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Investigation into the interactions during staged thermal co-conversion of kraft lignin and lignite Yuqian Huang, Luwei Li, Anqing Zheng, Zengli Zhao, and Haibin Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02209 • Publication Date (Web): 12 Oct 2017 Downloaded from http://pubs.acs.org on October 21, 2017
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Energy & Fuels
Investigation
into
the
interactions
during
staged
thermal
co-conversion of kraft lignin and lignite Yuqian Huang1,2,3, Luwei Li4, Anqing Zheng1,2,3, Zengli Zhao1,2,3, Haibin Li1,2,3 1.University of Chinese Academy of Sciences, Beijing 100049, PR China 2. CAS Key Laboratory of Renewable Energy, Guangzhou 510640, PR China. 3.Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, PR China. 4.School of Chemical Engineering, Nanjing University of Science & Technology, Nanjing 210094, PR China.
Corresponding author, Tel.:+86 02087057716. Fax: +86 02087057737. E-mail address:
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ABSTRACT The objective of this study is to investigate the interactions during co-pyrolysis of kraft lignin/lignite and subsequent gasification of co-pyrolysis-formed char. The co-pyrolysis behavior of kraft lignin and lignite was conducted in analytical pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS) and thermogravimetric analyzer (TGA). And then the CO2 gasification reactivity of pyrolysis-formed char was also examined in thermogravimetric analyzer (TGA). The experimental results showed that synergy was observed during co-pyrolysis of kraft lignin and lignite, resulting in promoting the formation of several lignin-derived compounds, such as 2,6-dimethyl-phenol, trans-isoeugenol, anisole, vanillin and guaiacylacetone. The CO2 gasification reactivity of lignite-derived chars were also obviously promoted due to the addition of lignin. The optimal proportion of kraft lignin in the blend may be 30% according to the experimental results from both pyrolysis and gasification. The synergy during co-conversion of kraft lignin and lignite are mainly attributed to the mutual catalytic effects of inherent alkali and alkaline earth metals in kraft lignin and lignite. The study may provide useful data for optimizing co-conversion process of kraft lignin and lignite.
Keywords: Lignite; Lignin; Pyrolysis; Gasification reactivity; Co-conversion; Synergy
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1. Introduction Owning to the increasing concerns with respect to limited fossil resources combined with climate change, biomass has been considered as a potential candidate to replace fossil fuels for liquid fuels, chemicals, heat and power. Biomass generally consists of cellulose, hemicellulose and lignin. Lignin is the second most abundant renewable carbon source after cellulose. It is an amorphous, randomly cross-linked network of three phenylpropane units with varying degrees of methoxylation, which are sinapyl, coniferyl, and p-coumaryl alcohol. These basic units are linked through various C-O-C (β-O-4, α-O-4, 4-O-5 and so on) and C–C bonds (β-5, β-1, β-β, 5-5 and so on).1,2 Currently, kraft lignin is the most industrially available lignin, and approximately 50 million tons of kraft lignin are produced annually as byproduct from the pulp and paper industry. Lignite is a low-rank coal with low calorific value and high oxygen content.3,4 Its structure is similar to the structure of lignin. Lignite is usually more condensed than lignin through the elimination of a certain number of hydroxyl/ether bonds and the formation of more C-C bonds. Considering that both of lignite and lignin have high oxygen content and low calorific value, and the inherent aromatic structures of lignin and lignite render them very suitable for phenolic and aromatic production. Staged thermal co-conversion of lignin and lignite is proposed for their clean and efficient utilization. In this process, lignin and lignite are first co-pyrolyzed for upgrading and simultaneously producing light tar (phenols and aromatics) at mild temperature. Their pyrolysis char is subsequently co-gasified at high temperature for syngas. Co-conversion of lignin and lignite offers several advantages that can be categorized in five aspects: (1) lignin is 第 3 页
