Spirulina hydrothermal carbonization: Effect on hydrochar properties and sulfur transformation

The effect of hydrothermal carbonization (Htc) on the hydrochar properties and sulfur conversion for microalgae was investigated. The sulfur species and distribution in solid and aqueous products produced from different temperature (180-300 °C) were evaluated. Results suggested that varying temperature significantly influenced the elemental composition, functional groups of hydrochar, and the sulfur species in the products. With temperature increased, the hydrochar had increased aromatic structure with low H/C and O/C ratio, and more conversion of organic sulfur into liquid as SO₄^2--S, which acquired the highest concentration (293.31 mg/L) at 300 °C. The thiophene-S, aromatic-S, and thiazole-S accounted for the main sulfur species in bio-oil, while components of thiophene-S and thiazole-S were decreased at high temperature. In addition, elevated temperature resulted in more sulfur forms (i.e. thiophene) in hydrochar and formation of more inorganic-S species like sulfate.