Potassium fixation and ash fusion behavior in biomass and sewage sludge co-gasification

Biomass gasification is an efficient method for utilizing biomass energy. However, the widespread adoption of this technology is limited by ash-related issues caused by potassium, which can disrupt the stable operation of gasification systems. This study explored the co-gasification of sewage sludge (SS) and corn cob (CC) to address this challenge. The high SiO2 and Al2O3 content in SS reacted with potassium to form the high-melting-point mineral leucite (KAlSi2O6), thereby facilitating potassium fixation in the gasification residues. At 1000 °C, the potassium retention in the gasification residues without SS addition was only 65.68%. However, when 15% and 20% SS were added, potassium retention increased significantly to 94.71% and 92.13%, respectively. The deformation temperature (DT) and softening temperature (ST) of the CC ash were 984 °C and 1041 °C, respectively. After adding 5% SS, both DT and ST increased to 1317 °C and 1349 °C, demonstrating a significant improvement in the ash fusion characteristics. Furthermore, FT-IR, XPS, and SEM-EDS analyses revealed that the melting of KCl and potassium silicate was the main cause of ash fusion and slagging. Additionally, the phosphorus in SS was found to exist in the molten phase as (PO4)3-. These findings confirm that SS can effectively reduce potassium volatilization and mitigate ash-related issues during biomass gasification. This study also offers new insights into the causes of biomass ash fusion and the fate of phosphorus, providing potential strategies for the recycling of gasification residues.