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Effects of Molybdenum Disulfide on Microwave Pyrolysis of Low-rank Coal Jun Zhou, Yunfei Chen, Lei Wu, Qiuli Zhang, Yonghui Song, and Xinzhe Lan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00836 • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 22, 2017
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Effects of Molybdenum Disulfide on Microwave Pyrolysis of Low-rank Coal Jun Zhoua,b,* , Yunfei Chena, Lei Wua, Qiuli Zhanga,b, Yonghui Songa,b, Xinzhe Lanb a
School of Metallurgical Engineering, Xi’ an University of Architecture and Technology, Xi’an 710055,
People's Republic of China b
Shaanxi Province Metallurgical Engineering and Technology Research Centre, Xi’an 710055, People's
Republic of China
ABSTRACT：To improve pyrolysis efficiency and tar quality, the effects of different amounts of additional molybdenum disulfide (MoS2) on the microwave pyrolysis of low-rank coal under the circulating gas atmosphere were studied in a coal-gas microwave co-pyrolysis experimental apparatus via self-research and development. Coal-gas analyzer, X-ray diffraction, combined scanning electron microscope and energy-dispersive spectrometer, Fourier transform infrared spectroscopy and gas chromatography-mass spectrometer were all used to conduct analysis and characterization of the pyrolysis products. The experimental results showed that both the yield of liquid products and valuable component (H2+CH4+CO) in the gas increased initially and then decreased with the increase of MoS2. When the optimum addition ratio of 8.3% MoS2 was selected, the yield of liquid products and light oil content were up to 28.4% and 67.55%, higher by 7.6% and 49.48% compared with that of non-MoS2, respectively. Gas heat value was up to 15.14MJ/m3, and the contents of CH4, CO, and H2 were 25.34%, 31.51%, and 22.19%, respectively. Valuable component content in the gas was up to 79.04%. Ash content in the bluecoke (i.e., solid product) was higher by 11.23% compared with that of non-MoS2, and n(H):n(C) value increased by 0.8%. The content of alkanes and single-ring aromatic compounds in the tar increased, so did the n(H):n(C) values in each of the light, intermediate and heavy component. It indicated that MoS2 could effectively promote the hydrocracking of coal tar and improve the stability of tar. KEYWORDS: Molybdenum disulfide, Low-rank coal, Self-circulating gas, Microwave pyrolysis
1. INTRODUCTION Low-rank coal is a kind of high quality power coal, and its efficient conversion and clean utilization have attracted much attention1-3. The low-temperature pyrolysis process is the best way to realize separation and transformation of gas, liquid, and solid states in coal4. Microwave pyrolysis of the coal has become a hot technology for its advantages such as quick heating rate, high tar yield and good gas quality5,6. Lan et al.7 found that CO and CH4 content in the microwave pyrolysis gas of low-rank coal can respectively reach more than 20%, and H2 content can reach above 40%. In addition, the calorific value is equal to that of the coke oven gas. The microwave pyrolysis characteristics of low-rank coal in H2 atmosphere were studied by Zhou et al.8 The yield of liquid products was up to 28.2%, and the content of hydrocarbon compounds in coal tar was 45.2%. To further improve the tar yield, a scientific research of coal was conducted using catalytic hydropyrolysis technology 9-12, which can not only improve the conversion rate and reduce reaction temperature for coal pyrogenation, but also control the component and distribution of
