Insight on oxygen-carrying and catalyst performance of sludge ash during Fe-rich sludge gasification

Abstract

Polyferric coagulants are employed in municipal wastewater treatment, resulting in sludges with higher iron concentrations. In this study, iron-rich sludge ash was utilized as an oxygen carrier to systematically explore its catalytic performance in sludge gasification. The oxygen carrying performance of sludge ash was investigated by thermogravimetric analysis (TGA) and multi-cycle testing in fixed bed, the oxygen release characteristics, oxygen carrying capacity and cycling stability of sludge ash were analyzed. TG analysis showed that sludge ash had peak oxygen release capacity at 775– 850 °C, with the highest oxygen release of 15.9 % in H₂ and 5.6 % in CO initially, the oxygen-carrying capacity under syngas fell between the two values during subsequent redox cycles, and the sludge was gasified in a chemical looping gasification (CLG) way. The oxygen-carbon (O/C) ratio, significantly affected syngas quality at the beginning of the redox cycle in sludge CLG. When the initial O/C ratio was 1:10, the oxygen carrier performance of sludge ash was optimal, and the hydrogen yield was reaching 63.1 %, and syngas calorific value of 15.4 MJ/Nm³ . With the increasing cycles, there was a decrease in syngas quality due to the reduction in sludge ash 's oxygen carrier performance, the sintering and pore structure degradation occurred. The optimal redox cycles with O/C ratio of 1:5, 1:10 and 1:20 were determined to be 3, 4, and 5 cycles, respectively. This study provides a theoretical foundation for sludge resource utilization and confirms the feasibility of sludge ash as a low-cost iron-based oxygen carrier in CLG technology.