Clay mineral as catalysis for controlling the nitrogen containing pollutants during sewage sludge pyrolysis

Pyrolysis technology is considered one of the most promising processes for the environmentally friendly disposal of sewage sludge (SS), as it can neutralize pathogens, reduce hazardous substances, and promote the immobilization of heavy metals. However, nitrogen-containing gases produced in SS pyrolysis can be converted to nitrogen oxides, causing serious environmental pollution. In this study, we investigated the evolution of the nitrogen (N) element in rapid pyrolysis of SS and explored the effect of clay minerals (attapulgite, montmorillonite, and kaolin) in regulating N conversion. The results showed that the higher temperature (800 °C) could promote the conversion of pyrroles/pyridines and NO_x precursors in char to N₂ (the conversion rate was 32.76 %), and clay minerals catalyzed the cleavage of N-containing macromolecules in the bio-oil, reducing the N content in bio-oil from 28.70 % to 6.23 %, and was conducted to realize the denitrification of bio-oil. Notably, the attapulgite (ATP) on N migration was more effective and could reduce the yield of NO_x precursors from 23.80 % to 10.55 % by capturing NH₄* and inhibiting the secondary reaction, while catalyzing the removal of N₂ from pyridine/pyrrole (N₂ production increased to 34.38 %). MgO and CaO in the clays played a major role in facilitating the conversion of char-N to N₂, and clay structures loading on the biochar surface promoted the catalysis of N-containing volatiles to N₂ by metal oxides. This study provides a viable and harmless approach to SS minimization.