Integrated Study of Organic Components for Hydrochar's Properties and the Nitrogen Transformation Mechanisms During Hydrothermal Carbonization of Sewage Sludge

Abstract

High nitrogen (N) content in the hydrochars derived from sewage sludge (SS) would result in the emission of undesirable pollutants during its combustion. However, the N transformation during hydrothermal carbonization (HTC) of SS is not yet fully understood. Hence, this study investigates the effects of organic components on N transformation during the HTC process and the properties of hydrochars. Six model components (protein, lipid, cellulose, hemicellulose, humic acid, and lignin) were mixed with SS and treated at a wide temperature range (150 °C-270 °C) for 1 h to amplify its effects. Except for protein and lipid, all the other components contributed to the N content and aromatization of the hydrochar to varying degrees, among which carbohydrates were the most beneficial to the production of Solid-N. The effects of hemicellulose and cellulose are strongest at 210 °C and 240 °C, respectively, and at the same time improved the aromaticity of hydrochar. The N retention caused by lignin occurred mainly in the Mannich reaction at 240 ℃. Humus was favorable for the transformation of N from Solid-N to Liquid-N to Gas-N at about 240 ℃. Compared with carbohydrates and lignin, humus had the best comprehensive performance in reducing N content and increasing aromaticity. The reaction temperatures of carbohydrates, lignin, and humus with increasing N content and aromaticity maintained a high degree of consistency. Although the protein pulled down the ER and yield of the hydrochar, evidence indicated that it favored the carbonization process. This finding can be used to assess the hydrochar N content derived from different SS, providing guidance for producing high-quality solid fuel through the HTC process.

Keywords: Sewage sludge, hydrochar, Nitrogen transformation, Hydrothermal carbonization, Solid fuel