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Effect of adjusting coal properties on HulunBuir lignite pyrolysis
FUPROC-05166; No of Pages 6 Fuel Processing Technology xxx (2016) xxx–xxx
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Research article
Effect of adjusting coal properties on HulunBuir lignite pyrolysis Cui-Ping Ye a,b, Zhen-Jian Yang a, Wen-Ying Li a,⁎, Hui-Ling Rong a, Jie Feng a a b
Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Taiyuan 030024, PR China College of Environmental Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China
a r t i c l e
i n f o
Article history: Received 20 July 2016 Received in revised form 30 September 2016 Accepted 2 October 2016 Available online xxxx Keywords: Lignite pyrolysis Thermal pretreatment Acid-washing Coal properties Tar
a b s t r a c t It is necessary to develop an effective method to suppress cross-linking reactions of oxygen-containing functional groups and minerals during lignite pyrolysis. The effect of thermal pretreatment, acid-washing, and acid-washing combined with thermal pretreatment on lignite pyrolysis at a low temperature with a fixed-bed reactor under N2 atmosphere was investigated. The chemical and physical characteristics of lignite with and without pretreatment were analyzed by Fourier transform infrared spectroscopy, scanning electron microscopy and X-ray diffraction. The molecular weight distributions of tars were estimated by gel permeation chromatography and the composition of gas products was characterized by using gas chromatography. The relation between the structure change and the pyrolysis performance is the key topic for discussion. The results show that there are more pores and surface cracks in coal after acid-washing, and the carboxyl groups increased. The yields of tar and gas were improved while the yields of char and pyrolysis water were reduced after pretreatment. The tar yields increased significantly, namely from 6.88 wt% for raw lignite to 7.23 wt% after thermal pretreatment, 8.57 wt% after acid-washing, and 9.27 wt% after acid-washing combined with thermal pretreatment, consequently. Meanwhile, the hexane soluble of tar increased from 75.57 wt% to 77.75 wt% after thermal pretreatment, while it decreased to 67.21 wt% or 69.70 wt% after acid-washing. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Traditional ways of utilizing lignite are combustion, gasification, liquefaction and pyrolysis. About 90% of lignite is used for power generation [1]. The lignite combustion not only wastes the hydrogen-rich materials which could be converted into high value-added oil, gas and chemical components, but also seriously pollutes the environment. In addition, the high moisture content of lignite requires dehydration process before burning, and therefore increases power costs. Lignite has high volatile matter contents and is relatively rich in hydrogen, therefore it is possible to obtain high value-added tar, char and gas simultaneously during pyrolysis at low temperature, and the products can be used comprehensively by a hybrid process [2]. Hence, multiproduct processing is a more efficient and cleaner way for lignite conversion. The breakup of the coal macromolecular network and the resulting product formation are controlled by the relative rates of bond-breaking, cross-linking and mass transport during coal pyrolysis. Cross-linking reactions are the most important factors because they determine the yield and the molecular weight distribution of tar, and the char's surface area and reactivity eventually [3–6]. The cross-linking reactions are mainly related to oxygen containing functional groups and minerals present ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (W.-Y. Li).
in coal. The total oxygen of lignite accounted for 15–30 wt%, and most is in the form of oxygen-containing functional groups, which can form hydrogen bonds with each other [7]. The cross-linking reaction of these hydrogen bonds or the intermediate products (molecules, ions, free radicals) in coal increase the formation of useless inorganic CO2 and water, and decrease the yields of tar and volatile matter during lignite pyrolysis [8,9]. Additionally, minerals in lignite containing calcium ion (Ca2+) can act as cross-linking points and suppress the tar formation during or prior to pyrolysis. Furthermore, the pore structure, surface area and pore volume of lignite increased, which should provide more active sites for tar molecules adsorption [10,11]. Therefore, the yields of tar decreased. Meanwhile, Ca2+ has a stable effect on carboxyl groups. So it is necessary to develop an effective method to suppress the cross-linking reactions of oxygen-containing functional groups and minerals during lignite pyrolysis. The methods used to suppress the cross-linking reactions during coal pyrolysis, include preheating [8], minerals removal [3], swelling [6] and O-alkylation [12–14]. However, all of these methods do not consider the effects of oxygen-containing functional groups and minerals simultaneously, and therefore, the enhancement of the yields and quality of tar is not obvious. The purpose of this study is to suppress cross-linking reactions of oxygen-containing functional groups and minerals during lignite pyrolysis, as well as to improve the yield and quality of tar. The effects of
http://dx.doi.org/10.1016/j.fuproc.2016.10.002 0378-3820/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: C.-P. Ye, et al., Effect of adjusting coal properties on HulunBuir lignite pyrolysis, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.002
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C.-P. Ye et al. / Fuel Processing Technology xxx (2016) xxx–xxx
1710 1610 1090 784 470
Trasmittance / %
40
H
30
R
2.2.2. Fourier transform infrared spectroscopy (FTIR) Spectra were performed by using a Bruker transform infrared spectrometer (Tensor 27) with spectral range of 400–4000 cm− 1, mixed samples with KBr as 1:100 ratio and flaked under pressure of 1 MPa. FTIR spectra were recorded with a 4 cm−1 spectral resolution. The spectrum is a result of total scan time of 16.
A AH
3600
3500
3400
3300
3200
AH
20
H 10
4000
3427
3500
2920
3000
2500
A
R
2850
2000
1500
3000 scanning electron microscope. Low vacuum (0.075 mm Hg), low accelerating voltage of 5 kV and magnification of 500× were used. The samples were sputter-coated with a layer of gold before SEM analysis.
1000
500
-1
Wavenumber / cm
2.2.3. X-ray powder diffraction (XRD) Data were recorded on a Rigaku Ultima IV diffractometer using Cu Kα radiation (λ = 1.54056 Å) operated at 40 kV and 40 mA over a 2θ range of 5–80° with a scan speed of 8° min−1. The interlayer spacing of aromatic layers (d002) in the samples was calculated from the peak position by means of Bragg's equation. The mean crystallite size along the c axis [16] was calculated from the width at the half maximum of the 002 peak using Scherrer's equation.
