Acetic Acid and Sodium Hydroxide-Aided Hydrothermal Carbonization of Woody Biomass for Enhanced Pelletization and Fuel Properties

Hydrothermal carbonization (HTC) at 200 and 250 degrees C was combined with acetic acid and NaOH (changing feedwater pH from 2 to 12) to treat woody biomass for enhancing its pelletization and fuel properties. The treated biomass (hydrochar) was characterized by composition analysis and Fourier transform infrared spectroscopy, after which the energy consumption of pelletization; tensile strength; fuel properties, including proximate, elemental analysis, and mass density; and combustion characteristics of corresponding pellets were analyzed. The results showed that both the acetic acid and NaOH-aided HTC processes decreased the energy consumption of hydrochar compression. Weakly basic or acidic feedwater decreased the tensile strength of hydrochar pellets, but it increased at pH 2 and 12. The changes in treatment pH from 4 to 9 had little effect on the O/C and H/C content of the hydrochar pellets, while the addition of acetic acid enhanced oxygen and hydrogen loss more than NaOH. The heating values of hydrochar pellets produced at pH 2 from 200 and 250 degrees C were 20.7 and 22.8 MJ/kg, respectively. Hydrochar pellets produced from HTC at 250 degrees C and pH 2 had higher mass density and lower equilibrium water content than those produced under other conditions. The combustion process was promoted with reduced ignition temperature, maximum weight loss rate, and burnout temperature. These findings demonstrate that acetic acid-aided HTC of woody biomass was more suitable than NaOH for producing hydrochar pellets for use as a biofuel.