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products, as well as realize the directional conversion of coal into high value-added chemical. Ding et al.13 used high-frequency electric furnace and thermal gravimetric analyzer with Na2CO3 as catalyst to study coal catalytic pyrolysis and gasification characteristics. They study found that the yield of H2 and CO comprised the main pyrolysis products, and adding Na2CO3 resulted in a 0-15% increase. Thus, Na2CO3 can improve coal and CO2 gasification reactivity. Jin et al.14used Mo/HZSM-5 as catalysts for the coupling of methane and coal pyrolysis, which resulted in the highest yield of coal tar at 21.5% under optimum pyrolysis conditions. Monsef-Mirzai et al.15 used CuO, Fe3O4, and metallurgical coke as microwave absorber for the microwave pyrolysis of pulverized coal. When coke was used as microwave absorber, the production of tar could reach 20%. When Fe3O4 was used as absorbent, the yield reached 27%, and some experiments using CuO as absorbent reported figures as high as 49%. Chareonpanichet et al.16 examined the USY zeolite effect on coal pyrolysis product component and distribution under the conditions with H2 pressure of 5 MPa and final pyrolysis temperature of 600℃. They found that the benzene, toluene and xylene (BTX) yield of pyrolysis products increased 9.3% because of USY catalyst adding. HY and HZSM-5, two types of zeolite catalysts, also produced lignite pyrolysis products in the increase of BTX and DTN (10-hydrogenated naphthalene, 4-hydrogenated naphthalene, naphthalene) to 14% and 20% respectively17,18. On the basis of the study of microwave co-pyrolysis of low-rank coal and self-circulating coal 19 gas , the common catalyst of molybdenum disulfide was introduced into this pyrolysis system in this study. The effect of different amounts of molybdenum disulfide on the yield and component of products was investigated by the co-pyrolysis experiments of low-rank coal, self-circulating coal gas and molybdenum disulfide. In particular, we used low-rank pulverized coal as experimental raw materials. The results have important reference value and promoting function for efficient conversion and utilization of low-rank pulverized coal.
2. MATERIALS AND METHODS 2.1. Materials and Samples. The coal samples used in this work were collected from the SJC coal mine located in Yulin city in the northwest area of China. Their proximate and ultimate analyses are shown in Table 1, while Table 2 shows the coal ash analysis. After crushing and screening, particle size decreased to 0.125 mm and below. These particles were placed in a vacuum drying box at the temperature of 100 ℃ for 12 h, then was kept in a sealed bag for standby. The molybdenum disulfide was bought from the market with an average particle size of about 1.33 µm. Table 1. Proximate and ultimate analyses of coal samples (wt.%, ad) Proximate analysis
Ultimate analysis
M
A
FC
V
C
H
N
St
O
3.41
2.64
56.16
37.79
76.38
4.71
0.99
0.26
11.61
SiO2
Fe2O3
Al2O3
CaO
MgO
TiO2
SO3
P2O5
K2 O
Na2O
53.1
7.60
24.76
6.60
1.52
1.18
2.62
0.33
0.02
0.08
Table 2. Analysis of coal ash component (wt.%)
2.2. Experiment Equipment and Methods. According to the MoS2 amount for 0%, 1.7%, 5.0%, 8.3%, and 11.7%, the MoS2 powder was fully and mechanically mixed (the number of samples was divided into 1#, 2#, 3#, 4#, and 5#) with a certain amount of coal powder. The powder mixture was pressed into the lump coals by the FYD desktop electric powder pressing machine. The machine had extrusion strength of 8 Mpa, coal shape with 30 mm diameter and 16 mm height
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cylinder. And then, the lump coals were put into the quartz tube reactor as shown in Figure 1. To produce the microwave pyrolysis experiments, we applied custom frequencies of 2450+50 MHz and a power of 1600 W for the microwave oven. Experimental conditions19 were set as follows: microwave power 800 W, pyrolysis time 40 min, and circulating gas flow rate 0.6 L/min. After pyrolysis, each product of bluecoke, tar and gas was collected and analyzed separately. The yield of each product and the conversion rate of coal were calculated in (1)-(3). Ychar =(W1-W2-Wc)/W0×100%
（1）
YL =(W3-W4)/W0×100%
（2）
CRC =( W0-W1+W2+Wc)/W0×100%
（3）
In the formula, Ychar—yield of the bluecoke (wt.%); YL—yield of the liquid products (wt.%); CRC—conversion rate of coal (wt.%); W0—mass of Coal sample, g; W1 —mass of the quartz tube（include bluecoke）after pyrolysis, g; W2—mass of the initial empty quartz tube, g; Wc—mass of the catalyzer in the coal sample, g; W3—mass of two absorption cooler and Electric tar precipitator（include cooling water, tar, pyrolysis water） after pyrolysis, g; W4—mass of two absorption cooler and Electric tar precipitator（include cooling water）before pyrolysis, g.