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a renewable resource, co-conversion of lignin and lignite could increase the energy diversity and security on a national scale; (2) lignin has been usually considered as carbon-neutral, co-conversion of lignin and lignite could reduce greenhouse gas (GHG) emissions; (3) The high amount of alkali and alkaline earth metals (AAEM) presented in kraft lignin and lignite could enhance pyrolysis and gasification reactivity of each other; (4) The relative high hydrogen content of lignin result in that lignin could serve as a hydrogen-donor for lignite conversion through hydrogen transfer reactions; (5) The similar distributions of liquid products from pyrolysis of lignin and lignite make their co-pyrolysis liquids easy to be upgraded in existing infrastructure from lignite pyrolysis.5-9 In recent years, more and more emphasis is placed on co-conversion of biomass and coal.10-12 The interaction between co-conversion of coal and biomass can be evaluated by comparing the experimental value with the calculated theoretical value. If the experimental value from co-conversion of biomass and coal is greater than, less than or equal to the sum of the values from individual conversion of biomass and coal, the interaction is respectively called synergistic effect, inhibiting effect or additive effect.13-15 Several literatures reported that there was an additive effect during co-pyrolysis of lignite and biomass. Those experiments were carried out in conventional thermobalance reactor with traditional slow pyrolysis, in which biomass and lignite particles devolatilized at different temperature.15-19 Liang Ding and co-workers14 suggested that the intimate contact and comparable gasification rate between biomass char and coal char resulted in the inhibiting effect during their co-gasification. Soncini et al. and HaykiriAcma et al. found that synergistic effect was only significant during 第 4 页
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co-pyrolysis of biomass and low-rank coal,20,21 in which the pyrolysis temperatures of them were about the same temperature range. Synergistic effects significantly promote the volatilization and lead to changes in product distribution, because of the catalytic roles of alkali and alkaline earth metallic species (AAEM) and the hydrogen transfer reactions between biomass and coal.13,22,23 There is an extensive body of information regarding the co-conversion of biomass and lignite. However, very few studies have reported the staged co-conversion of lignin and lignite. The interactions during co-pyrolysis and subsequent co-gasification of lignin and lignite are not well understood. Moreover, the high sodium content in kraft lignin can lead to serious ash-related operational problems, such as slagging, fouling, corrosion and erosion during its thermal utilization. exploring the volatility of sodium during thermal co-conversion of kraft lignin and lignite is a key issue that should be considered. Here, lignite and kraft lignin are co-pyrolyzed in Py-GC/MS and TG for identifying the product distribution and the volatility of sodium. And then the CO2 gasification reactivity of co-pyrolysis char are investigated by TG. 2.Experimental procedure 2.1. Materials Huolinhe lignite (referred to as HL hereafter) was obtained from Yankuang Group. Kraft lignin (KL) was purchased from Sigma-Aldrich. They were first ground and sieved below 200 mesh to make them sufficient contact with each other. Prior to experiments, the samples were dried at 105°C for 24h and then stored in a desiccator. In order to obtain demineralized lignin, kraft lignin was washed with 5 mol/L HCl for 24h, then the sample was rinsed repeatedly with deionized water until no Cl- ions were detected with silver nitrate. The demineralized lignin was 第 5 页
