Abstract
The objective of this study was to evaluate phenolic compounds produced from the catalytic pyrolysis of pine sawdust by commercial catalysts. Eight types of commercial catalysts consisting of SiO2, montmorillonite, α-Fe2O3, HZSM-5 (Si:Al = 25:1), ZnO, γ-Fe2O3, HZSM-5 (Si:Al = 50:1), and nano-HZSM-5 (Si:Al = 50:1) were screened in a fixed bed reactor at a reaction temperature of 500 °C and a vapor residence time of 3 s. All the tested commercial catalysts exhibited different catalytic performances for the adjustment of the composition of the bio-oil. HZSM-5 (Si:Al = 25:1) significantly increased hydrocarbon production in the bio-oil, which is helpful for improving its heating value. The different types of phenols were reduced significantly from 17 to 7 with nano-HZSM-5 (Si:Al = 50:1); however, the phenols content also decreased from 32.6% to 23.28% compared with non-catalytic pyrolysis. Meanwhile, the addition of nano-HZSM-5 (Si:Al = 50:1) to the raw material provided the highest amount of furans (up to 38.8%) among the tested commercial catalysts. The inexpensive ZnO and γ-Fe2O3 also were surprisingly effective for the reduction of the variety of phenolic compounds detected by GC/MS, reducing that number from 17 to 10.
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Reduction of the Variety of Phenolic Compounds in Bio-oil via the Catalytic Pyrolysis of Pine Sawdust
Duo Wang, Debin Li, Dongcan Lv, and Yunquan Liu*
The objective of this study was to evaluate phenolic compounds produced from the catalytic pyrolysis of pine sawdust by commercial catalysts. Eight types of commercial catalysts consisting of SiO2, montmorillonite, α-Fe2O3, HZSM-5 (Si:Al = 25:1), ZnO, γ-Fe2O3, HZSM-5 (Si:Al = 50:1), and nano-HZSM-5 (Si:Al = 50:1) were screened in a fixed bed reactor at a reaction temperature of 500 °C and a vapor residence time of 3 s. All the tested commercial catalysts exhibited different catalytic performances for the adjustment of the composition of the bio-oil. HZSM-5 (Si:Al = 25:1) significantly increased hydrocarbon production in the bio-oil, which is helpful for improving its heating value. The different types of phenols were reduced significantly from 17 to 7 with nano-HZSM-5 (Si:Al = 50:1); however, the phenols content also decreased from 32.6% to 23.28% compared with non-catalytic pyrolysis. Meanwhile, the addition of nano-HZSM-5 (Si:Al = 50:1) to the raw material provided the highest amount of furans (up to 38.8%) among the tested commercial catalysts. The inexpensive ZnO and γ-Fe2O3 also were surprisingly effective for the reduction of the variety of phenolic compounds detected by GC/MS, reducing that number from 17 to 10.
Keywords: Phenolic; Commercial catalysts; Catalytic pyrolysis
Contact information: School of Energy Research, Xiamen University, Xiamen, Fujian 361102, China;
*Corresponding authors: yq_liu@xmu.edu.cn
INTRODUCTION
Because of increasing concerns about the energy levels of fossil fuels, biomass pyrolysis technology has attracted more and more attention because it can convert renewable lignocellulosic biomass into high-energy density liquid fuel with the potential of substituting for petroleum fuel (Dobele et al. 2007). Biomass pyrolysis is a process in which biomass decomposes at moderate temperatures of about 500 °C and has short reaction times in the absence of oxygen to produce liquid products called bio-oil, along with solid chars and incondensable gases. Crude bio-oil is dark brown, acidic, thermally and chemically unstable, and contains high-value oxygenated compounds such as phenols, organic acids, and alcohols. For bio-oil products to be used in a wider variety of applications, it is usually upgraded physically, chemically, and catalytically. Among these processes, catalytic pyrolysis is considered as the most promising method because it produces chemicals and fuels compatible with the current petrochemical infrastructure (Dickerson and Soria 2013; Lu et al. 2012; Wang et al. 2013). Compared with crude bio-oil, upgraded bio-oil is much more stable, has a higher heating value, and its composition can be adjusted within limits depending on different catalysts (Bridgwater 2012).
