Insights into optimal gas-ash-energy nexus: Oxy-steam combustion of spent pot lining

Altering the atmosphere type may boost the physicochemical drivers of the combustion, thus contributing to the circularity and sustainable management of hazardous wastes. This study aimed to compare the combustions of spent pot lining (SPL) in the oxy-steam and air atmospheres. The SPL combustion with 21% (Oxy21), 30% (Oxy30), and 40% (Oxy40) O2 in the oxy-steam atmosphere improved the performance indices. Weighted mean activation energy (Em) fell from 148.36 kJ/mol in the air atmosphere to 118.04 kJ/mol in Oxy21, with the lowest Em value occurring in Oxy30 (referred to as energy-saving pathway). With the temperature rise, the pattern of fluorine distributions in the air atmosphere (the oxy-steam atmosphere) exhibited 65.60 → 92.27% (65.87 → 9.08%) of fluorine in bottom ash, 7.33 → 31.84% (22.31 → 72.83%) in off-gas, and 0.4 → 2.56% (11.82 → 22.22%) in fly ash. The increased O2 concentration slightly affected the combustion characteristic indices and fluorine distributions. Steam appeared to enhance the pore expansion. The advance decomposition of Na3AlF6, the new emergence of NaAlO2, and the disappearance of CaF2 characterized the oxy-steam combustion of SPL. The gas-ash-energy nexus was jointly optimized via artificial neural network. The oxy-steam combustion destroyed the stability of both soluble and insoluble fluorine, thus greatly releasing fluorine-bearing gas.