The redistribution and migration mechanism of nitrogen in the hydrothermal co?carbonization process of sewage sludge and lignocellulosic wastes

Blending lignocellulosic wastes (such as cornstalk, CS) into sewage sludge (SS) for hydrothermal carbonization (HTC) could contribute to the importance of the hydrothermal solid product (hydrochar) as a substitute for fossil fuel. However, the interactions between SS and CS changed the fate of Nitrogen (N), affecting the clean combustion utilization of hydrochar. This study focused on the influence of SS-CS interactions on the redistribution and migration behavior of N during the co-HTC process by tuning the mass ratio of SS to CS (SS:CS), reaction temperature, and residence time. Under the hydrothermal condition of 220 degrees C, 2 h, and SS:CS = 1:1, the high heating value of hydrochar and the energy recovery efficiency (ERE) respectively reached 15.89 MJ/kg and 71.19%. Further raising the temperature to 250 degrees C, the hydrochar was enhanced in the coalification degree, whereas ERE decreased to 61.86%. Part of the amino-N in sludge organics was fractured during the co-HTC process and reacted with carbohydrate and intermediate products, such as 5-hydroxymethylfurfural, which degraded from CS, to generate heterocyclic-N compounds (including pyridine, pyrrole, and pyrazine). The remaining amino-N formed pyridine-N, pyrrole-N, and quaternary-N through various solid-solid conversions. The heterocyclic-N polymerized and formed melanoidins, which thereafter polymerized with aromatic clusters to form the N-containing polyaromatic char. Therefore, the N retention rate (NRR) was enhanced and showed a synergistic effect. NRR was increased by raising the proportion of CS or extending time, reaching 57.02% at SS:CS = 1:1 and 8 h. Conversely, rising temperatures resulted in a downward trend of NRR with a phased increase at 220 degrees C-250 degrees C. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study investigates the influence of sewage sludge-cornstalk interactions on the redistribution and migration behavior of nitrogen during the co-hydrothermal carbonization process. It was found that the retention rate of nitrogen can be enhanced by increasing the proportion of cornstalk or extending the reaction time, indicating a synergistic effect in nitrogen retention. Additionally, the study reveals the formation of heterocyclic-N compounds and N-containing polyaromatic char during the co-hydrothermal carbonization process.