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Hydrothermal liquefaction of lignite, wheat straw and plastic waste in sub-critical water for oil: Product distribution
Journal of Analytical and Applied Pyrolysis 110 (2014) 382–389
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Hydrothermal liquefaction of lignite, wheat straw and plastic waste in sub-critical water for oil: Product distribution Baofeng Wang ∗ , Yaru Huang, Jinjun Zhang School of Chemistry & Material Science, Shanxi Normal University, Linfen 041004, PR China
a r t i c l e
i n f o
Article history: Received 19 June 2014 Accepted 14 October 2014 Available online 23 October 2014 Keywords: Lignite Wheat straw Plastic waste Hydrothermal liquefaction Sub-critical water
a b s t r a c t Product distribution and character of the products during hydrothermal liquefaction of Jingou lignite, wheat straw and plastic waste in sub-critical water are investigated in an autoclave. The effects of blending ratio of lignite, wheat straw and plastic waste, temperature, initial nitrogen pressure and additives on product distributions are also studied. The results indicate that blending ratio of lignite, wheat straw and plastic waste, liquefaction temperature, initial pressure and additives all could influence the product distributions during hydrothermal liquefaction. When the blending ratio of Jingou lignite, wheat straw and plastic waste is 5:4:1, there exists synergism effect for oil yield, and the oil yield and the gas yield are all the highest at this ratio. The total conversion increases when the temperature increases from 260 ◦ C to 300 ◦ C and then decreases. For the oil yield, when the temperature increases from 260 ◦ C to 280 ◦ C, oil yield decreases, while when the temperature increases from 280 ◦ C to 320 ◦ C, the oil yield increases. Moreover, adding tourmaline during hydrothermal liquefaction of lignite, wheat straw and plastic waste could get higher oil yield, higher total conversion and higher quality oil than that when adding the traditional catalyst. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Coal liquefaction can make clean coal into oil, and it also can reduce pollution. However, up to now, the cost of coal liquefaction is still very high. Biomass, such as wheat straw, is a kind of cheap, abundant and renewable energy source [1–5]. Furthermore, plastic wastes are also abundant. However, there are many difficulties for people to use them efficiently and cleanly and most of the biomass and plastic wastes are abandoned. The main processing mode today for plastic wastes is incineration or landfill deposition [6,7]. As we have known, the major elements of plastic wastes and biomass are carbon and hydrogen, and they have higher H/C ratio than coal. Then they can be used as rich hydrogen additive during coal liquefaction [8–11]. Moreover, the traditional solvent used in liquefaction is tetranap, which is one of the organic solvent. Using tetranap not only makes the cost high, but also harms the human beings. As we have known, water is cheap and clean, and it can form a single phase with liquefied products in subcritical state; moreover, it also can separate naturally from hydrophobic products as a different phase at normal temperatures and pressures [12–18]. Furthermore, hydrogen supply during sub-critical water
extraction is effective for obtaining compounds with low oxygen content, but hydrogen is expensive. Fortunately, biomass and plastic waste can be cheap sources of hydrogen when they are added in sub-critical water. Previous studies showed that there exists synergistic effect in the co-liquefaction of coal and rice straw [19]; and literatures also showed that co-liquefaction of plastic wastes with aromatic-rich materials such as coal could yield synergistic effects and increase the production of oil [20]. Besides that, studies also showed that the addition of biomass to plastic waste high density polyethylene (HDPE) liquefaction could make the reaction conditions milder, and enhance the conversion of HDPE at lower temperature, implying the synergistic effect of biomass and HDPE [21]. Thus, co-liquefaction of coal, biomass and plastic waste in subcritical water is proposed. In this paper, the products distribution during co-liquefaction of lignite, wheat straw and plastic waste was investigated, and the characters of the products were also studied. This study will help us to understand further the process and mechanism of co-liquefaction of coal, biomass and plastic waste. 2. Materials and methods 2.1. Samples
∗ Corresponding author. Tel.: +86 357 205 1192; fax: +86 357 209 2427. E-mail address: [email protected] (B. Wang). http://dx.doi.org/10.1016/j.jaap.2014.10.004 0165-2370/© 2014 Elsevier B.V. All rights reserved.
Jingou lignite (JG), wheat straw (WS) and plastic waste polyethylene terephthalate (PET) we used are all from Linfen in Shanxi
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Table 1 Proximate and ultimate analysis of the coal, wheat straw and plastic waste. Sample
Proximate analysis (wt.%)
Jingou lignite Wheat straw Plastic waste a
Ultimate analysis (wt.%, ad)
Mad
Aad
Vad
Vdaf
FCad
C
H
N
Oa
S
11.70 7.94 0.14
25.75 5.33 0.04
22.45 77.26 90.8
32.43 89.08 90.96
7.67 9.47 9.02
49.66 40.48 61.88
3.06 5.60 4.29
0.83 0.85 0.03
43.58 52.93 33.76
2.87 0.14 0.04
By difference.
Province of China. The samples were all first air-dried and then crushed, grounded and sieved from 0.15 mm to 0.25 mm. Table 1 is the results of proximate and ultimate analysis of the coal, wheat straw and plastic waste.
Asphaltene yield (A, %) =
wA × 100% wJG + wWS + wPET
Preasphaltene yield (PA, %) = 2.2. Experimental setup and procedure The co-liquefaction experiments were carried out in a 250 ml autoclave. Each time 10.00 g JG lignite, wheat straw, plastic waste, or mixtures of the three samples were put into the reactor together with 80 ml deionized water. Before the liquefaction experiment, the reactor was filled with nitrogen to the desired initial pressure (2.0–5.0 MPa) and sealed; then the reactor was heated to the desired temperature at 10 ◦ C/min by a furnace and then stayed for required time. After reaction the reactor was cooled to room temperature and depressurized to atmospheric pressure. The residue was taken out, dried and further analyzed. 2.3. Fractionation of liquefaction products The gas products were analyzed by Algien 7890 A gas chromatograph. The solid and liquid products were extracted by hexane, benzene and tetrahydrofuran (THF), here the reagents (bought from Guangfu Fine Chemical Research Institute of Tianjin, China) are all analytically pure agents. After extraction, the products were separated into oils (hexane soluble), asphaltene (hexane insoluble but benzene soluble), preasphaltene (benzene insoluble but tetrahydrofuran soluble), and residues (tetrahydrofuran insoluble) as shown in Fig. 1 [8].
