High fixed carbon and low ash hydrochar production from sewage sludge with addition of phloroglucinol: Theoretical modeling of reaction mechanism using density functional theory

The addition of phloroglucinol in hydrothermal carbonization of sewage sludge resulted in production of advanced sludge hydrochar with improved chemical structure and composition, which create conditions for its higher-value applications. Experimental results showed that the formation of some nanoparticle clusters rather than sheet sedimentary was observed due to enhancement of aromatic structures. Carbon content of hydrochar was increased by 15.42% and ash content was reduced by 25.68% when compared to single hydrothermal carbonization of sewage sludge. Moreover, reaction mechanism of phloroglucinol was investigated using density functional theory based on consideration of 5-hydroxymethylfurfural (5-HMF) and its hydration product levulinic acid as key active intermediates from degradation of saccharides that are one of the primary components of sewage sludge. Electrophilic substitution between phloroglucinol and 5-HMF is much easier than intermolecular dehydration due to the lower energy barrier and higher reaction rate constant. The precursors from electrophilic substitution preferentially occupied the core of hydrochar whereas that from intermolecular dehydration narrowly existed at outer surface. Reaction of phloroglucinol with levulinic acid also occurred easily by the electrophilic substitution, but which was limited by hydration of 5-HMF to form levulinic acid. Reaction pathway of phloroglucinol was mainly controlled by kinetics rather than thermodynamics.