Fig. 1. FTIR spectra of different coal sample.
thermal pretreatment, acid-washing and acid-washing combined with thermal pretreatment on structure and pyrolysis behaviors of lignite were investigated. 2. Experiments 2.1. Samples and pretreatment methods A lignite sample from HulunBuir, China was used in the experiment. The proximate analysis of lignite is 11.44 wt% moisture (Mad), 13.11 wt% ash (Aad), 33.08 wt% volatile matter (VMad), and 56.16 wt% fixed carbon (FCdaf). The ultimate analysis of lignite is 74.05 wt% Cdaf, 4.11 wt% Hdaf, 19.97 wt% Odaf (by difference), 1.26 wt% Ndaf, and 0.65 wt% St,daf, respectively. The lignite was ground into 0.25–0.43 mm fraction and dried in a vacuum oven at 110 °C for N10 h before use. 2.1.1. Acid-washing pretreatment Hydrochloric acid/hydrofluoric acid (HCl/HF) chemical purification method [15] was used for demineralization. Typically, 40 g of lignite was weighted, and then 6 mol/L HCl (400 mL) was added. The mixture was heated and stirred in a water bath at 70 °C for 2 h. After that, the lignite sample was transferred to a PTFE beaker, 40% HF (400 mL) was added, heated at 70 °C for 4 h and filtered. Another 400 mL HCl (6 mol/L) was added and the sample was heated at 70 °C for 2 h. Finally, the sample was filtered and washed with deionized water until the filtrate was neutral. The sample was dried at 110 °C under vacuum for N 10 h before use. Ash content of raw coal (sample R) and acid-washed coal (sample A) was determined using the programmable temperature furnace with reference to GB/T212–2008 of China. The temperature of furnace was raised from 90 °C to 500 °C within 35 min and maintained 30 min, and then raised to 815 ± 10 °C and kept for another 60 min. It was found that the ash content of sample A was b 0.6 wt% and the ash removal rate was N 95 wt%. 2.1.2. Thermal pretreatment A quartz reactor with lignite (sample R, about 7 g) or acid-washed coal (sample A, about 6 g) was put into the furnace at 170 °C under N2 atmosphere and kept for 30 min. The sample was cooled to room temperature and kept in the dryer under N2 atmosphere for further analysis. 2.2. Experimental analysis instrument 2.2.1. Scanning electron microscope (SEM) Morphology analysis of raw coal (sample R) and pretreated coal samples (samples H, A, and AH) were carried out with a HITACHI TM-
2.2.4. Gas chromatography (GC) GC analysis was performed on a Haixin GC-950 (Shanghai, China), with a TDX-01 (3 mm × 2 m) column packed with carbon molecular sieve coupled with a thermal conductivity detector (TCD). The temperatures of column and detector were set to 100 °C and the inlet temperature was kept to 150 °C. Gas injection volume was 1 mL. Area normalization was used for quantitative analysis. The protection gas was N2 and the bulk gas production includes H2, CO, CO2, and CH4, while the others are trace gaseous hydrocarbons, such as ethane, neglected here. 2.2.5. Gel permeation chromatography (GPC) GPC was employed to analyze the tar by using a Waters liquid pump 600, equipped with a diode array detector 996. The columns used are HT2 and HT3 (packed with polystyrene-divinyl benzene), with tetrahydrofuran (HPLC, Omni technology) as the mobile phase at the flow rate of 1 mL min−1. The column temperature was kept at 35 °C. 2.3. Pyrolysis program of coals The pyrolysis experiments were conducted on a self-built smallscale fixed-bed pyrolysis platform, which includes gas supply, temperature control system, reaction and products collection system [17,18]. The reactor is a quartz tube with a length of 60 mm, φ60 × 2 mm. At the beginning of the experiment, the dried lignite was fed into the reactor under the N2 atmosphere with a flow rate of 100 mL min−1 for protection. Thermal pretreated samples pyrolysis experiment was conducted after preheating instantly. When the furnace was heated to 550 °C, the reactor with coal sample (about 6 g or 7 g) was placed into furnace for fast pyrolysis and maintained for 30 min. When the reaction was completed, the reactor was quickly removed from the furnace and cooled in air. The gaseous products through condensation system were collected with airbags. The liquid product was cooled in liquid nitrogen
Table 1 Curve-fitting for the 1500–1850 cm−1 zone for FTIR of HulunBuir lignite. Center/cm−1
Assignment
Width/cm−1
Height/×10−2
Area
1772 1727 1701 1666 1609 1571 1549 1510
Esters Aromatic COOH Conjugated C_O Highly conjugated C_O Aromatic C_C COO\ \, aromatic ring stretch COO\ \, aromatic ring stretch COO\ \, aromatic ring stretch
26 51 36 59 55 39 28 21
1.21 6.90 2.77 6.01 9.19 3.55 0.89 0.13
0.40 4.41 1.25 4.45 6.31 1.76 0.31 0.03
Reduced Chi Squared = 3.85 × 10−9.
Please cite this article as: C.-P. Ye, et al., Effect of adjusting coal properties on HulunBuir lignite pyrolysis, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.002
C.-P. Ye et al. / Fuel Processing Technology xxx (2016) xxx–xxx Table 2 Ratio of C_O/Car, COOH/Car, and COO−/Car in different sample.
002
C_O/Cara
COOH/Carb
COO−/Carc
R H A AH
1.45 1.46 1.73 1.67
0.47 0.45 0.74 0.70
1.27 1.13 0.23 0.33
b c
Integrated area between 1766 and 1662 cm−1/integrated area of 1610 cm−1 band. Integrated area 1732 cm−1/integrated area of 1610 cm−1 band. Integrated area between 1585 and 1514 cm−1/integrated area of 1610 cm−1 band.
100
A ♣
♣
H ♣
♦
♣
R bath and washed with isopropanol and tetrahydrofuran from the outlet of reactor, and evaporated on a rotary evaporator to remove the solvents and water. Then the derived tar was divided into hexane soluble and hexane insoluble. The weight fraction yields were calculated on dry ash-free basis [18]. Ychar ¼
♦
10
Wwater
20
30
40
2θ /
Wchar −W A Wtar 100%; Ytar ¼ 100% W ð1−AÞ W ð1−AÞ
Ywater ¼ Wð1−AÞ 100%, Ygas = 1 − Ychar − Ytar − Ywater
♣ SiO2 ♦ CaCO3
AH
Intensity / a.u.
Sample
a
3
50
60
70
80
ο
Fig. 3. XRD spectra of different coal sample.
3. Results and discussions 3.1. Effect of pretreatment on chemical structure of coal
Wchar: the weight of char, g; W: the weight of coal, g; A: the content of ash of coal, g; Wtar: the weight of tar, g; Wwater: the weight of pyrolysis water, g; Y char , Ytar, Y water and Ygas: the yield of char, tar, water and gas, %, respectively.
Oxygen-containing functional groups and mineral matter play important roles in thermal conversion of brown coals. According to the literature [19], carboxyl groups and C\\O bonds in ethers, esters, alcohols
Fig. 2. SEM photographs of different coal sample.
