Elucidating hydrochar morphology and oxygen functionality change with hydrothermal treatment temperature ranging from subcritical to supercritical conditions

Hydrochar, used for numerous applications including solid fuel, adsorbent medium, anode material, and soil amendment, is the major solid product or unwanted byproduct resulting when biomass undergoes for subcritical or supercritical water treatment, respectively. Regardless of the reaction temperature, hydrochar forms from biomasses at different reaction kinetics. Our hypothesis in this study is morphology and oxygen functionality of hydrochar change with reaction temperature, especially from a subcritical to supercritical region. To test this hypothesis, hydrochar was produced from microcrystalline cellulose at 220 °C–420 °C with 40 °C interval with a reaction time of 30 min. The effects of subcritical and supercritical conditions of the hydrochar characteristics were investigated using a series of analytical tools including ultimate and proximate analyses, Brunauer-Emmett-Teller (BET) surface area, scanning electron microscopy (SEM), X-Ray Diffraction (XRD) analysis, Fourier-transform infrared spectroscopy (FTIR), and Boehm Titration. Results showed that oxygen-containing acidic functional groups reached the highest (1495 ± 25 μmol/g) at 260 °C and reduced afterward with an increase of temperature, although the elemental oxygen was reduced with the increase of reaction temperature. The particle size of hydrochar increases with the increase of temperature in the subcritical region but decreases in the supercritical region.