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Degradative solvent extraction of low-rank coals by the mixture of low molecular weight extract and solvent as recycled solvent
Fuel Processing Technology 173 (2018) 48–55
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Degradative solvent extraction of low-rank coals by the mixture of low molecular weight extract and solvent as recycled solvent
T
Xian Lia,b, Zong Zhanga, Lei Zhanga, Xianqing Zhua, Zhenzhong Hua, Weixiang Qiana, ⁎ Ryuichi Ashidac, Kouichi Miurad, Hongyun Hua, Guangqian Luoa, Hong Yaoa, a
State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People's Republic of China b College of Chemistry and Chemical Engineering, Xinjiang University, Urumchi, Xinjiang 830046, People's Republic of China c Department of Chemical Engineering, Kyoto University, Kyoto-Daigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan d Institute of Advanced Energy, Kyoto University, Kyoto-Daigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Low-rank coal Degradative solvent extraction Solvent recycle Ash content
The authors have proposed a degradative solvent extraction method to dewater, upgrade and separate low-rank coals to upgraded coal, high molecular weight extract (Deposit), low molecular weight extract (Soluble), and a little amount of liquid and gas products. For the practical application of this extraction method, the solvent must be recycled. Furthermore, the separation of the Soluble and solvent, which was implemented by distillation, consumed much energy and added the cost of the whole system significantly. In order to solve these issues, the utilization of the Soluble and solvent as the recycled mixture solvent was proposed and investigated in this work. The results showed that the yields of the Deposit increased by more than two times with the mixture solvent recycle, and got almost stable after 5–7 extraction cycles. This made up for the loss of the part of the extraction product (Soluble) to a large extent. Furthermore, the ratios of O/C, N/C, and S/C of the Deposit decreased with the mixture solvent recycle, indicating that the Deposit was further upgraded. The aromaticity and reactivity of the Deposit decreased and increased with the mixture solvent recycle, respectively. The Cal/Car of the Deposits ranged from 0.35 to 0.84. The ash content of the Deposit increased slightly from 0.25% to 0.29%, and then got almost constant after 4 times of the mixture solvent recycle. Only 0.2–0.4% of the main inorganic elements were transformed from raw coal to the Deposit. The main inorganic elements in the Deposit were Si, Al and Fe. The relatively high contents of some inorganic elements in the Deposit were caused by their high contents and existing form in the raw coal. Thus, it was showed that using the Soluble and solvent as recycled mixture solvent is a feasible way for the practical application of the degradative solvent extraction of low-rank coals.
1. Introduction The utilization of low-rank coals, such as lignites and subbituminous coals, have got more and more attention [1]. But the high moisture content, high oxygen content, low heating value and obvious self-ignition tendency suppress their thermal conversion and utilization [2]. Furthermore, the low-rank coals contain much small molecular weight fractions and have relatively high chemical activity, compared to the high rank coals [2, 3]. So, the multipurpose utilization of the low-rank coals by poly-generation technology is considered to be more promising compared to the direct combustion and gasification [3]. The authors have proposed a degradative solvent extraction method which treats low-rank coals in a nonpolar solvent at around 350 °C using an autoclave to dewater, upgrade and separate them to three solid fractions
⁎
and a small amount of liquid and gas products [4–6]. The three solid fractions were the extraction residue which is the so-called upgraded coal, high molecular weight extract (named as Deposit) which extracted at the extraction temperature and precipitated as solid from the solvent at room temperature, and low molecular weight extract (Soluble) which was soluble in the solvent at room temperature. The solid Soluble can be obtained after removing the solvent by distillation. The Soluble and Deposit have the carbon content of higher than 80%, oxygen content of lower than 10%, rather low moisture content, and are almost ash free. It was found in our previous work that the two extracts can be used as a good binder for coke making [7–9], to produce high-quality liquid fuel and chemical by further liquefaction [10, 11], and as a precursor for high value-added carbon material preparation, such as carbon fiber [12, 13] and porous carbon materials [14]. The upgraded coal has much
Corresponding author. E-mail address: [email protected] (H. Yao).
https://doi.org/10.1016/j.fuproc.2018.01.005 Received 23 October 2017; Received in revised form 2 January 2018; Accepted 9 January 2018 0378-3820/ © 2018 Elsevier B.V. All rights reserved.
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filter was then quickly opened at the extraction temperature after the residence time, in order to allow the mixture of the extract and solvent to be transferred to a reservoir. The reservoir was maintained at ambient temperature by circulating cooling water. A part of the extract, which precipitated as solid from the solvent in the reservoir at ambient temperature, was the Deposit. Another part of the extract, which was still solubilized in the solvent, was the Soluble. All of the substance in the reservoir was collected and separated by filtration. The filter cake was obtained as the Deposit. The residue remained in the autoclave above the filter. The detailed process flow diagram can be found in previous work [20, 21]. The filtrate, which was the mixture of the solvent and Soluble, was used as the recycled solvent for next extraction of fresh coal. Around 10 mL of the filtrate was sampled and separated by rotary evaporator, in order to obtain solid Soluble and solvent for yield calculation and characterization. 10 mL of fresh solvent was then added to the recycled solvent to ensure the same amount of solvent used for each run. This change of the solvent was rather small and the effect for the extraction was ignored in this work. So, the Soluble concentration in the recycled solvent and the total Soluble yield were calculated. The un-extracted fraction, remained in the autoclave after the extraction, was the Residue (upgraded coal). The gaseous products were collected in a gas bag for analysis. The degradative solvent extractions with different ratio of coal to fresh solvent were also performed at the same condition. The Soluble, Deposit and Residue, obtained from the extraction with recycled solvent and with different ratio of coal to solvent, were further dried in a drying oven under vacuum at 150 °C for 6 h to remove the residual solvent completely. The gaseous product was analyzed quantitatively by a gas chromatograph (Agilent MicroGC3000). The yields of Soluble, Deposit, Residue and Gas were calculated by their weights. The yield of Liquid was calculated by mass balance.