wR × 100% wJG + wWS + wPET
Gas yield (G, %) = 100% − (O + A + PA + R)% Total conversion (TC,%) = 100% − R% where wJG is the mass of Jingou lignite, (g); wWS is the mass of wheat straw, (g); wPET is the mass of the plastic waste, (g); wO is the mass of the oil, (g); wA is the mass of the asphaltene, (g); wPA is the mass of the preasphaltene, (g); wR is the mass of the residue, (g). Moreover, in order to examine the synergistic effect during co-liquefaction of Jingou coal, wheat straw and plastic waste, the weighted mean values of the co-liquefaction conversion and liquefied products yield are calculated based on the individual liquefaction result of Jingou lignite, wheat straw and plastic waste. The calculated value is obtained as follows: assuming that there is no interaction between Jingou lignite, wheat straw and waste plastic during co-liquefaction, thus the liquefied products yield from co-liquefaction should be equal to the weighted mean value of the individual liquefaction of Jingou lignite, wheat straw and plastic waste, and this value is defined as calculated value (Cal.) [22]. 2.5. TG analysis
2.4. Calculation of liquefaction products yield The yields of liquefaction products were calculated as: Oil yield (O, %) =
Residue yield (R, %) =
wPA × 100% wJG + wWS + wPET
wO × 100%; wJG + wWS + wPET
Co-liquefaction products
Thermo gravimetric (TG) analysis was carried out on a SETARAM TGA92 analyzer. Each time 13.00 mg sample was placed into an alumina crucible and heated from 25 ◦ C to 300 ◦ C at 10 ◦ C/min under nitrogen with the flow rate of 60 ml/min, and then heated to 900 ◦ C at 15 ◦ C/min and finally stayed at 900 ◦ C for 30 min. Moreover, differential thermal gravity (DTG) analysis also was done. 3. Results and discussion 3.1. TG–DTG analysis of lignite, wheat straw and waste plastic and their mixtures
Solid and liquid
Gas
Benzene
Benzene soluble fractions
Benzene insoluble fractions THF
Hexane
Hexane soluble fractions(O)
Hexane insoluble fractions(A)
THF soluble fractions (PA)
THF insoluble fractions (R)
Fig. 1. Fractionation procedure of liquefied product.
Fig. 2 shows the TG/DTG curves of Jingou lignite, wheat straw and plastic waste and their mixtures. From Fig. 2(a) it can be observed that at 200 ◦ C the wheat straw begins to decompose and at 317 ◦ C the weight loss rate is the highest, and about 70% of wheat straw loses when the temperature is 500 ◦ C. The plastic waste (PET) begins to decompose at 400 ◦ C, and the maximum weight loss rate appears at 435 ◦ C; and about 84% of the weight looses when the temperature increases to 500 ◦ C. The weight of lignite first decreases at 100 ◦ C, and then about 45% of the weight looses when the temperature increases from 350 ◦ C to 800 ◦ C, and at 495 ◦ C the weight loss rate is the highest. From Fig. 2(a) one
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Wheat straw PET JG Lignite
Yield ( %)
70 60 50 40 30 20 10 0
O
A
PA
G
TC
Fig. 3. Product distributions from liquefaction of lignite, wheat straw and plastic waste via sub-critical water (initial pressure 2 MPa, temperature: 300 ◦ C, retention time: 30 min).
the asphaltene yield is 21.8% and 4.4%, the preasphaltene yield is 21.8% and 4.4% respectively, while the gas yield is 10.7% and 0.3% respectively. From the distribution of products shown in Fig. 3, one can know that wheat straw liquefy most easily. Compared with lignite, the wheat straw and plastic waste all have higher liquefaction activity at the same conditions, and wheat straw has the highest liquefaction activity. Table 1 shows that the H/C ratio of wheat straw is the highest. Then it can be understood easily why wheat straw has highest hydrogen donation ability, and why it also has highest oil yield and gas yield during liquefaction [9]. Fig. 2. The TG/DTG curves of Jingou lignite, wheat straw and plastic waste and their mixtures.
could infer that wheat straw can liquefy most easily. Compared with lignite, wheat straw and plastic waste all have higher liquefaction activity at the same conditions, and wheat straw has the highest liquefaction activity. Fig. 2(b) shows the TG/DTG curves of the mixture of Jingou lignite, wheat straw and plastic waste with the ratio of 5:4:1. In Fig. 2(b), Exp TG and Exp DTG denote the experimental value of TG and DTG respectively; while Cal TG and Cal DTG denote respectively the corresponding weighted mean value calculated according to the individual TG/DTG curve shown in Fig. 2(a). From Fig. 2(b) it can be observed that there are four processes during co-pyrolysis of lignite, wheat straw and waste plastic; and it also can be seen that the experimental weight loss during co-pyrolysis is lower than the corresponding calculated weight loss. For the DTG curve, the obvious difference appears at 324 ◦ C and 424 ◦ C. At 324 ◦ C, the calculated value of DTG is higher than the experimental value; while at 424 ◦ C, it is just opposite. The result implies that the process of co-pyrolysis of lignite, wheat straw and plastic waste is a complicated process and there are interactions between each other during co-pyrolysis. 3.2. Products distribution during co-liquefaction of lignite, wheat straw and waste plastic in sub-critical water 3.2.1. Product distributions from individual liquefaction of lignite, wheat straw and plastic waste Product distributions from individual liquefaction of Jingou lignite, wheat straw and plastic waste are shown in Fig. 3. From Fig. 3 it can be observed that the total conversion of wheat straw is the highest, and it is 79.2%; the oil yield is 17.4% and the asphaltene yield is 25.2%, the preasphaltene yield is 12.3% and the gas yield is 24.3%. While for plastic waste and lignite, the total conversion is 44.3% and 13.1% respectively; the oil yield is 9.1% and 6.7%, and
3.2.2. Product distributions during co-liquefaction of lignite, wheat straw and plastic waste at different blending ratio Fig. 4(a) shows the product distributions during co-liquefaction of Jingou lignite, wheat straw and plastic waste at different blending ratios at 300 ◦ C, with the residence time is 30 min and the initial nitrogen pressure is 2.0 MPa. From Fig. 4(a) it could be seen that with increasing the wheat straw content from 10% to 50%, the total conversion and the preasphaltene yield decreases, while the asphaltene yield increases slowly. When the blending ratio of Jingou lignite, wheat straw and waste plastic is 5:1:4, the total conversion and the preasphaltene yield are all the highest, but the oil yield is the lowest. When the blending ratio of Jingou lignite, wheat straw and waste plastic is 5:4:1, the oil yield and the gas yield are all the highest; while the total conversion and the preasphaltene yield are all the lowest during co-liquefaction. The results suggest that the ratio of 5:4:1 is the best blending ratio of Jingou lignite, wheat straw and plastic waste to get higher oil yield. Fig. 4(b) shows the comparison of the experimental value (Exp.) and calculated value (Cal.) of the total conversions and the yield of the products. From Fig. 4(b) one could see that during co-liquefaction of Jingou lignite, wheat straw and plastic waste, there exists synergism effect for total conversion and the preasphaltene yield; with increasing the content of wheat straw, the synergism effect becomes lower. While for oil yield, there exists synergism effect only when the ratio of Jingou lignite, wheat straw and plastic waste is 5:4:1, for other ratios, there is negative effect. 3.2.3. Product distributions during co-liquefaction of lignite, wheat straw and plastic waste at different temperature The effect of temperature on product distributions during coliquefaction of Jingou lignite, wheat straw and plastic waste in sub-critical water are also studied, and the initial nitrogen pressure is 2 MPa and the residence time is 30 min. The result is shown in Fig. 5. From Fig. 5 it could be found that with increasing the temperature from 260 ◦ C to 300 ◦ C, the total conversion also increases,
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50 J.G:W.S:PET=5: J.G:W.S:PET=5: J.G:W.S:PET=5: J.G:W.S:PET=5:
60 50
(a)
1: 4 2: 3 2.5: 2.5 4: 1
40
Yield (%)
Yield (%)
385
40 30 20
2MPa 3MPa 4MPa 5MPa
30
20
10
10 0
O
A
PA
G
TC
0
O
A
PA
G
TC
40 35
O A PA G TC
(b)
30
Exp. -Cal. (%)
Fig. 6. Product distributions at different initial pressure from co-liquefaction via sub-critical water (temperature: 300 ◦ C, blending ratio: 5:4:1, time: 30 min).