Please cite this article as: C.-P. Ye, et al., Effect of adjusting coal properties on HulunBuir lignite pyrolysis, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.002
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C.-P. Ye et al. / Fuel Processing Technology xxx (2016) xxx–xxx
Table 3 Comparative analysis of microcrystalline parameters in different sample. Sample
d002/nm
Lc/nm
R H A AH
0.40 0.38 0.40 0.37
1.61 1.56 1.38 1.45
start decomposing at 150 °C, which could lead to the coal structure change. So the effect of thermal pretreatment and acid-washing on the chemical structure of the lignite was studied. Preheating the lignite at N 250 °C leads to reduced tar and increased char yields [4]. Therefore, the preheating temperature of 170 °C was selected for this study. FTIR was applied to study the changes related to functional groups of the pretreated samples and the results are shown in Fig. 1. The lignite studied has different types of hydrogen bonds association according to the above data. This is due to the high content of oxygen-containing groups, and the peaks at about 3600–3200 cm−1 in FTIR spectra are corresponding to the hydrogen bonds mainly between the oxygen-containing functional groups, such as phenolic hydroxyl, carboxyl and others. The phenolic hydroxyl typically existed in the form of associating with hydrogen in the water molecule in low rank coals. The thermal pretreatment could release some water from coals. Furthermore, there is a decrease of the content of oxygen-containing functional groups in lignite after thermal pretreatment [20]. As a result, the intensity of hydrogen bond after thermal pretreatment (sample H, 170 °C under N2 for 30 min) should be broken or weakened. The oxygen-containing acidic functional groups are abundant in low-rank coals and favor the presence of exchangeable metal ions in the lignite. However, acidwashing could reduce the carboxylate and transform them into carboxyl groups (\\COOH). Consequently, the intensity of hydrogen bond was improved after acid-washing (sample A). Chen et al. [21] found that the thermal stability of the five hydrogen bonds in coal extracts follows the order of OH-ether O N self-associated OH ≈ cyclic OH N OH\\N N OH-π. The peak of sample A at 3600–3200 cm−1 was weakened after thermal pretreatment (see sample AH). It was likely due to the breakage of selfassociated hydroxyl hydrogen bond and cyclic OH tetramers hydrogen bond after thermal pretreatment. The peak at about 1090 cm− 1 belongs to ether bond (\\O\\) stretching vibration of aliphatic and cyclic ethers. The absorption was weakened after pretreatment. The above peak of sample A and sample AH shifted to a long wavenumber and broadened. Meanwhile, the strength of the peaks at 2920 cm−1 (CH3–) and 2850 cm−1 (CH2–) decreased, which suggest that the aliphatic groups were also reduced. The peaks at 784 cm−1 and 470 cm−1 disappeared after acid-washing and these peaks might correspond to minerals such as calcite or quartz. The peak at 1710 cm−1 is contributed by C_O stretching vibration from aldehydes, ketones, esters, and carboxylic acids [22], which increased in sample A and sample AH. According to the previous studies [23–28], the contents of hydroxyl groups, aliphatic hydrogen and carboxyl changed while the aromatic frames remained unchanged during pretreatment process. Therefore, the content of functional groups could be semi-quantified by the ratios of the peak areas of the functional
groups to that of the aromatic C_C bond (1610 cm−1). The peaks at 1766–1662 cm−1, 1732 cm−1, and 1585–1514 cm−1 are attributed to the C_O bond of carbonyl groups, carboxyl groups, and carboxylate radical (COO−) groups stretching vibration, respectively. Overlapping peak resolving was involved for peaks of carbonyl adsorption (1850– 1500 cm− 1) zones by curve fitting analysis using the PFM (peak fit module) of Origin software. The assignment of the bands in the infrared spectra was made according to literature [23]. Initial approximation of the number of bands and peak positions were obtained by examining second derivatives of the spectral data. Lorentzian functions were used as mathematical functions for band shapes. The carbonyl adsorption zones fitted 8 bands according to carboxyl group (1772–1666 cm−1, 4 bands), aromatic carbon (1609 cm−1, 1 band) and carboxylate groups (1571–1510 cm−1, 3 bands). The R2 values of the curve-fitting analysis in carbonyl adsorption (1850–1500 cm− 1) zone was 0.999. The results are shown in Tables 1 and 2. No significant change for the ratios of NC_O/Car and\\COOH/Car were observed, yet the ratio of COO−/ Car declined from 1.27 (sample R) to 1.13 (sample H) after thermal pretreatment. The condensation reaction of partial carboxylate radical occurred during thermal pretreatment. When release CO2, the metal ions originally coupled with \\COO functional group may bond with char substrate, and the ratio of COO−/Car declined. After acid-washing pretreatment, the ratios of NC_O/Car rose from 1.45 (sample R) to 1.73 (sample A), and the ratios of \\COOH/Car improved from 0.47 (sample R) to 0.74 (sample A). However, since more carboxyl groups formed, the ratios of COO−/Car declined. The ratios of NC_O/Car and \\COOH/Car in sample AH declined from 1.73 (sample A) to 1.67 (sample AH) or from 0.74 (sample A) to 0.70 (sample AH) due to the decomposition or cross-linking reaction of partial carboxyl groups during thermal pretreatment. On the contrary, the ratio of COO−/Car increased from 0.23 (sample A) to 0.33 (sample AH). 3.2. Effect of pretreatment on physical structure of coal The SEM photographs of coal samples with and without pretreatment are shown in Fig. 2. There is small difference of the surface morphology between raw coal (sample R) and thermal pretreated coal (sample H). The surface is rough and shows layer shape. The only difference is the interlayer spacing of sample H which became smaller. The surface morphology of coal was changed significantly after acid-washing (Fig. 2). There are more pores in acid-washed coal (sample A) and surface cracks occurred due to a large number of defects and channels formed in the sample after removal of minerals. Acid-washing combined with thermal pretreatment coal (sample AH) shows more defects than sample A, and the pore size increased also. Acid-washing pretreatment, not only made carboxylate turned to carboxylic acid, but also changed the coal macromolecular structure and made part of the channel open. The XRD patterns of coal samples with and without pretreatment are shown in Fig. 3. The characteristic peaks of the main minerals are quartz (SiO2) and calcite (CaCO3). After acid-washing by HCl-HF, the relevant characteristic peaks of minerals in sample A and sample AH disappeared, which are consistent with the FTIR results. The peak at 20–30° (2θ) corresponds to the aromatic microcrystalline structures
Table 4 Pyrolysis product distribution of different sample. Sample
Yield, daf/wt%a Char
R H A AH a b
62.35 61.51 61.23 61.33
Tar ± ± ± ±
0.21 0.53 0.68 0.48
6.88 7.23 8.57 9.27
Gasb
Water ± ± ± ±
0.06 0.11 0.36 0.08
12.37 11.01 11.10 10.57
± ± ± ±
0.48 0.22 0.54 0.25
18.40 20.25 19.10 18.83
n-Hexane soluble of tar ± ± ± ±
0.33 0.14 0.22 0.32
75.57 77.75 67.21 69.70
± ± ± ±
0.46 0.93 0.75 0.52
The data are on the basis of dry and ash free with the absolute error. By difference.
