Characterization of Mn and Cu oxides as regenerable sorbents for hot coal gas desulfurization

To enhance the formation of different mixed oxides, which potentially might stabilize copper in oxidation states 2+ and/or 1+, regenerable sorbents for hot coal gas desulfurization have been prepared by calcination at 950°C of MnO2 and CuO powders in different mole ratios. The fresh sorbents were sulfided at 600°C using a simulated coal gas from cylinders, reduced in identical operating conditions compared to sulfidation in the absence of H2S, and regenerated at 710°C in oxidising atmosphere. Successive sulfidation–regeneration cycles were carried out in a quartz fixed-bed reactor. The fresh, reduced, sulfided or regenerated sorbents were characterized by SEM-EDX, XRD, TPR, XPS and FTIR spectroscopy. The study shows that, under the reducing power of coal gas at the operating conditions used, manganese oxides cannot prevent the reduction of copper oxides and, consequently, the expected beneficial effect derived from copper oxide addition, reducing the equilibrium H2S concentration in the outlet gas to very low levels cannot be achieved. In addition, copper oxides do not prevent the tendency of manganese-based sorbents to form sulfate species in an oxidative atmosphere and the regeneration of these sorbents has to be carried out at higher temperature.