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also dried at 105°C for 24h and referred to as DL. The proximate and ultimate analysis results of the samples are shown in Table 1. Alkali metal contents of the samples were detected by ICP (OPTIMA 8000DV, PerkinElmer, USA) and illustrate in Table 2. The experimental results show that kraft lignin has a high proportion of sodium and potassium in comparison with lignite, and acid treatment can effectively remove AAEM contents from kraft lignin. Table 1 Proximate and ultimate analysis results of the samples used in this study Ultimate analysis(wt.%, daf a)
Proximate analysis(wt.%, db c)
Samples C
H
Ob
N
S
Volatile
FC b
Ash
HL
67.67
4.62
25.44
1.05
1.21
32.18
40.23
27.59
KL
59.41
5.30
32.03
0.09
3.17
45.14
34.42
20.44
DL
68.36
5.94
24.36
0.13
1.21
59.70
36.95
3.34
a
Dry ash free basis Oxygen and fixed carbon content are obtained by difference c Dry basis b
Table 2 Contents of AAEM species in the samples used in this study AAEM content(mg/kg, ad) Samples Na
K
Mg
Ca
HL
1096
1023
1476
10951
KL
93110
14193
76
292
DL
95
52
0
87
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marked as HL, 20%, 30%, 50% and KL. In addition, the blend of DL and HL at a ratio of 20:80 was used for comparison and signed as D20%. The mixtures were carefully ground to ensure that the two components were mixed evenly. 2.2. Fast pyrolysis Fast pyrolysis of samples was conducted on a CDS Pyroprobe 5200 (CDS Analytical Inc., USA) coupled to a gas chromatography-mass spectrometry (GC-MS, Agilent 7890A-5975C, USA). During fast pyrolysis, 2 mg of the samples were pyrolyzed at 600 °C with a heating rate of 104 K/s, and held at 600 °C for 20 s. The GC column used was Rxi@-5Sil MS 30 m×0.25 mm. The GC oven was held at 40°C for 2 min and then raise the temperature to 280°C at a ramp rate of 10°C/min, and held for 5 min. All experiments were carried out at least two times and averaged to compensate experimental reproducibility. To study the changes brought by co-conversion, a synergy factor (𝐹𝑆 ) is defined as: 𝐹𝑆 =
𝐴𝑎𝑐𝑡 𝐴𝑐𝑎𝑙
(1)
In the formula, 𝐴𝑎𝑐𝑡 is the relative actual peak area (actual peak area/mg biomass) of the specific compound, while 𝐴𝑐𝑎𝑙 is the relative calculated peak area (calculated peak area/mg biomass) according to the weight ratio of lignite and lignin, for example, the 𝐴𝑐𝑎𝑙 of the 20% is shown as follows: 𝐻𝐿 𝐾𝐿 𝐴20% 𝑐𝑎𝑙 = 0.8 ∗ 𝐴𝑎𝑐𝑡 + 0.2 ∗ 𝐴𝑎𝑐𝑡
(2)
And 𝐴𝐻𝐿 𝑎𝑐𝑡 is the relative actual peak area of the specific compound from pyrolysis of HL , 𝐴𝐾𝐿 𝑎𝑐𝑡 is the relative actual peak area of the specific compound from pyrolysis of KL. 2.3. Thermogravimetric analysis 第 7 页
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The weight loss (thermogravimetry, TG) and weight loss rate (differential thermogravimetry, DTG) of samples were carried out in a thermogravimetric analyzer (TGAQ50, TA, USA). And the procedure was as follows: about 10 mg samples were heated from 40 to 900°C at a ramp rate of 10 °C/min, and an argon gas flow of 40 mL/min was used as the purge gas. The theoretical weight loss of co-pyrolysis was calculated as follows: Wcal = (a ∗ WKL/DL )/100 + (1 − a/100) ∗ WHL
(3)
Where Wcal is the theoretical residual mass of the sample (%), WKL/DL and WHL is the residual mass of KL or DL and HL (%), respectively, a is the weight ratio (%) of lignin and lignite. 2.4. Pyrolysis in vertical furnace The vertical furnace is illustrated schematically in Fig.1. N2 was used as the carrier gas with a flow rate of 300 mL/min. About 0.5 g samples placed in the quartz container which was covered with quartz wool at the bottom, and the quartz container was hung at the top of a quartz tube reactor which had a length of 125cm and 5.37cm for its internal diameter. Then reactor was purged with N2 for 1h and the reaction zone (the center of the rector) was heated up to 600°C. When the reactor reached the desired temperature of 600°C, the quartz container was pushed into the reaction zone and held there for 30min. After pyrolysis, the quartz container was lifted to the top of reactor and cooled with a nitrogen flow for 60 min until it reached room temperature so that the pyrolysis char could be collected and weighed.
