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Effect of hydrothermal treatment on chemical structure and pyrolysis behavior of Eucalyptus wood Anqing Zheng, liqun Jiang, Zengli Zhao, Sheng Chang, Zhen Huang, Kun Zhao, Fang He, and Haibin Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b03005 • Publication Date (Web): 22 Feb 2016 Downloaded from http://pubs.acs.org on February 27, 2016
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Energy & Fuels
Effect of hydrothermal treatment on chemical structure and pyrolysis behavior of eucalyptus wood Anqing Zheng, Liqun Jiang, Zengli Zhao∗, Sheng Chang, Zhen Huang, Kun Zhao, Fang He, Haibin Li Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China
∗
Corresponding author, Tel.:+86 2087057721. Fax: +86 2087057737. E-mail address:
[email protected] ABSTRACT Hydrothermal treatment experiment of eucalyptus wood was carried out in a high-pressure batch reactor at temperatures between 160 and 190 oC. The effect on chemical structure and pyrolysis behavior of eucalyptus wood as a result of hydrothermal treatment was investigated by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric (TG) analysis, and pyrolysis gas chromatography mass spectroscopy (Py-GC/MS). From FTIR analysis, it was observed that hydrothermal treatment could effectively remove acetyl groups from eucalyptus wood. XRD analysis showed that crystallinity degree of eucalyptus wood was enhanced by hydrothermal treatment due to the degradation of hemicellulose and amorphous cellulose, and crystalline size of cellulose became larger owing to the removal of small
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crystallites. TG analysis suggested that no significant carbonization and cross-linking of chemical components in eucalyptus wood occurred during hydrothermal treatment and the thermal stability of eucalyptus wood was enhanced. Compared to raw eucalyptus wood, hydrothermally treated eucalyptus wood gave much higher levoglucosan yields, while lower yields of reactive compounds including ketones, aldehydes, and organic acids in Py-GC/MS experiment. This implied that hydrothermal treatment had positive impacts on biomass pyrolysis product distribution and could improve chemical composition of bio-oil produced from fast pyrolysis. Keywords: Eucalyptus wood; Hydrothermal treatment; Chemical structure; Pyrolysis behavior
1. Introduction Environmental pollution and fossil fuel shortage promote the development and use of environmental-friendly renewable energy resources. Biomass as one of main renewable energy resources has attracted increasing attention, and it can be converted into fuels and valuable chemicals by a great variety of conversion technologies consisting of biochemical and thermochemical methods1. In general, biomass needs to be pretreated prior to conversion into target products. Pretreatment steps have a significant effect on the efficiency of conversion process and overall economic profit margins. When biomass is subjected to pyrolysis, biologic2, chemical3 and thermal4 pretreatment methods have been used to promote thermal
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decomposition of biomass feedstock and improve quality of pyrolysis products. Recently, the growing attention has been paid to biomass torrefaction which is recognized as a promising pretreatment technology for fast pyrolysis4-5. Torrefaction as a thermal pretreatment method is generally carried out in an inert atmosphere in the temperature range of 200-300
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C , where biomass is partly decomposed, and
hemicellulose can be mostly removed from biomass. This process alters physical properties and chemical structure of biomass, which results in changes in pyrolysis behavior of torrefied biomass. When torrefied biomass is subjected to fast pyrolysis, moisture, organic acids, and other lightweight reactive compounds are less formed due to the removal of hemicellulose. Therefore, more stable bio-oil with less contents of moisture and reactive compounds is produced. Unfortunately, in comparison to the direct pyrolysis of raw biomass, the bio-oil yield based on torrefied biomass is significantly reduced owing to the cross-linking and carbonization of biomass major constituents (hemicellulose and lignin) during torrefaction5. In fact, the torrefaction-aid fast pyrolysis can be considered as a step-wise process to obtain staged products that will be less complex than products from direct fast pyrolysis. The thermal fractionation process of biomass will be desired to be accomplished by step-wise pyrolysis, where the main biomass constituents can be decomposed stepwise rather than simultaneously. Hydrothermal treatment which is also called as wet torrefaction is taken as another effective method for fractionating biomass in hot compressed liquid water7. The process is generally conducted in the temperature range
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of 150-260 oC under the saturated vapor pressure. By the process, most of hemicellulose fractions in biomass can be solubilized into aqueous compounds such as oligomeric and monomeric sugars, furfural, and organic acids, the lignin seal is broken, while cellulose can be almost completely recovered in the solid product. Thus effective fractionation of biomass is achieved8. The process significantly increases the accessibility of cellulose to enzyme and is used as a pretreatment method for subsequent enzymatic hydrolysis of cellulose9. When biomass is subjected to hydrothermal pretreatment prior to fast pyrolsyis, the desired improvement of bio-oil properties will also be achieved owing to removal of hemicelluose from biomass. Particularly, main biomass constituents probably undergo less cross-linking and carbonization reactions under relatively low temperature in hydrothermal treatment, which will reduce bio-oil yield penalties in subsequent fast pyrolysis of treated biomass. Therefore, hydrothermal treatment could also be utilized as a promising pretreatment technology for fast pyrolysis. Johnson et al. conducted hydrothermal treatment of straw and α-cellulose at temperatures ranging between 150 and 260 oC, and it was found that the hydrothermal treatment promoted the formation of anhydrosugars in fast pyrolysis of both straw and α-cellulose 10. Long et al. performed hydrothermal treatment of microcrystalline cellulose in the temperature range from 250 oC to 350 oC. The results showed that the pyrolysis performance of microcrystalline cellulose treated at 250 oC was remarkably improved11. Investigating the chemical structure and pyrolysis behavior of hydrothermally treated biomass is critical to evaluate the potential of hydrothermal treatment as a pretreatment
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method for fast pyrolysis to produce high quality bio-oil. In reviewing recent literatures, such an investigation has not been sufficiently performed. To reduce the severe degradation of cellulose, hydrothermal treatment of eucalyptus wood was carried out at the relatively low temperatures ranging from 160 to 190 oC for 5 min in this paper. The main aim was to examine the effects of hydrothermal treatment temperature on chemical structure and pyrolysis behavior of eucalyptus wood to provide fundamental data for bio-oil production from fast pyrolysis of hydrothermally treated biomass. 2. Experimental methods 2.1. Material preparation Eucalyptus wood used in this study was acquired from a local wood processing plant in Guangzhou, Guangdong Province, China. Prior to hydrothermal treatment experiment, the samples were milled to small particle size and subsequently screened to 40-60 mesh. Then these selected particles were oven-dried to around 6% moisture content. The results of elemental and proximate analysis of the samples were listed in Table 1. 2.2. Hydrothermal treatment experiment Hydrothermal treatment experiment of eucalyptus wood was carried out at 160, 170, 180 and 190 oC in a high-pressure batch reactor (autoclave) detailed in a previous paper12. A mixture of eucalyptus wood and water (eucalyptus wood: water = 1: 9 w/w) was put into the reactor, and then the reactor was sealed. Before heating the reactor, nitrogen gas was passed through it at a flow rate of 1 L/min for 20 min to eliminate the
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presence of oxygen. The reactor with magnetic agitator operating at 600 rpm was heated using an electric furnace up to the target temperatures within 10-15 min. When the reactor reached the target temperature, the electric furnace was moved away. After maintaining the reaction temperature for 5 min, the reactor was cooled quickly to
