy operated at an accelerating voltage of 200 kV. Before examination, the dispersions and dilution of sample were carried out with ethanol, treated with ultrasonic waves at ambient temperature for 15 min, and then dried onto copper grids for preparation. 2.5. Dielectric properties
2. Experimental procedure
The dielectric properties of coal chars were probed on the basis of the coaxial transmission/reflection line theory. Char samples were uniformly mixed with dissolved paraffin at a weight ratio of 1:1 and die-pressed into a toroidal shape with 7.0 mm outer diameter, 3.04 mm inner diameter and approximately 2.0 mm thickness. The test ring was fixed to the input section of reflection access of an Agilent N5244A vector network analyzer, and electromagnetic parameters, i.e., the complex permittivity and the complex permeability were determined in the range of 2–18 GHz. The test apparatus is clearly shown in Fig. 1.
2.1. Raw coal and preparation of coal char samples
3. Results and discussions
Pulverized coal originating from a utility boiler was selected for this investigation. Proximate analysis and ultimate analysis of the raw material are presented in Table 1. Table 2 summarizes the chemical composition of coal ash. Coal chars were prepared at various temperatures by a horizontal tube furnace. The furnace reactor was electrically preheated at a heating rate of 10 °C/min to the temperature range of 850–1600 °C. About 2 g coal powder was spread uniformly in an alundum copple and then pushed into the center of the heated tube. After held at the desired temperature for 60 min, the coal in the copple was
3.1. Phase and crystal structure of coal chars by XRD Phase qualitative analysis can be made from XRD patterns of chars prepared at different temperature. As shown in Fig. 2, there are two major aspects of changes observed during heat treatment. The diffraction intensity, of the main peak corresponding to the (0 0 2) reflection of graphite, is very low but becomes higher with increasing temperature, indicating coal chars composed of turbostratic structure carbon, especially for those generated at relatively low temperature, while increasing temperature encourages
Table 1 Proximate and ultimate analysis of raw coal. Sample
Raw coal
Proximate analysis (%, mass, ad)
Ultimate analysis (%, mass, daf)
Volatile
Ash
Fixed carbon
C
H
N
S
O
34.49
7.43
53.28
78.63
3.79
2.28
0.25
10.66
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Table 2 Analysis data of coal ash. Sample
Raw coal
Ash component (%) SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
TiO2
K2O
Na2O
P2O5
43.6
15.58
10.36
17.82
2.5
4.53
0.62
1.12
0.51
0.46
Fig. 1. Test apparatus of the dielectric properties of coal char sample.
Fig. 3. Variation of structural parameters of coal chars at various temperature. Fig. 2. XRD diffraction patterns for chars prepared at different temperature.
structure conversion from amorphous carbon to more microcrystalline carbon. Another noticeable transformation is the performance of silicon and its derivative. The SiO2 substance is from the raw coal (Table 2), and still remains in coal chars after heat treatment at 850–1150 °C, through its corresponding peak in XRD spectra. Thereafter the disappearance of the SiO2 peak at 1300–1450 °C implies its transformation of silicon compound glass phase. The produced phase of SiC identified from the relevant peak is attributed to the reaction by silicon with carbon at 1600 °C. By means of Bragg’s law and conventional Scherrer equation, the obtained structure parameters from the XRD (0 0 2) profiles reveal the dependence on treatment temperature, as shown in Fig. 3. The interlayer spacing (d002) remains almost unchanged