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Reactor moving handle Flow controller 550°C
Gas in Sample Quartz wool Gas cylinders
N2
Heating furnace Thermocouple
Computer
Sealed reaction tube Temperature controller Gas out
Fig. 1 Schematic diagram of vertical furnace. 2.5 Gasification reactivity The pyrolysis char from the vertical furnace were quantitatively analyzed by a thermogravimetric analyzer (STA449 F3, GER) to evaluate its gasification reactivity. About 10 mg of char was heated at a ramp rate of 10 °C/min to 1150 °C and held for 30 min for an adequate conversion under a continuous CO2 flow of 120 ml/min. Char conversion (𝑥𝑡 ) and gasification rate (𝑟𝑡 , min-1) were calculated by the following formula, respectively:
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𝑤0 −𝑤𝑡
𝑥𝑡 = 𝑤
(4)
0 −𝑤𝑎𝑠ℎ
𝑟𝑡 = − 𝑤
1
0 −𝑤𝑎𝑠ℎ
𝑑𝑤𝑡 𝑑𝑡
=
𝑑𝑥𝑡 𝑑𝑡
(5)
In the formula, 𝑤0 means initial weight, 𝑤𝑡 means weight at time t, and 𝑤𝑎𝑠ℎ means the weight of ash left after the thermogravimetric procedure. 3. Results and discussion 3.1. The interaction during fast co-pyrolysis of kraft lignin and lignite in Py-GC/MS The Py-GC/MS experiments do not allow the collection of any liquid products, and thus the actual pyrolytic oil yields cannot be determined. However, the change in the yield of specific compound can be estimated by its corresponding chromatographic peak area relative to the amount of feed (peak area/mg biomass). The main identified compounds from fast pyrolysis of kraft lignin and lignite are shown in Table 3. Both kraft lignin and lignite mainly yielded aromatics, phenols and ketones. There are lots of phenolic hydroxyl groups and guaiacols in lignite and lignin, respectively.24-26 Therefore, phenols were the most abundant pyrolysis products for all seven samples and it was consistent with the result of Xiaona Lin et al..27 The predominant compounds included 4-methyl-1,2-benzenediol, 2-methoxy-4-vinylphenol, creosol, cresol, phenol, guaiacol, homocresol and so on. Cresol, phenol and catechol were the three most abundant compounds during fast pyrolysis of HL, while guaiacol, creosol, homocresol, 2-methoxy-4-vinylphenol were the four most abundant compounds during fast pyrolysis of KL. As shown in Table 3, it is obvious that kraft lignin produced more aromatics and phenols than huolinhe lignite. It could be due to more condensed structure and less C-O bond content of lignite. The aromatic and phenolic production were effectively improved by the demineralization 第 10 页
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of kraft lignin, suggesting that alkali metals (Na and K) play negative catalytic roles for most of the phenolic production during pyrolysis of lignin. Table 3 The production of light tar from fast co-pyrolysis of kraft lignin and lignite (5×104/mg) Component Aromatics 1,2,3-tMe- Benzene Indene 2-Me-Naphthalene Toluene Xylene Phenols 3-Me-1,2-Bdo 4-Me-1,2-Bdo 4,5-dMe-1,3-Bdo 2-Mo-4-vinylphenol 2-Mo-5- Mph Catechol Creosol Cresol Phenol 2,4,6-tMe-Phenol 2,6-dMe-Phenol 2-Et-Phenol Guaiacol 2-Mo-3-Me-Phenol 2-Mo-4-Pr-Phenol 2-Me-Phenol 2,4-dMe-Phenol 3,5-dMe-Phenol 3-Et-5-Me-Phenol 4-eEt-Phenol Homocresol trans-Isoeugenol Anisole Vanillin Ketones 2,3-dMe-2-Cp-1-one 2-Me-2-Cp-1-one 2-Cp-1-one Guaiacylacetone Apocynin
HL
20%
30%
50%
7.1±0.2 9.2±0.5
KL
D20%
DL
8.2±1.4
9.5
8.7
8.6±0.4
5.3±0.4
8.2
5.9
3.7±1.3
7.8±0.4
4.4±0.3
9.0
ND*
7.3±0.2
6.1±0.5
5.5±0.2
3.9±0.2
1.6±0.1
5.7
1.1±1.1
20.2±1.3
18.7±3.0
18.8±0.3
17.5±0.9
52.3±12.4
23.1±0.1
39.8±4.8
31.3±1.1
29.5±1.4
27.9±0.9
23.8±1.4
27.3±0.1
27.2
14.5±2.4
34.6±1.9
32.6±1.0
38.6±1.6
43.5±0.5
72.5±1.1
83.0±0.5
154.2±43.6
18.9
14.9±0.1
17.9±0.3
19.4±0.5
41.7±0.5
113.0±3.0
362.6±17.4
ND
4.6±0.5
8.1
10.6±0.9
23.8±0.3
12.6±0.1
38.6
ND
17.5±0.1
28.1±1.5
44.6±0.3
116.2±0.4
61.5±1.9
313.4±11.6
ND
3.8±0.4
6.7±0.4
11.2±0.2
25.7±0.5
2.8±0.1
8.0±0.1
61.2±2.4
ND
ND
ND
ND
ND
ND
ND