better combustion and gasification characteristics [15, 16], and can be used as a good precursor for activated carbon preparation [17], compared to the raw coals. So, the degradative solvent extraction of lowrank coal is believed to be one of the promising multi-purpose utilization approaches of the low-rank coals. For the practical application of this degradative solvent extraction method, the solvent must be recycled to minimize the solvent needed [18]. Furthermore, the best approach for the separation of the Soluble and solvent is distillation technology, such as flash separator. But it consumes much energy and adds the complexity and cost of the whole system significantly, because the solvent commonly used have rather high boiling point [19]. In order to solve these issues, the authors proposed to use the mixture of the Soluble and solvent as the recycled solvent for the degradative solvent extraction. By this way, it not only realized the solvent recycling, but also avoided the separation of the Soluble and solvent. When using the mixture of the Soluble and solvent as the recycled solvent for the degradative solvent extraction, the Soluble will no longer be the target product during each extraction experiment. So, the Soluble, which is one of the main products of the degradative solvent extraction, was lost by this way. Also, the effect of the Soluble in the recycled solvent on the subsequent extraction was not clear. In our previous work, we preliminarily studied the degradative solvent extraction of bituminous coal by the recycled mixture of the Soluble and solvent using a flow-type reactor [18]. It was found the recycled solvent can enhance the extraction yield. But the reason is not clear. The physicochemical property of the main product, such as Deposit, was not characterized in detail. Furthermore, the authors are currently focusing on the degradative solvent extraction of low-rank coals, rather than bituminous coal. It was found that the two type coals have different reaction behavior and product property for the degradative solvent extraction. Thus, in this work the degradative solvent extraction of low-rank coals by the mixture of the Soluble and solvent as recycled solvent was studied in an autoclave. The effects of solvent recycling and the Soluble concentration in the recycled solvent on the yields and properties of the extraction products were investigated in detail. The mechanism of this process was also discussed preliminarily.
2.2. Product characterization The proximate analyses of the raw coals were carried out by a muffle furnace according to GB/T212-2008 standard procedure. The elemental analysis was performed by an elemental analyzer (CHN EL-2, Vario). Thermal decomposition behaviors of solid products were characterized by a thermogravimetric (TG) analyzer (Diamond TG/DTA, PerkinElmer). About 10 mg sample was heated up to 900 °C at the heating rate of 10 K/min in flowing pure nitrogen of 80 mL/min. The chemical structure analyses of the solid products were performed using a Bruker VERTEX-70 FTIR spectrometer ranging from 4000 to 400 cm−1 at a resolution of 4 cm−1. The chemical compositions of the minerals in raw coals and products were estimated by X-ray fluorescence (XRF, PANalytical B.V., Zetium) with a Rh target X-ray tube (60 kV, 160 mA).
2. Experimental 2.1. Degradative solvent extraction procedure Three typical low-rank coals, which are Huolinhe lignite (HLH), Hefeng subbituminous coal (HF) and Naomaohu subbituminous coal (NMH), were used for the degradative solvent extraction. The proximate and ultimate analyses were shown in Table 1. Non-polar solvent 1-methylnaphthalene (1-MN) was used as the solvent for the extraction. The detailed procedure of the degradative solvent extraction was already described in our previous papers [5,20]. It was carried out by using a specially designed autoclave. There was a filter (0.5 μm diameter opening) equipped at the bottom of the autoclave. About 20 g of dried raw coal and 300 mL of 1-MN were placed in the autoclave. After purged adequately by N2, the autoclave was heated up to 350 °C at the heating rate of 5 °C/min, and maintained for 60 min. A valve below the
3. Results and discussion 3.1. Product yields of the extraction with solvent cycle The product yield distributions of the extraction using initial solvent were firstly investigated and shown in Fig. 1. It shows that the NMH coal gave highest Soluble and Deposit yields. On the contrary, the lowest Soluble and Deposit yields were obtained from HLH coal. The highest gaseous product yield was obtained from HLH coal. The difference of the yield distributions for the three coals was attributed to the property of the raw coals. Besides, the figure also shows that the Soluble yield was always higher than the Deposit yield for the three coals, consisting with our previous work [5]. The lower figure in Fig. 2 shows the yield changes of Solubles and Deposits against the extraction cycle. The gaseous product, Liquid and Residue yields were not shown in Fig. 2. It was because they were rather close to the yields of the extraction using initial solvent which was shown in Fig. 1. The total yields of Soluble and Deposit were also almost
Table 1 Proximate and ultimate analysis of the raw coals.
HLH HF NMH a
Proximate analysis
Ultimate analysis
(wt%)
(wt%, daf)
Mad
Adb
Vdaf
FCdaf
C
H
N
Oa
27.1 6.4 15.8
20.1 14.7 6.4
45.6 46.9 41.8
54.4 53.1 58.2
63.7 75.4 75.7
4.7 6.4 5.0
1.3 1.4 1.0
30.2 16.8 18.3
Calculated by difference.
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Fig. 1. Product yield distributions of the extraction using initial solvent.
constant against the extraction cycle, implying that the total extraction yield was not affected by the Soluble in the solvent. However, the Soluble yield decreased significantly with the extraction cycle. On the contrary, the Deposit yield increased with the extraction cycle. This should be because that a part of the Soluble was converted to Deposit during the extraction with recycled solvent. The part of the Soluble which converted to Deposit easily was the high molecular weight fraction, which had the solubility in the solvent was relatively low. The conversion from Soluble to Deposit was gradually stopped after about 5–7 extraction cycles. The soluble yields were about two times of the Deposit yields for the first extraction cycle for the three coals, as shown in Fig. 1 and Fig. 2. However, the Deposit yields were more than two times of the Soluble yields after 5–7 extraction cycles for HLH and HF coals. So, the main extract switched from Soluble to Deposit with the extraction cycle. This made up the loss of the part of the extraction product (Soluble) to a large extent. The Soluble dissolved in the solvent should be the main reason for the conversion from Soluble to Deposit during the extraction using recycled solvent. So, the changing tendencies of Soluble and Deposit concentrations in the solvent during the extraction at 350 °C were then calculated, and shown in the upper figure of Fig. 2. It shows that the Soluble concentration increased with extraction cycle, and reached almost constant after 7 cycles, which may be the saturation state of the Soluble in the solvent. The high molecular weight fraction of the Soluble, which got saturation state earlier, can precipitate as Deposit during the extraction with solvent recycling. This should be the reason for the conversion from the Soluble to the Deposit. The final concentrations of the Solubles were respectively around 4.1% and 5.0% for HLH and NMH coals, higher than that for HF coal which was only 2.5%. The concentration level was in agreement with our previous research results performed by a flow-type extractor for bituminous coal, which was ranging from 1.3% to 6.0% [18]. The Deposit concentrations increased slightly with the extraction cycle, and reached almost constant after 4 or 5 cycles. But it was judged that the Deposit did not yet reach saturation state, because the Deposit precipitation to Residue did not appeared clearly. Besides, the Deposit concentrations of the three coals were similar to each other, ranging from 1.0% to 1.2%.
