Article Cite This: Energy Fuels 2017, 31, 11947-11953
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Thermoplastic Properties of Coal and Coal Extract Chia-Ching Chu,† Chih-Min Chang,† Der-Her Wang,‡ and Thou-Jen Whang*,† †
Department of Chemistry, National Cheng Kung University, Tainan 70101, Taiwan Steel Research Department, China Steel Corporation, Kaohsiung 81233, Taiwan
‡
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ABSTRACT: The aim of this study is to find a simple, effective, and non-destructive way to estimate the fluidity of coal. Three kinds of metallurgical coals were blended in a specific ratio for fluidity measurement by a Gieseler plastometer and coke strength by the Roga test. One of the metallurgical coals was extracted with four kinds of solvents. The solvent-extracted extracts further underwent heat treatment at different temperatures and were then added to the blended coal to examine the change of fluidity and coke strength. Both the heat-treated and untreated coal extracts were further investigated by elemental analysis, thermogravimetric analysis, solid-state 13C nuclear magnetic resonance, and X-ray diffractometry, and it was found that the heat treatment will increase the degree of graphitization. The aromatic carbon content was found to correlate significantly with the fluidity of coal, and the relationship among the ratio of aromatic carbon/aliphatic carbon, aromaticity, and fluidity was studied in this work. Finally, X-ray diffractometry is suggested to be a non-destructive method to estimate the fluidity of coal.
1. INTRODUCTION Coal is primarily composed of the macromolecular structure of polycyclic aromatic hydrocarbons (PAHs) with some heteroatom groups, and admixes with a trace amount of mineral matter.1 Its secondary network structure is derived from aromatic ring stacking, aliphatic side-chain entanglement, hydrogen bonding, cation bridges, and electron donor− acceptor interaction. The aggregation of the coal structure is believed to be correlated closely with coal ranks, which is governed by the non-covalent bonding interactions, such as a complicated network structure. Polar solvents containing nitrogen or oxygen (for example, amine, phenol, pyridine, tetrahydrofuran, dimethylformamide, etc.) can be used to extract raw coal, and the network structure of coal has been widely investigated by means of the solvent extraction method. Besides, polar solvents have been proven to be effective for liberating the non-covalent bonding interactions from the network structure of coal.2 There are several methods for studying the structure and composition of coal, including destructive methods, e.g., elemental analysis (EA) and thermogravimetric analysis (TGA), and non-destructive methods, e.g., nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) spectroscopy. Solid-state 13C NMR (13C SSNMR) is a powerful instrument and technique that enables us to investigate the coal structure without destruction of the coal sample and to measure carbon aromaticity ( fa) as well as the fraction of aromatic carbon to aliphatic carbon.2−6 The chemical shift of SSNMR can also be simulated by theoretical calculation, which involves the structural information, and the simulation results are used to modify the chemical structure model of coal or coal extract.7,8 XRD is also a non-destructive technique that has been used to characterize carbonaceous materials for decades. Coal is a typical example of the carbonaceous material, and its structural information can be obtained by XRD for a better understanding of coal chemistry. In general, the X-ray diffractogram of coal shows broad and diffuse peaks because coal has a © 2017 American Chemical Society
