Biofuel and bioenergy production from diverse biomass sources using thermochemical technologies over the last decades has been investigated. The thermochemical conversion pathways comprise dry processes (i.e., torrefaction, combustion, gasification, and pyrolysis), and wet processes (i.e., liquefaction, supercritical water gasification, and hydrothermal carbonisation). It has been found that the thermochemical processes can convert diverse biomass feedstocks to produce bioenergy sources such as direct heat energy, as well as solid, liquid and gaseous biofuels for instance biochar, bio-oil and syngas. However, some of these processes have limitations that impede their large-scale utilisation such low energy efficiency, high costs, and generation of harmful chemicals that cause environmental concerns. Efforts are being made extensively to improve the conversion technologies in order to reduce or solve these problems for energy efficiency improvement. In this review, the emerging developments in the thermochemical techniques for producing biofuel and bioenergy from biomass are presented and evaluated in terms of their technological concepts and projections for implementation. It is suggested that an integration of torrefaction or hydrothermal carbonisation with combustion and/or gasification may optimise biomass energy use efficiency, enhance product quality, and minimise the formation of noxious compounds.

The present study addresses the production of hydrochars from brown seaweed (Fucus serratus) (FS-HCs), coconut shell (CS-HCs), and oak (Oak-HCs) as potential adsorbents using hydrothermal carbonisation (HTC). The effect of HTC processing temperature on the physicochemical adsorbent characteristics of the hydrochars is investigated at different temperatures (200, 220, 250 °C) using a hydrothermal batch reactor. Increasing HTC temperature causes the formation of many spheres in CS-HCs and Oak-HCs, increasing their porosity, except FS-HCs. The surface area of the hydrochars increases with increasing HTC temperature; 10.93–12.78 m²/g for FS-HCs, 2.18–21.94 m²/g for CS-HCs, except for Oak-HCs which decreases from 4.89 to 3.09 m²/g. Increasing HTC temperature decreases volatile matter content in the hydrochars, increases fixed carbon content, and decreases H/C ratio (except for FS-HCs) and O/C ratio of the hydrochars. For all the hydrochars, increasing the HTC temperature results in a slight decrease in zeta potential magnitude, with negatively charged surfaces, making them potential adsorbents for cationic pollutants. The study confirms that the HTC process improves key chemical and physical characteristics of the hydrochars compared to the original biomass, and that the physicochemical adsorbent characteristics are enhanced as the processing temperature increases.

Surface structure and chemical properties of adsorbents are important factors required to understand the mechanism of adsorption. The purpose of this study was to produce hydrochars from biomass using hydrothermal carbonisation (HTC) and to analyse their sorption capacities. The biomass used in this study were coco-peat (CP), coconut shell (CS), eggshell (ES), rice husk (RH) and lemon peel (LP). The operating conditions for HTC were 200 °C and 20 h residence time. The characterisation methods consisted of Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), Fourier Transform Infrared Ray (FTIR) Spectroscopy, and Brunauer, Emmett and Teller (BET). The results showed that HTC improved the sorption capacities of the biomass wastes. It was found that hydrochars were crispy and flaky with more micro- and meso-porous structures, indicating that lignin and other components were denatured due to carbonisation. This led to the creation of more active sites for sorption and pollutant binding. The hydrochars showed a percentage increase in carbon content and a decrease in oxygen content with traces of other elements, compared to their corresponding raw biomass. The major functional groups identified were –OH and –COOH. The surface area of the hydrochars which include CP (2.14 m²/g), CS (14.04 m²/g), ES (0.50 m²/g), RH (15.74 m²/g), and LP (6.89 m²/g) were significantly improved compared with those of the raw biomass. The study showed that the hydrochars produced from the biomass wastes have the potential to be used as adsorbents.