3.2. Product yields of the extraction with different ratio of solvent to coal The results above show that the Soluble and Deposit concentrations changed with the extraction cycle. This should be the reason for the Soluble and Deposit yield changes. In order to further investigate the relationship between their concentrations and yields, the extractions of HLH and HF coals with different ratio of fresh solvent to coal were performed. The results, shown in Fig. 3, show that the Soluble and Deposit yields increased slightly with the decrease of the ratio of
Fig. 2. Changes of the Soluble and Deposit concentrations in the solvent (upper figure) and the Soluble and Deposit yields (lower figure) with extraction cycle.
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Table 2 Elemental compositions of the Solubles and Deposits obtained by the extraction with recycled solvent. Sample
C
H
N
O
S
HLH-Raw 1-D 3-D 5-D HF-Raw 1-S 3-S 5-S 7-S 1-D 3-D 5-D 7-D NMH-Raw 1-S 3-S 5-S 7-S 1-D 3-D 5-D 7-D
65.14 82.44 83.37 83.56 75.43 81.94 82.56 82.21 82.41 78.25 79.51 79.10 79.10 63.89 80.91 81.49 81.86 82.22 78.91 78.99 79.04 79.29
3.13 4.51 4.64 4.74 6.45 7.88 8.03 8.23 8.19 5.89 5.97 6.00 6.15 5.46 7.56 7.71 7.78 7.82 5.94 6.05 6.09 6.15
1.38 1.59 1.38 1.32 1.39 0.80 0.69 0.69 0.66 1.56 1.43 1.39 1.35 0.90 0.64 0.56 0.52 0.52 0.99 0.98 0.96 0.96
30.35 11.46 10.61 10.38 16.45 9.05 8.39 8.53 8.47 13.94 12.81 13.27 13.13 29.52 10.32 90.70 9.33 8.93 13.74 13.63 13.57 13.28
– – – – 0.29 0.34 0.34 0.35 0.27 0.36 0.28 0.24 0.27 0.23 0.57 0.55 0.52 0.52 0.43 0.35 0.34 0.32
S: Soluble; D: Deposit. The sample was named by number of extraction cycle–extract, e.g. 1-D means the Deposit obtained by the first extraction cycle.
contents of the Solubles and Deposits slightly increased with the solvent cycle. On the contrary, the oxygen contents of them decreased slightly. For further comparing the elemental compositions of the extraction products clearer, the mole ratios of hydrogen to carbon (H/C), oxygen to carbon (O/C), sulfur to carbon (S/C) and nitrogen to carbon (N/C) of the Solubles and Deposits from HF and NMH coals were shown in Fig. 4 and Fig. 5. Fig. 4 shows that all of the O/C, S/C and N/C decreased with the extraction cycle for the Deposits, especially for S/C of HF coal which decreased about 33%. In other words, the heteroatoms (O, N and S) contents in the Deposits decreased with the solvent cycle. It was also the case for the Solubles as shown in Fig. 5. On the contrary, the H/C of the Solubles and Deposits were all increased with the extraction cycle, implying that the proportion of the aliphatic carbon in the Solubles and Deposits increased with the extraction cycle. On the whole, the elemental compositions of the Solubles and Deposits changed slightly, in agreement with the discussion above. The Deposit would be the target product for the optimized degradative solvent extraction method proposed in this work. The results above indicate that the proposed optimization can not only increase the Deposit yield, but also further upgrade the Deposit through promoting the heteroatoms (O, N and S) removal.
Fig. 3. Changes of the Soluble and Deposit concentrations in the solvent (upper figure) and the Soluble and Deposit yields (lower figure) with different ratios of fresh solvent to coal.
solvent to coal, and started to decrease when the ratio decreased from 5 to 4. The corresponding Soluble concentrations also increased significantly and started to decrease when the ratio decreased to from 5 to 4. The Soluble should get saturation state at the highest concentration point. However, the highest Soluble concentration for HLH coal was 2.4%, lower than that for the extraction using recycled solvent. It was also the case for HF coal. This was caused by the physicochemical property change of the Soluble obtained by the extraction with recycled solvent, e.g. the high molecular weight fraction of the Soluble precipitated as Deposit during the extraction with recycled solvent, as mentioned above. So the soluble left, which was the light molecular weight fraction, has high solubility in the solvent.
3.4. Chemical structure of the extraction products The FTIR analysis for the Solubles and Deposits were then performed to investigate the chemical structure change with the extraction cycle. The FTIR spectra were shown in Fig. 6. The spectra of the Soluble and Deposit were different. The Solubles had weaker OH stretching bands at 3100–3600 cm−1, and more distinct peaks attributed to aliphatic CeH, CeH2 and CeH3 stretching vibration at 2850–2960 cm−1. The Deposits had sharper peak attributed to C]C stretching vibration at 1530–1670 cm−1. But the spectra for the Solubles or Deposits obtained with extraction cycle were rather similar to each other at first glance. To more deeply investigate the chemical structure change of the extraction products, the ratio of the peak area ranged from 2850 to 2960 cm−1 to the peak area ranged from 1530 to 1670 cm−1 was calculated. It was considered that this ratio represented the ratio of aliphatic carbon to aromatic carbon (Cal/Car). The Cal/Car of all the Solubles and Deposits, shown in Fig. 6, increased slightly with the
3.3. Elemental composition of the extraction products The physicochemical properties of the Soluble and Deposit should be changed with the extraction cycle, as discussed above. So, the elemental compositions of them were firstly compared and shown in Table 2. The data for HLH coal was not complete. The table shows that the Soluble and Deposit had significantly higher carbon contents and lower oxygen contents, compared to those of the raw coals. The carbon 51
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Fig. 4. Mole ratios of hydrogen to carbon (H/C), oxygen to carbon (O/C), sulfur to carbon (S/C) and nitrogen to carbon (N/C) of the Deposits obtained by the extraction with recycled solvent.