heterogeneous and amorphous structure. Moreover, coal has similar XRD peak positions as graphite, which has sharp and clear XRD peaks; that is to say, coal has an intermediate structure between the graphite and the amorphous carbonaceous material, and the XRD peaks become sharper and less diffuse with the increase of the coal rank.9 The classification of the carbon-related peaks at approximately 2θ = 20° and 26°, viz., γ and π bands, respectively, can be analyzed by XRD in slow-step scan mode. The γ band is believed to be due to aliphatic chains, and the π band is due to aromatic carbons.10 In this study, a slightly caking coal (which is later denoted as coal III) was extracted with four kinds of solvents and the solvent-extracted extracts were then further examined by EA to identify the variation of carbon and hydrogen contents. A Gieseler plastometer was used to measure the fluidity of the coal samples with the addition of the coal extract, and the coke strength of these coal samples was also determined by the Roga test. The fluidity of coal can be estimated by the crucible swelling number (CSN) analysis. However, in contrast to the CSN analysis, the XRD is a non-destructive method and could avoid personal subjective judgment of the shape of coke. To establish a simple and non-destructive method to estimate the approximate fluidity of coal, both the XRD and SSNMR techniques were introduced into this study to explore the relationship among the ratio of aromatic carbon/aliphatic carbon (CAr/CAl), aromaticity ( fa value), and fluidity. It was found that CAr/CAl and fa value have a negative relationship with fluidity and the aromatic carbon content and aliphatic carbon content can be determined by the XRD method. Therefore, XRD is suggested to be a simple, effective, and nondestructive way to estimate the fluidity of coal. Received: August 7, 2017 Revised: October 1, 2017 Published: October 2, 2017 11947
DOI: 10.1021/acs.energyfuels.7b02327 Energy Fuels 2017, 31, 11947−11953
Article
Energy & Fuels Table 1. Fundamental Analysis of the Raw Coals EA (wt %, daf)a raw coal
C
H
N
S
Ob
C/H ratio
RMc(wt %)
PTd(°C)
VMe(%)
ash content (%)
maximum fluidity (ddpm)
coal I coal II coal III
78.20 81.42 73.60
5.35 4.51 5.26
2.13 1.84 1.67
0.65 0.74 0.64
13.67 11.49 18.83
1.22 1.50 1.17
64.93 83.78 66.21
477 512 464
33.2 17.4 34.9
6.22 10.09 9.54
433 15 120
a
Dry and ash free. bBy difference. cRemaining material = ash left after TGA. dPyrolysis temperature = sample begins to pyrolyze in TGA. eVolatile material.
Table 2. Elemental Analysis of Coal III Extracts Extracted with Various Solvents and Further Treated at Different Pretreatment Temperaturesa NMP QN NMP/QN CCO a
C/H C/O C/H C/O C/H C/O C/H C/O
25 °C
150 °C
300 °C
450 °C
600 °C
residueb
14.39 6.85 14.32 5.80 13.26 7.99 16.95 11.83
13.68 4.55 14.39 5.67 14.01 6.09 17.01 12.93
15.44 4.53 15.54 4.40 15.85 4.37 18.11 7.86
19.56 4.93 20.05 5.13 19.87 6.16 20.75 5.03
27.10 5.12 28.48 6.37 28.31 6.47 30.69 9.93
13.74 3.21 15.08 4.39 18.25 2.44 15.06 4.02
All values are in weight ratio. bResidue = thermal extraction residue of coal. standard conditions. A fixed amount of standard anthracite is mixed carefully with the coal sample and then heated to 850 °C for 15 min. After cooling, the produced coke is screened and tested for mechanical strength by treating the coke in a tumbling drum at a specific time, which is called the abrasion test. The RI can be calculated from the test results; the higher the remaining coke, the higher the RI. The Roga test is described in detail elsewhere.12,15 2.6. 13C SSNMR. The 13C SSNMR measurements were carried out at 100 MHz on a Bruker Avance 400 NMR spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) with magic angle spinning (MAS) at 5 kHz. The measurements were performed under the following conditions: typically 2000−4000 scans were accumulated for the single-pulse excitation technique with a relaxation delay of 5 s and a 90° 13C pulse of 5.5 μs. 2.7. X-ray Diffractometry. An X-ray powder diffractometer (Shimadzu XRD-6100, Kyoto, Japan) was used to carry out the slow-step scan XRD analysis. The powder coal samples were placed on an aluminum sample holder and analyzed using Cu Kα (40 kV and 30 mA) radiation with a Bragg−Brentano optical system. The scan was performed in the 2θ range of 15−70° with a scanning speed of 0.02°/ step and a counting time of 2 s at each step.