Co-hydrothermal carbonization of sewage sludge and corn straw: Physicochemical properties and gasification performance via process simulation using Aspen plus

The addition of corn straw (CS) to sewage sludge (SS) for co-hydrothermal carbonization (co-HTC) can increase the importance of hydrochar as an alternative to fossil fuels. This study aims to achieve a synergistic, proper, and innovative treatment of SS and CS through combined HTC and gasification. Therefore, the physicochemical properties of co-hydrochar samples prepared at different HTC temperatures and CS ratios were investigated, and the CO2 gasification performances at these conditions were also analyzed. The results showed that the hydrochar exhibited a substantial increase in specific surface area (SSA) and a decrease in the oxygen-to-carbon and hydrogen-to-carbon ratios compared to the feedstock. This suggests that HTC has the potential to improve fuel quality. Specifically, the SSA of the hydrochar was found to be 1.1–3.6 times higher than that of the feedstock. The reduction of the CO group in the co-hydrochar indicated the occurrence of the Maillard reaction during the co-HTC process. The content of alkali and alkaline earth metals in the co-hydrochar was high at the CS ratios of 25% and 50%. Owing to the addition of CS, the content of heavy metals in the co-hydrochar decreased, reducing the risks of environmental hazards caused by the utilization of hydrochar. The synergistic effect of hydrochar on the gasification performance increased with the CS content. The cold gas efficiency reached the highest value of 90.93% at the reaction temperature of 260 ℃ and CS mixing ratio of 25%. The results of this study indicate that the co-HTC process of SS and CS, combined with the gasification technology, is a promising method that can be used to optimize sewage treatment while minimizing the associated environmental risks.