extraction cycle. In other words, the aromaticity of the Solubles and Deposits decreased slightly with the extraction cycle. This is in agreement with the H/C of the Solubles and Deposits shown in Fig. 4 and
Fig. 5. The Cal/Car of the Solubles ranged from 1.63 to 2.17, much higher than those of the Deposits which ranged from 0.35 to 0.84. It implies that the Soluble aromaticity was lower than the Deposit
Fig. 5. Mole ratios of hydrogen to carbon (H/C), oxygen to carbon (O/C), sulfur to carbon (S/C) and nitrogen to carbon (N/C) of the Solubles obtained by the extraction with recycled solvent.
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Fig. 6. The FTIR spectra and ratio of aliphatic carbon to aromatic carbon (Cal/Car) of Solubles and Deposits obtained by the extraction with recycled solvent.
3.6. Inorganic element contents in the Solubles and Deposits
aromaticity.
One of the main purposes of the degradative solvent extraction of coal is to produce ash-free product. So the contents of the main inorganic elements (Fe, Ca, Mg, Ca, Si, Al, Ti and Cl) in the Deposits obtained from HF coal were investigated and shown in Fig. 8. The transformation rates of the elements from raw coal to the Deposits were also calculated and shown in Fig. 8. The total contents of the Fe, Ca, Mg, Ca, Si, Al, Ti and Cl in the Deposits increased slightly from 0.25% to 0.29%, and then got almost constant after about 4 times of the extraction recycle. Only 0.2–0.4% of the main inorganic elements were transformed from raw coal to the Deposit. Because most of them remained in the Residues [5]. The contents and transformation rates of all the inorganic elements investigated increased slightly at the first several extraction cycles, and then got almost stable or decreased at the last several extraction cycles, except Cl. The main inorganic elements in the Deposit were Si, Al and Fe, which had the total contents ranging from 0.15% to 0.22%. The contents of Ca, Mg and Cl were 0.01–0.03%. The contents of Ti and K were lower than 0.008%. However, the situation was different for the transformation rate, as shown in the figure. The Fe, Cl and Mg had the transformation rates of higher than 1.0%,
3.5. Thermal decomposition behavior of the extraction products The thermal decomposition behaviors of the Solubles and Deposits were investigated by a TGA and shown in Fig. 7. The TG curves show that the starting temperature points of weight loss for the Solubles and Deposits were around 250 °C and higher than 300 °C, respectively. The final weight loss of all of the Solubles and Deposits increased slightly with extraction cycle, indicating the volatile matter contents of them increased slightly with extraction cycle. The DTG curves show that the small peak at around 300 °C became larger with the extraction cycle for both Solubles and Deposits. It was judged that the thermal decomposition of the Solubles and Deposits at 300 °C did not start, because they were obtained at 350 °C by degradative solvent extraction. So, the weight loss peak at 300 °C should be attributed to the volatilization of the small molecules. This indicates that more small molecules were formed in the Solubles and Deposits during the extraction with extraction cycle. This is in agreement with the elemental composition and FTIR analysis results of the Solubles and Deposits.
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Fig. 7. TG and DTG curves of the Solubles and Deposits.
Fig. 8. The main inorganic element (Fe, Ca, Mg, Ca, Si, Al, Ti and Cl) contents in the Deposits and their transformation rates from raw coal to Deposit.
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significantly higher than those of Si, Al and Ca. It should be attributed to the existing form of the inorganic minerals in the raw coal. Thus, the relatively high inorganic element content in the Deposit is caused by two factors. The first is the high content in the raw coal, such as Si and Al. The second is the difference of the existing form of them. The slight increase of the inorganic element content in the Deposit should be caused by the accumulation of them in the recycled mixed solvent, and the subsequent transformation from the recycled mixed solvent to Deposit. It is necessary to deeply study the transformation mechanism of the inorganic element during the extraction with extraction cycle.
[2]
[3]
[4]
[5]
4. Conclusions
[6]
The utilization of the mixture of Soluble and solvent as the recycled solvent for the degradative solvent extraction of low-rank coal was investigated in this work. It was found that the yields of the Deposit increased by more than two times with the extraction cycle. It is because of that a part of the Soluble precipitated as Deposit with the extraction cycle. The ratios of O/C, N/C, and S/C of the Deposit decreased with the extraction cycle, indicating that the Deposit was further upgraded. The aromaticity and the reactivity of the Deposit decreased and increased with the extraction cycle, respectively. Furthermore, the ash content of the Deposit increased slightly from 0.25% to 0.29%, and then got almost constant after about 4 times of the extraction cycle. The main inorganic elements in the Deposit were Si, Al and Fe. The relatively high contents of some inorganic elements in the Deposit were caused by their high contents and existing forms in the raw coal. Thus, it was shown that using the mixture of Soluble and solvent as recycled solvent is a feasible way for the practical application of the degradative solvent extraction of low-rank coals.
[7]
[8]
[9] [10] [11]
[12]
[13]
[14] [15]
Acknowledgments [16]
The authors gratefully acknowledge the financial supports provided by the National Natural Science Foundation of China (21776109, U1510119, 21306059, U1503194) and the International Science & Technology Cooperation Program of China (2015DFA60410). This research was also supported by the Foundation of State Key Laboratory of Coal Conversion (Grant No. J17-18-301), and the State Key Laboratory of Coal Combustion (FSKLCCA1602, FSKLCCA1702). A portion of the experiment was conducted at the facilities of the Analytical and Testing Center of Huazhong University of Science and Technology.