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Three kinds of metallurgical coals supplied by China Steel Corporation (CSC) were selected for this study. A high-volatile bituminous caking coal, a low-volatile bituminous caking coal, and a high-volatile bituminous slightly caking coal are denoted as coals I, II, and III, respectively. EA was carried out using an elemental analyzer (Elementar vario EL III, Hanau, Germany). The analysis of the three raw coals is summarized in Table 1.11 A standard coal blend (SCB), which is composed of 35 wt % coal I, 40 wt % coal II, and 25 wt % coal III, was prepared for the thermoplastic test and the Roga test.12 2.2. Solvents and Extraction Procedure. N-Methyl-2-pyrrolidinone (NMP), quinoline (QN), a 1:1 (by volume) mixture of NMP/ QN, and coal tar creosote oil (CCO) were separately used as extraction solvents. The CCO was provided by CSC and was obtained from the fraction of the coal tar distillation. Coal III was extracted with different solvents by the following procedure: 50 g of the coal sample and 250 g of the extraction solvent were placed into a round-bottom flask equipped with a reflux condenser. Thermal extraction was carried out using a heating mantle at 360 °C for 3 h under an inert nitrogen atmosphere. After extraction, the coal component was precipitated by adding an excess amount of nhexane into the extract mixture.13 The mixture was then filtered and washed with dichloromethane. The extraction solvent was subsequently removed using the vacuum distillation method, and then the rest of the pitch-like extract was dried in vacuo at 200 °C for 6 h. The glass-like dried extract was then scraped and gathered from the flask and was prepared for further investigation.14 A portion of the coal extract was further heated at a specific temperature for around 30 min, which is called preheating treatment, to eliminate interference, such as aliphatic compounds and remaining solvents. 2.3. TGA. The coal extract sample was tested using a thermogravimetric analyzer (TA Instruments Q500, New Castle, DE, U.S.A.). The sample was placed on a platinum sample holder and was heated under an inert gas flow (40 mL/min nitrogen) from 40 to 800 °C with 30 °C/min temperature ramp. 2.4. Thermoplasticity. The thermoplastic properties were measured using a Gieseler plastometer (R.B. Automazione S.R.L. Plastometer PL 2000, Genova, Italy) in a temperature range from 300 to 550 °C with a temperature ramp of 3 °C/min and recorded the fluidity in dial divisions per minute (ddpm). 2.5. Roga Index (RI): Determination of the Caking Power. The RI indicates the caking properties of the coal sample under
3. RESULTS AND DISCUSSION 3.1. EA of the Coal Extract. The results of EA are shown in Table 2, which indicates that the hydrogen content in each sample decreases as the preheating treatment temperature increases. In general, the bond strength of the aliphatic hydrocarbons (C−H) is weaker than that of the aromatic hydrocarbons (Ar−H); therefore, the aliphatic C−H bonds will be broken easier as the temperature increases, which results in a reduction of the hydrogen content. The carbon/oxygen ratio (C/O) does not vary as much as the carbon/hydrogen ratio (C/H) under various preheating treatment temperatures. The decrease of the carbon content suggests that a portion of the carbon atoms reacts with atmospheric oxygen to form carbon dioxide (CO2). The carbon content varies with different preheating treatment temperatures because a higher treatment temperature results in the decrease of the hydrogen content and, thus, the increase of the carbon content. Therefore, the relative increase of the carbon content implies the decrease of the hydrogen content of the coal extract. 11948
DOI: 10.1021/acs.energyfuels.7b02327 Energy Fuels 2017, 31, 11947−11953
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Energy & Fuels The C/H ratio shows an obviously increasing trend when the preheating treatment temperature is increasing; moreover, a sudden increase of the C/H ratio is observed when the preheating treatment temperature is higher than 300 °C, which indicates that a significant breaking of the aliphatic C−H bonds takes place at this temperature. 3.2. TGA. Coal III extract extracted with different solvents was analyzed using TGA, as shown in Figure 1a. The QN-
Figure 2. Thermoplastic properties of the SCB with a 5 wt % addition of coal III extract: (a) extracted with different solvents and (b) QNextracted extract after the preheating treatment at different temperatures.