Please cite this article as: C.-P. Ye, et al., Effect of adjusting coal properties on HulunBuir lignite pyrolysis, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.002
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3.3. Pyrolysis behavior of coal samples with and without pretreatment The distribution of pyrolysis products of coal samples with and without pretreatment are shown in Table 4 (the absolute error is included). The data represent average values from two pyrolysis experiments. The relative errors of yields for char, tar, water, gas, and the hexane soluble of tar are below 1.1%, 4.0%, 5.0%, 1.8%, and 1.2%, respectively. The relative errors of yields of tar and water are higher than those of other products due to the tar collection difficulty and the accumulated error of water. As expected, the total conversion and tar yields increased while the yields of water and char declined. The tar yields increased after thermal pretreatment from 6.88 wt% (sample R) to 7.23 wt% (sample H). Acidwashing has a great impact on the yields of pyrolysis products, especially for tars. The corresponding tar yield increased about 1.69 weight percentage points from 6.88 wt% (sample R) to 8.57 wt% (sample A). The tar yield of sample AH reached 9.27 wt% and increased approximately 2.39 weight percentage points. The increase in the tar yield resulted from the thermal pretreatment was due to the breakage and/or the release of the hydrogen bond prior to the pyrolysis. The metal ions in the carboxylic compounds, such as Ca2 +, are actually the cross-linking points [15]. The metal ions acted as a cross-linking point continuously during coal pyrolysis process. As a result, the formation and release of
1.0
Absorbance/ a.u.
0.8
R 0.6
A AH
0.4
H
0.2
0.0 16
18
20
22
Elution time / min Fig. 4. GPC of tars from different coal sample's pyrolysis.
24
80
H2
67.17
70
CH4
CO
CO2
65.93 54.30
60
53.48
50
Yield/ w%
(002 band). The widening of the 002 band for the lignite attributed to small aromatic microcrystalline structure. It was found that 002 bands of raw coal (sample R) became bald and the peak shifted to left after acid-washing (sample A) and acid-washing combined with thermal pretreatment (sample AH). This observation reveals that the microcrystalline structures became smaller in the pretreated coals. Table 3 shows the d002 (interlayer spacing of aromatic layer) and Lc (stacking height) values for coal samples with and without pretreatment. The stacking height of coal decreased from 1.61 nm (sample R) to 1.38 nm (sample A), while the layer spacing was constant about 0.40 nm after acid-washing treatment. Thus acid-washing influences the stacking height only. It suggests that most of the minerals embedding between the aromatic layers, and acid-washing had little effects on the microcrystalline structures. Both the layer spacing and stacking height of sample H and sample AH declined. There are more carboxyl groups in sample A, which should form hydrogen bonds again. Part of these bonds was broken and part of the carboxyl was decomposed after thermal pretreatment, and more pores and defects were produced. These pores and defects can be seen on the SEM image (Fig. 2). Hydrogen bonds among oxygen-containing groups such as phenolic hydroxyl and carboxyl groups participate powerfully in forming the three-dimensional network structure of coal. The stacking height dropped also because of the suppressing effects of thermal and acid-washing on hydrogen bond crosslinking.
5
35.15
34.18
40 30
21.43
21.87
20 10.04
10.37
10.67
10.74
10 1.36
0.84
1.83
0.64
0 R
H
A
AH
Fig. 5. Gas distribution of different coal sample's pyrolysis.
the tar precursors are more difficult during pyrolysis and suppressed the further tar formation. These bonds are rarely cleaved upon thermal treatment and solvent extraction. Acid-washing to remove the exchangeable metal ions of the minerals is the only way to break these bonds. Therefore, removing the minerals and making partly open channel structure is an advantage for formation and release of tar. In addition, the molecular weight distributions of pyrolysis tars determined by GPC are shown in Fig. 4. The main peaks of GPC profiles of sample A and sample AH shift to shorter retention time. It indicates that the molecular weight of tar become larger after acid-washing pretreatment. However, the peaks of sample H shift to longer retention time, which is corresponding to smaller molecular weight. It is worthy of taking note to the molecular weight of major components in sample AH, which is larger than sample R, but smaller than sample A. Therefore, the molecular weight of tar declined after thermal pretreatment for both samples R and A. The GPC results also confirmed that the thermal pretreatment of coal samples could improve the quality of tars, and the hexane soluble of tar increased nearly 2.18 or 1.43 weight percentage points and from 75.57 wt% (sample R) to 77.75 wt% (sample H), and from 67.27 wt% (sample A) to 69.70 wt% (sample AH), respectively. The pyrolysis gas composition of coal samples with and without pretreatment is shown in Fig. 5 (the error bars are based on absolute error). The data represent the average values from two GC analyses. The relative error of all data is below 4.3%. Thermal pretreatment has less effect on the gas composition, and the yields of CO2 dropped slightly from 67.17 wt% (sample R) to 65.93 wt% (sample H). Acid-washing or acidwashing combined with thermal pretreatment strongly influence the yields of CO2, which decreased significantly to 54.3 wt% (sample A) or 53.48 wt% (sample AH). The contents of water declined simultaneously (Table 4) from 12.37 wt% (sample R) to 11.01 wt% (sample H), 11.10 wt% (sample A), and 10.57 wt% (sample AH), respectively. Moreover, the H2 yields are below 1.83 wt% and the yields of CH4 are about 10 wt%. The activities of hydroxyl and the carbonyl groups were stronger than those of other groups, which had an important effect on CO [29]. Furthermore, the FTIR results (Fig. 1 and Table 2) showed that the content of carbonyl increased after pretreated and the ether bond became unstable. Consequently, the yield of CO, derived from the carbonyl and ether decomposition, increased from 21.43 wt% (sample R) to 21.87 wt% (sample H), 34.18 wt% (sample A), and 35.15 wt% (sample AH), respectively. Therefore, acid-washing combined with thermal pretreatment was found to be much higher efficient than that of any other single pretreatment methods such as thermal pretreatment or acid-washing. This combined pretreatment could let the target of improving simultaneously the yield and quality of coal tar come true by reducing the cross-
Please cite this article as: C.-P. Ye, et al., Effect of adjusting coal properties on HulunBuir lignite pyrolysis, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.002
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C.-P. Ye et al. / Fuel Processing Technology xxx (2016) xxx–xxx
linking points of metal ions and suppressing the hydrogen bond crosslinking reactions between the oxygen functional groups during pyrolysis. 4. Conclusions The thermal pretreatment of coal had little effect on the coal structure, and there are more pores and defects formed after acid-washing, which make the tar release easily during pyrolysis. An advanced pretreatment method is the acid-washing combined with thermal pretreatment for lignite fast pyrolysis at a lower temperature. This method has remarkable effects on water reduction and could improve yield and quality of coal tar by reducing the cross-linking points of metal ions and the hydrogen bond cross-linking reactions between the oxygen functional groups during pyrolysis. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (U1361202). The authors would thank Dr. Stainslav Vassilev for proofreading this paper, who is financially supported by the High-end foreign experts' project in China (GDW20161400003). References [1] BP, BP Statistical Review of World Energy, June 2014. [2] J.G. Wang, X.S. Lu, J.Z. Yao, W.G. Lin, L.J. Cui, Experimental study of coal topping process in a downer reactor, Ind. Eng. Chem. Res. 44 (2005) 463–470. [3] P.R. Solomon, M.A. Serio, G.V. Despande, E. Kroo, Cross-linking reactions during coal conversion, Energy Fuel 4 (1990) 42–54. [4] Z. Cai, H.W. Wu, J.-i. Hayashi, C.-Z. Li, Effects of thermal pretreatment in helium on the pyrolysis behaviour of Loy Yang brown coal, Fuel 84 (2005) 1586–1592. [5] J. Ibarra, I. Cervero, M. Garcia, R. Moliner, Influence of cross-linking on tar formation during pyrolysis of low-rank coals, Fuel Process. Technol. 24 (1990) 19–25. [6] Z.Q. Wang, Z.Q. Bai, Suppressing cross-linking reactions during pyrolysis of lignite pretreated by pyridine, J. Fuel Chem. Technol. 36 (2008) 641–645. [7] D.T. Li, W. Li, H.K. Chen, B.Q. Li, The adjustment of hydrogen bonds and its effect on pyrolysis property of coal, Fuel Process. Technol. 85 (2004) 815–825. [8] K. Miura, K. Mae, K. Sakurada, K. Hashimoto, Flash pyrolysis of coal following thermal pretreatment at low temperature, Energy Fuel 6 (1992) 16–21. [9] P.W. Dong, J.R. Yue, S.Q. Gao, G.W. Xu, Influence of thermal pretreatment on pyrolysis of lignite, J. Fuel Chem. Technol. 40 (2012) 897–905.