70.6±1.2
86.1±0.5
108.8±2.2
208.6±2.5
302.9±13.2
1136.3±21.0
155.8±5.0
143.9±2.6
139.8±2.5
123.0±5.5
137.5±0.9
152.9±1.2
121.3±11.0
112.4±3.3
118.0±1.6
113.6±2.4
108.6±4.1
111.5±0.6
105.4±0.7
87.2±8.9
3.8±0.2
4.5
5.1
5.7±0.1
10.1
6.7±0.2
13.2±0.7
ND
9.7±1.8
12.0±1.0
13.1±0.6
26.6±0.2
12.5±0.2
26.4±1.6
8.2±0.4
9.4±0.2
10.2±0.1
10.7±0.4
15.6±0.2
6.6±0.2
8.4±1.2
ND
228.7±5.0
339.8±14.2
511.2±5.9
1050.2±15.2
172.3±11.1
864.4±14.7
ND
5.0±0.6
8.9±0.7
15.4±0.2
33.8±0.9
7.7±0.4
89.3±0.6
ND
2.4±0.1
9.0±0.5
12.0±1.6
29.1±0.6
14.0±0.7
65.7±0.2
48.7±1.8
60.2±1.2
66.1±0.5
69.3±2.9
111.4±0.7
57.3±0.5
88.0±7.2
19.2±0.4
18.5±0.3
18.7±0.2
18.5±0.8
27.6±0.2
24.7±0.4
79.0±7.7
2.3
4.9±0.6
8.3±0.2
10.3±0.7
29.8±0.3
3.5±0.4
8.6±1.0
6.8±0.1
6.2
6.6±0.1
6.0±0.2
9.2±0.1
6.9±0.2
ND
13.8±0.5
12.5±0.4
12.0±0.4
11.2±0.4
8.9±0.1
13.4±0.1
14.8
ND
28.5±0.2
41.4±2.2
60.7±1.0
136.6±2.8
62.6±3.5
302.2±0.4
ND
23.0±0.2
28.3±0.9
36.0±0.4
76.9±0.9
48.4±1.4
184.8±10.3
ND
2.2±0.9
5.5±0.2
9.2±0.1
6.6±0.8
ND
ND
ND
11.1±0.3
13.7±0.7
18.0±1.0
35.9±0.2
24.0±1.7
162.5±4.6
7.8±0.1 5.3±0.2
9.1±0.3 6.4±0.2
9.7 7.1±0.1
10.4±0.2 7.8±0.3
13.3±0.1 14.3±0.6
7.6±0.1 5.8
4.1±0.4 7.8±1.3
3.1±0.1
4.3
4.9±0.1
6.1±0.1
12.4±0.2
4.0±0.1
9.9±0.4
ND
15.6±0.3
22.7±0.4
32.8±1.0
60.9
45.6±3.0
214.4±0.6
ND
18.2±0.4
27.8±1.4
50.0±1.0
94.1±0.4
44.7±1.8
214.8±1.0
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Others 5.3±0.1 4.4±0.1 4.0±0.3 2.9±0.1 ND 4.5 ND 1-Decene 8.9± 0.7 7.8± 0.1 7.6± 0.1 7.3± 0.2 9.2± 0.2 9.7± 0.1 12.3± 0.6 2,3-Dh-Benzofuran ND 19.6±0.5 30.3±1.1 46.9±0.7 97.2±0.5 ND 2.0±0.2 1,2-dMo-Benzene Abbreviation: tMe: trimethyl; Me: methyl; dMe: dimethyl; Mo: methoxy; Et: ethyl; Pr: propyl; Cp: Cyclopenten; Dh: dihydro; dMo: dimethoxy: Bdo: Benzenediol Mph: methylphenol *ND means Not Detectd
The synergy factors (𝐹𝑆 ) of main compounds from co-pyrolysis of lignin and lignite are tabulated in Table 4. It is found that there were interactions during co-pyrolysis of lignin and lignite. 𝐹𝑆 of 1,2,3-trimethyl-benzene, indene, creosol, 2,6-dimethyl-phenol, trans-isoeugenol, anisole, vanillin and guaiacylacetone were greater than 1, whereas 𝐹𝑆 of the other compounds were less than or close to 1. It is evident that these phenols (creosol, 2,6-dimethyl-phenol, trans-isoeugenol, anisole, vanillin and guaiacylacetone) were not observed from individual pyrolysis of lignite. FS of these compounds were also promoted in co-pyrolysis of lignite and demineralized lignin. The results could be explained by that the abundant alkali metal species (Na and K) in kraft lignin could inhibit the formation of these phenols, whereas the abundant alkaline earth metal species (Ca and Mg) in lignite could facilitate the release of these phenols. In addition, the hydrogen transfer reaction between lignin and lignite might be partly responsible for these results. As the proportion of kraft lignin in the blend increased from 20 to 50%, FS of these phenolic compounds decreased, except anisole. The decrease in FS could be ascribed to the increasing alkali metal content resulted from elevating proportion of kraft lignin in the blend. FS of anisole exhibted the opposite variation trend. It could be attributed to the fact that anisole is an alkali metal catalyzed product from lignin pyrolysis. The result was further verified by that there was no anisole produced from pyrolysis of demineralized lignin. 第 12 页