25 20 15 10 5 0 -5 -10 -15 -20
5:1:4
5:2:3
2:1:1
5:4:1
J:W:P
J:W:P
J:W:P
J:W:P
--
Fig. 4. Product distributions and comparisons of the experimental and calculated value of the product yields and total conversion during co-liquefaction (initial pressure: 2 MPa, temperature: 300 ◦ C, time = 30 min).
and the total conversion is as high as 46.4% when the temperature is 300 ◦ C; and then decreases when the temperature increases from 300 ◦ C to 320 ◦ C. For the oil yield, when temperature increases from 260 ◦ C to 280 ◦ C, oil yield decreases, while when the temperature increases from 280 ◦ C to 320 ◦ C, the oil yield increases. For the asphaltene yield, when the temperature increases, the tendency of the change is just the same as that of the oil yield. For the preasphaltene yield, with increasing the temperature from 260 ◦ C to 280 ◦ C, the preasphaltene yield increases; and then decreases when the temperature increases from 280 ◦ C to 320 ◦ C. For the gas yield, with increasing the temperature from 260 ◦ C to 320 ◦ C,
Fig. 5. Product distributions at different temperatures from co-liquefaction via subcritical water (liquefaction time: 30 min, initial nitrogen pressure: 2 MPa blending ratio: 5:4:1).
the gas yield increases continuously. Literatures [23,24] reported that during liquefaction, coal, wheat straw and waste plastic waste could be broken down to form some fragments firstly and then the fragments were degraded to form smaller compounds by dehydration, dehydrogenation, deoxygenation and decarboxylation. These compounds, once produced, would be rearranged through condensation, cyclization and polyinerization under inert atmosphere or hydrogenation under H2 or H2 -rich gases, leading to formation of new compounds. Increasing temperature will enhance the reaction that materials pyrolyze to form preasphaltene and asphaltene and then preasphaltene and asphaltene decompose to from oil. Thus, the higher the temperature, the higher the oil yield and gas yield. 3.2.4. Product distributions during co-liquefaction of lignite, wheat straw and plastic waste at different initial nitrogen pressure The influences of initial nitrogen pressure on product distributions are also investigated. Fig. 6(a) shows the effect of initial nitrogen pressure on distribution of products during coliquefaction of coal, wheat straw and plastic waste. The total conversion decreases from 46.8% to 40.8% slowly when the pressure increases from 2.0 MPa to 5.0 MPa. The yield of oil increases from 13.8% to 15.8% when the pressure increases from 2.0 MPa to 3.0 MPa, and then decreases from 15.8% to 10.0% when the pressure increases from 3.0 MPa to 5.0 MPa. According to the results obtained from this work, it can be seen that a lower initial nitrogen pressure during co-liquefaction of coal, wheat straw and plastic waste, such as 2 MPa, may get higher total conversion, but at 3 MPa, may get higher oil yield. However, the maximum asphaltene yield which is 23.4% is obtained at 4.0 MPa during co-liquefaction of the three samples. That implies that the initial nitrogen pressure influences differentially on the total conversion and the products yield. Based on the results, if taking into account the technical and economical difficulties that may be conquered at higher pressure, it will be better to do the experiment at lower pressure. 3.2.5. Product distributions during co-liquefaction of lignite, wheat straw and plastic waste when adding different additives The influence of different additives on product distributions during co-liquefaction in sub-critical water is also investigated. The initial pressure is 2 MPa, the temperature is 300 ◦ C and the residence time is 30 min. Fig. 7 shows the product distributions during co-liquefaction when adding different additives. It can be observed from Fig. 7 that adding FeS, Fe2 O3 + S, FeS + S or tourmaline as catalysts during co-liquefaction makes the total conversion and the yield of oil, preasphaltene and asphaltene all decrease. However, from the results one also can see that compare with the oil yield when adding FeS, Fe2 O3 + S or FeS + S, adding tourmaline could get
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50
Without catalyst Adding tourmaline Adding Fe2O3 + S
Yield (%)
40
Adding FeS+S Adding FeS
30
20
10
0
O
A
PA
G
TC
Fig. 7. Product distributions from co-liquefaction via sub-critical water when adding different additives (temperature: 300 ◦ C, blending ratio: 5:4:1, residence time: 30 min, initial pressure: 2 MPa).
residues from co-liquefaction in sub-critical water is higher than that from co-liquefaction in tetralin. The results just coincide with the results of the total conversion in Figs. 7 and 8. Adding the additives made the total conversion decrease, which means that part of the volatile material did not evolve, and then caused the ash content in the residue lower. It is just the same for the ash content of the residue from co-liquefaction in tetralin. Fig. 9 is the TG/DTG curves of the residues from co-liquefacton in different solvents. From Fig. 9 one could see that the weight loss and the weigh loss rate of the residue from co-liquefaction in tetralin were all higher that that of the residue from co-liquefaction in sub-critical water, and it just coincided with the results of the total conversion. The higher the total conversion, the lower the weight loss and weight loss rate of the residue. Fig. 10 is the
50 Via tetralin Via sub-critical water 40
Yield (%)
Fig. 9. The TG/DTG curves of residues when co-liquefacton in differet solvents.
