Hydrothermal gasification (HTG) experiments were carried out to extract hydrogen from biomass. Although extensive research has been conducted on hydrogen production with HTG, limited research exists on the use of biochar as a raw material. In this study, woodland residues (wood chip) and biochar from wood-chip pyrolysis were used in HTG treatment to generate hydrogen. This research investigated the effect of temperature (300–425 °C) and biomass/water (0.5–10) ratio on gas composition. A higher temperature promoted hydrogen production because the water–gas shift reaction and steam-reforming reaction were promoted with an increase in temperature. The methane concentration was related positively to temperature because of the methanation and hydrogenation reactions. A lower biomass/water ratio promoted hydrogen production but suppressed carbon-monoxide production. Most reactions that produce hydrogen consume water, but water also affects the water–gas shift reaction balance, which decreases the carbon-monoxide concentration. By focusing on the practical application of HTG, we attempted biochar treatment by pyrolysis (temperature of heating part: 700 °C), and syngas was obtained from hydrothermal treatment above 425 °C.

This work describes batch-type hydrothermal liquefaction (HTL) treatments of conifer wood chips at 180–425 °C, under either air or nitrogen atmosphere. Such experiments allow efficient extraction of 5-hydroxymethyl furfural (HMF) and other valuable chemical substances, such as glycolic acid and acetic acid, from the lignocellulosic biomass. These compounds and their decomposition products present in the samples after HTL are analyzed and quantified using spectroscopic and chromatographic techniques. In general, the relatively higher-pressure nitrogen atmospheric condition is more suitable for obtaining the desired products, relative to the air atmosphere. Based on the quantitative results, the optimal temperatures for producing acetic acid, glycolic acid, and HMF are 300 °C, 250 °C, and 180 °C, respectively. The interesting relationship between HMF yield and temperature is also discussed; as the temperature increases, the yield of HMF first decreases and then increases. This phenomenon is explained by the exothermic nature of the HMF decomposition reaction, which is inhibited by excessively high temperature (in the range from 380 to 425 °C). At moderately high temperatures (optimized conditions; 300 °C), the generation rate of HMF exceeds its decomposition rate, resulting in a high yield of HMF. Based on the results of the experiments conducted in this study, the decomposition mechanism describing HTL treatment of wood chips can be elucidated. This study therefore provides guidance for future work involving HMF extraction from lignocellulosic biomass.

A correction to this paper has been published: https://doi.org/10.1007/s42452-021-04649-7

To realize the utilization of the valorization of buckwheat waste (BW), a two-stage thermal-chemical process was explored and evaluated to produce saccharides and biochar. During the first stage, BW underwent a hydrothermal extraction (HTE) of varying severity to explore the feasibility of saccharides production; then, the sum of saccharides yields in the liquid sample were compared. A higher sum of saccharides yields of 4.10 % was obtained at a relatively lower severity factor (SF) of 3.24 with a byproducts yield of 1.92 %. During the second stage, the contents of cellulose, hemicellulose, and lignin were analyzed in the residue after HTE. Enzymatic hydrolysis from the residue of HTE was inhibited. Thus, enzymatic hydrolysis for saccharides is not suitable for utilizing the residue after HTE of BW. These residues with an SF of 3.24 were treated by pyrolysis to produce biochar, providing a higher biochar yield of 34.45 % and a higher adsorption ability (based on methyl orange) of 31.11 % compared with pyrolysis of the raw BW. Meanwhile, the surface morphology and biomass conversion were analyzed in this study. These results demonstrate that the two-stage thermal-chemical process is efficient for treating BW and producing saccharides and biochar. This work lays a foundation for the industrial application of BW, and for improving the economic benefits of buckwheat cultivation.

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