Figure 1. Experimental apparatus of coal microwave catalytic pyrolysis20: (1) microwave device, (2) ceramic insulation sleeve, (3) quartz tube reactor, (4) first-order absorption cooler, (5) circulating cooling pump, (6) two-stage absorption cooler, (7) electrical tar precipitator, (8) T-branch pipe, (9) gas collecting cabinet, (10) intelligent temperature controller, (11) thermocouple, (12) intake tube, (13) loop valve, (14) explosion proof air pump, (15) outlet valve, (16) outlet pipe.
2.3. Analysis and Testing. Gas component analysis was performed with a Gasboard-3100P type gas analyzer (Wuhan Technology Co., Ltd., China). Micro morphology and component of the solid product were determined respectively by a Supra 55 sapphire-type scanning electron microscope (SEM) at a resolution of 1.6 nm and accelerating voltage of 30 kV and X-ray diffraction (XRD) analysis (Japan Science College D/Max2000). The X-ray diffractometer (XRD) was performed using a XRD-7000 diffractmeter (Japan, Shimadzu) with Cu Ka radiation at 40.0 kV and 30.0 mA. The scan range of 2θ was from 5° to 80°. Coal tar components were analyzed by gas chromatography-mass spectrometry (GC-MS) using GC (6890)-MS (5973) (Agilent Co., Ltd., United States).The GC was fitted with a RXI-5 ms column (0.25 µm film) with internal dimensions of 30 m × 0.25 µm × 0.25 µm. High-purity helium (99.999%) was used as the carrier gas at a flow rate of 1.14 mL·min−1. Groups in the bluecoke and coal tar were analyzed using a Fourier transform infrared spectroscopy (FTIR) (German Bruker vertex 70).
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3. RESULTS AND DISCUSSION 3.1. Effect on the yields of pyrolysis products. Figures 2 and 3 show the coal sample heating curve and change in pyrolysis product yield of different amounts of additional MoS2. 1000
Temperature/℃
800
600
0% MoS2 1.7% MoS2 5.0% MoS2 8.3% MoS2 11.7% MoS2
400
200
0 0
10
20
30
40
Time/min
Figure 2. Temperature-rising curves at different blend ratios of MoS2 72.1
Yield of pyrolysis products/wt.%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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61.8
Liquid products Bluecoke Conversion of coal
51.5
41.2
30.9
20.6
10.3 0.0
1.5
3.0
4.5
6.0
7.5
9.0
10.5
12.0
Ratio of MoS2/wt.%
Figure 3. Yield of pyrolysis products at different blend ratios of MoS2
As can be seen from Figure 2, compared to other MoS2 addition ratio, the MoS2 ratio was 8.3% and 11.7% for coal samples in the 0-25 min heat up faster. However, the end temperature of the pyrolysis reaction was relatively low. The two added ratios of coal samples’ heating rate were the same. The pyrolysis reaction at the end of the MoS2 added was 11.7% for coal pyrolysis, which was 8.3% higher at nearly 25°C. Hence, the yield of the liquid product obtained from microwave pyrolysis of low-rank coal in the recycle gas atmosphere increased first, and then decreased with the increase in the amount of added MoS2. When MoS2 ratio was 8.3%, the yield of the liquid product was 28.4%, higher than that of the 7.6% MoS2. When the content of MoS2 was less than 8.3%, the yield of coke and coal conversion rate were kept at 64% and 36% respectively. When MoS2 increased to 11.7%, the yield of liquid products decreased rapidly to 21.4%, and the coke yield rapidly increased to 68.5%. This result occurred because heating rate and pyrolysis temperature in the process of coal pyrolysis were mainly influenced by secondary reaction of primary volatiles21,22. Compared to the MoS2 addition ratio of 8.3%, when MoS2 was 0%, 1.7% and 5.0% of coal heating rate were relatively small and final pyrolysis temperature was relatively higher. Primary volatile points from the coal substrate could not escape quickly and promote the part of secondary reaction occurrence. Thus, the yield of liquid products was reduced. When MoS2 was more than 11.7%, final temperature of coal pyrolysis was higher. Higher pyrolysis temperature accelerated the secondary cracking effect of tar with increasing heating rate. The rapid escape of volatile competition resulted in a dominant position and the yield of liquid products derived from the pyrolysis decreased. MoS2 active component surface absorbed large amounts of
