Speciation and transformation of nitrogen for sewage sludge hydrothermal carbonization-influence of temperature and...

Hydrothermal carbonization (HTC) is an effective means of energizing high-water-content biomass that can be used to convert sewage sludge (SS) into hydrochar and reduce nitrogen content. To further reduce the emission of NO_x during the combustion of hydrochar and seek proper disposal method of liquid product, the mechanism of nitrogen conversion was studied in the range of 180-320 °C and 30-90 min. At 180-220 °C, 42.15-52.91% of the nitrogen in SS was transferred to liquid by hydrolysis of proteins and inorganic salts. At 240-280 °C, the nitrogen in hydrochar was mainly in the form of heterocyclic -N (quaternary-N, pyrrole-N, and pyridine-N). The concentration of NH₄⁺-N increased from 6.82 mg/L (180 °C) to 26.58 mg/L (280 °C) due to the enhancement of the deamination reaction. At 300-320 °C, pyrrole-N (from 15.92% to 9.38%) and pyridine-N (from 5.52% to 3.73%) in the hydrochar were converted to the more stable quaternary-N (from 0.31% to 4.28%). Meanwhile, the NH₄⁺-N and amino-N in the liquid decomposed into NH₃. Prolonging the carbonization time promoted the hydrolysis of proteins, the conversion of heterocyclic -N, and the production of NH₃. Under optimal reaction conditions (280 °C and 60 min), the nitrogen in the SS is converted to stable forms and the energy balance meets the requirements of circular-economy. The results show that temperature determines the nitrogen form and the carbonization time affects the nitrogen distribution. So HTC has the potential to reduce NO_x emissions from SS energy utilization processes.