organic species, which were extracted with CCO (see Table 3), can readily condense into the coal structure. Thus, the SCB with an addition of the CCO-extracted extract shows lower fluidity than that with an addition of the NMP-extracted extract. The thermoplastic property of the SCB with an addition of QN-extracted extract shows different behavior in comparison to those with an addition of extract extracted with NMP or CCO. QN is a heterocyclic organic compound, which has similar polarity to NMP. It can be an effective solvent for coal extraction because of the ability to relax the network structure or entangling structure of coal at a high temperature.16 Therefore, the QN-extracted extract has a large amount of coal organic species, so that adding it to the SCB exhibits excellent fluidity. The QN-extracted extracts after the preheating treatment were added to the SCB as well, which were analyzed by a Gieseler plastometer, as shown in Figure 2b. The fluidity of the SCB increases with the addition of the coal extracts; however, the fluidity decreases with the increase of the preheating treatment temperature because of the pyrolysis of organic matter or the evaporation of low-molecular-weight species. The extract after the heat treatment, which has a higher degree of graphitization, exhibits lower ability to improve the fluidity of coal. 3.4. Effect of an Addition of Coal Extract on the Coke Strength. The coke strength can be enhanced by adding coal extracts, as described in detail elsewhere.11 The enhancement of the coke strength is proportional to the fluidity of the coal extract because the high-fluidity materials have a wider temperature range of thermoplasticity, which act as the solvent-like liquid substances and blend well with the coal. The occurrence of condensation and polymerization during the high-temperature treatment is important for strengthening the coke strength.19,20 To elucidate the relationship between the coke strength and the thermoplasticity, the extract extracted with QN or CCO after the preheating treatment at different temperatures was added to the SCB, as shown in Figure 3. The coke strength decreases with the increase of the preheating treatment temperature of the coal extract, owing to the elimination of high-fluidity materials during the preheating
Figure 1. TGA of coal extract: (a) extracted with various solvents and (b) extracted with NMP after the preheating treatment at different temperatures.
extracted extract has more residual material than the other extracts, and the pyrolysis apparently appears to occur at approximately 450 °C, which implies that the QN-extracted extract contains more high-molecular-weight material than the others.16 The NMP-extracted extract after the preheating treatment was also analyzed using TGA, as shown in Figure 1b. The numeric value after the solvent abbreviation (e.g. NMP-150) represents the preheating treatment temperature of the coal extract. The weight loss of the coal extract decreases with an increasing preheating treatment temperature, which indicates that the low-molecular-weight materials are removed from the coal extract during the preheating treatment. The coal extract treated at a higher temperature leads to a lower weight loss in TGA, which means that the high-temperature-treated coal extract has a higher residual of the high-molecular-weight materials. 3.3. Thermoplasticity Measurement. Thermoplastic properties of the SCB and the SCB with a 5 wt % addition of coal III extract extracted with different solvents are shown in Figure 2a. The increasing fluidity of the SCB with an addition of NMP-extracted extract suggests that the NMP is a highly polar solvent, which can readily extract coal organic species from coal material during the thermal extraction process.17,18 These coal organic species extracted with NMP are believed to act as a lubricant in the coal structure, so that the fluidity increases. CCO is mainly composed of PAHs, which contains catacondensed benzenoids (e.g., naphthalene and 1-methylnaphthalene) and pericondensed benzenoids (e.g., phenanthrene and acenaphthene) and a minor amount of polar substances (e.g., quinoline), so that it also has an excellent extraction ability. In addition, plenty of the aromatic coal 11949
