Gasification performance of wet hydrochar from co-hydrothermal carbonization of high-moisture sludge and fungus bran

Abstract

Rational utilization of waste biomass is an important renewable energy alternative to address environmental issues caused by fossil fuels and climate change. Hydrothermal carbonization can improve the quality of municipal sewage sludge (SS) fuel by reducing water content, removing chlorine, and increasing energy density. This paper proposes an in-situ steam gasification method for syngas production. The influences of original water vapor content of hydrochar (0–30 %); hydrothermal conditions including severity factor (SF), fungus–bran mixing ratio (FBMR), and the proportion of citric acid mixed promoting agent (CAPA); and gasification temperature (700–900°C) on the composition of syngas. The effects of pure hydrochar gasification and mixed gasification on syngas composition were examined. Response surface methodology was used to optimize and maximize the higher heating value (HHV) of syngas. In addition, life cycle assessment was used to evaluate the environmental impact of the wet hydrochar gasification process. The vapor atmosphere generated automatically by heating wet hydrochar promoted steam reforming and water-gas shift reactions, driving them towards hydrogen production. Excess steam consumed a large amount of energy, leading to decreased reactor temperature, which in turn reduced gasification reaction activity and production of various gas components. As the SF increased, the syngas yield tended to decrease and the gasification reaction index decreased. Conversely, as the FBMR increased, the contents of all syngas components increased, along with the gasification reaction index. Additionally, as the gasification temperature increased, the HHV of the syngas increased. The quality of the syngas produced by mixed gasification was significantly better than that produced by pure hydrochar gasification. Compared to the syngas from the gasification of dry SS hydrochar, the HHV of the syngas from wet SS hydrochar increased by 4.4 %. At optimal conditions of SF 0.1, FBMR 20 %, and CAPA 20 %, the HHV of the syngas from wet hydrochar gasification increased by 28.1 %. Under these optimal conditions generated the highest quality syngas; at a gasification temperature of 900℃, the environmental impact was minimal. The lower quality of syngas and economic costs of drying hydrochar are considered disadvantages of traditional dry hydrochar gasification. However, the high-quality syngas produced from wet hydrochar in this study addressed these issues, laying a solid foundation for future industrial scale applications.