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hydrogen molecules. The dissociation of molecular hydrogen indicated a strong reduction of the hydrogen atom and a spillover mechanism of hydrogen diffusion into the interior of the coal particle, which resulted in bond breaking and free radical hydrogen saturation23,24. Low-rank coal and MoS2 pyrolysis showed a synergistic effect25 when MoS2 content was more than the saturated adsorption amount of hydrogen molecules. This effect covered much of the MoS2 particles on the coal surface, the pyrolysis process in bluecoke pore structure generated a carbon deposition, and MoS2 surface on the part of the active site was masked. Other effects included catalytic performance and reduction of emission volatility, which increased mass transfer resistance. A secondary reaction indicated that the synergistic effect gradually weakened and even showed inhibitory effect. As MoS2 content increased further, pyrolysis liquid yield did not rise nor decline. Based on increases from the pyrolysis of liquid product yield and reduction of consumption of MoS2 according to the experimental conditions of MoS2, the best adding proportion was 8.3%. 3.2. Effects on pyrolysis gas components. To explore the MoS2 content of low-rank coal pyrolysis gas components in low-rank coal during microwave pyrolysis, we used online gas analyzer synchronous detection of different MoS2 amounts of gas in the main group content changes. Table 3 shows the main component in pyrolysis gas by adding different contents of MoS2. Table 3. Main components in pyrolysis gas (vol.%) Component content
Ratio of
Heat value
MoS2/wt.%
CO2
CH4
CO
CnHm
H2
CH4+CO+H2
/MJ·m−3
0
7.33
13.88
19.95
0.97
39.13
72.96
12.34
1.7
11.07
14.74
20.33
1.00
38.53
73.60
12.56
5.0
7.89
20.08
26.05
0.84
29.34
75.47
14.06
8.3
10.72
25.34
31.51
1.11
22.19
79.04
15.14
11.7
9.63
19.37
27.46
1.17
19.09
65.92
10.38
Table 3 shows that MoS2 content increased and the content of CH4 and CO gas indicated the same trend of increasing first and decreasing after. As H2 content decreased, gas in the group price point (CH4+CO+H2) content increased initially and then decreased, which corresponded to the calorific value of the gas increasing then decreasing. When MoS2 content was 8.3%, the calorific value of gas was the highest at 15.14 MJ/m3, the contents of CH4 and CO gas were the highest at 25.34% and 31.51% respectively and H2 content was 22.19%, and a set of valuable content (CH4+CO+H2) was up to 79.04% in the gas. CH4 is mainly derived from the condensation of the macromolecular structure in coal, degradation, and side chain and bridge bond cleavage26. Molybdenum ion in MoS2and the oxygen atoms of an oxygen functional group in coal formed high activity of a surface complex formation that promoted contained oxygen functional groups for decomponent. The chain cleavage of fused ring aromatic and naphthenic hydrocarbon compounds branched into small molecule gases. In addition, MoS2 promoted weak bond fracture of the coal and hydrogen adsorption and dissociation. Pyrolysis of coal generated free radical fragments with hydrogen radical binding, resulting in an increase in the yield of CH4. According to Figure 2, compared with an additional MoS2 lower than 8.3%, when MoS2 was added to 11.7% coal pyrolysis, the final temperature was higher, coal maturity increased, but the aliphatic content
