Kinetic Analysis of Lignin Hydrothermal Conversion in Sub- and Supercritical Water

Hydrothermal conversion of lignin was conducted in subcritical temperatures of 300–370 °C and residence times of 0.5–10 s at a pressure of 25 MPa. This additional work (subcritical conditions, 300–370 °C) was used as a comparison with our previous study (supercritical conditions, 390–450 °C) to further investigate the temperature effect on lignin decomposition. Subsequently it was our aim to examine its overall reaction kinetics at wider temperature range, and conclusively we determined each of the reaction natures (radical or ionic reaction) as deduced previously in our original proposed lignin conversion pathways. Due to the change in the properties of water under sub- and supercritical conditions, ionic reactions were expected to deviate from Arrhenius behavior. This was caused by the affected stability of the ions in the reaction from the change in the dielectric constant and ionic product. However, radical reactions could not be expected to be affected by the change in the dielectric constant or ioni...

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