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As shown in Table 4, the synergy factors of some phenols, such as most of the alkylphenols, was obviously less than 1, indicating that inhibiting effect of these phenols was also found during co-pyrolysis of kraft lignin and lignite. In contrast, the synergy factors of the phenols with unsaturated branch chains, such as trans-isoeugenol, vanillin and guaiacylacetone was greater than 1. The results suggested that AAEM in alkali lignin could catalyze the decarboxylation, decarbonylation and the cleavage of unsaturated alkyl branch chains during pyrolysis of lignin. Table 4 Synergy factor of main compounds from co-pyrolysis of lignin and lignite. Component Aromatics 1,2,3-trimethyl-Benzene Indene 2-methyl-Naphthalene Toluene Xylene Phenols 3-methyl-1,2-Benzenediol 4-methyl-1,2-Benzenediol 4,5-dimethyl-1,3-Benzenediol 2-Methoxy-4-vinylphenol 2-Methoxy-5-methylphenol Catechol Creosol Cresol Phenol 2,4,6-trimethyl-Phenol 2,6-dimethyl-Phenol 2-ethyl-Phenol Guaiacol 2-methoxy-3-methyl-Phenol 2-methoxy-4-propyl-Phenol 2-methyl-Phenol 2,4-dimethyl-Phenol 3,5-dimethyl-Phenol 3-ethyl-5-methyl-Phenol 4-ethyl-Phenol Homocresol trans-Isoeugenol
20%
30%
50%
D20%
1.09 1.04 0.98 0.69 0.95
1.26 1.10 0.96 0.62 0.92
0.69 1.13 0.87 0.48 0.80
0.90 1.20 0.91 0.93 0.95
0.76 0.62 0.94 0.74 0.72 0.00 1.66 0.93 1.03 0.88 1.79 0.96 1.07 0.72 0.41 0.97 0.87 0.62 0.85 0.96 1.02 1.47
0.83 0.68 1.12 0.79 0.86 0.00 1.36 0.92 1.00 0.88 1.49 0.96 1.06 0.87 1.02 0.97 0.85 0.77 0.87 0.96 1.00 1.21
0.80 0.63 0.88 0.76 0.86 0.00 1.03 0.83 0.96 0.80 0.97 0.89 0.96 0.90 0.81 0.85 0.78 0.63 0.74 0.97 0.88 0.92
1.39 1.26 1.59 0.96 1.72 0.00 1.30 1.00 0.96 1.15 2.31 0.79 0.97 0.42 1.04 0.99 0.77 0.95 1.24 0.94 1.01 1.28
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Anisole Vanillin Ketones 2,3-dimethyl-2-Cyclopenten-1-one 2-methyl-2-Cyclopenten-1-one 2-Cyclopenten-1-one Guaiacylacetone Apocynin Others 1-Decene 2,3-dihydro-Benzofuran 1,2-dimethoxy-Benzene
1.66 1.52
2.72 1.25
2.74 0.99
0.72
1.00 0.89 0.85 1.26 0.95
1.02 0.87 0.81 1.22 0.97
0.97 0.78 0.77 1.06 1.05
1.05 0.97 0.87 1.04 1.02
1.01 0.85 0.99
1.05 0.84 1.02
1.08 0.80 0.95
1.03 0.99 0.00
3.2. Interactions during co-pyrolsis of kraft lignin and lignite in TGA (a)
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HL 20% Cal-20% 30% Cal-30% 50% Cal-50% KL
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Fig. 2 TG curves for (a) the full temperature range, (b) below 600°C, (c) above 600°C and DTG curves for full temperature range (d) of all the raw samples. As shown in Fig. 2, the weight loss of co-pyrolysis of kraft lignin and lignite in TGA can be
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divided into five stages. The first DTG peak of all samples centered at 80°C was due to the evaporation of moisture. The maximum weight loss rates of kraft lignin and lignite were observed at about 360°C and 450°C, respectively. According to the pyrolysis product analysis from Py-GC/MS and literature’s results, the weight loss was mainly ascribed to the release of phenols, aromatics and non-condensable gas from the thermal decomposition of lignin and lignite. The last two DTG peaks were found at around 740°C and 820°C, and they were probably caused by the thermal degradation and volatilization of Na2CO3 and Na2SO4.28,29 It is observed that the last two DTG peaks were significantly weakened in the kraft lignin/lignite blends. This might be ascribed to that Na in kraft lignin could react with the inorganic salts in lignite at high temperature to form thermal stable compounds of sodium with low volatility. When the pyrolysis temperature was below 600°C, 30% and 50% exhibited more weight loss than their theoretical weight loss, whereas 20% displayed less weight loss than its theoretical value. These results may due to the insufficient lignin content in the blend which is unable to provide enough active sites of alkali metals and hydrogen donor to generate a significant synergy at low temperature.30 It is worthy of note that 30% showed higher weight loss rate than 20% and 50% at around 450°C. The DTG peak centered at 450°C was primarily attributed to the devolatilization of lignite. Hence, the results indicated that 30% was the optimal ratio to maximize the catalytic effects of alkali metals for the pyrolysis of lignite. The results could be explained by that the proper amounts of alkali metals can catalyze the pyrolysis of lignite, however, further increasing the amount of alkali metals can inhibit the devolatilization reaction and favor the crosslinking and polycondensation reactions of lignite. It is concluded that the 第 15 页