Most of the organics in bio-oil are vital industrial chemical compounds primarily derived from petroleum. The separation and application of these chemicals from bio-oil has attracted increasing attention in recent years. Phenolic compounds in the bio-oil are important raw materials and additives in industry (Lin and Juang 2009; Hu et al. 2013). A number of researchers have tried to separate phenol from bio-oil as a substitute for petroleum-based phenol (Gonza´ et al. 2004; Boucher et al. 2000; Kang et al. 2011). However, because lignin is one of the most complicated aromatic chemicals in earth, its pyrolysis products contain a variety of phenolic compounds. The presence of dozens of phenolic compounds in the crude bio-oil not only increases the difficulty of separation, but also decreases the value of the separated phenolic product. Compared with petroleum-based phenol, the cost of separating phenol from bio-oil is still unbearable. Lu et al. (2010) tested CaO as a catalyst in a Py-GC/MS instrument, and the results showed that CaO was an effective catalyst for reducing the number of different phenols. Azuara et al. (2013) verified that γ-Al2O3 catalyst has a good effect for maximizing the levels of phenols.
In the present study, eight commercial catalysts consisting of SiO2, montmorillonite, α-Fe2O3, HZSM-5 (Si:Al = 25:1), ZnO, γ-Fe2O3, HZSM-5 (Si:Al = 50:1), and nano-HZSM-5 (Si:Al = 50:1) in a fixed bed pyrolysis reactor was screened in order to find a highly effective and low-cost catalyst to reduce the number of different phenols in bio-oil and improve the value of the phenolic compounds separated from bio-oil. Experiments performed in a fixed bed reactor have been developed by several researchers (Putun et al. 2005; Adam et al.2006; Luo et al. 2010; Wang et al. 2010) to screen the commercial catalysts. Traditional zeolites were used as a baseline to evaluate the catalytic performance of other catalysts.
EXPERIMENTAL
Materials and Catalysts
The pine sawdust used in this investigation as a raw material was obtained in China. Prior to use, the sample was dried at 105 °C in an oven for 24 h. Table 1 lists the characteristics of the sawdust. All the catalysts used in this study were commercially available. The catalytic materials were calcined at 500 °C for 4 h and stored in a desiccator before use.
Table 1. Physicochemical Characteristics of the Pine Wood Used in This Study
Apparatus and Experimental Parameters
As shown in Fig. 1, the catalytic pyrolysis system was composed of a 316 stainless steel, 1-in.-diameter, 24-in.-long reactor tube with a complete vacuum sealing assembly and a solid-tube furnace. The furnace was equipped with a single set-point temperature controller for a maximum working temperature of 1200 °C. As a carrier gas, N2with a flow rate of 1.2 L/min was introduced into the reactor to ensure the absence of oxygen during the experiments. Pine sawdust mixed with catalysts or silica sand, for the catalytic and non-catalytic tests, respectively, was placed in the center of the tube and was vertically supported by silica wool. When the reaction temperature reached the desired value, 9 g of pine sawdust mixed with 1.8 g of catalyst or silica sand was placed into the reactor. The volatiles produced in the pyrolysis process were swept out and entered the downstream condensation system, which was composed of three flasks dipped into an insulation box filled with ice. The condensable components in the volatiles were condensed and collected in the flasks. Table 1 shows the experimental parameters for catalytic pyrolysis.
Fig. 1. Schematic diagram of experimental setup
In a typical experiment, each sample was run for 30 min to ensure that all of the pyrolysis vapor flowed through the cold flasks. The pyrolysis vapor flowed into the condenser, and the catalytic pyrolysis process ended after 30 min. After the experiments, the liquid collected in the flasks and the solid that remained in the reactor were both quantified by weight. The gaseous product was quantified as the difference. Some earlier studies have also used the similar method (Putun et al. 2005; Adam et al. 2006) to screen the catalysts. The experimental parameters are shown in Table 2.
Table 2. Experimental Parameters of the Pyrolysis and Catalytic Pyrolysis of Pine Sawdust
Characterization
The chemical composition of the bio-oil was analyzed using a GC/MS-QP2010 SE (Shimadzu, Japan) device, and compounds were detected using a Mass Selectivity (MS) detector equipped with an Rtx-5MS (30 m × 0.25 mm × 0.25 μm) capillary column. The amount injected was 1 μL, and the split ratio was 25:1. Helium gas was employed as a carrier gas at a constant flow rate of 1.2 mL/min. The heating of the column started at 50 °C, where it was held for 5 min; then, the temperature was increased to 250 °C at a rate of 4 °C/min, at which point it was held for 5 min. The temperature of EI was 280 °C. The mass spectra obtained by GC-MS were interpreted based on an automatic library search (Wiley & NIST).