[17]
[18] [19]
[20]
[21]
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Degradative solvent extraction of low-rank coals by the mixture of low molecular weight extract and solvent as recycled solvent
T
Xian Lia,b, Zong Zhanga, Lei Zhanga, Xianqing Zhua, Zhenzhong Hua, Weixiang Qiana, ⁎ Ryuichi Ashidac, Kouichi Miurad, Hongyun Hua, Guangqian Luoa, Hong Yaoa, a
State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People's Republic of China b College of Chemistry and Chemical Engineering, Xinjiang University, Urumchi, Xinjiang 830046, People's Republic of China c Department of Chemical Engineering, Kyoto University, Kyoto-Daigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan d Institute of Advanced Energy, Kyoto University, Kyoto-Daigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Low-rank coal Degradative solvent extraction Solvent recycle Ash content
The authors have proposed a degradative solvent extraction method to dewater, upgrade and separate low-rank coals to upgraded coal, high molecular weight extract (Deposit), low molecular weight extract (Soluble), and a little amount of liquid and gas products. For the practical application of this extraction method, the solvent must be recycled. Furthermore, the separation of the Soluble and solvent, which was implemented by distillation, consumed much energy and added the cost of the whole system significantly. In order to solve these issues, the utilization of the Soluble and solvent as the recycled mixture solvent was proposed and investigated in this work. The results showed that the yields of the Deposit increased by more than two times with the mixture solvent recycle, and got almost stable after 5–7 extraction cycles. This made up for the loss of the part of the extraction product (Soluble) to a large extent. Furthermore, the ratios of O/C, N/C, and S/C of the Deposit decreased with the mixture solvent recycle, indicating that the Deposit was further upgraded. The aromaticity and reactivity of the Deposit decreased and increased with the mixture solvent recycle, respectively. The Cal/Car of the Deposits ranged from 0.35 to 0.84. The ash content of the Deposit increased slightly from 0.25% to 0.29%, and then got almost constant after 4 times of the mixture solvent recycle. Only 0.2–0.4% of the main inorganic elements were transformed from raw coal to the Deposit. The main inorganic elements in the Deposit were Si, Al and Fe. The relatively high contents of some inorganic elements in the Deposit were caused by their high contents and existing form in the raw coal. Thus, it was showed that using the Soluble and solvent as recycled mixture solvent is a feasible way for the practical application of the degradative solvent extraction of low-rank coals.
1. Introduction The utilization of low-rank coals, such as lignites and subbituminous coals, have got more and more attention [1]. But the high moisture content, high oxygen content, low heating value and obvious self-ignition tendency suppress their thermal conversion and utilization [2]. Furthermore, the low-rank coals contain much small molecular weight fractions and have relatively high chemical activity, compared to the high rank coals [2, 3]. So, the multipurpose utilization of the low-rank coals by poly-generation technology is considered to be more promising compared to the direct combustion and gasification [3]. The authors have proposed a degradative solvent extraction method which treats low-rank coals in a nonpolar solvent at around 350 °C using an autoclave to dewater, upgrade and separate them to three solid fractions
⁎
and a small amount of liquid and gas products [4–6]. The three solid fractions were the extraction residue which is the so-called upgraded coal, high molecular weight extract (named as Deposit) which extracted at the extraction temperature and precipitated as solid from the solvent at room temperature, and low molecular weight extract (Soluble) which was soluble in the solvent at room temperature. The solid Soluble can be obtained after removing the solvent by distillation. The Soluble and Deposit have the carbon content of higher than 80%, oxygen content of lower than 10%, rather low moisture content, and are almost ash free. It was found in our previous work that the two extracts can be used as a good binder for coke making [7–9], to produce high-quality liquid fuel and chemical by further liquefaction [10, 11], and as a precursor for high value-added carbon material preparation, such as carbon fiber [12, 13] and porous carbon materials [14]. The upgraded coal has much
Corresponding author. E-mail address: [email protected] (H. Yao).
https://doi.org/10.1016/j.fuproc.2018.01.005 Received 23 October 2017; Received in revised form 2 January 2018; Accepted 9 January 2018 0378-3820/ © 2018 Elsevier B.V. All rights reserved.
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filter was then quickly opened at the extraction temperature after the residence time, in order to allow the mixture of the extract and solvent to be transferred to a reservoir. The reservoir was maintained at ambient temperature by circulating cooling water. A part of the extract, which precipitated as solid from the solvent in the reservoir at ambient temperature, was the Deposit. Another part of the extract, which was still solubilized in the solvent, was the Soluble. All of the substance in the reservoir was collected and separated by filtration. The filter cake was obtained as the Deposit. The residue remained in the autoclave above the filter. The detailed process flow diagram can be found in previous work [20, 21]. The filtrate, which was the mixture of the solvent and Soluble, was used as the recycled solvent for next extraction of fresh coal. Around 10 mL of the filtrate was sampled and separated by rotary evaporator, in order to obtain solid Soluble and solvent for yield calculation and characterization. 10 mL of fresh solvent was then added to the recycled solvent to ensure the same amount of solvent used for each run. This change of the solvent was rather small and the effect for the extraction was ignored in this work. So, the Soluble concentration in the recycled solvent and the total Soluble yield were calculated. The un-extracted fraction, remained in the autoclave after the extraction, was the Residue (upgraded coal). The gaseous products were collected in a gas bag for analysis. The degradative solvent extractions with different ratio of coal to fresh solvent were also performed at the same condition. The Soluble, Deposit and Residue, obtained from the extraction with recycled solvent and with different ratio of coal to solvent, were further dried in a drying oven under vacuum at 150 °C for 6 h to remove the residual solvent completely. The gaseous product was analyzed quantitatively by a gas chromatograph (Agilent MicroGC3000). The yields of Soluble, Deposit, Residue and Gas were calculated by their weights. The yield of Liquid was calculated by mass balance.
