Sequential hydrothermal carbonization and CO2 gasification of sewage sludge for improved syngas production with mitigated emissions of NOx 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 hy-drothermal carbonization (HTC) and CO2 gasification system has been proposed to effectively improve the gasification efficiency and considerably reduce the emissions of NOx 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 NOx precursors was greatly reduced due to formed non-active quaternary-N in HC, while HTC with CaO favored the mitigated NOx 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 NOx precursors, especially the reinforced transformation of NH3 to N2. The fundamental knowledge could ultimately help to achieve significant abatement of NOx precursors during CO2 gasification for improved syngas production.