Coupling influences of organic components and temperature on nitrogen transformation and hydrochar characterization during hydrothermal carbonization of sewage sludge

Nitrogen (N) in sewage sludge (SS) should be reduced if it is to be used to produce clean solid fuels. However, the N transformation during hydrothermal carbonization (HTC) of SS is not yet fully understood. Since the composition of SS is complex, it is wise to study a model compound, which should have typical functional groups of organic compo-nents. Hence, in this study, six model components (protein, lipid, cellulose, hemicellulose, humic acid, and lignin) representing the main organic components in SS were mixed with SS and treated at 150-270 center dot C for 1 h. The influence of the organic component and reaction temperature on hydrochar yield, hydrochar characterization, and N distribu-tion in the products was investigated. Except for proteins and lipids, all the other components were found to contribute to the N content and aromatization of the hydrochar. Humus shows the best comprehensive performance in terms of both reducing the N content and increasing the aromaticity. The strongest effects of hemicellulose and cellulose on N retention in hydrochar are found to occur at 210 center dot C and 240 center dot C, respectively. The N retention caused by lignin is correlated with the Mannich reaction at 240 center dot C, while humus significantly promotes N transformation at 240 center dot C. For carbohydrates, lignin, and humus, the temperatures required for increasing the N content and aromaticity maintain a high degree of consistency. Although protein pulls down the energy recovery (ER) and yield of the hydrochar, obser-vations indicate that it favors the carbonization process. This finding can be used for estimating the N content and qual-ity of hydrochar and provides references for future research targeting the upgrading of hydrochar.

Automated summary

This study investigates the influence of model compounds on nitrogen transformation during the hydrothermal carbonization (HTC) of sewage sludge (SS). It is found that the organic components can reduce nitrogen content and increase the aromaticity of the hydrochar, with humus showing the best performance. Hemicellulose and cellulose have the strongest effects on nitrogen retention, while lignin and humus promote nitrogen transformation. Although protein decreases energy recovery and yield, it favors the carbonization process. This research provides references for estimating nitrogen content and quality of hydrochar, and for future research on hydrochar upgrading.