30
20
10
0 O
A
PA
G
TC
Fig. 8. The distribution of product during co-liquefaction in different solvents.
higher oil yield. Shui et al. [10] found that the activity of catalysts in co-liquefaction of rice straw and coal is different from that in individual liquefaction of coal and rice straw. Our result shows that the catalysts in co-liquefaction of coal and biomass could not show catalysis in co-liquefaction of lignite, wheat straw and plastic waste; and new catalyst should be explored for co-liquefaction of coal, biomass and plastic waste. 3.3. Character of the products from co-liquefaction of lignite, wheat straw and waste plastic in sub-critical water 3.3.1. Character of the solid residue Table 2 is the ash content of the residues at different conditions. From Table 2 one could see that the ash content of the residues when adding additives (FeS, Fe2 O3 + S or tourmaline) during co-liquefaction in sub-critical water is lower than that without additive; and it also could be seen that the ash content of the Table 2 Ash content of the residues (300 ◦ C, 2 MPa, 30 min). Solvent
Additive
Aad (wt./%)
Sub-critical water Tetralin Sub-critical water Sub-critical water Sub-critical water
a
82.66 77.52 79.61 79.50 79.53
a
Without additive.
a
FeS Fe2 O3 + S Tourmaline
Fig. 10. The TG/DTG curves of residue when adding different additives during coliquefaction.
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Fig. 11. SEM micrographs of the samples and the residues.
TG/DTG curves of the residues when adding different additives during co-liquefaction in sub-critical water. From Fig. 10 one could see that the weight loss and weight loss rate of the residues changes slightly when adding additives. In order to know further the detail of the residues, SEM and FTIR analysis were also done. Fig. 11 is the SEM micrographs of the residues from individual liquefaction of
the lignite, wheat straw and plastic wastes and co-liquefaction of the three samples, with the ratio of lignite, wheat straw and plastic waste is 2:1:1. From Fig. 11 one could know that the surface topography of the samples changes obviously during co-liquefaction. Fig. 12 is the FTIR spectra of the residues at different conditions. From Fig. 12 one could know that the functional group of the
Table 3 Ultimate analysis of preasphaltenes. Preasphaltene Solvent
Additive
Sub-critical water Tetranap Sub-critical water Sub-critical water Sub-critical water
b
a b
By difference. Without additive.
b
FeS Fe2 O3 + S Tourmaline
C (%)
H (%)
O (%)a
N (%)
S (%)
H/C
68.800 71.775 63.935 66.125 64.560
7.149 7.692 5.503 6.344 5.804
23.554 19.695 30.013 26.404 29.214
0.345 0.490 0.300 0.832 0.265
0.152 0.348 0.249 0.295 0.157
1.247 1.286 1.033 1.151 1.079
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Table 4 Various parameters of the oil based on 1 H NMR. Sample
Solvent
Additive
ω(H␣ )/%
ω(H )/%
ω(H␥ )/%
ω(Har )/%
WS JG PET JG:WS:PET (5:4:1) JG:WS:PET (5:4:1) JG:WS:PET (5:4:1) JG:WS:PET (5:4:1) JG:WS:PET (5:4:1)
Sub-critical water Sub-critical water Sub-critical water Sub-critical water Sub-critical water Sub-critical water Sub-critical water Tetranap
– – – – Tourmaline FeS Fe2 O3 + S –
10.74 2.39 2.38 7.78 1.43 1.98 6.79 3.81
57.99 71.60 67.85 74.52 73.38 73.02 66.87 74.94
23.87 23.86 28.57 8.05 22.46 24.75 11.81 24.48
7.40 2.15 1.20 14.64 0.73 0.25 14.53 0.57
residue from co-liquefaction via sub-critical water is different from that which is from co-liquefaction via tetralin. Moreover, adding additives also could change the functional group of the residue.
3.3.3. Analysis of the oil Table 4 is the various parameters based on 1 H NMR of the oil. From Table 4 one could know that adding tourmaline and FeS in sub-critical water can make the content of Har lower, while it makes the content of H higher. That is to say, adding tourmaline and FeS during co-liquefaction of lignite, wheat straw and plastic waste in sub-critical water could make the quality of the oil better.
1 2
T (%)
3 4 5
2
T (%)
3.3.2. Analysis of the preasphaltene Table 3 is the result of ultimate analysis of preasphaltenes. From Table 3 one could see that compare with the oxygen content of the preasphaltene from tetranap, the oxygen content of the preasphaltene from sub-critical water is higher, that is to say co-liquefaction in sub-critical water is good for more oxygen to turn into preasphaltenes. Moreover, adding FeS, Fe2 O3 + S or tourmaline during co-liquefaction in sub-critical water also makes the oxygen content higher, which means that adding FeS, Fe2 O3 + S or tourmaline during co-liquefaction in sub-critical water could make more oxygen to turn into preasphaltenes instead of oil. Furthermore, one also could see that adding FeS, Fe2 O3 + S or tourmaline could make the H/C ratio of the preasphaltenes lower. Fig. 13 is the FTIR spectra of the preasphaltenes at different conditions. From Fig. 13 one could see that the FTIR spectra of the residue from co-liquefaction in tetranap is different from that of the residue from co-liquefaction in sub-critical water at 3400 cm−1 and 2920 cm−1 , which is the bond of OH and C H. Moreover, the spectra of the residue from co-liquefaction in sub-critical water when adding FeS, Fe2 O3 + S or tourmaline becomes stronger at 3400 cm−1 and 2920 cm−1 , and it just coincides with the results in Table 3.
1
3 4 5
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm ) Fig. 13. FTIR spectra of the preaspaltenes at different conditions.
4. Conclusions The result shows that the process of co-pyrolysis of lignite, wheat straw and plastic waste is a complicated process and there are interactions between each other during co-pyrolysis. Just as copyrolysis, co-liquefaction of lignite, wheat straw and plastic waste also is very complicated. During the process of co-liquefaction, coal, wheat straw and waste plastic waste could be broken down to form some fragments; at the same time wheat straw and plastic waste also could supply hydrogen. Increasing temperature will enhance the reaction that materials pyrolyze to form preasphaltene and asphaltene and then preasphaltene and asphaltene decompose to form oil. Thus, the higher the temperature, the higher the oil yield and gas yield. The result also indicates that when the blending ratio of Jingou lignite, wheat straw and plastic waste is 5:4:1, there exists synergism effect for oil yield, and the oil yield and the gas yield are all the highest at this ratio. Moreover, the initial nitrogen pressure influences differentially on the products yield and there is an optimal initial nitrogen pressure for each product to get higher yield. Interestingly, compare with the traditional catalyst for liquefaction, adding tourmaline during co-liquefaction of lignite, wheat straw and plastic waste in sub-critical water could get higher oil yield, higher total conversion and higher quality oil.
Acknowledgements
4000
3500
3000
2500
2000
1500
1000
-1
Wavenumber (cm ) Fig. 12. FTIR spectra of the residues at different conditions.
500
Financial supports of this work by Research Fund for the Doctoral Program of Higher Education for new teachers of China (20091404120002), Shanxi Province Science Foundation for Youths of China (2011021008-1) and the Soft Science Program of Shanxi Province (2011041015-01) are gratefully acknowledged.
