Short CommunicationRapid and solvent-saving liquefaction of woody biomass using microwave–ultrasonic assisted technology
Graphical abstract
Introduction
Catalytic liquefaction is a promising technology for converting biomass to produce sustainable liquid biofuels and chemicals (Yazdani et al., 2015). Traditional biomass liquefaction has usually been carried out in a stirred tank reactor using inorganic acid/alkali as a catalyst at a temperature higher than 150 °C for 60 min or longer. A high mass ratio of solvent/biomass of larger than 6 (Li et al., 2015) has commonly been applied in traditional liquefaction. The large dosage of solvent supplies good transfer property of heat and mass for the desired liquefaction yield. However, the use of large amounts of solvents requires high energy consumption to recycle the excess solvent from the liquefaction products, bringing with it a considerable increase in the cost of the liquefaction. Therefore, it is urgent to develop a rapid and solvent-saving liquefaction method to serve as a substitute technology for traditional biomass liquefaction.
Traditional biomass liquefaction, as a liquid–solid heterogeneous catalytic reaction, is very complicated, because it not only involves heat and mass transfer between the inner and interphase of the solvent and the biomass particles, but also involves complicated degradation reactions in the hemicellulose, lignin and cellulose. These components may undergo their degradation only while the acid/alkali catalyst diffuses from the solvent phase into the interior of the biomass particles and as the temperature reaches the level required for degradation reactions. However, the transfer resistance of heat and mass during traditional liquefactions of lignocellulosic biomass is often very large due to such biomass’ typically small pore diameter and low thermal conductivity. The heat and mass transfer are the controlling step during traditional biomass liquefaction, especially when only a little solvent is used during the liquefaction. Intensifying the transfer process can save solvent and allow for more rapid liquefaction.
In the past two decades, microwave technology has begun to be used to intensify heat transfer in the field of rapid biomass pyrolysis (Mushtaq et al., 2015, Wang et al., 2015) and biomass liquefaction (Zhuang et al., 2012, Zhang and Zhao, 2010). These efforts indicated that biomass could be liquefied efficiently and rapidly with the incorporation of microwave under suitable reaction conditions, which had good reference significances for the process intensification of biomass liquefaction. Ultrasonic waves as a process intensification technology have also been used in some liquid–solid mass transfer processes. Ultrasonic waves can clean the surface of solid particles in fluids (solvents) by producing impact forces, and can even cause solid particles to break (Hu et al., 2014) or collapse due to the instant high temperature and pressure resulting from the production of the cavity effect when a relatively high ultrasonic power and frequency are used. Therefore, the use of ultrasonic waves as a substitution for mechanical stirring or another intensification technology has been applied to some physicochemical processes for porous biomass, such as enzyme hydrolysis (Shi et al., 2013) and alkali treatment (Subhedar and Gogate, 2014). Until recently, ultrasonic wave technology begins to be introduced into biomass liquefaction.
Combining the above analyses and our previous work (Lu et al., 2013a, Lu et al., 2014), we speculated that if microwave and ultrasonic wave were introduced simultaneously into biomass liquefaction to intensify heat and mass transfer, the result would be a solvent-saving and rapid liquefaction of abundant and renewable woody biomass that would enhance its commercial potential. In recent years, microwave–ultrasonic assisted technology (MUAT) has been used in some treatment processes for biomass with excellent results in process intensification (Lu et al., 2013b). However, this new method for biomass liquefaction has not to date been reported in the published literature. In this paper, the liquefaction process of fir sawdust using MUAT was investigated using sulfuric acid as the catalyst in a solvent blend of polyethylene glycol 400 (PEG 400) and glycerol. The influences of the various parameters on the liquefaction yield were studied and the products were characterized by means of field emission scanning electron microscopy (FESEM), thermal gravimetric analysis (TGA), and elementary analysis.
Section snippets
Materials
Samples of fir sawdust were collected from a wood processing factory (Fuzhou, China), oven-dried at 388 K for 12 h, and then screened into several fractions. The fraction with mean particle size of 1.265 mm (10–20 mesh) was selected and stored in a desiccator for experiments. The composition of the raw material was determined according to China’s GB/T 2677 standard as follows: 40.1% cellulose, 26.7% hemicellulose, 31.3% acid-insoluble lignin, and 1.87% ash. All of the chemicals were of analytical
Effect of ultrasonic on the liquefaction yield
The liquefaction yield of fir sawdust was investigated, respectively, under microwave-assisted (MW-assisted) and microwave-ultrasonic wave-assisted (MW–UW-assisted), and is shown in Fig. 1a. When the mass ratio of solvent/sawdust ranges from 2:1 to 5:1, the yield of MW–UW-assisted liquefaction can be seen to be higher than that of MW, indicating the presence of a non-negligible intensified effect. However, the yield difference between MW-assisted and MW–UW-assisted liquefaction is negligible at
Conclusion
The liquefaction time was shortened from 60 to 20 min and the solvent consumption was cut in half via MUAT. The influences of some parameters on the yield were similar to that those occurring in traditional liquefaction, suggesting that the MUAT intensified the heat and mass transfer, but did not alter the mechanism or pathway. Ultrasonic waves at low power could not inhibit the polycondensates from depositing and covering the micro-surface of sawdust. This work is a successful attempt to apply
Acknowledgements
The authors are thankful for the financial support from the commonweal specialty industry foundation of the State Forestry Administration of China (Grant No. 201504603), the National Natural Science Foundation of China (Grant Nos. 31100431 and 21506031) and the Foundation of Education Department of Fujian Province (Grant No. JB13038).
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