The Role Elucidating of Eps in Thermal Hydrolysis and Nitrogen Conversion of Sewage Sludge During Hydrothermal Carbonization Process, and its Effect on Hydrochar's Properties

Abstract

The extracellular polymeric substances (EPS) were stripped at different strengths. Then, hydrothermal carbonization (HTC) was conducted to observe the effect of EPS structure on the nitrogen (N) migration mechanism during the HTC process. The EPS and the cell membrane were the main factors that hindered the organic matter hydrolysis and N-removal. Under hydrothermal conditions, the N-removal efficiency N in extracellular was higher than that of intracellular. The N retention efficiency of hydrochar showed a decreasing trend as the reaction temperature increased from 180 °C to 270 °C. Compared with the SS after deep stripping of EPS (from 40.00% to 28.75%), the presence of EPS led to a more obvious decrease in N retention efficiency (from 59.24% to 34.39%) and was more sensitive to the reaction temperature. The cell membrane was the key factor hindering the conversion of N to the aqueous products at low temperature (180 °C). The N/C of hydrochar at 180 °C without cell membrane protection was 0.06, which dropped to half that of SS (0.11). The presence of cell membrane caused the C and N contents of hydrochar to be insensitive to reaction temperature, whereas the increase of reaction temperature weakened the influence of EPS on hydrochar's properties. The N-containing components in the cell were more easily converted into stubborn N-containing heterocycles, such as heterocyclic-N and quaternary-N, which hindered the deep N-removal of hydrochar. This study revealed the influence of EPS structure on N-migration in hydrothermal environment and gave a significant reference for the preparation of clean solid fuels.

Keywords: Keywords: hydrothermal carbonization, nitrogen migration, Sewage sludge, solid fuels, extracellular polymeric substances