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Contents lists available at ScienceDirect
Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap
Hydrothermal liquefaction of lignite, wheat straw and plastic waste in sub-critical water for oil: Product distribution Baofeng Wang ∗ , Yaru Huang, Jinjun Zhang School of Chemistry & Material Science, Shanxi Normal University, Linfen 041004, PR China
a r t i c l e
i n f o
Article history: Received 19 June 2014 Accepted 14 October 2014 Available online 23 October 2014 Keywords: Lignite Wheat straw Plastic waste Hydrothermal liquefaction Sub-critical water
a b s t r a c t Product distribution and character of the products during hydrothermal liquefaction of Jingou lignite, wheat straw and plastic waste in sub-critical water are investigated in an autoclave. The effects of blending ratio of lignite, wheat straw and plastic waste, temperature, initial nitrogen pressure and additives on product distributions are also studied. The results indicate that blending ratio of lignite, wheat straw and plastic waste, liquefaction temperature, initial pressure and additives all could influence the product distributions during hydrothermal liquefaction. When the blending ratio of Jingou lignite, wheat straw and plastic waste is 5:4:1, there exists synergism effect for oil yield, and the oil yield and the gas yield are all the highest at this ratio. The total conversion increases when the temperature increases from 260 ◦ C to 300 ◦ C and then decreases. For the oil yield, when the temperature increases from 260 ◦ C to 280 ◦ C, oil yield decreases, while when the temperature increases from 280 ◦ C to 320 ◦ C, the oil yield increases. Moreover, adding tourmaline during hydrothermal liquefaction of lignite, wheat straw and plastic waste could get higher oil yield, higher total conversion and higher quality oil than that when adding the traditional catalyst. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Coal liquefaction can make clean coal into oil, and it also can reduce pollution. However, up to now, the cost of coal liquefaction is still very high. Biomass, such as wheat straw, is a kind of cheap, abundant and renewable energy source [1–5]. Furthermore, plastic wastes are also abundant. However, there are many difficulties for people to use them efficiently and cleanly and most of the biomass and plastic wastes are abandoned. The main processing mode today for plastic wastes is incineration or landfill deposition [6,7]. As we have known, the major elements of plastic wastes and biomass are carbon and hydrogen, and they have higher H/C ratio than coal. Then they can be used as rich hydrogen additive during coal liquefaction [8–11]. Moreover, the traditional solvent used in liquefaction is tetranap, which is one of the organic solvent. Using tetranap not only makes the cost high, but also harms the human beings. As we have known, water is cheap and clean, and it can form a single phase with liquefied products in subcritical state; moreover, it also can separate naturally from hydrophobic products as a different phase at normal temperatures and pressures [12–18]. Furthermore, hydrogen supply during sub-critical water
extraction is effective for obtaining compounds with low oxygen content, but hydrogen is expensive. Fortunately, biomass and plastic waste can be cheap sources of hydrogen when they are added in sub-critical water. Previous studies showed that there exists synergistic effect in the co-liquefaction of coal and rice straw [19]; and literatures also showed that co-liquefaction of plastic wastes with aromatic-rich materials such as coal could yield synergistic effects and increase the production of oil [20]. Besides that, studies also showed that the addition of biomass to plastic waste high density polyethylene (HDPE) liquefaction could make the reaction conditions milder, and enhance the conversion of HDPE at lower temperature, implying the synergistic effect of biomass and HDPE [21]. Thus, co-liquefaction of coal, biomass and plastic waste in subcritical water is proposed. In this paper, the products distribution during co-liquefaction of lignite, wheat straw and plastic waste was investigated, and the characters of the products were also studied. This study will help us to understand further the process and mechanism of co-liquefaction of coal, biomass and plastic waste. 2. Materials and methods 2.1. Samples
∗ Corresponding author. Tel.: +86 357 205 1192; fax: +86 357 209 2427. E-mail address: [email protected] (B. Wang). http://dx.doi.org/10.1016/j.jaap.2014.10.004 0165-2370/© 2014 Elsevier B.V. All rights reserved.
Jingou lignite (JG), wheat straw (WS) and plastic waste polyethylene terephthalate (PET) we used are all from Linfen in Shanxi
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Table 1 Proximate and ultimate analysis of the coal, wheat straw and plastic waste. Sample
Proximate analysis (wt.%)
Jingou lignite Wheat straw Plastic waste a
Ultimate analysis (wt.%, ad)
Mad
Aad
Vad
Vdaf
FCad
C
H
N
Oa
S
11.70 7.94 0.14
25.75 5.33 0.04
22.45 77.26 90.8
32.43 89.08 90.96
7.67 9.47 9.02
49.66 40.48 61.88
3.06 5.60 4.29
0.83 0.85 0.03
43.58 52.93 33.76
2.87 0.14 0.04
By difference.
Province of China. The samples were all first air-dried and then crushed, grounded and sieved from 0.15 mm to 0.25 mm. Table 1 is the results of proximate and ultimate analysis of the coal, wheat straw and plastic waste.
Asphaltene yield (A, %) =
wA × 100% wJG + wWS + wPET
Preasphaltene yield (PA, %) = 2.2. Experimental setup and procedure The co-liquefaction experiments were carried out in a 250 ml autoclave. Each time 10.00 g JG lignite, wheat straw, plastic waste, or mixtures of the three samples were put into the reactor together with 80 ml deionized water. Before the liquefaction experiment, the reactor was filled with nitrogen to the desired initial pressure (2.0–5.0 MPa) and sealed; then the reactor was heated to the desired temperature at 10 ◦ C/min by a furnace and then stayed for required time. After reaction the reactor was cooled to room temperature and depressurized to atmospheric pressure. The residue was taken out, dried and further analyzed. 2.3. Fractionation of liquefaction products The gas products were analyzed by Algien 7890 A gas chromatograph. The solid and liquid products were extracted by hexane, benzene and tetrahydrofuran (THF), here the reagents (bought from Guangfu Fine Chemical Research Institute of Tianjin, China) are all analytically pure agents. After extraction, the products were separated into oils (hexane soluble), asphaltene (hexane insoluble but benzene soluble), preasphaltene (benzene insoluble but tetrahydrofuran soluble), and residues (tetrahydrofuran insoluble) as shown in Fig. 1 [8].