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in coal by pyrolysis of gaseous products decreased27. Under microwave irradiation, the bluecoke surface can produce the effect of “hot” or “micro” plasma to excite methane cracking for CHn (n=1, 2, 3) free radical28. CO in gas comes mainly from carbonyl, phenolic hydroxyl, ether, and oxygen-containing heterocyclic cracking29,30. Owing to the increase in MoS2, the number of hydrogen molecules in active hydrogen atoms increased, resulting in a decreased of H2 content. 3.3. Effects on solid product (bluecoke) components 3.3.1. Proximate and ultimate analyses of bluecoke. Table 4 presents the ultimate and proximate analysis of the bluecokes obtained from coal co-pyrolysis under the addition ratio of MoS2 at 0% and 8.3%. Table 4. Proximate and ultimate analyses of bluecoke(wt.%, ad) Proximate analysis
Ratio of
Ultimate analysis n(H):n(C)
MoS2/wt.%
M
A
FC
V
C
H
N
St
O*
0
1.58
6.70
89.52
2.20
88.26
1.16
0.72
0.22
0.43
15.8
8.3
1.90
15.93
76.41
5.76
75.33
1.04
0.84
4.34
0.62
16.6
The table 4 shows that with and without the addition of MoS2, microwave pyrolysis derived bluecoke compared to the MoS2 increased by 1.32%, 11.23%, and 3.56% ratio of 8.3% coke moisture, ash, and volatile content, respectively. Fixed carbon content reduced by 3.11%, and n (H):n (C) value was increased by 0.8%. MoS2 is hexagonal, has good chemical and thermal stability, and does not decompose in bluecoke. In addition, in the pyrolysis process of coal, with the evolution of volatile coal matter, CaO in coal solid sulfur CaS generation, coal mineral residue in bluecoke, and solid coke ash content increased significantly. MoS2 proportion with an 8.3% coal pyrolysis temperature without adding MoS2 resulted in an increase in low income coal volatility content. The n (H):n (C) value increase showed that unsaturated hydrocarbons and their derivatives in bluecoke structure were reduced. The unsaturated structure number of all the double bonds, triple bonds and aromatic rings in bluecoke reduced, as well as the number of aromatic carbon. The number of aliphatic carbon increased. 3.3.2. XRD analysis of bluecoke. Figure 4 shows the XRD pattern of bluecokes obtained from the pyrolysis of low-rank coal with different MoS2 contents. MoS2 MoS2
Si02
MoS2
11.7% MoS2
MoS2
CaS
Intensity/cps
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8.3% MoS2 5.0%MoS2 1.7% MoS2 0% MoS2 10
20
30
40
50
60
70
80
2θ/°
Figure 4. XRD spectra of bluecoke at different blend of MoS2
The end of pyrolysis reaction indicated solid product MoS2was exist. With the MoS2 content increase, each added amount (except 0%) did not change the peak position, MoS2 diffraction peak intensity gradually increased, and peak width became larger. These effects suggested that grain size became larger, specific surface area decreased, and MoS2 in semi-solid dispersion degree became smaller. CaO in coal sulfur coal generated CaS and SiO2 in coal pyrolysis after transferring to bulecoke.
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3.3.3. Combined analysis of SEM and energy dispersive spectrometer (EDS). SEM and EDS analysis results of the bluecokes obtained from coal co-pyrolysis under the addition ratio of MoS2 at 0% and 8.3% are shown in Figure 5 and Table 5.
（a）coal+0%MoS2
（b）coal+8.3%MoS2 Figure 5. SEM photographs of bluecoke mixed with MoS2 Note: the numbers “1-3” denote different surface regions of the bluecoke. Table 5. Element component of bluecoke measured by EDS(/wt.%) Contact surface
C
O
Mo
S
Al
Fe
Ca
Si
Coal+0%MoS2-1
39.67
3.02
--
4.75
1.83
2.50
41.74
6.49
coal+0%MoS2-2
84.56
9.21
--
0.07
0.93
1.71
1.62
1.90
coal+0%MoS2-3
46.49
5.74
--
3.09
1.12
2.75
35.94
4.87
coal+8.3%MoS2-1
10.77
1.21
86.83
1.19
--
--
--
--
coal+8.3%MoS2-2
61.42
4.70
27.70
0.99
0.45
1.28
2.63
0.83
coal+8.3%MoS2-3
84.23
8.85
1.97
0.09
0.23
0.45
0.26
3.92
Note: coal+0%MoS2-1 denotes“1” surface region of coal+0%MoS2. The other surface regions of carbonaceous materials were named accordingly as coal+0%MoS2-1.
