Transformation of nitrogen during hydrothermal carbonization of sewage sludge: Effects of temperature and Na/Ca acetates addition

This study reports the effects of temperature and Ca/Na acetates addition on the transformation of nitrogen during hydrothermal carbonization of sewage sludge at 160–250 °C. The nitrogen species in the hydrochar, aqueous, oil and gas products from sludge hydrothermal carbonization at different temperatures are well characterized, with a focus on the amino acid species in various products. Temperature is found to greatly affect the nitrogen transformation during sludge hydrothermal carbonization. At 160 °C, 47.3% of nitrogen is transformed into the aqueous product. When the temperature increases to 250 °C, only 27.1% of nitrogen is retained in the hydrochar, while 69.2 and 6.7% of nitrogen is present in the aqueous and oil products, respectively. During hydrothermal carbonization, the protein-N is first converted into the polypeptide-N in the aqueous product, followed by its further decomposition into the NH+ 4-N. This leads to a high content of the NH+ 4-N in the aqueous product, especially at increased temperatures. The labile protein-N is also transformed into the heterocylic-N (especially the pyrrole-N) in the hydrochar as the temperature increases. Among all nitrogen species in the aqueous product, the polypeptide-N consisting of amino acids with the alkyl group is the most stable. Moreover, the addition of NaAc and CaAc₂ reduces the nitrogen retention in the hydrochar, mainly due to enhanced hydrolysis of the protein-N. While for CaAc₂ addition, the deamination of the polypeptide-N is also enhanced, leading to a higher NH+ 4-N in the aqueous product. Our results show that the type of amino acid in protein is important to determine the nitrogen transformation pathways, and acetate addition is an important strategy for enhancing nitrogen removal in the hydrochar during hydrothermal carbonization.