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

Abstract

<p>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 CO<sub>2</sub> gasification system has been proposed to effectively improve the gasification efficiency and considerably reduce the emissions of NO<sub>x</sub> precursors (i.e., NH<sub>3</sub> and HCN) in SS treatment. Yield and quality of syngas were comprehensively investigated during CO<sub>2</sub> gasification in terms of various temperatures with different dosages of CaO additive in HTC, and varied reaction temperature and CO<sub>2</sub> percentage in gasification process. On the whole, CO<sub>2</sub> 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/Nm<sup>3</sup>. Production of NO<sub>x</sub> precursors was greatly reduced due to formed non-active quaternary-N in HC, while HTC with CaO favored the mitigated NO<sub>x</sub> precursors in syngas owing to enriched pyrrole-N and quaternary-N therein. Catalytic tar decomposition and Boudouard reaction in CO<sub>2</sub> 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 CO<sub>2</sub> atmosphere can maximize syngas yield, lower portion of CO<sub>2</sub> (ca. 20 %) was more beneficial to drastically mitigate emissions of NO<sub>x</sub> precursors, especially the reinforced transformation of NH<sub>3</sub> to N<sub>2</sub>. The fundamental knowledge could ultimately help to achieve significant abatement of NO<sub>x</sub> precursors during CO<sub>2</sub> gasification for improved syngas production.</p>