Research Article pubs.acs.org/journal/ascecg
Hydrolysis Kinetics and Structure Changes of Wood Meal in Subcritical Water Wei Yang, Hui Wang, Jie Zhou, and Shengji Wu*
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College of Materials and Environmental Engineering, Hangzhou Dianzi University, Xiasha University Park, Hangzhou, Zhejiang 310018, China
ABSTRACT: Hydrolysis kinetics and structure changes of wood meal were investigated at a temperature range of 170−330 °C. The weight of wood-meal residue began to decrease at 180 °C, reachng its lowest value at 319 °C. Chars were generated in the subcritical water treatment, resulting in an increase in the lignin weight in the wood-meal residue. Three zones were observed in the entire hydrolysis process, which can be described by Coats−Redfern models, yielding the activation energies and preexponential factors for the hydrolysis zones with a high correlation coefficient. The first-order kinetic model was the most effective one to express the hydrolysis kinetics of wood meal, yielding an activation energy of 98.8 kJ mol−1 in Zone 1, 42.8 kJ mol−1 in Zone 2, and 126.7 kJ mol−1 in Zone 3. The crystalline structures of the wood-meal residue showed no great changes below 283 °C, but were completely destroyed at 319 °C. The hydrolysis behaviors of the cellulose and hemicellulose in the wood meal in the subcritical water treatment involves two steps, i.e., the surface hydrolysis and destruction of the hydrogen and glycosidic bonds. The optimal temperature for hydrolysis of wood meal to produce solid fuel and valuable chemicals is 283 °C. KEYWORDS: Hydrolysis kinetics, Kinetic parameters, Subcritical water, Wood meal
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INTRODUCTION With the increasing concerns about resource scarcity and environmental pollution, the production of energy from renewable resources (such as solar, wind, hydro, and biomass) has attracted more and more interest in recent years. Among all of the renewable resources, biomass, with an annual production of 1.46 × 1011 tons, is considered a promising candidate to replace fossil fuels in the near future, because it is the only resource that has the potential to produce biofuel and bio-based materials.1−5 Besides, biomass is a carbon-neutral fuel, with an annual CO2 fixation of 5.6 × 1010 tons, and a mere one-eighth of the biomass that is biosynthesized every year can provide all the energy needed for human use.1,6 However, biomass is not suitable for direct combustion as a biofuel, because of its low heating value, low energy density, and high volatility.7 Therefore, a pretreatment process is needed to upgrade the fuel properties of biomass or to convert it to valuable chemicals, such as biocrude and biogases.8,9 Subcritical water treatment is considered as an environmentally friendly process to pretreat biomass, because it uses only water as a reaction medium and therefore has low © 2017 American Chemical Society
environmental impact. Water that maintains its liquid state in the temperature range of 100−374 °C under pressurized conditions is called subcritical water. Such water has two unique properties, compared with water at ambient temperature: a low relative dielectric constant and a high ion product. For example, when the temperature increases from room temperature to 250 °C,10,11 the relative dielectric constant of 80, in ordinary water, is reduced to ∼27, in subcritical water, which are values that are similar to those of acetone and methanol at room temperature. Meanwhile, the value of the ion product increases to 10−11, indicating that subcritical water can act as an acidic and/or alkali catalyst for chemical reactions.12 In addition, subcritical water treatment also showed some advantages, compared with the other thermal processes, such as pyrolysis, gasification, and dry torrefaction: (i) elimination of both a predrying process and the need for a carrier gas, combined with lower treatment temperReceived: January 30, 2017 Revised: March 9, 2017 Published: March 9, 2017 3544
DOI: 10.1021/acssuschemeng.7b00300 ACS Sustainable Chem. Eng. 2017, 5, 3544−3552
Research Article
ACS Sustainable Chemistry & Engineering
total carbohydrate and reducing sugar contents of the hydrolysates, components, functional groups, and structural changes of wood-meal residues obtained at some specific temperatures.
ature, made subcritical water treatment a lower-cost technique; and (ii) the acidic properties of gaseous products (sulfur and nitrogen oxides) could avoid the noxious air pollution.13 Hence, subcritical water has been widely employed in hydrolysis and in the hydrothermal carbonization of biomass for biofuel production.7,9,13−16 For example, Hata et al.11 and Wiboonsirikui et al.9 successfully extracted functional substances from rice bran with subcritical water. Yan et al.17 and Lyman et al.18 improved the fuel properties of loblolly pine through hydrothermal carbonization treatment. In our previous research, we also hydrothermally carbonized various types of biomass to prepare biofuel, and we found that the biomass species affected the fuel properties of the prepared biofuel.7 To clarify the hydrolysis behavior of biomass, Yong and Matsumura investigated the hydrolysis kinetics of lignin in subcritical water, and they proposed possible hydrolysis routes for lignin in subcritical and supercritical water.19 They reported that ionic reactions were likely to occur in subcritical water, whereas radical reactions were favored in supercritical water. They also suggested that all of the reactions that occurred during lignin hydrolysis obeyed a first-order model and Arrhenius behavior. Sasaki et al. evaluated the hydrolysis behavior of cellulose in subcritical and supercritical water. They suggested that the hydrolysis of cellulose consisted of surface peeling when the temperature was
