Pyrolytic characteristics of sweet potato vine

Accepted Manuscript Pyrolytic characteristics of sweet potato vine Tipeng Wang, Xiaochen Dong, Zaixing Jin, Wenjing Su, Xiaoning Ye, Qiang Lu PII: DOI: Reference:

S0960-8524(15)00682-3 http://dx.doi.org/10.1016/j.biortech.2015.05.018 BITE 14989

To appear in:

Bioresource Technology

Received Date: Revised Date: Accepted Date:

7 April 2015 7 May 2015 8 May 2015

Please cite this article as: Wang, T., Dong, X., Jin, Z., Su, W., Ye, X., Lu, Q., Pyrolytic characteristics of sweet potato vine, Bioresource Technology (2015), doi: http://dx.doi.org/10.1016/j.biortech.2015.05.018

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Pyrolytic characteristics of sweet potato vine Tipeng Wang*, Xiaochen Dong, Zaixing Jin, Wenjing Su, Xiaoning Ye, Qiang Lu* National Engineering Laboratory for Biomass Power Generation Equipment, North China Electric Power University, 2 Beinong Lu Beijing 102206, China *

Corresponding author.

E-mail address: [email protected] (T. Wang), [email protected] ( Q. Lu) Tel: +86-10-61772060; Fax: +86-10-61772031

Abstract. To utilized biomass for optimum application, sweet potato vine (SPV) was studied on its pyrolytic characteristics by TGA and Py-GC/MS analysis as a representative of biomass with low lignin content and high extractives content. Results indicated that lignin, cellulose, hemicellulose and extractives contents were 7.85 wt. %, 33.01 wt. %, 12.25 wt. % and 37.12 wt. %, respectively. In bio-oil, sugars content firstly increased from 8.76 wt. % (350oC) to 13.97 wt. % (400oC) and then decreased to 9.19 wt. % (500oC); linear carbonyls and linear acids contents decreased from 16.58 wt. % and 17.45 wt. % to 5.26 wt. % and 4.03 wt. %, respectively; furans content increased from 7.10 wt. % to 15.47 wt. %. The content 11.86 wt. % of levoglucose at 400oC, 15.41 wt. % of acetic acid at 350oC and 6.94 wt. % of furfural at 500oC suggested good pyrolysis selectivity of SPV. Keywords. Pyrolysis; Py-GC/MS; Furfural; Sweet potato vine; Levoglucose

1. Introduction To reduce the greenhouse gas emissions and improve national energy security, utilization of renewable energy had been given increasing attentions in past decades. As an important renewable energy, biomass could be converted into clean fuels and

Tipeng Wang, Xiaochen Dong, Zaixing Jin, Wenjing Su, Xiaoning Ye, Qiang Lu

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chemicals by thermochemical processes. Fast pyrolysis was one of the most promising thermochemical technologies and could convert the biomass into solid (bio-char) and volatile (bio-gas and bio-oil) products. It is well known that biomass mainly consists of cellulose, lignin, hemicellulose, extractives and minerals. In pyrolysis, cellulose could be converted into sugar (levoglucose,

LG;

levoglucosenone,

LGO,

etc.),

linear

carbonyls

(1-hydroxy-2-propanone, HA; hydroxyacetaldehyde, HAA, etc.), furans (furfural; 5-hydroxymethyl furfural, HMF, etc.), linear ketones (1-hydroxy, acetone, etc.) and linear acids (acetic acid, AA, etc.) (Dong et al, 2012; Shen et al, 2010); hemicellulose could be converted into linear carbonyls, linear ketones, linear acids and furans (Dong et al, 2012; Lu et al, 2011; Wu et al, 2003); extractives could be converted into furans (Lu et al, 2011; Wu et al, 2003); and lignin could mainly be degraded into phenols (phenol, 4-vinyl phenol, etc). Meanwhile, some chemical reactions such as oxidation reactions, condensation reactions, esterification reactions and so on were also occurred among the formed chemicals. So, hundreds of organic compounds were included in the bio-oil and most of them contents were low, which was disadvantage to separation. To obtain economically a specific chemical by pyrolysis method, it has to improve the pyrolysis selectivity of biomass. The feedstock property is one of the most influential factors that affect on the products distribution as well as catalyst and pretreatment. Corncob was a better feedstock than other types of biomass to produce furfural due to its richest pentosan content (Lu et al, 2011). The chemical inhomogeneity of rice straw and corn stalk fractions resulted in significant differences

