Insight into the influence mechanism of inherent ash in pyrolysis recycling of sewage sludge: Dual roles of catalysis and physical barrier

Pyrolysis can convert sewage sludge (SS) into high-value products, while reducing its environmental hazards. However, SS contains significant amount of inherent ash (20–70 %), which can complicate the pyrolysis process. Understanding the impact of inherent ash is crucial for optimizing SS pyrolysis. In this study, the HCl-HF stepwise dissolution method was used to decouple the organic matter and ash in SS. The effects of inherent ash on the pyrolysis process and the physicochemical properties of the products were investigated within the temperature range of 300–700°C. Results showed that the inherent ash exhibited both physical barrier and catalytic effects during pyrolysis. Inert substances in the ash hindered heat transfer and reduced the physical volatilization of fatty acids at lower temperatures, increasing the activation energy for pyrolysis. At the end of the reaction (α=0.9), the activation energy for raw sludge (RS) was about 450 kJ/mol, compared to 300 kJ/mol for de-ashed sludge (DS). At the same time, metal compounds in the ash catalyzed C-C bond cleavage and decarboxylation reactions, promoting the transformation of biochar/bio-oil into syngas, thereby increasing CO₂ and CH₄ yields. At 700°C, the CO₂ and CH₄ yield of RS (based on organic matter) was 299.34 mg/g and 106.79 mg/g, while 166.16 mg/g and 9.30 mg/g for DS. The inherent ash also stabilized C/N elements in biochar and promoted secondary reactions, enhancing nitrogen-containing heterocycles and oxygen/nitrogen compounds in bio-oil. This study provides insights into the role of inherent ash in SS pyrolysis and aids in optimizing process parameters for high-value SS utilization.