Characterization of Hydrochars Produced by Hydrothermal Carbonization of Lignin, Cellulose, <scp>d</scp>-Xylose, and Wood Meal

TLDR

Hydrothermal carbonization of cellulose, lignin, d‑xylose, and wood meal was performed at 225–265 °C, revealing a proposed mechanism where furfural intermediates form during d‑xylose conversion, lignin yields polyaromatic and phenolic hydrochars, and hemicellulose transformation drives microsphere formation on cellulose and wood meal surfaces. The hydrochars produced 45–60 % yield, with lignin giving the highest yield, and exhibited stability below 300 °C, aromatic structures, high C content (63–75 %), C recovery (80–87 %), energy recovery (78–89 %), C/O ratios 2.3–4.1, C/H ratios 12–15, and HHVs of 24–30 MJ kg⁻¹, representing 45–91 % higher heating values than the original feedstocks, while microspheres formed on cellulose and wood meal surfaces.

Abstract

Hydrothermal carbonization of cellulose, lignin, d-xylose (substitute for hemicellulose), and wood meal (WM) was experimentally conducted between 225 and 265 °C, and the chemical and structural properties of the hydrochars were investigated. The hydrochar yield is between 45 and 60%, and the yield trend of the feedstock is lignin > WM > cellulose > d-xylose. The hydrochars seem stable below 300 °C, and aromatic structure is formed in all of these hydrochars. The C content, C recovery, energy recovery, ratio of C/O, and ratio of C/H in all of these hydrochars are among 63–75%, 80–87%, 78–89%, 2.3–4.1, and 12–15, respectively. The higher heating value (HHV) of the hydrochars is among 24–30 MJ/kg, with an increase of 45–91% compared with the corresponding feedstock. The carbonization mechanism is proposed, and furfural is found to be an important intermediate product during d-xylose hydrochar production, while lignin hydrothermal carbonization products are made of polyaromatic hydrochar and phenolic hydrochar. The formation of microspheres on the surface of cellulose and WM hydrochars is discussed, and transformation of the hemicellulose should be the reaction for WM microsphere production.