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Research article
Effect of adjusting coal properties on HulunBuir lignite pyrolysis Cui-Ping Ye a,b, Zhen-Jian Yang a, Wen-Ying Li a,⁎, Hui-Ling Rong a, Jie Feng a a b
Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Taiyuan 030024, PR China College of Environmental Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China
a r t i c l e
i n f o
Article history: Received 20 July 2016 Received in revised form 30 September 2016 Accepted 2 October 2016 Available online xxxx Keywords: Lignite pyrolysis Thermal pretreatment Acid-washing Coal properties Tar
a b s t r a c t It is necessary to develop an effective method to suppress cross-linking reactions of oxygen-containing functional groups and minerals during lignite pyrolysis. The effect of thermal pretreatment, acid-washing, and acid-washing combined with thermal pretreatment on lignite pyrolysis at a low temperature with a fixed-bed reactor under N2 atmosphere was investigated. The chemical and physical characteristics of lignite with and without pretreatment were analyzed by Fourier transform infrared spectroscopy, scanning electron microscopy and X-ray diffraction. The molecular weight distributions of tars were estimated by gel permeation chromatography and the composition of gas products was characterized by using gas chromatography. The relation between the structure change and the pyrolysis performance is the key topic for discussion. The results show that there are more pores and surface cracks in coal after acid-washing, and the carboxyl groups increased. The yields of tar and gas were improved while the yields of char and pyrolysis water were reduced after pretreatment. The tar yields increased significantly, namely from 6.88 wt% for raw lignite to 7.23 wt% after thermal pretreatment, 8.57 wt% after acid-washing, and 9.27 wt% after acid-washing combined with thermal pretreatment, consequently. Meanwhile, the hexane soluble of tar increased from 75.57 wt% to 77.75 wt% after thermal pretreatment, while it decreased to 67.21 wt% or 69.70 wt% after acid-washing. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Traditional ways of utilizing lignite are combustion, gasification, liquefaction and pyrolysis. About 90% of lignite is used for power generation [1]. The lignite combustion not only wastes the hydrogen-rich materials which could be converted into high value-added oil, gas and chemical components, but also seriously pollutes the environment. In addition, the high moisture content of lignite requires dehydration process before burning, and therefore increases power costs. Lignite has high volatile matter contents and is relatively rich in hydrogen, therefore it is possible to obtain high value-added tar, char and gas simultaneously during pyrolysis at low temperature, and the products can be used comprehensively by a hybrid process [2]. Hence, multiproduct processing is a more efficient and cleaner way for lignite conversion. The breakup of the coal macromolecular network and the resulting product formation are controlled by the relative rates of bond-breaking, cross-linking and mass transport during coal pyrolysis. Cross-linking reactions are the most important factors because they determine the yield and the molecular weight distribution of tar, and the char's surface area and reactivity eventually [3–6]. The cross-linking reactions are mainly related to oxygen containing functional groups and minerals present ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (W.-Y. Li).
in coal. The total oxygen of lignite accounted for 15–30 wt%, and most is in the form of oxygen-containing functional groups, which can form hydrogen bonds with each other [7]. The cross-linking reaction of these hydrogen bonds or the intermediate products (molecules, ions, free radicals) in coal increase the formation of useless inorganic CO2 and water, and decrease the yields of tar and volatile matter during lignite pyrolysis [8,9]. Additionally, minerals in lignite containing calcium ion (Ca2+) can act as cross-linking points and suppress the tar formation during or prior to pyrolysis. Furthermore, the pore structure, surface area and pore volume of lignite increased, which should provide more active sites for tar molecules adsorption [10,11]. Therefore, the yields of tar decreased. Meanwhile, Ca2+ has a stable effect on carboxyl groups. So it is necessary to develop an effective method to suppress the cross-linking reactions of oxygen-containing functional groups and minerals during lignite pyrolysis. The methods used to suppress the cross-linking reactions during coal pyrolysis, include preheating [8], minerals removal [3], swelling [6] and O-alkylation [12–14]. However, all of these methods do not consider the effects of oxygen-containing functional groups and minerals simultaneously, and therefore, the enhancement of the yields and quality of tar is not obvious. The purpose of this study is to suppress cross-linking reactions of oxygen-containing functional groups and minerals during lignite pyrolysis, as well as to improve the yield and quality of tar. The effects of
http://dx.doi.org/10.1016/j.fuproc.2016.10.002 0378-3820/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: C.-P. Ye, et al., Effect of adjusting coal properties on HulunBuir lignite pyrolysis, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.002
2
C.-P. Ye et al. / Fuel Processing Technology xxx (2016) xxx–xxx
1710 1610 1090 784 470
Trasmittance / %
40
H
30
R
2.2.2. Fourier transform infrared spectroscopy (FTIR) Spectra were performed by using a Bruker transform infrared spectrometer (Tensor 27) with spectral range of 400–4000 cm− 1, mixed samples with KBr as 1:100 ratio and flaked under pressure of 1 MPa. FTIR spectra were recorded with a 4 cm−1 spectral resolution. The spectrum is a result of total scan time of 16.
A AH
3600
3500
3400
3300
3200
AH
20
H 10
4000
3427
3500
2920
3000
2500
A
R
2850
2000
1500
3000 scanning electron microscope. Low vacuum (0.075 mm Hg), low accelerating voltage of 5 kV and magnification of 500× were used. The samples were sputter-coated with a layer of gold before SEM analysis.