better combustion and gasification characteristics [15, 16], and can be used as a good precursor for activated carbon preparation [17], compared to the raw coals. So, the degradative solvent extraction of lowrank coal is believed to be one of the promising multi-purpose utilization approaches of the low-rank coals. For the practical application of this degradative solvent extraction method, the solvent must be recycled to minimize the solvent needed [18]. Furthermore, the best approach for the separation of the Soluble and solvent is distillation technology, such as flash separator. But it consumes much energy and adds the complexity and cost of the whole system significantly, because the solvent commonly used have rather high boiling point [19]. In order to solve these issues, the authors proposed to use the mixture of the Soluble and solvent as the recycled solvent for the degradative solvent extraction. By this way, it not only realized the solvent recycling, but also avoided the separation of the Soluble and solvent. When using the mixture of the Soluble and solvent as the recycled solvent for the degradative solvent extraction, the Soluble will no longer be the target product during each extraction experiment. So, the Soluble, which is one of the main products of the degradative solvent extraction, was lost by this way. Also, the effect of the Soluble in the recycled solvent on the subsequent extraction was not clear. In our previous work, we preliminarily studied the degradative solvent extraction of bituminous coal by the recycled mixture of the Soluble and solvent using a flow-type reactor [18]. It was found the recycled solvent can enhance the extraction yield. But the reason is not clear. The physicochemical property of the main product, such as Deposit, was not characterized in detail. Furthermore, the authors are currently focusing on the degradative solvent extraction of low-rank coals, rather than bituminous coal. It was found that the two type coals have different reaction behavior and product property for the degradative solvent extraction. Thus, in this work the degradative solvent extraction of low-rank coals by the mixture of the Soluble and solvent as recycled solvent was studied in an autoclave. The effects of solvent recycling and the Soluble concentration in the recycled solvent on the yields and properties of the extraction products were investigated in detail. The mechanism of this process was also discussed preliminarily.
2.2. Product characterization The proximate analyses of the raw coals were carried out by a muffle furnace according to GB/T212-2008 standard procedure. The elemental analysis was performed by an elemental analyzer (CHN EL-2, Vario). Thermal decomposition behaviors of solid products were characterized by a thermogravimetric (TG) analyzer (Diamond TG/DTA, PerkinElmer). About 10 mg sample was heated up to 900 °C at the heating rate of 10 K/min in flowing pure nitrogen of 80 mL/min. The chemical structure analyses of the solid products were performed using a Bruker VERTEX-70 FTIR spectrometer ranging from 4000 to 400 cm−1 at a resolution of 4 cm−1. The chemical compositions of the minerals in raw coals and products were estimated by X-ray fluorescence (XRF, PANalytical B.V., Zetium) with a Rh target X-ray tube (60 kV, 160 mA).
2. Experimental 2.1. Degradative solvent extraction procedure Three typical low-rank coals, which are Huolinhe lignite (HLH), Hefeng subbituminous coal (HF) and Naomaohu subbituminous coal (NMH), were used for the degradative solvent extraction. The proximate and ultimate analyses were shown in Table 1. Non-polar solvent 1-methylnaphthalene (1-MN) was used as the solvent for the extraction. The detailed procedure of the degradative solvent extraction was already described in our previous papers [5,20]. It was carried out by using a specially designed autoclave. There was a filter (0.5 μm diameter opening) equipped at the bottom of the autoclave. About 20 g of dried raw coal and 300 mL of 1-MN were placed in the autoclave. After purged adequately by N2, the autoclave was heated up to 350 °C at the heating rate of 5 °C/min, and maintained for 60 min. A valve below the
3. Results and discussion 3.1. Product yields of the extraction with solvent cycle The product yield distributions of the extraction using initial solvent were firstly investigated and shown in Fig. 1. It shows that the NMH coal gave highest Soluble and Deposit yields. On the contrary, the lowest Soluble and Deposit yields were obtained from HLH coal. The highest gaseous product yield was obtained from HLH coal. The difference of the yield distributions for the three coals was attributed to the property of the raw coals. Besides, the figure also shows that the Soluble yield was always higher than the Deposit yield for the three coals, consisting with our previous work [5]. The lower figure in Fig. 2 shows the yield changes of Solubles and Deposits against the extraction cycle. The gaseous product, Liquid and Residue yields were not shown in Fig. 2. It was because they were rather close to the yields of the extraction using initial solvent which was shown in Fig. 1. The total yields of Soluble and Deposit were also almost
Table 1 Proximate and ultimate analysis of the raw coals.
HLH HF NMH a
Proximate analysis
Ultimate analysis
(wt%)
(wt%, daf)
Mad
Adb
Vdaf
FCdaf
C
H
N
Oa
27.1 6.4 15.8
20.1 14.7 6.4
45.6 46.9 41.8
54.4 53.1 58.2
63.7 75.4 75.7
4.7 6.4 5.0
1.3 1.4 1.0
30.2 16.8 18.3
Calculated by difference.
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Fig. 1. Product yield distributions of the extraction using initial solvent.
constant against the extraction cycle, implying that the total extraction yield was not affected by the Soluble in the solvent. However, the Soluble yield decreased significantly with the extraction cycle. On the contrary, the Deposit yield increased with the extraction cycle. This should be because that a part of the Soluble was converted to Deposit during the extraction with recycled solvent. The part of the Soluble which converted to Deposit easily was the high molecular weight fraction, which had the solubility in the solvent was relatively low. The conversion from Soluble to Deposit was gradually stopped after about 5–7 extraction cycles. The soluble yields were about two times of the Deposit yields for the first extraction cycle for the three coals, as shown in Fig. 1 and Fig. 2. However, the Deposit yields were more than two times of the Soluble yields after 5–7 extraction cycles for HLH and HF coals. So, the main extract switched from Soluble to Deposit with the extraction cycle. This made up the loss of the part of the extraction product (Soluble) to a large extent. The Soluble dissolved in the solvent should be the main reason for the conversion from Soluble to Deposit during the extraction using recycled solvent. So, the changing tendencies of Soluble and Deposit concentrations in the solvent during the extraction at 350 °C were then calculated, and shown in the upper figure of Fig. 2. It shows that the Soluble concentration increased with extraction cycle, and reached almost constant after 7 cycles, which may be the saturation state of the Soluble in the solvent. The high molecular weight fraction of the Soluble, which got saturation state earlier, can precipitate as Deposit during the extraction with solvent recycling. This should be the reason for the conversion from the Soluble to the Deposit. The final concentrations of the Solubles were respectively around 4.1% and 5.0% for HLH and NMH coals, higher than that for HF coal which was only 2.5%. The concentration level was in agreement with our previous research results performed by a flow-type extractor for bituminous coal, which was ranging from 1.3% to 6.0% [18]. The Deposit concentrations increased slightly with the extraction cycle, and reached almost constant after 4 or 5 cycles. But it was judged that the Deposit did not yet reach saturation state, because the Deposit precipitation to Residue did not appeared clearly. Besides, the Deposit concentrations of the three coals were similar to each other, ranging from 1.0% to 1.2%.