DOI: 10.1021/acs.energyfuels.7b02327 Energy Fuels 2017, 31, 11947−11953
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Energy & Fuels
Table 3. Aromatic/Aliphatic Carbon Ratio (CAr/CAl), Carbon Aromaticity ( fa), and Integrated Intensity of 13C SSNMR of Various Intervalsa integrated intensity of various chemical shift intervals sample
0−25
25−35
35−65
CAl 0−65
100−120
120−145
145−160
CAr 100−160
CAr/CAl
fa
graphite anthracite coal III NMP NMP(R)b NMP-150 NMP-300 NMP-450 NMP-600 NMP/QN NMP/QN(R)b QN QN(R)b CCO CCO(R)b
10.54 16.88 21.17 23.30 22.34 25.79 20.66 18.10 8.55 28.18 22.69 27.17 19.17 20.39 20.73
8.89 3.04 18.28 20.41 18.62 20.14 17.74 9.13 7.53 19.59 14.71 17.84 19.97 11.18 19.46
6.07 11.68 17.10 15.54 16.24 14.57 12.22 9.10 7.08 12.32 8.44 11.14 15.96 11.57 19.71
25.50 31.60 56.55 59.25 57.20 60.50 50.62 36.33 23.16 60.09 45.84 56.15 55.10 43.14 59.90
14.91 29.61 12.73 12.00 12.78 11.46 12.39 15.45 17.08 13.39 17.74 9.29 11.63 18.53 9.31
57.25 32.37 25.54 25.58 25.13 24.36 32.52 43.45 55.16 23.11 32.40 30.71 28.80 32.55 27.26
2.33 6.42 5.17 3.17 4.89 3.69 4.47 4.78 4.59 3.41 4.01 3.85 4.48 5.77 3.52
74.50 68.40 43.44 40.75 42.80 39.51 49.38 63.68 76.83 39.91 54.15 43.85 44.91 56.85 40.09
2.92 2.16 0.77 0.69 0.75 0.65 0.98 1.75 3.32 0.66 1.18 0.78 0.82 1.32 0.67
0.74 0.68 0.43 0.41 0.43 0.40 0.49 0.64 0.77 0.40 0.54 0.44 0.45 0.57 0.40
a All values are in percentage, unless otherwise indicated. The percentage of SSNMR signal of different chemical shift intervals. CAr/CAl and fa are in ratio form. b(R) = thermal extraction residue of coal.
represents quaternary aromatic carbon; and (3) 145−160 ppm represents aromatic carbon connected to oxygen (Ar−O). The 13C SSNMR integrated intensities of NMP-extracted extract with treating at preheating treatment temperatures of 150, 300, 450, and 600 °C are shown in Table 3. The aromaticity (fa value) is defined as the intensity ratio of aromatic carbon to total carbon, which is fa = CAr/(CAr + CAl).23,24 Both the fa value and the ratio of aromatic carbon/ aliphatic carbon (CAr/CAl) increase with the increase of the preheating treatment temperature. Figure 4 illustrates the
Figure 3. Results of the Roga test of SCB with a 5 wt % addition of coal III extract extracted with QN (black solid squares) and CCO (red solid circles) after preheating treatment at different temperatures.
treatment; in addition, as mentioned in the previous section, the heat-treated extract has a higher degree of graphitization and, therefore, exhibits a lower ability to condense with coal. 3.5. 13C SSNMR Analysis. With respect to the 13C SSNMR spectra of coal, the chemical shifts represent different functional groups.7,21,22 The chemical shift interval of 0−65 ppm represents aliphatic carbon, which can be approximately subdivided into three intervals: (1) 0−25 ppm represents methyl carbon (−CH3); (2) 25−35 ppm represents methylene carbon of the linear alkyl chain or cycloalkane (−CH2−)n; and (3) 35−65 ppm represents more complex aliphatic structures, such as aliphatic carbon connected to oxygen (−OCH3 and −O−CH2−) and nitrogenated carbon. The chemical shift interval of 100−160 ppm represents aromatic carbon, which can be roughly subdivided into three intervals: (1) 100−120 ppm represents protonated aromatic carbon; (2) 120−145 ppm
Figure 4. CAr/CAl ratio (red solid circles) and the fa value (blue solid squares) of the coal extract extracted with NMP and then treated at different pretreatment temperatures.