RESULTS AND DISCUSSION
Influence of Commercial Catalysts on the Distribution of Pyrolysis Products
The pyrolysis and catalytic pyrolysis of pine sawdust were carried out at a reaction temperature of 500 °C and a pyrolysis vapor residence time of 3 s in a fixed bed reactor. The yields are shown in Fig. 2. As expected, the organics in the bio-oil varied with the different commercial catalysts used; the highest yield, of 33.1% bio-oil and 33.8% char, was obtained for the pyrolysis of the pure pine sawdust. Only 26.4% bio-oil was obtained with theγ-Fe2O3 catalyst, and 28.4% char was obtained with HZSM-5 (Si:Al = 25:1). The highest yield of gas (44.4%) was obtained with the ZnO catalyst. More biomass was converted to gas, which indicated that the solid char experienced further decomposition and secondary reactions of the liquid fraction of volatiles under the action of the catalysts in the reactor (Wang et al 2006). Meanwhile, the low effective liquid collection equipment could be another reason for the high content of gases. The differences among bio-oil, char, and gas generated using the commercial catalysts showed that different catalytic mechanisms occurred during the process of the catalytic pyrolysis of pine sawdust.
Fig. 2. The effect of catalysts on the product yields (9 g of pine sawdust, 500 °C, 20 wt.% catalyst addition). Cat. 1: SiO2; Cat. 2: montmorillonoid; Cat. 3: α-Fe2O3; Cat. 4: HZSM-5 (Si:Al = 25:1); Cat. 5: ZnO; Cat. 6: γ-Fe2O3; Cat. 7: HZSM-5 (Si:Al = 50:1); Cat. 8: nano-HZSM-5 (Si:Al = 50:1))
Influence of Catalysts on Product Distribution of Bio-oil
Table 3 shows the difference in bio-oil composition for the tested catalysts. The identified compounds were classified into 9 groups: phenols, aldehydes, acids, esters, ketones, alcohols, furans, hydrocarbons, and others. Although the chromatographic peak area does not represent the actual content of the bio-oil, it is considered linear with its quantity (Lu et al. 2010). Compounds with high molecular weights that could not be analyzed by the GC-MS system were classified as others. Different catalysts were found to be important factors for product distribution and yield. Ketones and aldehydes are responsible for aging reactions in the bio-oil and greatly affect its quality (Lu et al. 2009; Zhao et al. 2010). High acid contents are corrosive and decrease the stability of the bio-oil (Huber et al. 2006). Esters in general reduce the heating value of the bio-oil. All of these compounds are thus considered undesirable when using bio-oil for energy production purposes.
The highest yield of the phenols was observed when using HZSM-5 (Si:Al = 25:1) as a catalyst, followed by HZSM-5 (Si:Al = 50:1). These yields were higher than those with other catalysts and non-catalytic pyrolysis, which indicated that a molecular sieve catalyst can significantly increase the phenolic composition of the bio-oil (Foster et al. 2012; Gopakumar et al. 2012). Stefanidis et al. (2011) obtained similar conclusions. In addition, it can be seen that the addition of nano-HZSM-5 (Si:Al = 50:1) to raw material provided the highest percentage of furans (up to 38.8%) among the tested commercial catalysts, which is attractive for further investigation on extracting furans from upgraded bio-oil.
