Revealing the intensification effect of circulating hydrothermal pretreatment: A sustainable route for biorefinery

Abstract

Hydrothermal pretreatment (HTP) has received huge progress in converting biomass waste into available products at a large scale, but challenges in massive water input and wastewater output still block sustainable biorefinery future. Herein, a circulating hydrothermal technique was proposed by reusing the process wastewater into the next HTP process to enhance energy efficiency and suppress the environmental burdens. During hydrothermal liquid (HL) recycling, the variation of lignin chemical structure, cellulose accessibility, and soluble components were investigated for an in-depth understanding of what happened to lignocellulose. An enhanced hemicellulose dissolution (from 78.1 % to ∼ 85 %) was achieved by the aqueous phase recycling. The reactions of dehydration and polymerization occurred during the circulating HTP process, which contributed to the generation of pseudo-lignin on the cellulose surface, causing a slight weakening of cellulose accessibility. Meanwhile, the organic acid-catalyzed hydrothermal process can facilitate the cleavage of lignin β-O-4 ether linkage and condensation reaction, endowing its potential amphiphilic properties. An in-depth analysis of the evolution of HL components substantiated the potential intensification effect on biomass deconstruction by the generation of hemicellulose-derived organic acids. Life cycle assessment revealed that the circulating HTP technique resulted in about a 29 % reduction in primary energy depletion and an order of magnitude reduction in CO2 emissions as opposed to that of the conventional one. This work unveils the structure evolutions of lignocellulose clearly during the HL recycled process, and also highlights a feasible route for the sustainable biorefinery.