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in the pyrolysis behaviors and product properties (Worasuwannarak et al, 2007; Wang et al, 2014a). The pyrolysis behaviors of woods were different among tree species due to the differences of cellulose, lignin and hemicellulose contents (Amutio et al, 2013; Chang et al, 2013; Hosoya et al, 2007; Shebani et al, 2008). The product properties indicated that algal biomass was not a good feedstock for pyrolysis (Yanik et al, 2013). Therefore, studying the pyrolysis behaviors of different biomass for optimum applications is important. Sweet potato is one of the major food crops in China and its production is about 75 % of the total yield in the world (FAO, 2012). As an agricultural residue, parts of the sweet potato vines (SWV), including leaves, stalks and stems, are mainly used to as animal feed and the rest are considered as a waste and underutilized. Different from other biomass such as corn stalk, wood and grass and so on, more extractives and less lignin are included in the SPV (Aregheore, 2004), which could suggest high pyrolysis selectivity to specific chemicals. However, up to now, the pyrolysis behaviors of the SPV are still un-reported. In this paper, chemical components of the SPV were analyzed, and their pyrolysis behaviors were evaluated by TGA with a final temperature

of

600°C

under

nitrogen

atmosphere

and

pyrolysis-gas

chromatograph/mass spectrometry (Py-GC/MS) analysis.

2. Materials and methods 2.1 Materials and analysis SPV including leaves, stalks and stems was harvested and collected from a local farm in Beijing suburb in 2014 and stored in plastic bags. The moisture content ranges Tipeng Wang, Xiaochen Dong, Zaixing Jin, Wenjing Su, Xiaoning Ye, Qiang Lu

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from 8 to 12 wt. %. The SPV was milled and the fractions under 20 meshes were used for the pyrolysis experiments. The chemical components were analyzed according to ASTM E1758-01 and the proximate analysis of the samples was performed according to GB/T 28731-2012. Every experiment was repeated three times and the average value was obtained. 2.2 TG analysis The pyrolysis progress of the SPV was conducted in a Thermo-Gravimetric (TGA, Perkin Elmer STA 6000, USA) from 30°C to 600°C at a heating rate of 10°C/min under nitrogen environment (99.999%) at a flow rate of 50 mL/min. 2.3 Py-GC/MS analysis The fast pyrolysis was conducted in a CDS Pyroprobe 5200HP pyrolyser (Chemical Data Systems) connected with a GC/MS (Perkin Elmer, Clarus 560). The procedures were the same as reported previously in the literature (Wang et al, 2014b).

3. Results and discussion 3.1 Properties of samples The results of the SPV chemical components were: acid-insoluble lignin 7.85±0.96 wt. %, cellulose 33.01± 2.78 wt. %, hemicellulose 12.25± 1.16 wt. %, extractives 37.12 wt. %. The extractives content was obtained by difference, 100– lignin content– cellulose content– hemicellulose content ashes content. The results of the SPV proximate analysis were: ash 9.77± 0.54 wt. %, volatiles 79.73±5.23 wt. %, fixed carbon 8.52±0.57 wt. % and water 2.98± 0.18 wt. %. Compared with results from other biomass such as corn stalk (Wang et al, 2014a), wood (Chang et al, 2013; Yuan Tipeng Wang, Xiaochen Dong, Zaixing Jin, Wenjing Su, Xiaoning Ye, Qiang Lu