wR × 100% wJG + wWS + wPET
Gas yield (G, %) = 100% − (O + A + PA + R)% Total conversion (TC,%) = 100% − R% where wJG is the mass of Jingou lignite, (g); wWS is the mass of wheat straw, (g); wPET is the mass of the plastic waste, (g); wO is the mass of the oil, (g); wA is the mass of the asphaltene, (g); wPA is the mass of the preasphaltene, (g); wR is the mass of the residue, (g). Moreover, in order to examine the synergistic effect during co-liquefaction of Jingou coal, wheat straw and plastic waste, the weighted mean values of the co-liquefaction conversion and liquefied products yield are calculated based on the individual liquefaction result of Jingou lignite, wheat straw and plastic waste. The calculated value is obtained as follows: assuming that there is no interaction between Jingou lignite, wheat straw and waste plastic during co-liquefaction, thus the liquefied products yield from co-liquefaction should be equal to the weighted mean value of the individual liquefaction of Jingou lignite, wheat straw and plastic waste, and this value is defined as calculated value (Cal.) [22]. 2.5. TG analysis
2.4. Calculation of liquefaction products yield The yields of liquefaction products were calculated as: Oil yield (O, %) =
Residue yield (R, %) =
wPA × 100% wJG + wWS + wPET
wO × 100%; wJG + wWS + wPET
Co-liquefaction products
Thermo gravimetric (TG) analysis was carried out on a SETARAM TGA92 analyzer. Each time 13.00 mg sample was placed into an alumina crucible and heated from 25 ◦ C to 300 ◦ C at 10 ◦ C/min under nitrogen with the flow rate of 60 ml/min, and then heated to 900 ◦ C at 15 ◦ C/min and finally stayed at 900 ◦ C for 30 min. Moreover, differential thermal gravity (DTG) analysis also was done. 3. Results and discussion 3.1. TG–DTG analysis of lignite, wheat straw and waste plastic and their mixtures
Solid and liquid
Gas
Benzene
Benzene soluble fractions
Benzene insoluble fractions THF
Hexane
Hexane soluble fractions(O)
Hexane insoluble fractions(A)
THF soluble fractions (PA)
THF insoluble fractions (R)
Fig. 1. Fractionation procedure of liquefied product.
Fig. 2 shows the TG/DTG curves of Jingou lignite, wheat straw and plastic waste and their mixtures. From Fig. 2(a) it can be observed that at 200 ◦ C the wheat straw begins to decompose and at 317 ◦ C the weight loss rate is the highest, and about 70% of wheat straw loses when the temperature is 500 ◦ C. The plastic waste (PET) begins to decompose at 400 ◦ C, and the maximum weight loss rate appears at 435 ◦ C; and about 84% of the weight looses when the temperature increases to 500 ◦ C. The weight of lignite first decreases at 100 ◦ C, and then about 45% of the weight looses when the temperature increases from 350 ◦ C to 800 ◦ C, and at 495 ◦ C the weight loss rate is the highest. From Fig. 2(a) one
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80
Wheat straw PET JG Lignite
Yield ( %)
70 60 50 40 30 20 10 0
O
A
PA
G
TC
Fig. 3. Product distributions from liquefaction of lignite, wheat straw and plastic waste via sub-critical water (initial pressure 2 MPa, temperature: 300 ◦ C, retention time: 30 min).
the asphaltene yield is 21.8% and 4.4%, the preasphaltene yield is 21.8% and 4.4% respectively, while the gas yield is 10.7% and 0.3% respectively. From the distribution of products shown in Fig. 3, one can know that wheat straw liquefy most easily. Compared with lignite, the wheat straw and plastic waste all have higher liquefaction activity at the same conditions, and wheat straw has the highest liquefaction activity. Table 1 shows that the H/C ratio of wheat straw is the highest. Then it can be understood easily why wheat straw has highest hydrogen donation ability, and why it also has highest oil yield and gas yield during liquefaction [9]. Fig. 2. The TG/DTG curves of Jingou lignite, wheat straw and plastic waste and their mixtures.
could infer that wheat straw can liquefy most easily. Compared with lignite, wheat straw and plastic waste all have higher liquefaction activity at the same conditions, and wheat straw has the highest liquefaction activity. Fig. 2(b) shows the TG/DTG curves of the mixture of Jingou lignite, wheat straw and plastic waste with the ratio of 5:4:1. In Fig. 2(b), Exp TG and Exp DTG denote the experimental value of TG and DTG respectively; while Cal TG and Cal DTG denote respectively the corresponding weighted mean value calculated according to the individual TG/DTG curve shown in Fig. 2(a). From Fig. 2(b) it can be observed that there are four processes during co-pyrolysis of lignite, wheat straw and waste plastic; and it also can be seen that the experimental weight loss during co-pyrolysis is lower than the corresponding calculated weight loss. For the DTG curve, the obvious difference appears at 324 ◦ C and 424 ◦ C. At 324 ◦ C, the calculated value of DTG is higher than the experimental value; while at 424 ◦ C, it is just opposite. The result implies that the process of co-pyrolysis of lignite, wheat straw and plastic waste is a complicated process and there are interactions between each other during co-pyrolysis. 3.2. Products distribution during co-liquefaction of lignite, wheat straw and waste plastic in sub-critical water 3.2.1. Product distributions from individual liquefaction of lignite, wheat straw and plastic waste Product distributions from individual liquefaction of Jingou lignite, wheat straw and plastic waste are shown in Fig. 3. From Fig. 3 it can be observed that the total conversion of wheat straw is the highest, and it is 79.2%; the oil yield is 17.4% and the asphaltene yield is 25.2%, the preasphaltene yield is 12.3% and the gas yield is 24.3%. While for plastic waste and lignite, the total conversion is 44.3% and 13.1% respectively; the oil yield is 9.1% and 6.7%, and
3.2.2. Product distributions during co-liquefaction of lignite, wheat straw and plastic waste at different blending ratio Fig. 4(a) shows the product distributions during co-liquefaction of Jingou lignite, wheat straw and plastic waste at different blending ratios at 300 ◦ C, with the residence time is 30 min and the initial nitrogen pressure is 2.0 MPa. From Fig. 4(a) it could be seen that with increasing the wheat straw content from 10% to 50%, the total conversion and the preasphaltene yield decreases, while the asphaltene yield increases slowly. When the blending ratio of Jingou lignite, wheat straw and waste plastic is 5:1:4, the total conversion and the preasphaltene yield are all the highest, but the oil yield is the lowest. When the blending ratio of Jingou lignite, wheat straw and waste plastic is 5:4:1, the oil yield and the gas yield are all the highest; while the total conversion and the preasphaltene yield are all the lowest during co-liquefaction. The results suggest that the ratio of 5:4:1 is the best blending ratio of Jingou lignite, wheat straw and plastic waste to get higher oil yield. Fig. 4(b) shows the comparison of the experimental value (Exp.) and calculated value (Cal.) of the total conversions and the yield of the products. From Fig. 4(b) one could see that during co-liquefaction of Jingou lignite, wheat straw and plastic waste, there exists synergism effect for total conversion and the preasphaltene yield; with increasing the content of wheat straw, the synergism effect becomes lower. While for oil yield, there exists synergism effect only when the ratio of Jingou lignite, wheat straw and plastic waste is 5:4:1, for other ratios, there is negative effect. 3.2.3. Product distributions during co-liquefaction of lignite, wheat straw and plastic waste at different temperature The effect of temperature on product distributions during coliquefaction of Jingou lignite, wheat straw and plastic waste in sub-critical water are also studied, and the initial nitrogen pressure is 2 MPa and the residence time is 30 min. The result is shown in Fig. 5. From Fig. 5 it could be found that with increasing the temperature from 260 ◦ C to 300 ◦ C, the total conversion also increases,
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70
50 J.G:W.S:PET=5: J.G:W.S:PET=5: J.G:W.S:PET=5: J.G:W.S:PET=5:
60 50
(a)
1: 4 2: 3 2.5: 2.5 4: 1
40
Yield (%)
Yield (%)
385
40 30 20
2MPa 3MPa 4MPa 5MPa
30
20
10
10 0
O
A
PA
G
TC
0
O
A
PA
G
TC
40 35
O A PA G TC
(b)
30
Exp. -Cal. (%)
Fig. 6. Product distributions at different initial pressure from co-liquefaction via sub-critical water (temperature: 300 ◦ C, blending ratio: 5:4:1, time: 30 min).
