Carbon sequestration for high-quality sludge-based carbon preparation via K/Na bi-molten salts pyrolysis

Pyrolysis carbonization of sewage sludge is employed to achieve carbon sequestration and access carbon resources, while the quality of the obtained sludge-based carbon (SBC) is poor due to high ash contents and volatile organic matter. Here, carbonization in KOH/Na₂CO₃ (K/Na) bi-molten salts was developed for SBC preparation, improvement of carbon exploitation from biomass, and to reduce the contents of ash and volatile organic matter. The results showed that the surface area and pore volume of SBC under optimized conditions reached 1631 m² g^-1 and 1.312 cm³ g^-1 at 700 °C, respectively, with a K/Na bi-molten salts/sludge ratio of 2:1 (K:Na = 5:5). Moreover, over fivefold the higher surface area and 43.61% amount of carbon element could be obtained, with a decrease in the mass loss rate for sludge pyrolysis of 25%. The mechanism behind the higher surface area of the SBC was identified and divided into three stages: intense dehydration and dehydrogenation caused by molten salt-enhanced polycondensation of protein and polysaccharide (200-400 °C), strongly reduced carbon-oxygen structure after deoxygenation reactions (400-600 °C), aromatization and cyclization of long-chain fatty acids triggered by deamidation of tar catalyzed by molten salts (600-900 °C). Eventually, 14.63% carbon was sequestered for the high-surface-area SBC prepared by K/Na bi-molten salts system.