relationship between the CAr/CAl ratio and the fa value of the NMP-extracted extract after the preheating treatment. The fa value shows a positive correlation with the CAr/CAl ratio, which reveals that the degree of graphitization of the coal extract increases with the increase of the preheating treatment temperature. The chemical shift interval of 120−145 ppm shows an obvious increase of quaternary aromatic carbon, which means that the PAHs become the main structure of the 11950
DOI: 10.1021/acs.energyfuels.7b02327 Energy Fuels 2017, 31, 11947−11953
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Energy & Fuels
stacking).29 The coal rank is correlated to the peak intensity at 2θ positions of 20° and 26°, which is the rank ratio I26/I20; besides, this ratio is also proportional to the fa value.30 According to Figure 6a, coal II has an obviously stronger intensity of the π band in comparison to coal I, which implies
coal extract. Four kinds coal extracts, which were extracted with NMP, QN, NMP/QN, and CCO, respectively, were further investigated by 13C SSNMR, as shown in Table 3. The CCOextracted extract has more aromatic content than the others and has the ability to increase the coke strength and decrease the fluidity of coal, which proves that CCO can be an effective solvent for coal extraction. A larger CAr/CAl ratio leads to higher aromaticity. The CAr/ CAl ratio exceeds 1.00 (viz., fa value of >0.50) when the preheating treatment temperature is higher than 300 °C; moreover, EA also shows a sudden increase of the C/H ratio at the same temperature. The initial pyrolysis temperature is clearly visible in Figure 1b when the preheating treatment temperature is below 300 °C. As the preheating treatment temperature increases, pyrolysis gradually diminishes as a result of the high-temperature preheating treatment.25 Thermoplasticity analysis of the SCB with a 5 wt % addition of the QNextracted extract after the preheating treatment is shown in Figure 2b, which also shows a sudden decrease of the fluidity when the preheating treatment temperature is 300 °C. 3.6. XRD Analysis. Coal is mainly composed of a wide variety of organic substances, and its core structure is PAHs with side chains; furthermore, it also has a significant amount of amorphous carbon, which is considered to be responsible for the background intensity of XRD patterns of coal. As the temperature increases during the coking process, the bond of the aliphatic/aromatic side chain connected to aromatic carbon will break and then condense into the coal to be a microcrystalline structure, which is similar to the graphite structure; therefore, graphite is usually used as a reference material.26 With regard to the XRD pattern of graphite, which is a commercial graphite being used here (99+%, Acros Organics, Geel, Belgium), the diffractogram is shown in Figure 5a. The
Figure 6. XRD pattern of (a) coals I and II (0.10°/step) and (b) coal I before and after the heat treatment at 850 °C (0.02°/step).