Figure 5(a) shows that the space structure of the bluecoke obtained from coal pyrolysis without adding MoS2 is loose. The bigger hole resulted in the collapse of part of the structure, which shed many coal particle attachments. Most attachments showed silver with metallic materials unevenly distributed in the concave convex rough surface of the bluecoke. This effect was the result of poor adhesion of low-rank coal and volatile and higher oxygen content under the condition of microwave heating. Heating, expansion, and rupture of volatile resulted in the increase of the number of holes with medium pore scale on the surface of coal particles. In the coke formation process of coal particles between uneven contraction and bluecoke structure, a layer-by-layer structure also existed in uneven expansion and residual stress did not eliminate the peeling phenomenon31, which developed a large hole in the coke structure. The detection of EDS results showed that in high brightness “1” regions from the bluecoke structure without adding
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MoS2, Ca, Si, and S elements content reached 41.74%, 6.49%, and 4.75% respectively. C content was only 39.67%. In weak brightness “2” regions, Ca, Si, and S elements content respectively reached only 1.62%, 1.9%, and 0.07%. C element content was as high as 84.56%. It indicated that the white metallic substances attached to the surface of the bluecoke may be the mixed crystal of CaS and SiO2. Figure 5(b) reflects that when more than 8.3% of MoS2 was added, the bluecoke structure became more compact. The detection of EDS results showed Mo element content was as high as 86.83% in high brightness “1” regions, while C element content was only 10.77%. In weak brightness “3” regions, Mo element content was only 1.97% and C element content reached 84.23%. This indicated that the white metallic material attached to the surface of the bluecoke might be molybdenum sulfide. 3.3.4. FTIR analysis of bluecoke. Figure 6 shows the infrared spectra analysis of the bluecokes obtained from coal co-pyrolysis under the addition ratio of MoS2 at 0% and 8.3%. 8.3% MoS2
1.001
0.990
0.979
Absorbance
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0% MoS2 0.968
0.957
0.946
0.935
4000
3500
3000
2500
2000
Wavenumber/cm
1500
1000
500
-1
Figure 6. FTIR spectra of bluecoke at different blend ratios of MoS2
The figure 6 reflected that the bluecoke had complex functional absorption band diagram. Referring to IR absorption peaks of coal32, the absorption peak at 3440 cm-1 belongs to the stretching vibration of hydrogen association -OH, -NH and phenolic. In addition, low-rank coal nitrogen content was very low, which indicated that the influence of the -NH groups was very small. The aromatic C-H stretching vibration absorption occurred in 3040 cm-1-3100 cm-1, and 2920 cm-1-2860 cm-1 belongs to naphthenic and aliphatic -CH3 hydrogen absorption peaks. The hydrogen association of carbonyl and aromatic hydrocarbons and polycyclic aromatic layers of C=C skeletal stretching vibration were near 1600 cm-1. This result may be attributed to the hydrogen bond of carbonyl and aromatic ring C=C absorption peaks, which showed overlapping results. Hence, condensation of aromatic ring in CH2 could have occurred33. The absorption characteristics of 1380 cm-1 absorption peak belonged to -CH3, and C-O 1300 cm-1-1000cm-1 stretch was mainly for phenol, alcohol, ether, ester, and ether. Absorption peaks at 900 cm-1-650 cm-1 were mainly a variety of substituted aromatic compounds. Most of the absorption peaks were from aromatic rings. From Figure 6, 3440, 1600, and 1380 cm-1 were the obvious absorption peaks, indicating that the blucoke was in the presence of aromatic compounds34. The content of C=O, -OH, C-O functional groups and aromatic C=C skeleton were higher. 3.4. Effect on tar component 3.4.1. FTIR analysis. Figure 7 shows the infrared spectra analysis of the coal tar obtained from coal co-pyrolysis under the addition ratio of MoS2 at 0% and 8.3%.
