Performance of zinc oxide based sorbents for hot coal gas desulfurization in multicycle tests in a fixed-bed reactor

Zinc oxide based sorbents doped with CuO or TiO2 have been studied in 5-cycles tests in a fixed-bed reactor, as regenerable sorbents for desulfurization of coal gas at high temperature (600°C). TiO2 increases the stability of ZnO and zinc ferrite, under the reducing power of coal gas, and the sorbent porosity. However, it also increases the H2S concentration in the outlet gas before breakthrough, and the sorbent reactivity is not substantially modified. In addition, the presence of TiO2 makes more difficult the formation of mixed oxides and the behaviour of the fresh sorbents is usually different than that of the first regenerated samples. The presence of CuO increases sorbent reactivity and the efficiency for the first 1–3 cycles is excellent. Unfortunately, neither CuO nor TiO2 can prevent the excessive decay in performance of the studied sorbents as the number of cycles increases. This feature appears correlated, not with structural changes as shown by XRD, but with a decrease of the sorbent porosity due to progressive thermal sintering. The presence of iron oxides in the sorbent composition causes different behaviour with the appearance, after breakthrough, of COS in sulfidation and H2S and elemental sulphur in regeneration.