| Summary: | This paper presents hydrothermal carbonization (HTC) of cow and pig manure in an acidic environment and compares the results with HTC performed without acid. HTC experiments were carried out at 170 °C for 10 min. The presence of an acid increased the extent of carbonization and energy densification for both types of manure. The heating value of the hydrochar was increased by an amount ranging from 9–21%, with sulfuric acid having a greater impact than acetic acid. The increased fuel densification may provide for more economic use as a fuel. Both acids increased the leaching of ionic species including PO<sub>4</sub>, Ca, K, Mg, and Fe. The results demonstrated conversion greater than 90% of total phosphorous to phosphate in hydrothermal conditions. HTC with sulfuric acid resulted in increased nitrogen leaching for both types of manure and resulted in the generation of ammonia. Further analysis of the aqueous products signified the formation of acetic acid, furfural, and 3-aminopyrazine 1-oxide compounds for the sulfuric acid-catalyzed hydrothermal treatment. Analysis of the residual gas confirmed the presence of a trace amount of H<sub>2</sub>S, NO<sub>2</sub>, and SO<sub>2</sub> formation. The addition of sulfuric acid increased NO<sub>2</sub> significantly. Finally, detailed mass balances are presented for carbon and macronutrients (NPK).
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