Sequential hydrothermal carbonization and CO₂ gasification of sewage sludge for improved syngas production with mitigated emissions of NO_x precursors

Due to high moisture and protein contents in sewage sludge (SS), conventional thermal treatment of SS is energy-intensive and a large amount of harmful nitrogen-containing gases are emitted. In this study, a sequential hydrothermal carbonization (HTC) and CO₂ gasification system has been proposed to effectively improve the gasification efficiency and considerably reduce the emissions of NO_x precursors (i.e., NH₃ and HCN) in SS treatment. Yield and quality of syngas were comprehensively investigated during CO₂ gasification in terms of various temperatures with different dosages of CaO additive in HTC, and varied reaction temperature and CO₂ percentage in gasification process. On the whole, CO₂ gasification of SS derived hydrochar (HC) demonstrated obvious reduction of tar formation from 10.9 to 6.8 % and enhancement of calorific value of formed syngas from 14.1 to 15.7 MJ/Nm³. Production of NO_x precursors was greatly reduced due to formed non-active quaternary-N in HC, while HTC with CaO favored the mitigated NO_x precursors in syngas owing to enriched pyrrole-N and quaternary-N therein. Catalytic tar decomposition and Boudouard reaction in CO₂ gasification were responsible for the distinct reduction of tar formation, and notable increase of carbon conversion ratio and syngas yield. Although facilitated catalytic gasification in CO₂ atmosphere can maximize syngas yield, lower portion of CO₂ (ca. 20 %) was more beneficial to drastically mitigate emissions of NO_x precursors, especially the reinforced transformation of NH₃ to N₂. The fundamental knowledge could ultimately help to achieve significant abatement of NO_x precursors during CO₂ gasification for improved syngas production.