Abstract
Bio-oil, obtained from the pyrolysis of biomass, is identified as a potential material to produce transportation fuels and value-added chemicals. However, physical and chemical properties of bio-oil change versus time, known as “aging”, and the instability of bio-oil brings a critical hurdle to the commercial application of bio-oil. Therefore, expanding and deepening the understanding of aging mechanism of bio-oil is the key for later efficient application of bio-oil. In addition, the extreme complexity of pyrolysis bio-oil composition brings great difficulties in studying the aging mechanism. Thus, this study tries to better understand the aging mechanism by evaluating the aging performance for 39 model compound aging tests performed at 80℃ for 72 hours. Four kinds of reactions (self-condensation, esterification, aldol condensation, and phenol and aldehyde reaction) were investigated to understand the contribution of various compounds and reactions during aging process. It has been found that acid played an important role in aging process, as it acted as the reactant in esterification reaction and acted as the catalysis for aldol condensation and phenol and aldehyde reaction. Acids and alcohols reacted via esterification reaction, resulting in the decline of aliphatic C-O bonds. Due to the absence of acid, aromatic compounds were relatively stable in these tests. In comparison, aldehydes and HMF were active since self-condensation reactions for these chemicals were observed in absence of acid. Moreover, with the aid of acid, HMF showed high tendency towards polymerization, result in the increasing water content and molecule weight during accelerated aging process.
Highlights
Biomass is viewed as a promising renewable energy source, which can produce various forms of biofuels and biomaterials without contributing to new CO2 in the atmosphere (Ragauskas, 2006; Yang et al, 2015)
Hydrolysis was the main reaction for levoglucosan to form glucose, this product appeared only when the temperature exceeded 90◦C (Hu et al, 2013), which suggests that levoglucosan reacted with acids as the alcohol at 80◦C
To elucidate the roles of different components of bio-oil and various reactions that may occur in the bio-oil aging process, pyrolysis model compounds have been employed to explore more information about the aging process
Summary
Biomass is viewed as a promising renewable energy source, which can produce various forms of biofuels and biomaterials without contributing to new CO2 in the atmosphere (Ragauskas, 2006; Yang et al, 2015). The application of bio-oil is limited due to a number of undesirable properties; one is its aging problem, which results in chemical and physical changes of bio-oil such as high and changing water content, acidity, and molecular weight. Increasing water content is observed during the aging process, as water is a by-product of the condensation reaction (Joseph et al, 2016) Reactive substances such as aldehydes, alcohols, and olefins reacting via the condensation reaction result in chemical changes of bio-oil (Alsbou and Helleur, 2014). This knowledge plays a very important role in expanding our understanding of aging and studying the aging process of bio-oil. Applying model compounds in bio-oil aging study helps provide more insights into the bio-oil aging mechanism at the molecular level, which few studies focused on
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