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synergy was not a monotonically increasing function of the proportion of kraft lignin in the blend. The results are consistent with the earlier findings from Py-GC-MS. The pyrolysis residue of 20%, 30% and 50% was respectively higher than their corresponding calculated theoretical value, suggesting that alkali metals could promote the crosslinking and polycondensation reactions of lignite to form more char. (a)
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400
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Temperature (℃)
Temperature (℃)
Fig. 3 TG (a) and DTG (b) curves of raw and demineralized samples. As shown in Fig.3, in the range of temperature between 410-880 °C, the weight loss of demineralized lignin was higher than that of kraft lignin, and the gap between their weight loss first increased and then decreased with increasing temperature in this range. It is demonstrated that alkali metals played negative roles in the pyrolysis of lignin. Alkali metals can suppress the cleavage of C-C and C-O bonds in lignin to generate low-molecular-weight-compounds and permanent gases in this temperature range. Hence, the weight loss of D20% was greater than that of 20%. Compared with kraft lignin, the maximum weight loss rate of demineralized lignin increased obviously and shifted towards higher temperature, implying that the removal of AAEM can effectively improve the thermal stability of lignin, and promote the reaction rate of 第 16 页
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devolatilization of lignin. The results were mainly explained by the complete decomposition of AAEM compounds in lignin at this temperature. When the temperature reached 900 °C, the weight loss of demineralized lignin was equal to that of kraft lignin. It is found that the DTG peaks assigned to the decomposition of Na2CO3 and Na2SO4 were not observed in the DTG curves of demineralized lignin, indicating that the AAEMs in demineralized lignin were effectively removed by acid washing. 3.3. CO2 gasification reactivity of chars derived from co-pyrolysis of kraft lignin and lignite
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Fig. 4 (a) Conversions and (b) gasification reaction rates of different chars derived from co-pyrolysis of kraft lignin and lignite. The conversions of different chars derived from pyrolysis of kraft lignin and lignite are drawn in Fig.4 (a). As can be seen from Fig.5 (a), the rank order of char conversions was HL20%>30%>50%>KL,
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as well as Tr=max . These results demonstrated that alkali metals in kraft lignin can reduce the active energy of CO2 gasification of char, leading to the decreased in Tx=o.5 and Tr=max. The gasification reaction rates of different chars are given in Fig.4(b). The CO2 gasification rates of different chars exhibited the same trend as their char conversions. The rank order of CO2 gasification rates of chars was HL20%, when char conversion was larger than 0.2. It is should be noted that the instantaneous gasification reactivity of 20% was slightly lower than that of HL. During CO2 gasification of char derived from 20%, insufficient Na from kraft lignin could react with the inorganic salts (including alkaline earth metals) in lignite at high temperature to form thermal stable compounds of sodium, thus reducing their catalytic activity in CO2 gasification. As the proportion of kraft lignin further increased, the excess alkali metals could strongly catalyze the CO2 gasification of chars. In order to quantitatively compare the gasification reactivity of different chars, The gasification reactivity index 𝑟𝑡𝑜 is defined as below: 1