3.2. Product yields of the extraction with different ratio of solvent to coal The results above show that the Soluble and Deposit concentrations changed with the extraction cycle. This should be the reason for the Soluble and Deposit yield changes. In order to further investigate the relationship between their concentrations and yields, the extractions of HLH and HF coals with different ratio of fresh solvent to coal were performed. The results, shown in Fig. 3, show that the Soluble and Deposit yields increased slightly with the decrease of the ratio of
Fig. 2. Changes of the Soluble and Deposit concentrations in the solvent (upper figure) and the Soluble and Deposit yields (lower figure) with extraction cycle.
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Table 2 Elemental compositions of the Solubles and Deposits obtained by the extraction with recycled solvent. Sample
C
H
N
O
S
HLH-Raw 1-D 3-D 5-D HF-Raw 1-S 3-S 5-S 7-S 1-D 3-D 5-D 7-D NMH-Raw 1-S 3-S 5-S 7-S 1-D 3-D 5-D 7-D
65.14 82.44 83.37 83.56 75.43 81.94 82.56 82.21 82.41 78.25 79.51 79.10 79.10 63.89 80.91 81.49 81.86 82.22 78.91 78.99 79.04 79.29
3.13 4.51 4.64 4.74 6.45 7.88 8.03 8.23 8.19 5.89 5.97 6.00 6.15 5.46 7.56 7.71 7.78 7.82 5.94 6.05 6.09 6.15
1.38 1.59 1.38 1.32 1.39 0.80 0.69 0.69 0.66 1.56 1.43 1.39 1.35 0.90 0.64 0.56 0.52 0.52 0.99 0.98 0.96 0.96
30.35 11.46 10.61 10.38 16.45 9.05 8.39 8.53 8.47 13.94 12.81 13.27 13.13 29.52 10.32 90.70 9.33 8.93 13.74 13.63 13.57 13.28
– – – – 0.29 0.34 0.34 0.35 0.27 0.36 0.28 0.24 0.27 0.23 0.57 0.55 0.52 0.52 0.43 0.35 0.34 0.32
S: Soluble; D: Deposit. The sample was named by number of extraction cycle–extract, e.g. 1-D means the Deposit obtained by the first extraction cycle.
contents of the Solubles and Deposits slightly increased with the solvent cycle. On the contrary, the oxygen contents of them decreased slightly. For further comparing the elemental compositions of the extraction products clearer, the mole ratios of hydrogen to carbon (H/C), oxygen to carbon (O/C), sulfur to carbon (S/C) and nitrogen to carbon (N/C) of the Solubles and Deposits from HF and NMH coals were shown in Fig. 4 and Fig. 5. Fig. 4 shows that all of the O/C, S/C and N/C decreased with the extraction cycle for the Deposits, especially for S/C of HF coal which decreased about 33%. In other words, the heteroatoms (O, N and S) contents in the Deposits decreased with the solvent cycle. It was also the case for the Solubles as shown in Fig. 5. On the contrary, the H/C of the Solubles and Deposits were all increased with the extraction cycle, implying that the proportion of the aliphatic carbon in the Solubles and Deposits increased with the extraction cycle. On the whole, the elemental compositions of the Solubles and Deposits changed slightly, in agreement with the discussion above. The Deposit would be the target product for the optimized degradative solvent extraction method proposed in this work. The results above indicate that the proposed optimization can not only increase the Deposit yield, but also further upgrade the Deposit through promoting the heteroatoms (O, N and S) removal.
Fig. 3. Changes of the Soluble and Deposit concentrations in the solvent (upper figure) and the Soluble and Deposit yields (lower figure) with different ratios of fresh solvent to coal.
solvent to coal, and started to decrease when the ratio decreased from 5 to 4. The corresponding Soluble concentrations also increased significantly and started to decrease when the ratio decreased to from 5 to 4. The Soluble should get saturation state at the highest concentration point. However, the highest Soluble concentration for HLH coal was 2.4%, lower than that for the extraction using recycled solvent. It was also the case for HF coal. This was caused by the physicochemical property change of the Soluble obtained by the extraction with recycled solvent, e.g. the high molecular weight fraction of the Soluble precipitated as Deposit during the extraction with recycled solvent, as mentioned above. So the soluble left, which was the light molecular weight fraction, has high solubility in the solvent.
3.4. Chemical structure of the extraction products The FTIR analysis for the Solubles and Deposits were then performed to investigate the chemical structure change with the extraction cycle. The FTIR spectra were shown in Fig. 6. The spectra of the Soluble and Deposit were different. The Solubles had weaker OH stretching bands at 3100–3600 cm−1, and more distinct peaks attributed to aliphatic CeH, CeH2 and CeH3 stretching vibration at 2850–2960 cm−1. The Deposits had sharper peak attributed to C]C stretching vibration at 1530–1670 cm−1. But the spectra for the Solubles or Deposits obtained with extraction cycle were rather similar to each other at first glance. To more deeply investigate the chemical structure change of the extraction products, the ratio of the peak area ranged from 2850 to 2960 cm−1 to the peak area ranged from 1530 to 1670 cm−1 was calculated. It was considered that this ratio represented the ratio of aliphatic carbon to aromatic carbon (Cal/Car). The Cal/Car of all the Solubles and Deposits, shown in Fig. 6, increased slightly with the
3.3. Elemental composition of the extraction products The physicochemical properties of the Soluble and Deposit should be changed with the extraction cycle, as discussed above. So, the elemental compositions of them were firstly compared and shown in Table 2. The data for HLH coal was not complete. The table shows that the Soluble and Deposit had significantly higher carbon contents and lower oxygen contents, compared to those of the raw coals. The carbon 51
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Fig. 4. Mole ratios of hydrogen to carbon (H/C), oxygen to carbon (O/C), sulfur to carbon (S/C) and nitrogen to carbon (N/C) of the Deposits obtained by the extraction with recycled solvent.
