This critical review summarises and analyses all the work conducted to date on the use of microwave-assisted hydrothermal processes for the conversion of biomass into hydrochar, bio-crude (bio-oil) and valuable chemicals.

Biomass is a renewable and carbon-neutral resource with good features for producing biofuels, biochemicals, and biomaterials. Among the different technologies developed to date to convert biomass into such commodities, hydrothermal conversion (HC) is a very appealing and sustainable option, affording marketable gaseous (primarily containing H₂, CO, CH₄, and CO₂), liquid (biofuels, aqueous phase carbohydrates, and inorganics), and solid products (energy-dense biofuels (up to 30 MJ/kg) with excellent functionality and strength). Given these prospects, this publication first-time puts together essential information on the HC of lignocellulosic and algal biomasses covering all the steps involved. Particularly, this work reports and comments on the most important properties (e.g., physiochemical and fuel properties) of all these products from a holistic and practical perspective. It also gathers vital information addressing selecting and using different downstream/upgrading processes to convert HC reaction products into marketable biofuels (HHV up to 46 MJ/kg), biochemicals (yield >90 %), and biomaterials (great functionality and surface area up to 3600 m²/g). As a result of this practical vision, this work not only comments on and summarizes the most important properties of these products but also analyzes and discusses present and future applications, establishing an invaluable link between product properties and market needs to push HC technologies transition from the laboratory to the industry. Such a practical and pioneering approach paves the way for the future development, commercialization and industrialization of HC technologies to develop holistic and zero-waste biorefinery processes.

The catalytic steam reforming of four different aqueous fractions of bio-oil has been carried out in a fixed bed reactor at 650 °C and atmospheric pressure using a Ni–Co/Al–Mg catalyst, employing a spatial time of 4 g catalyst min/g organics. The chemical analysis of the aqueous fractions revealed that the source of biomass (pine or poplar sawdust) and the pyrolysis unit significantly influenced the chemical composition of these liquids. Depending on their chemical composition, the initial H2 yield varied from 0.101 to 0.182 g H2/g organics and the initial CO2 yield from 0.814 to 1.28 g CO2/g organics during their catalytic reforming. Regarding catalytic stability, higher catalyst deactivation took place during the reforming of the two pine bio-oil aqueous fractions. The reforming results of the four aqueous fractions have been correlated to their chemical compositions using statistical empirical additive models developed using the Bayesian Information Criterion (BIC). This strategy enabled the identification of the chemical compounds responsible for the most significant variations observed during the reforming of the liquids. The different proportions of acetic acid and furfural in the liquids had the greatest impact on the reforming results. Acetic acid was identified as a compound with low reactivity and low coke formation. In contrast, furfural was found to have high reactivity and a high tendency to produce coke in the reforming process. Additional reforming experiments conducted with acetic acid, phenol, furfural, levoglucosan and guaiacol helped to confirm and explain the results obtained during the catalytic steam reforming of the aqueous fractions. (Résumé d'auteur

This work firstly addresses the microwave-assisted, catalytic, hydrothermal liquefaction (MAC-HTL) of a mixture of pine and spruce biomass, examining the effects of the temperature (150–250 °C), pressure (50–120 bar), time (0–2 h) and catalyst amount (Ni–Co/Al–Mg; 0–0.25 g catalyst/g biomass). This hydrothermal process turned out to be a very promising route for the production of bio-oil with suitable physicochemical properties to be used as a precursor for liquid bio-fuels and/or renewable platform chemicals. The statistical analysis of the results revealed that the operating conditions exerted a significant influence on the process; the overall biomass conversion and the yields of gas and bio-oil varied by 13–77%, 7–67% and 1–29%, respectively. This liquid consisted of a complex mixture of esters (0–30%), aldehydes (4–69%), ketones (0–35%), alcohols (0–14%), phenols (0–83%), acids (0–28%), cyclic compounds (0–38%), acetates (0–11%), ethers (0–27%) and furans (0–12%). The proportions of C, H and O in the liquid shifted by 2–70 wt%, 4–11 wt% and 27–87 wt%, which varied the Higher Heating Value (HHV) between 4 and 28 MJ kg⁻¹. The optimisation of the process revealed that it is possible to transform up to 27% of the original biomass into a phenol-rich (47%) bio-oil with a relatively high HHV (20 MJ kg⁻¹) using a temperature as low as 250 °C, at 80 bar, and employing 0.25 g catalyst/g biomass for 1.9 h. The properties of the liquid produced under optimum conditions make it suitable to be used as a renewable bio-fuel precursor, a bio-based source of aromatics and/or a sustainable phenolic-rich antioxidant additive. Therefore, this process might represent a promising improvement in biomass pre-processing technologies, helping the development of novel routes for biomass valorisation.