from 850 °C to 1000 °C and then decreases significantly from 3.83 Å to 3.55 Å at temperatures above 1000 °C. The stacking height of the crystallite (Lc) increases from 8.0 Å to 16.2 Å. In contrast to the d value (3.354 Å) of the single crystal of graphite, the large interlayer spacing, in conjunction with the low stacking height indicates that carbon structure of coal chars is poor crystallinity. Furthermore, the average layer number (Nc) was calculated from d002 and Lc by the equation N c ¼ Lc =d002 and shown in Fig. 3. The N c value is 2.09 for chars at 850 °C, just increasing to 4.55 for chars at 1600 °C. The little change of the stacking layers implies the graphitization is difficulty. Although calculated from the XRD (10) carbon bands, the lateral size of the crystallite (La) is not shown here. Based on possible large errors consideration from the diffuse peak and weak intensity of (10) bands at 44°,
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La may be unsuitable for analyzing the crystallite dimension in the a-axis direction. However, the crystallite structure is regularly developed after higher temperature treatment. The presence of mineral materials may have a non-negligible affect on the evolution of coal char structure, since a variety of metal compounds promote or restrain carbon structural ordering [28,30]. The increasing degree of graphitization in carbonaceous materials has been mainly attributed to the occurrence of catalytic ordering from metals including boron, calcium, aluminum, iron, silicon, etc., especially their carbides [31]. Among them, silicon and its derivative are considered as the main contribution to catalytic graphitization here, instead of iron, which, although was reported to act as effective catalyst to graphitic structure in previous literatures [25]. Two reasons for this can be identified. First, silicon catalytically forms only graphitic carbon, while iron causes formation of both graphitic and turbostratic carbon. Silicon is thought to be more effective than iron for acceleration of graphitization. Second but not least, it can be seen that silicon is the major constituent of inorganic matter in coal chars, whose content is much higher than that of any other metal. Phenomena of catalytic graphitization results from the carbide formation–decomposition mechanism [30,32], and the formation of silicon carbide detected through XRD patterns also suggests that it is no more than silicon which exists in carbide forms, thus the transition of mineral matters is believed to provide the motivation for the development of carbon crystallite structure. The XRD quantitative analysis reveals the overall change of coal chars during heat treatment, similar with other studies of char structure [18,23,25]. Moreover, silicon or its carbide accelerating the improvement of coal char structure is proposed in this paper, which has never been explicitly emphasized before. The change of the degree of ordering or graphitization during thermal process, as well as catalysis from mineral matters would be confirmed and elucidated in the subsequent sections.
in the 1580 cm1 position of Raman spectra, representing E2g symmetry vibration for ideal graphitic lattice. Any disorder, defect and imperfections would cause some additional bands, and bands at different position have been used for special materials and studies [38–41]. From the intensity of the acquired Raman spectra against Raman shift in the first-order region, five bands were assigned and curve-fitted for this study, as shown in Fig. 4. The D1, D2, D3 and D4 bands located approximately 1350, 1620, 1530 and
3.2. Structural order of coal chars by Raman Coal or coal char has a polycrystalline structure between shortrange order and long-range disorder. Although XRD analysis gives straightforward information on the crystallite structure, it may be debatable for characterizing highly disordered crystallite, e.g., chars produced at low temperatures [33–35]. As an effective method for characterizing physicochemical composition of carbonaceous material through vibratory structure, Raman spectroscopy (RS) has also been employed to analyze the evolution of char structure in this work [36,37]. It is well known that only one band exists
Fig. 4. Assignments of curve-fitted bands for a typical Raman spectrum in the firstorder region.
167
Fig. 5. Variation of the band area ratios against temperature.
