99/02831 Study on concentrations and sources of heavy metals in atmospheric particulate matters in Qingdao urban districts

Abstract

Cerium oxide has been studied as a candidate second-generation sorbent for high-temperature gas desulfurization. Although CeO2 will react with H2S, the reaction thermodynamics do not permit H2S target levels of about 20 ppmv to be reached. However, at high temperature in a strongly reducing gas, CeO2 is reduced to a non-stoichiometric oxide, CeOn (n<2), which is superior to CeO2 in removing H2S. The reduction and sulfidation reactions were studied in a fixed-bed reactor as a function of temperature, pressure, and feed gas composition and flow rate. With feed rates corresponding to about 1.4 s reactor residence time, the H2S concentration was reduced to less than 10 ppmv at temperatures as high as 850°C and to near 1 ppmv at 700°C. Effectively complete conversion of CeOn to Ce2O2S was achieved, and the sorbent showed excellent durability through 25 reduction–sulfidation–regeneration cycles. CeOn produced in a separate pre-reduction step was marginally more effective than when the reduction–sulfidation reactions occurred simultaneously. However, both approaches were capable of reducing the H2S to below the 20-ppmv target level. The cerium sorbent is most effective at temperatures and in reducing gas compositions where first-generation zinc-based sorbents cannot be used.