Characteristics of Distillation Residue from Rice Straw Fast Pyrolysis Oil

Characteristics of distillation residue from rice straw fast pyrolysis oil

Hao Li, Shuqian Xia, Peisheng Ma

Key Laboratory for Green Chemical Technology of State Education Ministry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People's Republic of China.

Because of the shortage of oil resources and the deteriorating environmental problems, the biomass fast pyrolysis oil deservedly offers an opportunity as alternatives to replace fossil fuels due to its renewable characteristics. Unfortunately, pyrolysis oil is an unstable product, because of the abundant reactive oxygenated compounds. During storage, the physical and chemical properties of the bio-oil were continuously changing. Therefore, it is very necessary for pyrolysis oil to improve its properties before they can be used in existing equipment.

To date, many techniques have been developed to improve in the physicochemical properties of bio-oil, including hydrodeoxygenation, catalytic cracking, distillation and emulsification.[1]Since the bio-oil contains a wide range of compounds with different boiling ranges, separation of the components by distillation has been regarded as a feasible method for upgrading pyrolysis oil. Capunitan and Capareda[2] investigated fractional distillation of bio-oil under atmospheric and vacuum conditions and obtained the useful information for distillate fraction. Additionally, other scholars also applied the effectiveness of distillation to improve the properties of the bio-oil by separation of the components.[3;4] Although the high-value chemicals or high-quality fuel were obtained from pyrolysis oil, the highest distillate yield of pyrolysis oil was approximately 65% and a fraction of solid residue was formed after distillation.[2] In additional, there were few studies about the solid residues from the distillation of pyrolysis oil. In fact, such solid residues should have high carbon contents and could be regarded as a kind of bio-char.[5] Therefore, the application of distillation residues could be developed until precise information regarding the properties of distillation residues from pyrolysis oil were obtained.

In this work, the non-volatile solid residues (called distillation residue) were obtained by the atmospheric distillation (p=101 kPa) and the vacuum distillation (p=4 kPa). The detail experimental procedure described in our previous paper.[6] In this work, the atmospheric distillation residue and vacuum distillation residue yields were 48.3% and 35.2%, respectively.

In order to obtain precise information, the distillation residues have been comprehensively and systematically characterized, including FTIR, ¹³C NMR, XRD, SEM, TG/DTG and elemental analysis. Table 1 illustrated the elemental compositions in atmospheric distillation residue and in vacuum distillation. The chemical compositions of the atmospheric distillation residues and vacuum distillation residues are CH_0.9724O_0.1964N_0.0124S_0.0003 and CH_0.9989O_0.2795N_0.0122S_0.0003, respectively. Compared with pyrolysis oil, there were significant increase in the carbon content and remarkable reduction in the oxygen content for the distillation residues. It may be due that the oxygenated volatile (especially water and small active molecular) were being removed from pyrolysis oil under distillation condition. Furthermore, the H/C atomic ratios in distillation residues are very close to 1, which indicates the degree of aromaticity in distillation residues is very high. The crystallinity and morphology of distillation residues were characterized by XRD and SEM, the results indicated the distillation residues are amorphous (seen in Figs. 1 and 2).

Table 1. The yields and elemental compositions of pyrolysis oil, atmospheric distillation residue and vacuum distillation residue.

	Prolysis oil	Atmospheric distillation residue	Vacuum distillation residue
Yields (wt%, wet basis)		48.3446	35.1794
Elemental analysis (wt%, wet basis)
C	34.53 a	73.29	67.98
H	6.170 a	5.939	5.659
N	1.04 a	1.06	0.97
S	0.626 a	0.520	0.060
O b	57.634 a	19.191	25.331
H/C mole ratio	2.144 a	0.972	0.976
O/C mole ratio	1.2518 a	0.196	0.260

^a ref[6], ^b calculated by difference.

Figure 1. XRD patterns of the atmospheric distillation residue and vacuum distillation residue.

Figure 2. Scanning electron micrograph of (a and b) the atmospheric distillation residue and (c and d) vacuum distillation residue.