25 20 15 10 5 0 -5 -10 -15 -20
5:1:4
5:2:3
2:1:1
5:4:1
J:W:P
J:W:P
J:W:P
J:W:P
--
Fig. 4. Product distributions and comparisons of the experimental and calculated value of the product yields and total conversion during co-liquefaction (initial pressure: 2 MPa, temperature: 300 ◦ C, time = 30 min).
and the total conversion is as high as 46.4% when the temperature is 300 ◦ C; and then decreases when the temperature increases from 300 ◦ C to 320 ◦ C. For the oil yield, when temperature increases from 260 ◦ C to 280 ◦ C, oil yield decreases, while when the temperature increases from 280 ◦ C to 320 ◦ C, the oil yield increases. For the asphaltene yield, when the temperature increases, the tendency of the change is just the same as that of the oil yield. For the preasphaltene yield, with increasing the temperature from 260 ◦ C to 280 ◦ C, the preasphaltene yield increases; and then decreases when the temperature increases from 280 ◦ C to 320 ◦ C. For the gas yield, with increasing the temperature from 260 ◦ C to 320 ◦ C,
Fig. 5. Product distributions at different temperatures from co-liquefaction via subcritical water (liquefaction time: 30 min, initial nitrogen pressure: 2 MPa blending ratio: 5:4:1).
the gas yield increases continuously. Literatures [23,24] reported that during liquefaction, coal, wheat straw and waste plastic waste could be broken down to form some fragments firstly and then the fragments were degraded to form smaller compounds by dehydration, dehydrogenation, deoxygenation and decarboxylation. These compounds, once produced, would be rearranged through condensation, cyclization and polyinerization under inert atmosphere or hydrogenation under H2 or H2 -rich gases, leading to formation of new compounds. Increasing temperature will enhance the reaction that materials pyrolyze to form preasphaltene and asphaltene and then preasphaltene and asphaltene decompose to from oil. Thus, the higher the temperature, the higher the oil yield and gas yield. 3.2.4. Product distributions during co-liquefaction of lignite, wheat straw and plastic waste at different initial nitrogen pressure The influences of initial nitrogen pressure on product distributions are also investigated. Fig. 6(a) shows the effect of initial nitrogen pressure on distribution of products during coliquefaction of coal, wheat straw and plastic waste. The total conversion decreases from 46.8% to 40.8% slowly when the pressure increases from 2.0 MPa to 5.0 MPa. The yield of oil increases from 13.8% to 15.8% when the pressure increases from 2.0 MPa to 3.0 MPa, and then decreases from 15.8% to 10.0% when the pressure increases from 3.0 MPa to 5.0 MPa. According to the results obtained from this work, it can be seen that a lower initial nitrogen pressure during co-liquefaction of coal, wheat straw and plastic waste, such as 2 MPa, may get higher total conversion, but at 3 MPa, may get higher oil yield. However, the maximum asphaltene yield which is 23.4% is obtained at 4.0 MPa during co-liquefaction of the three samples. That implies that the initial nitrogen pressure influences differentially on the total conversion and the products yield. Based on the results, if taking into account the technical and economical difficulties that may be conquered at higher pressure, it will be better to do the experiment at lower pressure. 3.2.5. Product distributions during co-liquefaction of lignite, wheat straw and plastic waste when adding different additives The influence of different additives on product distributions during co-liquefaction in sub-critical water is also investigated. The initial pressure is 2 MPa, the temperature is 300 ◦ C and the residence time is 30 min. Fig. 7 shows the product distributions during co-liquefaction when adding different additives. It can be observed from Fig. 7 that adding FeS, Fe2 O3 + S, FeS + S or tourmaline as catalysts during co-liquefaction makes the total conversion and the yield of oil, preasphaltene and asphaltene all decrease. However, from the results one also can see that compare with the oil yield when adding FeS, Fe2 O3 + S or FeS + S, adding tourmaline could get
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50
Without catalyst Adding tourmaline Adding Fe2O3 + S
Yield (%)
40
Adding FeS+S Adding FeS
30
20
10
0
O
A
PA
G
TC
Fig. 7. Product distributions from co-liquefaction via sub-critical water when adding different additives (temperature: 300 ◦ C, blending ratio: 5:4:1, residence time: 30 min, initial pressure: 2 MPa).
residues from co-liquefaction in sub-critical water is higher than that from co-liquefaction in tetralin. The results just coincide with the results of the total conversion in Figs. 7 and 8. Adding the additives made the total conversion decrease, which means that part of the volatile material did not evolve, and then caused the ash content in the residue lower. It is just the same for the ash content of the residue from co-liquefaction in tetralin. Fig. 9 is the TG/DTG curves of the residues from co-liquefacton in different solvents. From Fig. 9 one could see that the weight loss and the weigh loss rate of the residue from co-liquefaction in tetralin were all higher that that of the residue from co-liquefaction in sub-critical water, and it just coincided with the results of the total conversion. The higher the total conversion, the lower the weight loss and weight loss rate of the residue. Fig. 10 is the
50 Via tetralin Via sub-critical water 40
Yield (%)
Fig. 9. The TG/DTG curves of residues when co-liquefacton in differet solvents.