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1.05
0% MoS2
1.00 0.95 0.90 0.85
Absorbance
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0.80 0.75 0.70 0.65
8.3% MoS2
0.60 0.55 0.50 4000
3500
3000
2500
2000
1500
1000
500
Wavenumber/cm-1
Figure 7. FTIR spectra of the tar at different blend ratios of MoS2
Pyrolysis process in coal tar has a complex functional absorption band diagram. Compared to standard infrared spectral library, the absorption peak about 3440 cm-1 is attributable to hydrogen association -OH or -NH, and or phenolic stretching vibration. The aromatic C-H stretching vibration absorption occurs in 3040 cm-1-3100 cm-1 district. 2920 cm-1-2860 cm-1 belongs to naphthenic and aliphatic -CH3 hydrogen absorption peaks. Mononuclear aromatic C=C skeleton stretching vibration absorption occurs in 1500 cm-1-1480 cm-1 and 1610 cm-1-1590 cm-1 area35. The alkanes and single-ring aromatic substances in the tar obtained from coal co-pyrolysis under the addition ratio of 8.3% MoS2 are much more than that of non-MoS2. It indicated that MoS2 effectively promoted coal tar hydrogenation cracking. This result was consistent with that of Lee et al.36. 3.4.2. GC-MS analysis. Usually, the number of carbon atoms is 6–10 for light oil, 11–18 for intermediate oil, and greater than 19 for heavy oil37. The content of light, intermediate and heavy component in the tar at different blend ratios of MoS2 are summarized in Table 6. Figure 8 reflects the n(H):n(C) values in the tar dependences of carbon numbers at different blend ratios of MoS2. Table 6. Component content of light oil, intermediate oil and heavy oil in the tar at different blend ratios of MoS2(wt.%) Carbon number
0% MoS2
8.3% MoS2
C6
1.45
28.53
C7
2.74
23.93
C8
3.25
6.40
C9
3.80
1.82
C10
6.83
6.87
Total
18.07
67.55
C11
4.77
9.22
C12
7.88
5.71
C13
4.42
2.43
C14
5.32
3.39
C15
2.97
1.42
C16
5.45
3.42
C17
1.64
2.42
C18
3.44
1.97
Total
35.89
29.98
C19
4.26
1.06
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C20
5.47
0.92
C21
9.68
1.04
C22
3.93
0.32
C23
5.67
0.67
C24
8.45
0.24
C25
6.55
0.01
C>25
2.03
0.21
Total
46.04
4.47
Table 6 shows that in low-rank coal without adding MoS2, tar from light oil accounted for only 18.07%, and tar in the intermediate and heavy oil content reached up to 35.89% and 46.04% respectively. When the MoS2 added was 8.3%, the light oil content in tar reached 67.55%, was 49.48% higher than that of non-MoS2. The intermediate and heavy oil content in tar accounted for only 29.98% and 4.47% respectively. Adding MoS2 is conducive to the formation of light oil because MoS2 with adsorbed hydrogen molecules is effective for dissociating hydrogen free radical in the coal pyrolysis process and plays the role of hydrogen donor and hydrogen radical stabilizer38. Coal molecular structure bridge bonds the cracks of small molecular compounds and free radical fragments with hydrogen radical converging fast precipitation, which reduces probability of colloid curing and free radical condensation. Therefore, low-rank coal and MoS2 co-pyrolysis can make tar component appear “lightweight”. Given that minerals in coal have wave absorbing property and microwave heating speed is very fast, microwave pyrolysis experiments were conducted simultaneously. When product formation occurs, a secondary cracking is probable as well as increases in the amount of light tar oil39. 2.170
2.015
0% MoS2 8.3% MoS2
1.860
n(H):n(C)
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1.705
1.550
1.395
1.240
1.085
C5～10
C11～18 Carbon number
C>19
Figure 8. n(H):n(C) values in the tar dependences of carbon numbers at different blend ratios of MoS2
Figure 8 shows the increase in the number of carbon atoms; n (H):n (C) value also increased the coal tar fractions obtained from low-rank coal at different MoS2 ratios. The MoS2 ratio for 8.3% of the number of carbon atoms in the tar was obtained for 6-10 light oil in n (H):n (C) values compared to that without adding MoS2, which was higher by 0.125, carbon atoms for 11-18 intermediate n (H):n (C) value were higher than 0.708, and carbon atoms more than 19 of heavy oil in n (H):n (C) showed a value higher than 0.278. Increased coal tar components in n (H):n (C) values reduced unsaturation of the main components of tar component of hydrocarbons and their derivatives, unsaturated double bonds and triple bonds, and aromatic-ring structure to reduce the number. Hence, the stability of coal tar can be improved.
