Characterization and Evaluation of Hydrothermal Liquefaction Char from Alkali Lignin in Subcritical Temperatures

An evaluation of hydrothermal liquefaction (HTL) char is investigated in this work. Morphological studies, N₂ adsorption behavior, FTIR analysis, thermal behavior, and elemental composition are studied. The HTL char yield showed an increase with higher operating temperatures. It increased from 11.02% to 33% when the temperature increased from 573 K to 623 K. At lower temperatures, the residence time showed an impact on the yield, while close to the critical point, residence time became less impactful. Elemental analysis showed that both higher operating temperatures and longer residence times increased the nitrogen content of the chars from 0.32% to 0.51%. FTIR analysis suggested the char became more aromatic with the higher temperatures. The aliphatic groups present diminished drastically with the increasing temperature. Residence time did not show a significant impact as much as the temperature when considering the functional group elimination. An increase in operating temperatures and residence times produced thermally stable chars. HTL char produced at the lowest operating temperature and showed both the highest surface area and pore volume. When temperature and residence time increase, more polyaromatic char is produced due to carbonization.