Analytical evaluation of novel thermochemical-biological pathways for lignocellulose conversion into volatile fatty acids

Hybrid thermochemical-biological (HTB) biorefineries were recently proposed as a novel system for the conversion of lignocellulosic biomass into valuable products. This approach aims to couple the ability of thermochemistry to fast depolymerise refractory biomass into smaller molecules (e.g., pyrolysis), and the ability of microorganisms (e.g., microbial mixed cultures, MMC) to transform a variety of chemicals into a few targets products. Despite attempts were made to provide an evaluation of pyrolysis-anaerobic digestion (Py-AD), difficulties were found. These studies highlighted the need to increase the share of easily biodegradable molecules provided by thermochemistry to microbes. In this work, novel HTB pathways were proposed and analytically evaluated. Process flows were deeply investigated through a novel array of analytical techniques, providing a new chemical picture of different process combinations. Specifically, hydrothermal carbonization-intermediate/fast pyrolysis (HTC-Int-Py-AD and HTC-Fast-Py-AD), and slow pyrolysis-gasification (Py-Gs-AD) were evaluated in order to increase the delivery and quality of substrate within the thermochemical step. Gas and liquid products obtained through the proposed paths were biologically upgraded to volatile fatty acids (VFA). Results revealed an improved conversion of biomass into biological products, especially for the Py-Gs-AD route. The combination of hydrothermal carbonization and intermediate pyrolysis provided liquids with an increased share of sugars which resulted in fermentable, however did not significantly increase the overall yield of products with respect to the direct Py-AD. Noticeably, HTC-Fast-Py-AD path was found to be not suitable for the unequivocal higher toxicity of fast pyrolysis products with or without hydrothermal carbonization. The best results achieved were 50% chemical energy of biomass (on a chemical oxygen demand basis) converted into products within Py-Gs-AD pathway, and 40% of the latter with HTC-Int-Py-AD.