𝑟𝑡𝑜 = ∫ 𝑟 𝑑𝑥 0
The gasification reactivity index, representing the average gasification reactivity of different chars, is given in Fig.5 (b). There is a strong positive linear correlation between the gasification reactivity index and the proportion of kraft lignin in the blend, implying that the gasification reactivity indexs of chars linearly increased as elevating proportion of kraft lignin in the blend.
1000 0.6
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Fig. 6 (a) conversions and (b) gasification rates of chars derived from demineralized lignins. 第 20 页
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(a)
Char conversion (X)
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The conversions and gasification rates of chars derived from demineralized lignin are graphed in Fig.6. As shown in Fig. 6 (a), when the gasification temperature was below 1175°C, the rank order of char conversion was KL>20%>HL>D20%>DL. DL exhibited lowest char conversion during all samples, suggested that alkali metals played important catalytic roles in CO2 gasification of char. The removal of alkali metals from lignin can dramatically reduce the conversion of its derived char. Therefore, the char conversion of D20% was obviously lower than that of 20%. It is important to note that the char conversion of D20% was equal to its theoretical value, suggesting that alkali metals were responsible for the synergy during CO2 gasification. The gasification rate of demineralized lignin is shown in Fig.6 (b). The rank order of the maximum gasification rates of different chars was KL>DL>HL>20%>D20%. Tx=o.5 and Tr=max of DL and D20% are given in Table 5. Tx=o.5 and Tr=max of DL (1056.40 and 1091.73°C) were far greater than those of HL(899.44 and 908.57°C). Tx=o.5 and Tr=max of DL20 were close to those of HL, and they were slightly lower than Tx=o.5 and Tr=max of 20%. These results again demonstrated that the catalytic activity of alkali metals in kraft lignin was the main reason for improving CO2 gasification reactivity of the char derived from co-pyrolysis of kraft lignin and lignite. 4.Conclusions Co-pyrolysis behavior of kraft lignin/lignite and subsequent gasification reactivity of co-pyrolysis-formed char were conducted in this study. The results demonstrated that synergy was found during co-conversion of kraft lignin and lignite. Co-pyrlysis of kraft lignin and lignite can effectively enhance the formation of several lignin-derived compounds, such as 第 21 页
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2,6-dimethyl-phenol, trans-isoeugenol, anisole, vanillin and guaiacylacetone. And Na in kraft lignin may react with the inorganic salts in lignite at high temperature to form thermal stable compounds of sodium with low volatility. The Char conversions and CO2 gasification rates of co-pyrolysis-formed chars were also obviously promoted due to the catalytic effects of alkali metals. And the optimal proportion of kraft lignin in the blend was 30%. Higher proportion of kraft lignin in the blend can inhibit the formation of the phenols mentioned above during co-pyrolysis, while lower proportion of kraft lignin can reduce the CO2 gasification reactivity of co-pyrolysis-formed chars. These findings can provide some guidance for the co-conversion of kraft lignin and lignite.
Acknowledgements
The authors gratefully acknowledge the National Natural Science Foundation of China (Grants 21406227 and 51376186), the Natural Science Foundation of Guangdong Province, China (Grant 2014A030313672) and the Science and Technology Planning Project of Guangdong Province, China (Grants 2014B020216004 and 2015A020215024) for financial support of this work.
References
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