Table 3. Comparison of Bio-oil Compositions between Pyrolysis and Catalytic Pyrolysis of Pine Sawdust (9 g of Pine Sawdust, 500 °C, 20 wt.% Catalyst Addition)
Cat. 1: SiO2; Cat. 2: montmorillonoid；Cat. 3: α-Fe2O3; Cat. 4: HZSM-5 (Si:Al = 25:1); Cat. 5: ZnO; Cat. 6: γ-Fe2O3; Cat. 7: HZSM-5 (Si:Al = 50:1); Cat. 8: nano-HZSM-5 (Si:Al=50:1)
Influence of Catalysts on the Phenolic Compounds of Bio-oil
The distribution of phenolic compounds in bio-oil is shown in Table 4. There were 17 phenolic compounds present in bio-oil obtained from the pyrolysis of pine sawdust. All the tested commercial catalysts were effective for the reduction of the number of different types of phenolic compounds, compared with non-catalytic pyrolysis. Significantly, the types of phenolic compounds decreased from 17 to 7 with nano-HZSM-5 (Si:Al = 50:1), and 40% of the phenolic compounds was 2-methoxyphenol, an important chemical compound used extensively in industrial applications. However, the contents of phenolic compounds also decreased from 32.6% to 23.28% compared with that obtained from non-catalytic pyrolysis. The number of different types of phenolic compounds decreased to 10 with catalytic pyrolysis using ZnO, γ-Fe2O3, or HZSM-5 (Si:Al = 50:1). Therefore, the nano-HZSM-5 catalyst was much more effective than these catalysts for reducing the variety of phenolic compounds in bio-oil. 2-Methoxyphenol, actually increased from 6.99% to 12.22% when using HZSM-5 (Si:Al = 50:1) as a catalyst. Among the tested catalysts, SiO2, montmorillonite and α-Fe2O3 seemed to be mild catalysts, as most of phenolic compounds were unaltered. The commercial ZnO and γ-Fe2O3 are very cheap compared with HZSM-5 and nano-HZSM-5 catalyst, and they exhibited good catalytic performance. It is worth noting that γ-Fe2O3 showed high catalytic performance, but α-Fe2O3 is a mild catalyst, which indicated the different crystal structure of Fe2O3 exhibited completely different catalytic performance for the upgrading of bio-oil.
Table 4. Chemical Composition of Phenols in Bio-oil from Pyrolysis of Pine Sawdust (9 g of Pine Sawdust, 500 °C, 20 wt.% Catalyst Addition)
Cat. 1: SiO2; Cat. 2: montmorillonite；Cat. 3: α-Fe2O3; Cat. 4: HZSM-5 (Si:Al = 25:1); Cat. 5: ZnO; Cat. 6: γ-Fe2O3; Cat. 7: HZSM-5 (Si:Al = 50:1); Cat. 8: nano-HZSM-5 (Si:Al = 50:1)
CONCLUSIONS
- The HZSM-5 (Si:Al = 25:1) catalyst significantly increased hydrocarbon production in the bio-oil, which is helpful for improving the heating value of the bio-oil.
- The number of different types of phenols was decreased significantly, from 17 to 7, with nano-HZSM-5 (Si:Al = 50:1); however, the contents of phenols also decreased from 32.6% to 23.28% compared with non-catalytic pyrolysis.
- The addition of nano-HZSM-5 (Si:Al = 50:1) to the raw material produced the highest amount of furans (up to 38.8%) among the tested commercial catalysts, which is attractive for further investigation on extracting furans from upgraded bio-oil.
- SiO2, montmorillonite, and α-Fe2O3 seemed to be mild catalysts, as most of phenolic compounds were unaltered.
- As a cheaper catalyst, the inexpensive ZnO and γ-Fe2O3 also were found to be surprisingly effective for the reduction of the variety of phenolic compounds which were detected by GC-MS, reducing the number from 17 to 10.
ACKNOWLEDGMENTS
The authors are grateful for the support of the Natural Science Foundation of China (NSFC, Grant No. 21206142; 21276214). The authors are also grateful for the kind support from the Committee of the 4th Conference on Biorefinery towards Bioenergy (ICBB2013) in Xiamen, China.
REFERENCES CITED
Adam, J., Antonakou, E., Lappas, A., Stocker, M., Nilsen, M. H., Bouzga, A., Hustad, J. E., and Oye, G. (2006). Microporous and Mesoporous Materials 96, 93-101.
Azuara, M., Fonts, I., Barcelona, P., Murillo, M. B., and Gea, G. (2013). “Study of catalytic post-treatment of the vapours from sewage sludge pyrolysis by means of γ-Al2O3,” Fuel 107, 113-121.
Boucher, M. E., Chaala, A., and Roy, C. (2000). “Bio-oils obtained by vacuum pyrolysis of softwood bark as a liquid fuel for gas turbines. Part I: Properties of bio-oil and its blends with methanol and a pyrolytic aqueous phase,” Biomass Bioenergy 19(5), 337–350.
Bridgwater, A. V. (2012). “Review of fast pyrolysis of biomass and product upgrading,” Biomass and Bioenergy 38, 68-94.
Dickerson, T., and Soria, J. (2013). “Catalytic fast pyrolysis: A Review,” Energies 6, 514-538.
