Oxygen-carrying characteristics of sludge ash in a dual-stage reactor during sewage sludge gasification

To achieve efficient resource-oriented disposal of sludge, this study employed sludge ash （SA） as the bed material and investigated its oxygen-carrying performance and action mechanism during sludge gasification in a dual-stage reactor. By varying the SA arrangement, reactor type, and temperature gradient in the dual-stage reactor, combined with TGA, GC, and XRD analyses, the evolution of SA’s oxygen-carrying characteristics and its influence on gasification efficiency were systematically studied. The results showed that SA achieved an oxygen release and uptake of 15.9% in the first H₂/Air cycle, which stabilized at approximately 12% after three cycles. However, the oxygen-carrying rate decreased to 2.3% after multiple CO/Air cycles, and its oxygen-carrying mechanism relies on the reduction-oxidation of Fe₂O₃ and Fe. In the dual-stage reactor gasification experiments, the fluidization state enhanced gas-solid mass transfer, resulting in higher proportions of H₂ and CO in the syngas and better carbon conversion efficiency compared to the fixed bed. The fluidized bed with dual-stage arranged SA exhibited the optimal performance, where the proportions of H₂ and CO reached 79.5% in the late reaction stage, and the carbon conversion efficiency peaked due to the synergistic effects of CH₄ oxidation and tar cracking by the dual-stage oxygen carriers. When the temperature of the lower section was 600 ℃, the lattice oxygen release of SA was reduced, resulting in the oxygen-carrying performance being inferior to that of the 850 ℃ condition.