that coal II contains more aromatic carbon than coal I. The rank ratios of coals I and II are 0.96 and 1.35, respectively; thus, the fluidity of coal I (433 ddpm) is evidently higher than that of coal II (15 ddpm). As mentioned above, the abundance of aromatic carbon has a significant influence on the fluidity of coal; i.e., the coal exhibiting lower fluidity consists mainly of the larger size of the aromatic cluster with fewer substituents on the aromatic ring and has a higher density of cross-linking structure. Also, in contrast to low-fluidity coal, high-fluidity coal has a relatively lower aromaticity.31,32 Figure 6b shows the XRD patterns of coal I before and after the high-temperature preheating treatment; both the (002) and (100) peaks can be seen clearly in the spectra. The (002) and (100) peaks represent the mean stacking of the aromatic layers and the mean size of the aromatic layers, respectively.9,33 Coal I after treated at 850 °C for 30 min exhibits the intensified (002) and (100) peaks along with the decrease of the γ band, which means that the degree of graphitization increases under carbonization conditions.34 The rank ratios of coal I and coal I after the heat treatment at 850 °C are 0.92 and 1.61, respectively, which also reveals that the aromaticity increases after the preheating treatment. The similar tendency of the γ and π band intensity variations and the trend of the rank ratio corresponding to the preheating treatment temperature were observed in the NMP-extracted extract, as shown in Figure 7. In addition, the peaks of the γ and π bands become sharper with the increase of the preheating treatment temperature, which indicates that the degree of graphitization increases with the formation of the polycyclic aromatic structure as a result of the intramolecular condensation. Thermal cracking of the covalent bond in the coal structure may also take place during the heat treatment, such as the cleavage of the methylene group, the alkyl side chain of aromatic carbon, and the oxygen-containing functional groups.10,26,34 These thermal cracking reactions reduce the
Figure 5. XRD pattern of (a) graphite and its (002), (100), (101), and (004) planes and (b) anthracite and its γ and π bands.
(002) and (100) peaks are observed at 2θ ≅ 26° and 43°, respectively,27 and the 2θ positions can be confirmed by comparing to the crystal structure database.28 Graphite has a layer structure and shows a high degree of crystallization in comparison to coal. The diffraction pattern of anthracite, which is shown in Figure 5b, has a broad hump at diffraction angles from around 20° (so-called γ band, derived from aliphatic chains) to 26° (so-called π band, derived from aromatic ring 11951
DOI: 10.1021/acs.energyfuels.7b02327 Energy Fuels 2017, 31, 11947−11953
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Thou-Jen Whang: 0000-0003-0111-7107 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the CSC (Kaohsiung, Taiwan) under Contract RE99011 is acknowledged. Chia-Ching Chu acknowledges the careful reading and helpful suggestions on details of the paper provided by Rui-En Hsu.
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Figure 7. XRD pattern of the NMP-extracted extract of coal III after the preheating treatment at different temperatures. Note that the numeric value after the solvent abbreviation (e.g., NMP-150) represents the preheating treatment temperature of the coal extract.
abundance of aliphatic carbon, thereby increasing the rank ratio, the aromaticity, and the degree of graphitization.
4. CONCLUSION Solvent-extracted extract of coal III after the preheating treatment was investigated by EA and TGA, which shows that the preheating treatment leads to the increase of the carbon content as a result of the decrease of the hydrogen content. The coal extract after the preheating treatment possesses the higher molecular weight residual material and a higher degree of graphitization, which exhibits less ability to increase the fluidity of coal and less improvement in the coke strength when added to the SCB. 13C SSNMR and XRD are used here to elucidate the relationship between aromaticity and fluidity. The fluidity decreases with the increase of integrated intensities of SSNMR for the chemical shift interval of aromatic carbon, which indicates that the higher aromaticity results in lower fluidity. A positive relationship between the ratio of aromatic carbon/aliphatic carbon (CAr/CAl) and the aromaticity (fa value) is found. Besides, the CAr/CAl ratio and the fa value can be calculated from the diffraction angle 2θ positions of 20° and 26° on the basis of XRD spectra.30 In summary of the results of the analysis, the fluidity of coal is found to be governed by the fa value, which is affected by the degree of graphitization; therefore, the XRD method is expected to be a simple, effective, and non-destructive way to estimate the fluidity of coal in coal chemistry.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b02327. 13 C SSNMR spectra for the graphite, anthracite, coal III, solvent-extracted coal extracts, and thermal extraction residue of coal, along with the integrated intensities for each subdivided interval (PDF) 11952
DOI: 10.1021/acs.energyfuels.7b02327 Energy Fuels 2017, 31, 11947−11953
Article
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DOI: 10.1021/acs.energyfuels.7b02327 Energy Fuels 2017, 31, 11947−11953