Dobele, G., Urbanovich, I., Volpert, A., Kampars, V., and Samulis, E. (2007). “Fast pyrolysis – Effect of wood drying on the yield and properties of bio-oil,” BioResources 2(4), 699-706.
Foster, A. J., Jae, J., Cheng, Y. T., Huber, G. W., and Lobo, R. F. (2012). “Optimizing the aromatic yield and distribution from catalytic fast pyrolysis of biomass over ZSM-5,” Applied Catalysis A: General423, 154-161.
Gonzá, J., Cruz, J. M., Domínguez, H., and Parajó, J. C. (2004). “Production of antioxidants from Eucalyptus globulus wood by solvent extraction of hemicellulose hydrolysates,” Food Chemistry84(2), 243-251.
Gopakumar, S. T., Adhikari, S., Chattanathan, S. A., and Gupta, R. B. (2012). “Catalytic pyrolysis of green algae for hydrocarbon production using H+ZSM-5 catalyst,” Bioresource Technology 118, 150-157.
Hu, G., Li, J., and Zeng, G. (2013). Recent development in the treatment of oily sludge from petroleum industry: A review,” J. Hazardous Materials 261(15), 470-490.
Huber, G., Iborra, S., and Corma, A. (2006). “Synthesis of transportation fuels from biomass: Chemistry, catalysts, and engineering,” Chem. Rev. 106(9), 4044-4098.
Kang, S., Li, X., Fan, J., and Chang, J. (2011). “Classified separation of lignin hydrothermal liquefied products,” Industrial & Engineering Chemistry Research 50(19), 11288-11296.
Lin, S. H., and Juang, R. S. (2009). “Adsorption of phenol and its derivatives from water using synthetic resins and low-cost natural adsorbents: A review,” Journal of Environmental Management 90(3), 1336-1349.
Lu, Q., Li, W. Z., and Zhu, X. F. (2009). “Overview of fuel properties of biomass fast pyrolysis oils,” Energy Convers. Manage. 50(5), 1376-1383.
Lu, Q., Zhang, X. M., Zhang, Z. B., Zhang, Y. Zhu, X. F., and Dong, C. Q. (2012). “Catalytic fast pyrolysis of cellulose mixed with sulfated titania to produce levoglucosenone: Analytical Py-GC/MS study,” BioResources 7(3), 2820-2834.
Lu, Q., Zhang, Z., Dong, C., and Zhu, X. (2010). “Catalytic upgrading of biomass fast pyrolysis vapors with nano metal oxides: An analytical Py-GC/MS study,” Energy 3(11), 1805-1820.
Luo, S., Xiao, B., Hu, Z., Liu, S., Guan, Y., and Cai, L. (2010). “Influence of particle size on pyrolysis and gasification performance of municipal solid waste in a fixed bed reactor,” Bioresource Technology 101, 6517-6520.
Putun, E., Uzun, B. B., and Putun, A. E. (2005). “Fixed-bed catalytic pyrolysis of cotton-seed cake: Effects of pyrolysis temperature, natural zeolite content and sweeping gas flow rate,” BioResource Technology 97, 701-710.
Stefanidis, S. D., Kalogiannis, K. G., Iliopoulo, E. F., Lappas, A. A., and Pilavachi, P. A. (2011). “In-situ upgrading of biomass pyrolysis vapors: Catalyst screening on a fixed bed reactor,” Bioresource Technology 102(17), 8261-8267.
Wang, D., Li, D., and Liu, Y. (2013). “High quality bio-oil production via catalytic pyrolysis of pine sawdust,” BioResources 8(3), 4142-4154.
Wang, J., Zhang, M., Chen, M., Min, F., Zhang, S., Ren, Z., and Yan, Y. (2006). “Catalytic effects of six inorganic compounds on pyrolysis of three kinds of biomass,” Thermochimica Acta 444(1), 110-114.
Wang, P., Zhan, S., Yu, H., Xue, X., and Hong, N. (2010). “The effects of temperature and catalysts on the pyrolysis of industrial wastes (herb residue),” Bioresource Technology 101, 3236-3241.
Zhao, Y., Deng, L., Liao, B., Fu, Y., and Guo, Q. X. (2010). “Aromatics production via catalytic pyrolysis of pyrolytic lignins from bio-oil,” Energy Fuel 24, 5735-5740.
Article submitted: Jan. 17, 2014; Peer review completed: March 3, 2014; Revised version received: May 10, 2014; Accepted: May 11, 2014; Published: May 16, 2014.