extraction cycle. In other words, the aromaticity of the Solubles and Deposits decreased slightly with the extraction cycle. This is in agreement with the H/C of the Solubles and Deposits shown in Fig. 4 and
Fig. 5. The Cal/Car of the Solubles ranged from 1.63 to 2.17, much higher than those of the Deposits which ranged from 0.35 to 0.84. It implies that the Soluble aromaticity was lower than the Deposit
Fig. 5. Mole ratios of hydrogen to carbon (H/C), oxygen to carbon (O/C), sulfur to carbon (S/C) and nitrogen to carbon (N/C) of the Solubles obtained by the extraction with recycled solvent.
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Fig. 6. The FTIR spectra and ratio of aliphatic carbon to aromatic carbon (Cal/Car) of Solubles and Deposits obtained by the extraction with recycled solvent.
3.6. Inorganic element contents in the Solubles and Deposits
aromaticity.
One of the main purposes of the degradative solvent extraction of coal is to produce ash-free product. So the contents of the main inorganic elements (Fe, Ca, Mg, Ca, Si, Al, Ti and Cl) in the Deposits obtained from HF coal were investigated and shown in Fig. 8. The transformation rates of the elements from raw coal to the Deposits were also calculated and shown in Fig. 8. The total contents of the Fe, Ca, Mg, Ca, Si, Al, Ti and Cl in the Deposits increased slightly from 0.25% to 0.29%, and then got almost constant after about 4 times of the extraction recycle. Only 0.2–0.4% of the main inorganic elements were transformed from raw coal to the Deposit. Because most of them remained in the Residues [5]. The contents and transformation rates of all the inorganic elements investigated increased slightly at the first several extraction cycles, and then got almost stable or decreased at the last several extraction cycles, except Cl. The main inorganic elements in the Deposit were Si, Al and Fe, which had the total contents ranging from 0.15% to 0.22%. The contents of Ca, Mg and Cl were 0.01–0.03%. The contents of Ti and K were lower than 0.008%. However, the situation was different for the transformation rate, as shown in the figure. The Fe, Cl and Mg had the transformation rates of higher than 1.0%,
3.5. Thermal decomposition behavior of the extraction products The thermal decomposition behaviors of the Solubles and Deposits were investigated by a TGA and shown in Fig. 7. The TG curves show that the starting temperature points of weight loss for the Solubles and Deposits were around 250 °C and higher than 300 °C, respectively. The final weight loss of all of the Solubles and Deposits increased slightly with extraction cycle, indicating the volatile matter contents of them increased slightly with extraction cycle. The DTG curves show that the small peak at around 300 °C became larger with the extraction cycle for both Solubles and Deposits. It was judged that the thermal decomposition of the Solubles and Deposits at 300 °C did not start, because they were obtained at 350 °C by degradative solvent extraction. So, the weight loss peak at 300 °C should be attributed to the volatilization of the small molecules. This indicates that more small molecules were formed in the Solubles and Deposits during the extraction with extraction cycle. This is in agreement with the elemental composition and FTIR analysis results of the Solubles and Deposits.
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Fig. 7. TG and DTG curves of the Solubles and Deposits.
Fig. 8. The main inorganic element (Fe, Ca, Mg, Ca, Si, Al, Ti and Cl) contents in the Deposits and their transformation rates from raw coal to Deposit.
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significantly higher than those of Si, Al and Ca. It should be attributed to the existing form of the inorganic minerals in the raw coal. Thus, the relatively high inorganic element content in the Deposit is caused by two factors. The first is the high content in the raw coal, such as Si and Al. The second is the difference of the existing form of them. The slight increase of the inorganic element content in the Deposit should be caused by the accumulation of them in the recycled mixed solvent, and the subsequent transformation from the recycled mixed solvent to Deposit. It is necessary to deeply study the transformation mechanism of the inorganic element during the extraction with extraction cycle.
[2]
[3]
[4]
[5]
4. Conclusions
[6]
The utilization of the mixture of Soluble and solvent as the recycled solvent for the degradative solvent extraction of low-rank coal was investigated in this work. It was found that the yields of the Deposit increased by more than two times with the extraction cycle. It is because of that a part of the Soluble precipitated as Deposit with the extraction cycle. The ratios of O/C, N/C, and S/C of the Deposit decreased with the extraction cycle, indicating that the Deposit was further upgraded. The aromaticity and the reactivity of the Deposit decreased and increased with the extraction cycle, respectively. Furthermore, the ash content of the Deposit increased slightly from 0.25% to 0.29%, and then got almost constant after about 4 times of the extraction cycle. The main inorganic elements in the Deposit were Si, Al and Fe. The relatively high contents of some inorganic elements in the Deposit were caused by their high contents and existing forms in the raw coal. Thus, it was shown that using the mixture of Soluble and solvent as recycled solvent is a feasible way for the practical application of the degradative solvent extraction of low-rank coals.
[7]
[8]
[9] [10] [11]
[12]
[13]
[14] [15]
Acknowledgments [16]
The authors gratefully acknowledge the financial supports provided by the National Natural Science Foundation of China (21776109, U1510119, 21306059, U1503194) and the International Science & Technology Cooperation Program of China (2015DFA60410). This research was also supported by the Foundation of State Key Laboratory of Coal Conversion (Grant No. J17-18-301), and the State Key Laboratory of Coal Combustion (FSKLCCA1602, FSKLCCA1702). A portion of the experiment was conducted at the facilities of the Analytical and Testing Center of Huazhong University of Science and Technology.
[17]
[18] [19]
[20]
[21]
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