Optimizing S Chemical Looping Combustion with Cu-Fe Combined Oxygen Carriers: Performance and Mechanistic Insights

This study focuses on the S-to-H₂SO₄ industry by investigating the chemical looping combustion (CLC) process utilizing Fe-based and Cu-based oxygen carriers (OCs), which are widely applied in CLC technology. The primary objective is to conduct combined CLC reactions of these two metal carriers in a three-zone temperature tube furnace, aiming to achieve a higher SO₂ yield than what is attainable by reacting a single metal carrier with S. The investigation reveals promising industrial applications, offering potential benefits in terms of reducing equipment costs, enhancing energy efficiency, and lowering the emissions of the H₂SO₄ production industry. Through a series of experiments, the study examines the effects of reaction temperature and material molar ratios on SO₂ generation. The solid reaction products were characterized using X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), and X-ray photoelectron spectroscopy (XPS). The experimental results indicate that the combined Cu-based and Fe-based OCs exhibit a higher SO₂ yield during the reduction stage compared to using either Fe-based or Cu-based OCs independently. Under optimal conditions, with a carrier gas flow rate of 300 mL/min, an Fe₂O₃/S molar ratio of 6:1 in the second temperature zone, and a reaction temperature of 900 °C, the total SO₂ yield in the third temperature zone reached approximately 85%. This was achieved at a reaction temperature of 850 °C, with an Fe₂O₃/S molar ratio of 6:1 in the first half of the zone and a CuO/S molar ratio of 12:1 in the second half of the zone. SEM-EDS analysis further revealed that the combined OCs showed no significant signs of agglomeration or sintering after 10 cycles of the reaction. However, Cu-based carrier particles increased in size by 50%, while Fe-based carrier particles remained relatively stable. Additionally, the low mass-to-atom ratio of S on the surface of OCs after the cyclic reaction suggests that the reduced-state OCs can be fully oxidized and regenerated following the release of SO₂ during oxidation.