30
20
10
0 O
A
PA
G
TC
Fig. 8. The distribution of product during co-liquefaction in different solvents.
higher oil yield. Shui et al. [10] found that the activity of catalysts in co-liquefaction of rice straw and coal is different from that in individual liquefaction of coal and rice straw. Our result shows that the catalysts in co-liquefaction of coal and biomass could not show catalysis in co-liquefaction of lignite, wheat straw and plastic waste; and new catalyst should be explored for co-liquefaction of coal, biomass and plastic waste. 3.3. Character of the products from co-liquefaction of lignite, wheat straw and waste plastic in sub-critical water 3.3.1. Character of the solid residue Table 2 is the ash content of the residues at different conditions. From Table 2 one could see that the ash content of the residues when adding additives (FeS, Fe2 O3 + S or tourmaline) during co-liquefaction in sub-critical water is lower than that without additive; and it also could be seen that the ash content of the Table 2 Ash content of the residues (300 ◦ C, 2 MPa, 30 min). Solvent
Additive
Aad (wt./%)
Sub-critical water Tetralin Sub-critical water Sub-critical water Sub-critical water
a
82.66 77.52 79.61 79.50 79.53
a
Without additive.
a
FeS Fe2 O3 + S Tourmaline
Fig. 10. The TG/DTG curves of residue when adding different additives during coliquefaction.
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Fig. 11. SEM micrographs of the samples and the residues.
TG/DTG curves of the residues when adding different additives during co-liquefaction in sub-critical water. From Fig. 10 one could see that the weight loss and weight loss rate of the residues changes slightly when adding additives. In order to know further the detail of the residues, SEM and FTIR analysis were also done. Fig. 11 is the SEM micrographs of the residues from individual liquefaction of
the lignite, wheat straw and plastic wastes and co-liquefaction of the three samples, with the ratio of lignite, wheat straw and plastic waste is 2:1:1. From Fig. 11 one could know that the surface topography of the samples changes obviously during co-liquefaction. Fig. 12 is the FTIR spectra of the residues at different conditions. From Fig. 12 one could know that the functional group of the
Table 3 Ultimate analysis of preasphaltenes. Preasphaltene Solvent
Additive
Sub-critical water Tetranap Sub-critical water Sub-critical water Sub-critical water
b
a b
By difference. Without additive.
b
FeS Fe2 O3 + S Tourmaline
C (%)
H (%)
O (%)a
N (%)
S (%)
H/C
68.800 71.775 63.935 66.125 64.560
7.149 7.692 5.503 6.344 5.804
23.554 19.695 30.013 26.404 29.214
0.345 0.490 0.300 0.832 0.265
0.152 0.348 0.249 0.295 0.157
1.247 1.286 1.033 1.151 1.079
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Table 4 Various parameters of the oil based on 1 H NMR. Sample
Solvent
Additive
ω(H␣ )/%
ω(H )/%
ω(H␥ )/%
ω(Har )/%
WS JG PET JG:WS:PET (5:4:1) JG:WS:PET (5:4:1) JG:WS:PET (5:4:1) JG:WS:PET (5:4:1) JG:WS:PET (5:4:1)
Sub-critical water Sub-critical water Sub-critical water Sub-critical water Sub-critical water Sub-critical water Sub-critical water Tetranap
– – – – Tourmaline FeS Fe2 O3 + S –
10.74 2.39 2.38 7.78 1.43 1.98 6.79 3.81
57.99 71.60 67.85 74.52 73.38 73.02 66.87 74.94
23.87 23.86 28.57 8.05 22.46 24.75 11.81 24.48
7.40 2.15 1.20 14.64 0.73 0.25 14.53 0.57
residue from co-liquefaction via sub-critical water is different from that which is from co-liquefaction via tetralin. Moreover, adding additives also could change the functional group of the residue.
3.3.3. Analysis of the oil Table 4 is the various parameters based on 1 H NMR of the oil. From Table 4 one could know that adding tourmaline and FeS in sub-critical water can make the content of Har lower, while it makes the content of H higher. That is to say, adding tourmaline and FeS during co-liquefaction of lignite, wheat straw and plastic waste in sub-critical water could make the quality of the oil better.
1 2
T (%)
3 4 5
2
T (%)
3.3.2. Analysis of the preasphaltene Table 3 is the result of ultimate analysis of preasphaltenes. From Table 3 one could see that compare with the oxygen content of the preasphaltene from tetranap, the oxygen content of the preasphaltene from sub-critical water is higher, that is to say co-liquefaction in sub-critical water is good for more oxygen to turn into preasphaltenes. Moreover, adding FeS, Fe2 O3 + S or tourmaline during co-liquefaction in sub-critical water also makes the oxygen content higher, which means that adding FeS, Fe2 O3 + S or tourmaline during co-liquefaction in sub-critical water could make more oxygen to turn into preasphaltenes instead of oil. Furthermore, one also could see that adding FeS, Fe2 O3 + S or tourmaline could make the H/C ratio of the preasphaltenes lower. Fig. 13 is the FTIR spectra of the preasphaltenes at different conditions. From Fig. 13 one could see that the FTIR spectra of the residue from co-liquefaction in tetranap is different from that of the residue from co-liquefaction in sub-critical water at 3400 cm−1 and 2920 cm−1 , which is the bond of OH and C H. Moreover, the spectra of the residue from co-liquefaction in sub-critical water when adding FeS, Fe2 O3 + S or tourmaline becomes stronger at 3400 cm−1 and 2920 cm−1 , and it just coincides with the results in Table 3.
1
3 4 5
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm ) Fig. 13. FTIR spectra of the preaspaltenes at different conditions.
4. Conclusions The result shows that the process of co-pyrolysis of lignite, wheat straw and plastic waste is a complicated process and there are interactions between each other during co-pyrolysis. Just as copyrolysis, co-liquefaction of lignite, wheat straw and plastic waste also is very complicated. During the process of co-liquefaction, coal, wheat straw and waste plastic waste could be broken down to form some fragments; at the same time wheat straw and plastic waste also could supply hydrogen. Increasing temperature will enhance the reaction that materials pyrolyze to form preasphaltene and asphaltene and then preasphaltene and asphaltene decompose to form oil. Thus, the higher the temperature, the higher the oil yield and gas yield. The result also indicates that when the blending ratio of Jingou lignite, wheat straw and plastic waste is 5:4:1, there exists synergism effect for oil yield, and the oil yield and the gas yield are all the highest at this ratio. Moreover, the initial nitrogen pressure influences differentially on the products yield and there is an optimal initial nitrogen pressure for each product to get higher yield. Interestingly, compare with the traditional catalyst for liquefaction, adding tourmaline during co-liquefaction of lignite, wheat straw and plastic waste in sub-critical water could get higher oil yield, higher total conversion and higher quality oil.
Acknowledgements
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3500
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2500
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Wavenumber (cm ) Fig. 12. FTIR spectra of the residues at different conditions.
500
Financial supports of this work by Research Fund for the Doctoral Program of Higher Education for new teachers of China (20091404120002), Shanxi Province Science Foundation for Youths of China (2011021008-1) and the Soft Science Program of Shanxi Province (2011041015-01) are gratefully acknowledged.
B. Wang et al. / Journal of Analytical and Applied Pyrolysis 110 (2014) 382–389
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