1000
500
-1
Wavenumber / cm
2.2.3. X-ray powder diffraction (XRD) Data were recorded on a Rigaku Ultima IV diffractometer using Cu Kα radiation (λ = 1.54056 Å) operated at 40 kV and 40 mA over a 2θ range of 5–80° with a scan speed of 8° min−1. The interlayer spacing of aromatic layers (d002) in the samples was calculated from the peak position by means of Bragg's equation. The mean crystallite size along the c axis [16] was calculated from the width at the half maximum of the 002 peak using Scherrer's equation.
Fig. 1. FTIR spectra of different coal sample.
thermal pretreatment, acid-washing and acid-washing combined with thermal pretreatment on structure and pyrolysis behaviors of lignite were investigated. 2. Experiments 2.1. Samples and pretreatment methods A lignite sample from HulunBuir, China was used in the experiment. The proximate analysis of lignite is 11.44 wt% moisture (Mad), 13.11 wt% ash (Aad), 33.08 wt% volatile matter (VMad), and 56.16 wt% fixed carbon (FCdaf). The ultimate analysis of lignite is 74.05 wt% Cdaf, 4.11 wt% Hdaf, 19.97 wt% Odaf (by difference), 1.26 wt% Ndaf, and 0.65 wt% St,daf, respectively. The lignite was ground into 0.25–0.43 mm fraction and dried in a vacuum oven at 110 °C for N10 h before use. 2.1.1. Acid-washing pretreatment Hydrochloric acid/hydrofluoric acid (HCl/HF) chemical purification method [15] was used for demineralization. Typically, 40 g of lignite was weighted, and then 6 mol/L HCl (400 mL) was added. The mixture was heated and stirred in a water bath at 70 °C for 2 h. After that, the lignite sample was transferred to a PTFE beaker, 40% HF (400 mL) was added, heated at 70 °C for 4 h and filtered. Another 400 mL HCl (6 mol/L) was added and the sample was heated at 70 °C for 2 h. Finally, the sample was filtered and washed with deionized water until the filtrate was neutral. The sample was dried at 110 °C under vacuum for N 10 h before use. Ash content of raw coal (sample R) and acid-washed coal (sample A) was determined using the programmable temperature furnace with reference to GB/T212–2008 of China. The temperature of furnace was raised from 90 °C to 500 °C within 35 min and maintained 30 min, and then raised to 815 ± 10 °C and kept for another 60 min. It was found that the ash content of sample A was b 0.6 wt% and the ash removal rate was N 95 wt%. 2.1.2. Thermal pretreatment A quartz reactor with lignite (sample R, about 7 g) or acid-washed coal (sample A, about 6 g) was put into the furnace at 170 °C under N2 atmosphere and kept for 30 min. The sample was cooled to room temperature and kept in the dryer under N2 atmosphere for further analysis. 2.2. Experimental analysis instrument 2.2.1. Scanning electron microscope (SEM) Morphology analysis of raw coal (sample R) and pretreated coal samples (samples H, A, and AH) were carried out with a HITACHI TM-
2.2.4. Gas chromatography (GC) GC analysis was performed on a Haixin GC-950 (Shanghai, China), with a TDX-01 (3 mm × 2 m) column packed with carbon molecular sieve coupled with a thermal conductivity detector (TCD). The temperatures of column and detector were set to 100 °C and the inlet temperature was kept to 150 °C. Gas injection volume was 1 mL. Area normalization was used for quantitative analysis. The protection gas was N2 and the bulk gas production includes H2, CO, CO2, and CH4, while the others are trace gaseous hydrocarbons, such as ethane, neglected here. 2.2.5. Gel permeation chromatography (GPC) GPC was employed to analyze the tar by using a Waters liquid pump 600, equipped with a diode array detector 996. The columns used are HT2 and HT3 (packed with polystyrene-divinyl benzene), with tetrahydrofuran (HPLC, Omni technology) as the mobile phase at the flow rate of 1 mL min−1. The column temperature was kept at 35 °C. 2.3. Pyrolysis program of coals The pyrolysis experiments were conducted on a self-built smallscale fixed-bed pyrolysis platform, which includes gas supply, temperature control system, reaction and products collection system [17,18]. The reactor is a quartz tube with a length of 60 mm, φ60 × 2 mm. At the beginning of the experiment, the dried lignite was fed into the reactor under the N2 atmosphere with a flow rate of 100 mL min−1 for protection. Thermal pretreated samples pyrolysis experiment was conducted after preheating instantly. When the furnace was heated to 550 °C, the reactor with coal sample (about 6 g or 7 g) was placed into furnace for fast pyrolysis and maintained for 30 min. When the reaction was completed, the reactor was quickly removed from the furnace and cooled in air. The gaseous products through condensation system were collected with airbags. The liquid product was cooled in liquid nitrogen
Table 1 Curve-fitting for the 1500–1850 cm−1 zone for FTIR of HulunBuir lignite. Center/cm−1
Assignment
Width/cm−1
Height/×10−2
Area
1772 1727 1701 1666 1609 1571 1549 1510
Esters Aromatic COOH Conjugated C_O Highly conjugated C_O Aromatic C_C COO\ \, aromatic ring stretch COO\ \, aromatic ring stretch COO\ \, aromatic ring stretch
26 51 36 59 55 39 28 21
1.21 6.90 2.77 6.01 9.19 3.55 0.89 0.13
0.40 4.41 1.25 4.45 6.31 1.76 0.31 0.03
Reduced Chi Squared = 3.85 × 10−9.
Please cite this article as: C.-P. Ye, et al., Effect of adjusting coal properties on HulunBuir lignite pyrolysis, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.002
C.-P. Ye et al. / Fuel Processing Technology xxx (2016) xxx–xxx Table 2 Ratio of C_O/Car, COOH/Car, and COO−/Car in different sample.
002
C_O/Cara
COOH/Carb
COO−/Carc
R H A AH
1.45 1.46 1.73 1.67
0.47 0.45 0.74 0.70
1.27 1.13 0.23 0.33
b c
Integrated area between 1766 and 1662 cm−1/integrated area of 1610 cm−1 band. Integrated area 1732 cm−1/integrated area of 1610 cm−1 band. Integrated area between 1585 and 1514 cm−1/integrated area of 1610 cm−1 band.
100
A ♣
♣
H ♣
♦
♣
R bath and washed with isopropanol and tetrahydrofuran from the outlet of reactor, and evaporated on a rotary evaporator to remove the solvents and water. Then the derived tar was divided into hexane soluble and hexane insoluble. The weight fraction yields were calculated on dry ash-free basis [18]. Ychar ¼
♦
10
Wwater
20
30
40
2θ /
Wchar −W A Wtar 100%; Ytar ¼ 100% W ð1−AÞ W ð1−AÞ
Ywater ¼ Wð1−AÞ 100%, Ygas = 1 − Ychar − Ytar − Ywater
♣ SiO2 ♦ CaCO3
AH
Intensity / a.u.
Sample
a
3
50
60
70
80
ο
Fig. 3. XRD spectra of different coal sample.