The FTIR and 13C NMR spectrum had been used to understand the chemical structures of the distillation residues (seen in Fig. 3). Table 2 lists the function groups that were identified from the FTIR spectra. The C-H stretching at 1607, 1512, 1449 and 624 cm-1 show the presence of aromatic compounds in distillation residue.[4] Meanwhile, the clear aromatic resonances (110¨C140 ppm) were shown in 13C NMR spectra, which illustrate the highly aromatic nature of distillation residue (especially vacuum distillation residue).[7] This phenomenon also consists with the results of the elemental analysis given in Table 1.

Figure 3. FTIR and ¹³C NMR spectra of the distillation residues.

Table 2. The FTIR functional groups of the atmospheric distillation residues and the vacuum distillation residues.

Frequency (cm-1)	Group	Class of compound
3421	O-H stretching	phenols and alcohols
2928	C-H stretching	Alkanes
1607	Aromatic ring stretching, C=C stretching	Aromatic compounds and alkenes
1512	Aromatic ring stretching	Aromatic compounds
1449	Aromatic ring stretching, C-H bond	Aromatic compounds
1371	Methyl C-H stretching	Alkanes
1116	C-O-C stretching	ethers
624	Aromatic C-H stretching	Aromatic compounds

In additional, the thermal degradation of the atmospheric distillation residue and vacuum distillation residue were studied by the thermogravimetric (TGA) and differential thermogravimetric (DTG) analysis (shown in Fig. 4). The comparison of the TG/DTG curves of the atmospheric and vacuum distillation residues illustrated the thermal decomposition process of the atmospheric distillation residue existed in two stages. At the first stage (< 200°æ), the atmospheric distillation residues may represent the evaporation of moisture and other small molecule residues which weren't completely removed from pyrolysis oil. The atmospheric distillation residues and vacuum distillation residues at the main thermal decomposition stage (200-500°æ) may attribute to the thermal degradation of saccharide and lignin derivative. The solid residues of the distillation residues were greater than 40% when the temperature reached nearly 850 °æ. Therefore, the distillation residues have a fine thermal stability.

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Figure 4. TG and DTG curves of (a) the atmospheric distillation residue and (b) the vacuum distillation residue under nitrogen purge of 50 mL/min and heating rate of 10 °æ/min.

In summary, the precise information on the non-volatile solid residues (called distillation residue) is very important for the efficient utilization of bio-oil though distillation process. It is the first time that the distillation residues obtained by the atmospheric distillation (p=101 kPa) and the vacuum distillation (p=4 kPa) have been comprehensively and systematically characterized, including FTIR, 13C NMR, XRD, SEM, TG/DTG and elemental analysis in this work. The experimental results suggested that the distillation residues have higher carbon content, the highly aromatic nature, the amorphous structure and fine thermal stability. According to all of the preceding results, the distillation residues seem to be the raw material for further production of aromatic compounds or as solid fuels and soil amendment.

Acknowledgements

The authors sincerely acknowledge the Tianjin Natural Science Foundation (13JCYBJC19300) and the National Basic Research (973) special preliminary study program (2014CB260408) for the financial support.

References

[1] L. Zhang, R. Liu, R. Yin, Y. Mei, Renewable and Sustainable Energy Reviews 24 (2013) 66-72.

[2] J. A. Capunitan, S.C. Capareda, Fuel 112 (2013) 60-73.

[3] J. L. Zheng, Q. Wei, Biomass and Bioenergy 35 (2011) 1804-1810.

[4] X. S. Zhang, G.X. Yang, H. Jiang, W.J. Liu, H.S. Ding, Sci. Rep. 3 (2013).

[5] K. H. Kim, J.Y. Kim, T.-S. Cho, J.W. Choi, Bioresource Technology 118 (2012) 158-162.

[6] H. Li, S. Q. Xia, Y. Li, P. S. Ma, C. Zhao, Stability evaluation of fast pyrolysis oil from rice straw, Chemical Engineering Science 2015, accept.

[7] C.A. Mullen, G.D. Strahan, A.A. Boateng, Energy & Fuels 23 (2009) 2707-2718.