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et al, 2015) and rice straw (Worasuwannarak et al, 2007), the contents of lignin, cellulose and hemicellulose in the SPV were lower significantly, respectively, however, extractives content was higher. According to the literature (Aregheore, 2004), more microminerals exited in the SPV, which resulted in the high ash content. 3.2 TG analysis The TG and DTG curves of the SPV were shown in Figure 1. The pyrolysis processes mainly occurred in four stages. The weight loss of 2.42 wt. % from 30oC to120 oC in the first stage was due to the water release. The weight loss of 5.33 wt. % from 120 oC to 190 oC in the second stage was attributed to the degradation of light volatile compounds such as fructose and small molecule acid. In the third stage, the weight loss was 8.58 wt. % from 190-255oC due to the degradation of hemicellulose. Compared with the results of hemecellulose from other biomass (Azargohar et al, 2014; Hosoya et al, 2007), the temperature range in the stage three was remarkably narrow. In fact, as a non-crystal carbohydrate component, hemicellulose is different in the molecular weight and chemical structure from one biomass to another, which could the main reason of the different pyrolysis temperature range. The weight loss in the fourth stage in a temperature range of 255- 400oC was attributed to the cellulose pyrolysis. However, the wide pyrolysis temperature range (200–700°C) of lignin could result in the overlap of its weight loss with the degradation of other components. Therefore, separate pyrolysis curve of lignin was not observed in Figure 1. 3.3 Py-GC/MS analysis Due to not allowing product collection, the exact bio-oil yield cannot be obtained by

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Py-GC/MS experiment. However, the yield changes of the detected products could be estimated through a comparison of the total chromatographic peak area. The identified chemical compounds by the GC/MS were listed in Table 1. As analyzed in the literature (Wang et al, 2014c), the chemicals were classified into six groups, including sugar (LG; 1,4:3,6-dianhydro-.alpha.-d-glucopyranos, DGP, etc.), linear carbonyls (HAA; pentanal, etc.), linear acids (AA; dodecanoic acid, etc.), linear ketones (acetone, etc.), phenols (phenol; 4-vinyl phenol, 4-VP, etc.), furans (furfural, FF; 5-hydroxymethyl furfural, HMF, etc.). The unidentified chemicals were included in the others. With an increase of pyrolysis temperature from 350oC to 500 oC, the sugars content firstly increased from 8.76 wt. % (350 oC) to 13.97 wt. % (400 oC) and then decreased to 9.19 wt. % (500oC); the changes of the linear carbonyls and linear acids contents were similar, decreasing from 16.58 wt. % and 17.45 wt. % to 5.26 wt. % and 4.03 wt. %, respectively; the furans content increased from 7.10 wt. % to 15.47 wt. %. Compared with the other four groups of chemicals, the effects of pyrolysis temperature on the contents of the linear ketones and phenols were insignificant, and the maximum variations were 3.48 wt. % and 2.38 wt. %, respectively. The results indicated that low pyrolysis temperatures (for example 350 oC) contributed to improving the pyrolysis selectivity of SPV to linear carbonyls, linear acids, and linear ketones; high temperatures (for example 500oC) increased the furans contents; moderated temperatures (for example 400oC) could be a good choice to enhance the sugar contents in the bio-oil.

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Noticeably, among the identified chemical compounds, the LG content of 11.86 wt. % at 400 oC and the AA content of 15.41 wt. % at 350 oC were higher than those from other biomass such as corn stalk, fir wood, rice husk and bagasse (Lu et al, 2011), and the FF content of 6.94 wt. % at 500 oC was also higher as compared with most of other biomass. This suggested a good pyrolysis selectivity of SPV to such chemicals. In pyrolysis, cellulose and hemicellulose could be degraded into sugars, linear acids, linear carbonyls, linear ketones and furans; lignin was be converted into phenols; and extractive compounds (fructose and starch, etc) could form sugars and furans (Dong et al, 2012; Lu et al, 2011; Shen et al, 2010; Wild et al, 2012). Therefore, the low lignin content of 7.85 wt. % of could be the main reason for the low phenols content; high extractive compounds content could result in the increase of the sugars especially LG and furans contents in the bio-oil. The results suggested that high extractive compounds content of SPV biomass were contributed to the enhancement of the pyrolysis selectivity to product sugars and furans.