4. Conclusion To improve pyrolysis efficiency and tar quality, the common catalyst of molybdenum disulfide was introduced into the microwave co-pyrolysis system of low-rank coal and self-circulating coal
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gas. The overall experimental results show that the molybdenum disulfide has an important influence on yield and component of pyrolysis products. With the increase of MoS2 addition, both the yield of liquid products and valuable component (H2+CH4+CO) in the gas increased initially and then decreased. When the optimum ratio of MoS2 to low-rank coal for 8.3% was selected, the yield of liquid products and light oil content were up to 28.4% and 67.55%, higher by 7.6% and 49.48% compared with that of non-MoS2, respectively. Gas heat value was up to 15.14MJ/m3, and the contents of CH4, CO, and H2 were 25.34%, 31.51%, and 22.19%, respectively. Valuable component content in the gas was up to 79.04%. Ash content in the bluecoke was 11.23% higher than that of non-MoS2, and n(H):n(C) value increased by 0.8%. The content of alkanes and single-ring aromatic compounds in the tar increased, so did the n(H):n(C) values in each of the light, intermediate and heavy component. It indicated that MoS2 could effectively promote the hydrocracking of coal tar and improve the stability of tar. This study has important reference value and promoting function for efficient conversion and utilization of low-rank pulverized coal. AUTHOR INFORMATION Corresponding Author * Telephone/Fax: 8629-82201248. E-mail:
[email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT The authors would like to express their gratitude for the financial support of the National High-Tech Research and Development Program of China (863 program No.2011AA05A202), as well as the Shaanxi Provincial Balanced-planning Science and Innovation Engineering Program of China (No. 2011KTDZ01-05-04). REFERENCES (1) Xie K. C.; Li W. Y.; Zhao W. Coal chemical industry and its sustainable development in China. Energy 2010, 35(11), 4349-4355. (2) Wang Y. J. Developing prospect for production of semi-coke, coal tar and retort oven gas. Fuel & Chemical Processes 2010, 41(1), 9-11. (3) Zhou Z. K.; Wang L. J. Development potentiality and tendency of coal derived clean energy in China. China Coal 2011, 37(5), 24-36. (4) Shang J. X.; Wang L. J.; Gan J. P. Prospect of the Shanbei comprehensive coal grading utilization technology. Coal Conversion 2011, 34(1), 92-96. (5) Lan X. Z.; Zhao X. C.; Ma H. Z. A method on fast microwave carbonization of coal in medium-and-low temperature. China Patent, 200810232680.4. (6) Tao X. X.; Xu N.; Xie M. H.; Tang L. F. Progress of the technique of coal microwave desulfurization. Journal of China Coal Society 2014, 1(1), 113-128. (7) Lan X. Z.; Pei J. J.; Song Y. H.; Su T. Analysis of low-rank coal microwave pyrolysis. Coal Conversion 2010, 33(3), 15-18. (8) Zhou J.; Yang Z.; Wu L.; Zhang Q. L.; Lan X. Z.; Shang W. Z. Study on microwave pyrolysis of low rank coal under H2 atmosphere. Coal Conversion 2015, 38(3), 22-26. (9) Ariunaa A.; LI B. Q.; LI W.; Purevsuren B.; Munkhjargal S.; Liu F. R.; Bai Z. Q.; Wang G. Coal pyrolysis
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