3. Results and discussions 3.1. Effect of pretreatment on chemical structure of coal
Wchar: the weight of char, g; W: the weight of coal, g; A: the content of ash of coal, g; Wtar: the weight of tar, g; Wwater: the weight of pyrolysis water, g; Y char , Ytar, Y water and Ygas: the yield of char, tar, water and gas, %, respectively.
Oxygen-containing functional groups and mineral matter play important roles in thermal conversion of brown coals. According to the literature [19], carboxyl groups and C\\O bonds in ethers, esters, alcohols
Fig. 2. SEM photographs of different coal sample.
Please cite this article as: C.-P. Ye, et al., Effect of adjusting coal properties on HulunBuir lignite pyrolysis, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.002
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C.-P. Ye et al. / Fuel Processing Technology xxx (2016) xxx–xxx
Table 3 Comparative analysis of microcrystalline parameters in different sample. Sample
d002/nm
Lc/nm
R H A AH
0.40 0.38 0.40 0.37
1.61 1.56 1.38 1.45
start decomposing at 150 °C, which could lead to the coal structure change. So the effect of thermal pretreatment and acid-washing on the chemical structure of the lignite was studied. Preheating the lignite at N 250 °C leads to reduced tar and increased char yields [4]. Therefore, the preheating temperature of 170 °C was selected for this study. FTIR was applied to study the changes related to functional groups of the pretreated samples and the results are shown in Fig. 1. The lignite studied has different types of hydrogen bonds association according to the above data. This is due to the high content of oxygen-containing groups, and the peaks at about 3600–3200 cm−1 in FTIR spectra are corresponding to the hydrogen bonds mainly between the oxygen-containing functional groups, such as phenolic hydroxyl, carboxyl and others. The phenolic hydroxyl typically existed in the form of associating with hydrogen in the water molecule in low rank coals. The thermal pretreatment could release some water from coals. Furthermore, there is a decrease of the content of oxygen-containing functional groups in lignite after thermal pretreatment [20]. As a result, the intensity of hydrogen bond after thermal pretreatment (sample H, 170 °C under N2 for 30 min) should be broken or weakened. The oxygen-containing acidic functional groups are abundant in low-rank coals and favor the presence of exchangeable metal ions in the lignite. However, acidwashing could reduce the carboxylate and transform them into carboxyl groups (\\COOH). Consequently, the intensity of hydrogen bond was improved after acid-washing (sample A). Chen et al. [21] found that the thermal stability of the five hydrogen bonds in coal extracts follows the order of OH-ether O N self-associated OH ≈ cyclic OH N OH\\N N OH-π. The peak of sample A at 3600–3200 cm−1 was weakened after thermal pretreatment (see sample AH). It was likely due to the breakage of selfassociated hydroxyl hydrogen bond and cyclic OH tetramers hydrogen bond after thermal pretreatment. The peak at about 1090 cm− 1 belongs to ether bond (\\O\\) stretching vibration of aliphatic and cyclic ethers. The absorption was weakened after pretreatment. The above peak of sample A and sample AH shifted to a long wavenumber and broadened. Meanwhile, the strength of the peaks at 2920 cm−1 (CH3–) and 2850 cm−1 (CH2–) decreased, which suggest that the aliphatic groups were also reduced. The peaks at 784 cm−1 and 470 cm−1 disappeared after acid-washing and these peaks might correspond to minerals such as calcite or quartz. The peak at 1710 cm−1 is contributed by C_O stretching vibration from aldehydes, ketones, esters, and carboxylic acids [22], which increased in sample A and sample AH. According to the previous studies [23–28], the contents of hydroxyl groups, aliphatic hydrogen and carboxyl changed while the aromatic frames remained unchanged during pretreatment process. Therefore, the content of functional groups could be semi-quantified by the ratios of the peak areas of the functional
groups to that of the aromatic C_C bond (1610 cm−1). The peaks at 1766–1662 cm−1, 1732 cm−1, and 1585–1514 cm−1 are attributed to the C_O bond of carbonyl groups, carboxyl groups, and carboxylate radical (COO−) groups stretching vibration, respectively. Overlapping peak resolving was involved for peaks of carbonyl adsorption (1850– 1500 cm− 1) zones by curve fitting analysis using the PFM (peak fit module) of Origin software. The assignment of the bands in the infrared spectra was made according to literature [23]. Initial approximation of the number of bands and peak positions were obtained by examining second derivatives of the spectral data. Lorentzian functions were used as mathematical functions for band shapes. The carbonyl adsorption zones fitted 8 bands according to carboxyl group (1772–1666 cm−1, 4 bands), aromatic carbon (1609 cm−1, 1 band) and carboxylate groups (1571–1510 cm−1, 3 bands). The R2 values of the curve-fitting analysis in carbonyl adsorption (1850–1500 cm− 1) zone was 0.999. The results are shown in Tables 1 and 2. No significant change for the ratios of NC_O/Car and\\COOH/Car were observed, yet the ratio of COO−/ Car declined from 1.27 (sample R) to 1.13 (sample H) after thermal pretreatment. The condensation reaction of partial carboxylate radical occurred during thermal pretreatment. When release CO2, the metal ions originally coupled with \\COO functional group may bond with char substrate, and the ratio of COO−/Car declined. After acid-washing pretreatment, the ratios of NC_O/Car rose from 1.45 (sample R) to 1.73 (sample A), and the ratios of \\COOH/Car improved from 0.47 (sample R) to 0.74 (sample A). However, since more carboxyl groups formed, the ratios of COO−/Car declined. The ratios of NC_O/Car and \\COOH/Car in sample AH declined from 1.73 (sample A) to 1.67 (sample AH) or from 0.74 (sample A) to 0.70 (sample AH) due to the decomposition or cross-linking reaction of partial carboxyl groups during thermal pretreatment. On the contrary, the ratio of COO−/Car increased from 0.23 (sample A) to 0.33 (sample AH). 3.2. Effect of pretreatment on physical structure of coal The SEM photographs of coal samples with and without pretreatment are shown in Fig. 2. There is small difference of the surface morphology between raw coal (sample R) and thermal pretreated coal (sample H). The surface is rough and shows layer shape. The only difference is the interlayer spacing of sample H which became smaller. The surface morphology of coal was changed significantly after acid-washing (Fig. 2). There are more pores in acid-washed coal (sample A) and surface cracks occurred due to a large number of defects and channels formed in the sample after removal of minerals. Acid-washing combined with thermal pretreatment coal (sample AH) shows more defects than sample A, and the pore size increased also. Acid-washing pretreatment, not only made carboxylate turned to carboxylic acid, but also changed the coal macromolecular structure and made part of the channel open. The XRD patterns of coal samples with and without pretreatment are shown in Fig. 3. The characteristic peaks of the main minerals are quartz (SiO2) and calcite (CaCO3). After acid-washing by HCl-HF, the relevant characteristic peaks of minerals in sample A and sample AH disappeared, which are consistent with the FTIR results. The peak at 20–30° (2θ) corresponds to the aromatic microcrystalline structures
Table 4 Pyrolysis product distribution of different sample. Sample
Yield, daf/wt%a Char
R H A AH a b
62.35 61.51 61.23 61.33
Tar ± ± ± ±
0.21 0.53 0.68 0.48
6.88 7.23 8.57 9.27
Gasb
Water ± ± ± ±
0.06 0.11 0.36 0.08
12.37 11.01 11.10 10.57
± ± ± ±
0.48 0.22 0.54 0.25
18.40 20.25 19.10 18.83
n-Hexane soluble of tar ± ± ± ±
0.33 0.14 0.22 0.32
75.57 77.75 67.21 69.70
± ± ± ±
0.46 0.93 0.75 0.52
The data are on the basis of dry and ash free with the absolute error. By difference.