4. Conclusions Pyrolysis behaviors of SPV were studied by TG and Py-GC/MS analysis. Results indicated that pyrolysis processes of SPV mainly occurred in four stages. With an increase of temperature from 350 oC to 500 oC, in bio-oil, sugars content firstly increased and then decreased; linear carbonyls and linear acids contents decreased and furans content increased; the effects of pyrolysis temperature on the contents of linear ketones and phenols were insignificant. The content of LG (11.86 wt. %), AA (15.41 wt. %) and FF (6.94 wt. %) suggested good pyrolysis selectivity of SPV. Low lignin content and extractives content could be the main reason. Tipeng Wang, Xiaochen Dong, Zaixing Jin, Wenjing Su, Xiaoning Ye, Qiang Lu

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Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (grant #51306053, #51206050 and #51206045), Beijing Higher Education Young Elite Teacher Project (grant #YETP0711) and 111 Project (grant #B12034).

References 1. Amutio, M., Lopez, G., Alvarez, J., Moreira, R., Duarte, G., Nunes, J., Olazar, M., Bilbao, J., 2013. Flash pyrolysis of forestry residues from the Portuguese Central Inland Region within the framework of the BioREFINA-Ter project. Bioresource Technol. 129, 512-518. 2. Aregheore, E.M., 2004. Nutritive value of sweet potato (Ipomoea batata L.Lam) forage as goat feed: voluntary intake, growth and digestibility of mixed rations of sweet potato and batiki grass. Small Rumi. Res. 51, 235-241 3. Azargohar, R., Nanda, S., Kozinski, J.A., Dalai, A.K., Dalai, A.K., Sutarto, R., 2014. Effects of temperature on the physicochemical characteristics of fast pyrolysis bio-chars derived from Canadian waste biomass. Fuel, 125, 90-100 4. Chang, S., Zhao, Z., Zheng, A., Li, X., Wang, X., Huang, Z., He, F., Li, H., 2013. Effect of hydrothermal pretreatment on properties of bio-oil produced from fast pyrolysis of eucalyptus wood in a fluidized bed reactor. Bioresource Technol. 138, 321-328. 5. Dong, C., Zhang, Z., Lu, Q., Yang, Y., 2012. Characteristics and mechanism study of analytical fast pyrolysis of poplar wood. Energy Conver. Manage. 57, 49-59. Tipeng Wang, Xiaochen Dong, Zaixing Jin, Wenjing Su, Xiaoning Ye, Qiang Lu

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6. FAO (Food and Agriculture Organization), 2012. Food and agricultural commodities production. Food and Agriculture Organization of the United Nations. 7. Hosoya, T., Kawamoto, H., Saka, S., 2007. Cellulose-hemicellulose and cellulose-lignin interactions in wood pyrolysis at gasification temperature. J. Anal. Appl. Pyrolysis 80,118-125 8. Lu, Q., Dong, C., Zhang, X., Tian, H., Yang, Y., Zhu, X., 2011. Selective fast pyrolysis of biomass impregnated with ZnCl2 to produce furfural: Analytical Py-GC/MS study. J. Anal. Appl. Pyrolysis 90，204-212 9. Shebani, A.N., Van Reenen, A.J., Meincken, M., 2008. The effects of wood extractives on the thermal stability of different wood species. Thermochim. Acta, 471, 43-50 10. Shen, D.K., Gu, S., Bridgwater, A.V., 2010. Study on the pyrolytic behavior of xylan-based hemicellulose using TG-FTIR and Py-GC-FTIR. J. Anal. Appl. Pyrolysis 87, 199-206. 11. Wang, T.P, Ye X.N., Yin J., Lu, Q., Zheng, Z.M., Dong, C.Q., 2014b. Effects of biopretreatment on pyrolysis behaviors of corn stalk by methanogen. Bioresource Technol. 164, 416-419. 12. Wang, T.P., Ye X.N., Yin J., Jin, Z.X., Lu, Q., Zheng, Z.M., Dong, C.Q.,2014c. Fast pyrolysis product distribution of biopretreated corn stalk by methanogen. Bioresource Technol. 169, 812-815 13. Wang, T.P., Yin, J., Lu, Q., Zheng, Z.M., 2014a. Effects of chemical inhomogeneity on pyrolysis behaviors of corn stalk fractions. Fuel, 129,111-115

