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Communication Green Solvent for Flash Pyrolysis Oil Separation Li Deng, Zhao Yan, Yao Fu,* and Qing-Xiang Guo* Key Laboratory for Biomass Clean Energy of Anhui ProVince, Department of Chemistry, UniVersity of Science and Technology of China, Hefei 230026, China ReceiVed March 15, 2009. ReVised Manuscript ReceiVed May 12, 2009 Until now, flash pyrolysis technology has been the most effective, energetically independent, and commercially available method for biomass conversion to liquid fuel.1 The resulting product pyrolysis oil also known as bio-oil has more potential to be used as a fossil resource substitute because the yields can be as high as 70% based on the dry biomass weight, the nominal capacity of 50 tons/day has been achieved, and the required high temperature (425-550 °C) can be kept by the exothermic pyrolysis reaction.2 The pyrolysis oil contains more than 400 organic components derived from cellulose, hemicellulose, and lignin. Because of their different thermal stabilities, the cracking reaction occurs at different extents, resulting in molecule-weight distribution of pyrolysis oil from 30 g/mol to more than 1000 g/mol and a boiling point (bp) from -19 to 386 °C.3 However, bio-oil cannot be separated by conventional distillation as conducted in the petroleum process because of its poor thermal stability. When distilling pyrolysis oil at 150 °C, the residues swell and block the vessel, seriously leaving about 40 wt % solid cokes.2,4 Thus, an effective separation method should be established prior to its application and upgrading. Gallivan et al. separated phenols, acids, and neutral compounds from pyrolysis oil by adding a desired amount of strong bases and acids.5 To avoid the waste of bases and acids and the resulting salt formation, ethyl acetate was used as an extractant by Chum et al.6 With the aim to separate pyrolysis oil efficiently and ecofriendly, we proposed a novel method to distillate pyrolysis oil employing a high bp solvent. Glycerol derived from animal fat and vegetable oil is a renewable liquid with a bp of 290 °C. Along with the fast development of biodiesel, the global capacity of glycerol is raising while the price of it is declining.7 Besides, glycerol is a good solvent with a high bp, high flash point (160 °C), no or low toxicity, and good solvency toward pyrolysis oil. Thus, it drew our attention and was selected as a solvent and heattransfer media for pyrolysis oil distillation. Scheme 1 demonstrates the separation process that we designed (see the Supporting Information for details of the process). The pyrolysis oil subjected to the process was finally separated into two fractions: distillate and pyrolytic lignin. The distillate contains many low-molecular-weight volatile compounds, which are able to be further separated and purified via a conventional method to * To whom correspondence should be addressed. Fax: +86-551-3606689. E-mail:
[email protected] (Y.F.);
[email protected] (Q.-X.G.). (1) Czernik, S.; Bridgwater, A. V. Overview of applications of biomass fast pyrolysis oil. Energy Fuels 2004, 18 (2), 590–598. (2) Mohan, D.; Pittman, C. U.; Steele, P. H. Pyrolysis of wood/biomass for bio-oil: A critical review. Energy Fuels 2006, 20 (3), 848–889. (3) Branca, C.; Di Blasi, C. Multistep mechanism for the devolatilization of biomass fast pyrolysis oils. Ind. Eng. Chem. Res. 2006, 45 (17), 5891– 5899. (4) Oasmaa, A.; Czernik, S. Fuel oil quality of biomass pyrolysis oilssState of the art for the end user. Energy Fuels 1999, 13 (4), 914–921. (5) Gavillan, R. M.; Erie, P.; Peter, K. M.; Plainsboro, N. J. U.S. Patent 4,233,465, 1980. (6) Chum, H. L.; Black, S. K. U.S. Patent 4,942,269, 1990. (7) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of transportation fuels from biomass: Chemistry, catalysts, and engineering. Chem. ReV. 2006, 106 (9), 4044–4098.
Scheme 1. Schematic Diagram of Pyrolysis Oil Separation Using Glycerol
Table 1. Results of Glycerol-Assisted Pyrolysis Oil Separation water content recovery rate temperature yield of of distillate yield of of crude (°C) distillate (wt %) (wt %) lignin (wt %) glycerol (%) 200a 225a 225b 225c 250a
45.81 50.75 55.00 56.78 58.74
a Using fresh glycerol. glycerol recovered twice.
56.30 64.37 58.32 58.55 62.90 b
26.34 27.12 41.53 42.83 27.13
87.99 86.78 79.04 73.24 83.22
Using glycerol recovered once.
c
Using
Table 2. Molecular Weight of Lignin Obtained at Different Conditions operating number average weight average temperature molecular molecular dispersity (°C) weight (Mn) (g mol-1) weight (MW) (g mol-1) (MW/Mn) 0a 200 225 250 a
644 764 757 791
1047 1330 1331 1443
1.63 1.74 1.76 1.82
Pyrolysis oil was added to water at 0 °C directly.
produce value-added chemicals. It also provides ideal feed for vapor-phase upgrading, with the aim to produce a high-grade fuel additive. The other product is pyrolytic lignin, which is precipitated from the glycerol mixture by water. The lignin can be converted to alkanes or a fuel additive by different ways, while it is also substituent for phenol to produce resin.7-10 The glycerol used in the distillation could be recovered easily by three steps: precipitation of lignin using water, filtration, and heating to remove water. In comparison to former methods, no acid and base were used; (8) Shabtai, J. S.; Zmierczak, W. W. U.S. Patent 5,959,167, 1999. (9) Miller, J. E.; Evans, L.; Littlewolf, A.; Trudell, D. E. Batch microreactor studies of lignin and lignin model compound depolymerization by bases in alcohol solvents. Fuel 1999, 78 (11), 1363–1366. (10) Kleinert, M.; Barth, T. Towards a lignincellulosic biorefinery: Direct one-step conversion of lignin to hydrogen-enriched biofuel. Energy Fuels 2008, 22 (2), 1371–1379. (11) Scholze, B.; Meier, D. Characterization of the water-insoluble fraction from pyrolysis oil (pyrolytic lignin). Part I. PY-GC/MS, FTIR, and functional groups. J. Anal. Appl. Pyrolysis 2001, 60 (1), 41–54.