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1150 cm1, stand for A1g symmetry vibration of disordered graphitic lattice, E2g symmetry vibration of disordered graphitic lattice, amorphous carbon with sp2–sp3 mixed bond forms, and ionic impurities or sp3-rich structures at the periphery of crystallites, respectively [42,43]. Since the identifiable bands from the shape and the height of the Raman peaks are relative to various structures in coal chars, the band area ratios, of the G band relative to the completely integrated area (denoted as IG =IAll ) and the Di bands to the G band (denoted as IDi =IG ), are applied to study the char structural evolution. The impact of treatment temperature on char structure is shown in Fig. 5. Through the temperature dependence of band area ratios, it can be seen that IG =IAll increases significantly from 0.07 to 0.32, while ID1 =IG , ID2 =IG and ID3 =IG decrease gradually with the increase of treatment temperature. Generally, ID4 =IG shows a reduction with a little fluctuation for chars at 1000 °C. The value of IG =IAll is small for chars at low treatment temperature, implying carbon in coal chars is chiefly amorphous, but these organic carbons with disordered structure would transform into periodically organized aromatic layers with temperature. Several major types of defect are also eliminated under severer heat treatment. The results signify well an agreement with the above analysis by
XRD, which are also consistent with the literature studies of char structural characteristics [36,43]. Besides, some researchers had deduced a relationship between La and ID =IG , and the parameters from different methods revealed linear [33,44]. But the relationship was not inferred here, in view of the disordered carbon structure and the possible calculated errors of La . Nevertheless, the XRD technique is still valid to estimate char crystallite parameters for studying the structural changes of crystallites during heat treatment, which has been confirmed via the consistency of the results from XRD and RS analysis. 3.3. Morphological evolution and phase transition by HRTEM HRTEM photographs, for their abilities in visualizing structural changes, are also significant for structural properties of coal chars. Small selected regions from HRTEM images of coal chars at different temperatures are shown in Fig. 6. The variety of the lattice fringes in HRTEM can be used to characterize the structural evolution [22,23,45]. With the help of element detection by EDS module of HRTEM apparatus, the 850 °C chars exhibit silicon-rich components, as well as abundant amorphous carbon from the observation of distorted layers (see Fig. 6a). The lattice crystals of carbon
Fig. 6. HRTEM images of chars at the temperature of (a) 850 °C, (b) 1000 °C, (c) 1150 °C, (d) 1300 °C, (e) 1450 °C, and (f) 1600 °C.
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169
present a high degree of turbostratic arrangements at lower temperature. As temperature increases, turbostratic crystallites in coal chars become recognizable and oriented gradually (see Fig. 6b–e). In the 1300 °C chars, carbon crystallite is found to stack in groups of several short graphene layers with orientation and arrangement. Furthermore, the 1600 °C chars display a better degree of crystallization, for long parallel-aligned layers perform a close-grained and unified stacking (see Fig. 6f). The monitored lattice fringes indicate that carbon crystallite structure has became more ordered during heat treatment. On the other hand, the crystallite phases of silicon compound were also identified constantly, which could support the XRD findings. Although revealing a relatively scattered distribution in lower temperature chars (e.g., Fig. 6a), carbon and silicon would migrate for structural transformation. Silicon compounds tend to concentrate with the compacted carbon layers stacking tighter and tighter, leading to an increasing rate of nucleation graphite. Meanwhile, silicon carbide is produced during the diffusion of carbon atoms within silicon-rich components, especially in liquid form above the ash fusion temperature. It is formation and decomposition of silicon carbide that is responsible for the mechanism of the acceleration of ordered carbon arrays. Since the crystallite structure cannot be fully mirrored only through some selected regions, no effort was made to obtain the crystallite size from HRTEM. However, HRTEM lattice fringe images show directly visual evidence of the structural evolution at a crystallite level, strongly proving the quantitative results from XRD and Raman. 3.4. Dielectric properties of coal chars When materials are exposed to microwave radiation, the interaction between materials and the electromagnetic field can be characterized with the complex permittivity ðe ¼ e0 je00 Þ and the complex permeability ðl ¼ l0 jl00 Þ, of which the real part is relative to energy storage and the imaginary part is associated to the dielectric loss [46,47]. Fig. 7 is the real part (e0 ) and imaginary part ðe00 Þ of the complex permittivity plotted as a function of frequency. One can observe that both the e0 and e00 curves display a gradual decrease with increasing frequency. However, the dependence of the measuring frequency differs for chars at various temperatures. Take, for example, Fig. 7b. The e00 values of chars at 850 °C vary from 6.5 to 2.8, while the e00 values of 1600 °C chars decrease sharply from 62.9 to 15.1 with frequency. The results show that lower temperature chars have a slight reduction, while higher temperature chars decrease significantly in the frequency range. On the other hand, the complex permittivity is essentially dependent on heat temperature. From Fig. 7a and b, it is observed that both the e0 and e00 values of chars increase significantly as heat temperature increases from 850 °C to 1600 °C. The complex permittivity represents macroscopic measurement of interaction between microscopic particle and electromagnetic wave, and the response of atomic or molecular structures to an alternating electric field commonly relied on polarization degree [48,49]. For a given dielectric material there are several basic mechanisms of polarization, such as space-charge, relaxation, dipolar orientation and electron-ion shift polarization. The sum of contributions for every mechanism determines the net polarizability.