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14. Wild, P.J.de, Huijgen, W.J.J., Heeres, H.J., 2012. Pyrolysis of wheat straw-derived organosolv lignin. J. Anal. Appl. Pyrolysis 93, 95-103. 15. Worasuwannarak, N., Sonobe, T., Tanthapanichakoon, W., 2007. Pyrolysis behaviors of rice straw, rice husk and corncob by TG-MS technique. J. Anal. Appl. Pyrolysis 78, 265-271 16. Wu, C., Chang, C., Tseng, C., Lin, J., 2003. Pyrolysis product distribution of waste newspaper in MSW. J. Anal. Appl. Pyrolysis 67, 41-53 17. Yanik, J., Stahl, R., Troeger, N., Sinag, A., 2013. Pyrolysis of algal biomass. J. Anal. Appl. Pyrolysis 103,134-141 18. Yuan, H.R., Xing, S.Y., Huhetaoli, Lu, T., Chen, Y., 2015. Influences of copper on the pyrolysis process of demineralized wood dust through thermogravimetric and Py–GC/MS analysis. J. Anal. Appl. Pyrolysis 112, 325-332

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Table 1 Py-GC/MS products of SPV at different temperature

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Figure 1 TGA/DTG diagram of SPV under nitrogen atmosphere

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Figure 1 110 1

stage 1

0

Weight loss (%)

90 80

DTG

stage 2

-1 -2

70

stage 3 -3

60

-4

50

TGA

40

stage 4

-5 -6

30 100

200

300

400

500

o

Temperature ( C)

Tipeng Wang, Xiaochen Dong, Zaixing Jin, Wenjing Su, Xiaoning Ye, Qiang Lu

Derivative weight (%/min)

100

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Table 1 Py-GC/MS products of SPV at different temperature No.

1

2

3

4

5

6

7

Name

Sugars levoglucose 1,4:3,6-dianhydro-.alpha.-d-glucopyranos Linear carbonyls pentanal hydroxyacetaldehyde butanal,2-methylLinear acids acetic acid dodecanoic acid Linear ketones acetone 2-cyclopenten-1-one, 2-hydroxy1,2-cyclopentanedione, 3-methyl2-cyclopenten-1-one, -ethyl-2-hydroxy1,6;2,3-dianhydro-4-o-acetyl-.beta.-d-gu Phenols phenol 4-vinyl phenol phenol 4-methylphenol,2-methoxyFurans methyl furan furfural 5-hydroxymethyl furfural Others

Product distributions at different temperature (%) 350oC

400oC

500oC

8.76

13.97 11.86 2.11 8.72 4.36 3.54 0.82 12.89 10.16 2.73 19.15 1.46 9.89 3.30 1.45 3.05 6.52 2.30 1.99 0.37 1.86 8.98 4.01 4.01 0.96 29.77

9.19 7.45 1.74 5.26 1.81 2.62 0.83 4.03 0.62 3.41 15.31 5.46 1.15 5.12 1.61 1.97 8.9 2.44 1.31 1.39 3.76 15.47 6.88 6.94 1.65 41.84

7.30 1.46 16.58 7.42 8.18 0.98 17.45 15.41 2.04 18.24 0.45 10.96 2.54 1.25 3.04 7.46 1.61 1.84 0.87 3.14 7.10 0.37 4.66 2.07 24.41

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Highlights ★ Sweet potato vine has good pyrolysis selectivity for sugars, linear carbonyls and

linear acids, linear ketones and furans. ★ At 350 oC, acetic acid content reached the maximum15.41 wt. % in bio-oil. ★ At 400 oC, levoglucose content reached the maximum11.86 wt. % in bio-oil. ★ At 500 oC, furfural content reached the maximum 6.94 wt. % in bio-oil.

Tipeng Wang, Xiaochen Dong, Zaixing Jin, Wenjing Su, Xiaoning Ye, Qiang Lu