Hydrothermal Carbonization (HTC) of Lignocellulosic Biomass

TLDR

Hydrothermal carbonization converts biomass into gases, water, and solid char through hydrolysis, dehydration, and decarboxylation in hot, pressurized water. The study proposes a two‑step HTC process—low‑ and high‑temperature stages—to simultaneously recover sugars and produce energy‑dense char. Using a 2 L Parr stirred vessel, the authors varied temperature (215–295 °C) and hold time (5–60 min) to assess effects on product composition and yield. Higher temperature and longer residence time increased gaseous and aqueous products and decreased char, but the char’s energy density rose—at 255 °C/30 min it was 39 % higher than raw biomass, while lower temperatures yielded sugars and higher temperatures produced more acetic acid.

Abstract

Hydrothermal carbonization (HTC) of biomass involves contacting raw feedstock with hot, pressurized water. Through a variety of hydrolysis, dehydration, and decarboxylation processes, gaseous and water-soluble products are produced, in addition to water itself and a solid char. In this experimental effort, a 2 L Parr stirred pressure vessel was used to apply the HTC process to a mixed wood feedstock. The effects of the reaction conditions on product compositions and yields were examined by varying temperature over the range of 215−295 °C and varying reaction hold time over the range of 5−60 min. With increasing temperature and time, the amounts of gaseous products and produced water increased, while the amount of HTC char decreased. The energy density of the char increased with reaction severity. At reaction conditions of 255 °C for 30 min, the HTC char had 39% higher energy density than the raw biomass feedstock. Aqueous solutions from HTC experiments at lower temperatures (215−235 °C) contained significant levels of sugars. At higher temperatures (255−295 °C), greatly reduced concentrations of sugars were observed, while concentrations of acetic acid increased. A two-step HTC process involving low- and high-temperature regimes may be advantageous to maximize both the recovery of sugars and production of energy-dense char.