10.1021/ef9002268 CCC: $40.75 2009 American Chemical Society Published on Web 05/28/2009
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Figure 1. Comparison of Py-GC-MS of pyrolytic lignin. Retention time: 14.1 min (phenol), 15.6 min (glycerol), 17.8 min (4-methylphenol), 18.3 min (2-methoxylphenol), 21.2 min (4-ethylphenol), 22.1 min (2-methoxyl-4-methylphenol), 23.2 min (benzofuran), 25.1 min (2-methoxyl-4-ethylphenol), 26.5 min (2-methoxyl-4-vinylphenol), and 31.0 min (eugenol).
therefore, no salt was formed, and volatile extractant was avoided in the separation as well. Therefore, it is clear that no waste or less if any is produced during the whole process, and safety could also be improved by employing glycerol. Table 1 shows the yield of distillate and pyrolytic lignin at different temperatures. The yield of the distillate ranges from 45 to 58 wt %. Although more distillate could be obtained at 250 °C, the loss of glycerol is also higher. The loss could be attributed to volatilization with the distillate and absorption on lignin during precipitation. The former one could be solved by reducing temperatures. As a result, the glycerol content in the distillate was decreased from 8% at 250 °C to less than 1% at 225 °C. The latter one is easy to be overcome by washing the lignin and recovering the glycerol in washing water. When the recovered crude glycerol was used to separate pyrolysis oil, the yields of both the distillate and lignin are higher than using fresh glycerol because the recovered one is saturated by some components, for example, lignin oligomers. According gas chromatography-mass spectrometry (GC-MS) analysis of the distillate (Table S1 in the Supporting Information), many typical components, such as acetic acid, acetol, furfural, and phenols, could be found. It is worth noting that acetic acid and furfural are important chemicals in industry and some phenols, such as guaiacol and eugenol, are value-added chemicals. In comparison to whole pyrolysis oil, it is clear that components with low boiling point were concentrated. Acetic acid was concentrated twice with the content of 8.88 wt % in the distillate. Similar results could be found in acetol, furfural, and phenols, and the content of three components were increased from 2.63, 1.02, and 0.22 wt % to 4.05, 1.82, and 0.26 wt %, respectively. However, the concentration of eugenol and syringol was reduced or eliminated while aldehydes vanilline and methylbenzaldehyde disappeared in the distillate, which could be attributed to their high activity to polymerize or condense. On the other hand, because of the poor volatility, levoglucosan was not observed in the distillate as well (see Table S1 in the Supporting Information for details). Lignin is the second most abundant compound in plant biomass, and under pyrolysis, it could be transferred to the liquid product in the form of phenols and oligomers. The lower oxygen content makes it a superior candidate to produce liquid fuel. Flash pyrolysis is a good pretreatment to transform the insoluble natural lignin to soluble pyrolytic lignin for further treatment. With this aspect, the lignin separated should not be polymerized significantly forming
an insoluble product. To investigate and assess the present separation method whether it meets this requirement, gel permeation chromatography (GPC) was employed to detect the molecular-weight distribution, which gives the first evidence for the polymeric nature of pyrolytic lignin. As shown in Table 2, it is evident that the lignin recovered from glycerol (lignin-gly) is similar to the lignin precipitated in ice water (lignin-w) according to Scholze’s method.11 The weight average molecular weight (MW) of lignin-w is 1047 g/mol, while the values of lignin-gly subjected to 200, 225, and 250 °C in glycerol are 1330, 1331, and 1443 g/mol, respectively. This means that slight polymerization occurred in glycerol, or in other words, glycerol is a good solvent for pyrolysis oil to stabilize lignin oligomers and inhibit intense polymerization. Besides, the lignin-gly could be dissolved in methanol or tetrahydrofuran adequately; therefore, no coke formation was observed. Analytical pyrolysis combined with GC-MS (Py-GC-MS) analysis indicates that the composition of lignin-gly is identical to lignin-w, except for the absorbed glycerol. The two chromatograms match very well, and only the intensities of the peaks show differences (Figure 1). Similar structures of the two types of lignin could also be proven by Fourier transform infrared (FTIR) spectroscopy (see the Supporting Information for details). In conclusion, a novel method for pyrolysis oil separation using glycerol was established. In the whole process, no salt was formed and the use of volatile solvent was avoided. By reusing glycerol, more than 95% of the products (distillate and pyrolytic lignin) could be obtained and glycerol could be easily recovered. The acetic acid can be concentrated 2-fold in the distillate. The pyrolytic lignin obtained from glycerol is polymerized slightly. Thus, glycerol is doubtless a renewable, good solvent for pyrolysis oil separation, and the method has potential to be developed into a commercial process. Acknowledgment. This work was supported by the National Basic Research Program of China (2007CB210205), the Knowledge Innovation Program of Chinese Academy of Science (KGCX2-YW3306), and the Excellent Young Scholars Foundation of Anhui Province (08040106829). Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. EF9002268