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C.-P. Ye et al. / Fuel Processing Technology xxx (2016) xxx–xxx
3.3. Pyrolysis behavior of coal samples with and without pretreatment The distribution of pyrolysis products of coal samples with and without pretreatment are shown in Table 4 (the absolute error is included). The data represent average values from two pyrolysis experiments. The relative errors of yields for char, tar, water, gas, and the hexane soluble of tar are below 1.1%, 4.0%, 5.0%, 1.8%, and 1.2%, respectively. The relative errors of yields of tar and water are higher than those of other products due to the tar collection difficulty and the accumulated error of water. As expected, the total conversion and tar yields increased while the yields of water and char declined. The tar yields increased after thermal pretreatment from 6.88 wt% (sample R) to 7.23 wt% (sample H). Acidwashing has a great impact on the yields of pyrolysis products, especially for tars. The corresponding tar yield increased about 1.69 weight percentage points from 6.88 wt% (sample R) to 8.57 wt% (sample A). The tar yield of sample AH reached 9.27 wt% and increased approximately 2.39 weight percentage points. The increase in the tar yield resulted from the thermal pretreatment was due to the breakage and/or the release of the hydrogen bond prior to the pyrolysis. The metal ions in the carboxylic compounds, such as Ca2 +, are actually the cross-linking points [15]. The metal ions acted as a cross-linking point continuously during coal pyrolysis process. As a result, the formation and release of
1.0
Absorbance/ a.u.
0.8
R 0.6
A AH
0.4
H
0.2
0.0 16
18
20
22
Elution time / min Fig. 4. GPC of tars from different coal sample's pyrolysis.
24
80
H2
67.17
70
CH4
CO
CO2
65.93 54.30
60
53.48
50
Yield/ w%
(002 band). The widening of the 002 band for the lignite attributed to small aromatic microcrystalline structure. It was found that 002 bands of raw coal (sample R) became bald and the peak shifted to left after acid-washing (sample A) and acid-washing combined with thermal pretreatment (sample AH). This observation reveals that the microcrystalline structures became smaller in the pretreated coals. Table 3 shows the d002 (interlayer spacing of aromatic layer) and Lc (stacking height) values for coal samples with and without pretreatment. The stacking height of coal decreased from 1.61 nm (sample R) to 1.38 nm (sample A), while the layer spacing was constant about 0.40 nm after acid-washing treatment. Thus acid-washing influences the stacking height only. It suggests that most of the minerals embedding between the aromatic layers, and acid-washing had little effects on the microcrystalline structures. Both the layer spacing and stacking height of sample H and sample AH declined. There are more carboxyl groups in sample A, which should form hydrogen bonds again. Part of these bonds was broken and part of the carboxyl was decomposed after thermal pretreatment, and more pores and defects were produced. These pores and defects can be seen on the SEM image (Fig. 2). Hydrogen bonds among oxygen-containing groups such as phenolic hydroxyl and carboxyl groups participate powerfully in forming the three-dimensional network structure of coal. The stacking height dropped also because of the suppressing effects of thermal and acid-washing on hydrogen bond crosslinking.
5
35.15
34.18
40 30
21.43
21.87
20 10.04
10.37
10.67
10.74
10 1.36
0.84
1.83
0.64
0 R
H
A
AH
Fig. 5. Gas distribution of different coal sample's pyrolysis.
the tar precursors are more difficult during pyrolysis and suppressed the further tar formation. These bonds are rarely cleaved upon thermal treatment and solvent extraction. Acid-washing to remove the exchangeable metal ions of the minerals is the only way to break these bonds. Therefore, removing the minerals and making partly open channel structure is an advantage for formation and release of tar. In addition, the molecular weight distributions of pyrolysis tars determined by GPC are shown in Fig. 4. The main peaks of GPC profiles of sample A and sample AH shift to shorter retention time. It indicates that the molecular weight of tar become larger after acid-washing pretreatment. However, the peaks of sample H shift to longer retention time, which is corresponding to smaller molecular weight. It is worthy of taking note to the molecular weight of major components in sample AH, which is larger than sample R, but smaller than sample A. Therefore, the molecular weight of tar declined after thermal pretreatment for both samples R and A. The GPC results also confirmed that the thermal pretreatment of coal samples could improve the quality of tars, and the hexane soluble of tar increased nearly 2.18 or 1.43 weight percentage points and from 75.57 wt% (sample R) to 77.75 wt% (sample H), and from 67.27 wt% (sample A) to 69.70 wt% (sample AH), respectively. The pyrolysis gas composition of coal samples with and without pretreatment is shown in Fig. 5 (the error bars are based on absolute error). The data represent the average values from two GC analyses. The relative error of all data is below 4.3%. Thermal pretreatment has less effect on the gas composition, and the yields of CO2 dropped slightly from 67.17 wt% (sample R) to 65.93 wt% (sample H). Acid-washing or acidwashing combined with thermal pretreatment strongly influence the yields of CO2, which decreased significantly to 54.3 wt% (sample A) or 53.48 wt% (sample AH). The contents of water declined simultaneously (Table 4) from 12.37 wt% (sample R) to 11.01 wt% (sample H), 11.10 wt% (sample A), and 10.57 wt% (sample AH), respectively. Moreover, the H2 yields are below 1.83 wt% and the yields of CH4 are about 10 wt%. The activities of hydroxyl and the carbonyl groups were stronger than those of other groups, which had an important effect on CO [29]. Furthermore, the FTIR results (Fig. 1 and Table 2) showed that the content of carbonyl increased after pretreated and the ether bond became unstable. Consequently, the yield of CO, derived from the carbonyl and ether decomposition, increased from 21.43 wt% (sample R) to 21.87 wt% (sample H), 34.18 wt% (sample A), and 35.15 wt% (sample AH), respectively. Therefore, acid-washing combined with thermal pretreatment was found to be much higher efficient than that of any other single pretreatment methods such as thermal pretreatment or acid-washing. This combined pretreatment could let the target of improving simultaneously the yield and quality of coal tar come true by reducing the cross-
Please cite this article as: C.-P. Ye, et al., Effect of adjusting coal properties on HulunBuir lignite pyrolysis, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.002
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Please cite this article as: C.-P. Ye, et al., Effect of adjusting coal properties on HulunBuir lignite pyrolysis, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.002