Influence of externally added organic or inorganic acids on evolution of property of hydrochar from hydrothermal carbonization of potato peel

Acidic sites play key roles to enhance hydrochar yield in hydrothermal carbonization (HTC) of biomass via catalyzing dehydration and polymerization reactions. Addition of external acidic sites might accelerate carbonization during HTC, affecting yields and properties of hydrochar. This was investigated herein by conducting HTC of potato peel at 180 °C with presence of organic acids (acetic acid, formic acid or lactic acid) or mineral acids (H₃PO₄, HCl and H₂SO₄). The results showed that lactic acid involved in conversion of reaction intermediates during HTC and increased the hydrochar yield by 10.8 %, while formic acid, HCl or H₂SO₄ catalyzed hydrolysis polymeric structures, dehydration of sugars to furans, and decomposition of furans to levulinic acid. This diminished production of hydrochar, especially HCl or H₂SO₄ that led to reduced yields of hydrochar from 36.1 % in water to 16.0 % or 20.0 %. Additionally, all acids catalyzed conversion of the peel-derived nitrogen-containing organics and ketones. The acids (excluded lactic acid and H₂SO₄) promoted slightly deoxygenation to make hydrochar carbon-rich, while lactic acid and H₂SO₄ enhanced oxygen content from 21.6 % in water to 26.4 % and 36.6 %, respectively. Cross-polymerization of reaction intermediates were not dominate reaction routes, and occurrence of carbonization reactions with especially H₂SO₄ was rather limited. H₂SO₄ as a catalyst resulted in “peeling” of the structures layer by layer, leading to much lower heating value than others (22.0 versus ca. 29 MJ/kg). Nonetheless, characterization of the hydrochars with in-situ IR technique showed that HCl or H₂SO₄ did catalyze aromatization to form more phenolic -OH with diminished abundance of C=O. The formation of aromatic structures resulted in superior capability for adsorption of methyl blue and improved combustion performance.