P ¼ Ps þ Pr þ Pd þ Pe
ð1Þ
The temperature dependence of the complex permittivity can be explained as follows. The structural transformation of coal chars is schematically illustrated in Fig. 8, according to the former subsection. In general, coal chars treated at low temperature exhibit disordered structure (Fig. 8a), providing little space-charge or
Fig. 7. The (a) real part, (b) imaginary part of the complex permittivity, and (c) the dielectric loss tangent plotted against frequency for coal chars.
orientation because of the canceling influences from different atoms and molecules of turbostratic structure. The low values of the complex permittivity are the overall result of weak polarization. Elevated temperatures increase the crystal growth, and crystallites with a group of several carbon layers could be induced to orient by microwave radiation (Fig. 8b). Electron-ion shift polarization is established quickly without energy loss, but its contribution to the complex permittivity is small in comparison to other polarization at measuring microwave frequency in this paper. Therefore, the strengthened orientation polarization, losing energy during the slow formation of relaxation process, is regarded as the dominant mechanism to the complex permittivity. In addition, structural order enhances sp2-hybridization carbon in a single plane and generates delocalized electrons in the graphite interplanar bonding. Electron migration along the layer plane direction and possible hopping between crystallites further contribute to dielectric loss (Fig. 8c). By analysis above, it is concluded that the degree of orientation polarization and electron conductivity could be proposed to the main contribution to the increase of the complex permittivity, and the behavior of structure ordering is eventually responsible for dielectric properties of coal chars.
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Fig. 8. Schematic illustration showing the structural order of coal chars according to these analyses by XRD, Raman and HRTEM (a) char disordered structure, (b) crystallites with several aligned layers, and (c) graphitic carbon planes.
Fig. 9 illustrates the frequency dependence of the real part (l0 ) and the imaginary part ðl00 Þ of the complex permeability. It is observed that the complex permeability does not give a constant expression with frequency for all the chars at different temperature, but the values are distributed in the range 0.95 < l0 < 1.15 and 0.1 < l00 < 0.1, fairly small compared to the complex permittivity. Calculated from the equation tan dm ¼ l00 =l0 , the magnetic loss tangent (tan dm) values are found to fluctuate just around 0. Thus, it suggests that the magnetic loss may contribute almost nothing to microwave application, compared with the dielectric loss. 4. Conclusions The most significant result of this paper is to demonstrate the assured influence of the crystallite structure on the dielectric properties of coal chars. Coal chars were produced at 850–1600 °C. The structural variation was inspected by XRD, RS and HRTEM. The following conclusions were drawn from the present investigation: (1) Increasing temperature made the crystallite structure of coal chars ordered, the fraction of organized carbon increased and aromatic layers periodically rearranged. In addition, mineral matters were examined to behave conversion and consumption. The results from different techniques provided concordant evidence of the structural evolution. (2) The complex permittivity of coal chars exhibited significant dependence on treatment temperature. Both the real and imaginary part increased at higher temperature. The dielectric loss tangent behaved similar with heat treatment conditions. The dielectric properties were strongly predominated by char structure resulting in various intensities of polarization, and only marginally influenced by the inorganic matters. (3) The complex permeability and the magnetic loss tangent of coal chars may be insignificant in this work. The dielectric properties of coal chars were mainly responsible for the attenuation properties in microwave application.
Fig. 9. The (a) real part, (b) imaginary part of the complex permeability, and (c) the magnetic loss tangent plotted against frequency for coal chars.
Acknowledgement The authors gratefully acknowledge the support from the plan of the National Natural Science Foundation (No. 51206116).
As a factor representing the fraction of stored energy loss per period of field oscillation, the dielectric loss tangent (tan de) is necessary in microwave application [46,50], which can be calculated by the equation tan de ¼ e00 =e0 and shown in Fig. 7c. It can be seen that tan de is distinctly influenced by heat temperature. From the relationship between the e0 and e00 , the increase of tan de with temperature can be inferred for chars. Moreover, the dielectric loss tangent is directly related to attenuation factor of material in a microwave field, so chars treated at higher temperature imply good attenuating properties for microwave application.
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