Abstract
Hydrothermal depolymerization of lignin-rich streams (LRS) from lignocellulosic ethanol was successfully carried out in a lab-scale batch reactors unit. A partial depolymerization into oligomers and monomers was achieved using subcritical water as reaction medium. The influence of temperature (300–350–370 °C) and time (5–10 minutes) was investigated to identify the optimal condition on the monomers yields in the lighter biocrude (BC1) and aqueous phase (AP) fractions, focusing on specific phenolic classes as well as carboxylic acids and alcohols. The effect of base catalyzed reactions (2–4 wt. % of KOH) was compared to the control tests as well as to acid-catalyzed reactions obtained with a biphasic medium of supercritical carbon dioxide (sCO2) and subcritical water. KOH addition resulted in enhanced overall depolymerization without showing a strong influence on the phenolic generation, whereas sCO2 demonstrated higher phenolic selectivity even though no effect was observed on the overall products mass yields. In conclusion, a comparison between two different biocrude collection procedures was carried out in order to understand how the selected chemical extraction mode influences the distribution of compounds between BC1 and AP.
Highlights
Lignin is one of the main constituents of lignocellulosic biomass, and making up to 15–35 wt. %of the total organic matter weight, carrying the highest specific energy content compared to cellulose and hemicellulose [1,2,3]
The hydrothermal conversion of Lignin-rich streams (LRS) resulted in the generation of five main products: DEE-soluble biocrude (BC1), DMK-soluble biocrude (BC2), aqueous phase (AP) with water-soluble organics (WSO), char or solid residue (SR) and gases
In another work [26] the authors reported the results of the uncatalyzed experimental campaign only in terms of yields, elemental analysis and molecular weight: here, a detailed characterization of BC1 and AP was carried out to observe how the mechanism of lignin depolymerization is influenced by reaction conditions
Summary
Lignin is one of the main constituents of lignocellulosic biomass, and making up to 15–35 wt. %of the total organic matter weight, carrying the highest specific energy content compared to cellulose and hemicellulose [1,2,3]. The global amount of lignin estimated in the Earth’s surface is 300 billion tons and annually increases by around 20 billion tons [4]. Lignin separation from cellulose and hemicellulose takes place extensively in the second generation lignocellulosic ethanol biorefineries and pulp and papers industry [5]. In order to estimate the potential of lignin from lignocellulosic ethanol biorefinery, an European study from E4 Tech [6] investigated scenarios for the growth of Energies 2020, 13, 1241; doi:10.3390/en13051241 www.mdpi.com/journal/energies. Energies 2020, 13, 1241 biorefinery industry under a favorable supporting policy as the one foreseen by the Directive (EU). From 2022 to 2025, 3–4 new plants per year across Europe are expected, but from 2025 to 2030, this rate should increase to an average of 6–7 new plants commencing production annually
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