Interaction of SO2 with a Cu–Mn oxide oxygen carrier during chemical looping with oxygen uncoupling

Abstract

Chemical looping with oxygen uncoupling (CLOU) is a variant of the chemical looping combustion (CLC) process, where fuel combustion occurs via metal oxides, known as oxygen carriers. The purpose of this study is to investigate how SO₂ interacts with a Cu–Mn oxide oxygen carrier during CH₄ combustion in the CLOU process. The oxygen carrier was reduced via CH₄–N₂ gas mixture with and without SO₂ in a batch fluidized-bed reactor. The results indicate that SO₂ does not affect the oxygen release capacity of a Cu–Mn oxide. However, it negatively affects the CH₄ conversion by increasing CO formation as well as the unreacted CH₄ amount. This adverse effect becomes more pronounced as the SO₂ concentration increases. Increasing the temperature from 850 to 950 °C improves the conversion of CH₄. During a reduction cycle, SO₂ oxidizes to SO₃, creating a competing demand for oxygen between fuel and SO₂, and lowers CH₄ conversion if there is not enough oxygen for both reactions. Hence, decreasing the feed CH₄ concentration with a fixed oxygen carrier amount improves CH₄ conversion in the presence of SO₂. Additionally, characterization of SO₂-exposed oxygen carriers shows that sulfate species form on the surface, possibly causing a further reduction in CH₄ conversion. However, the presence of SO₂ does not cause a permanent deactivation of the particles and they can be fully regenerated during the oxidation cycle.