Iron oxide supported on alumina is a promising catalyst/absorbent for use in the simultaneous removal of NO, and SO, from power plant stack gases. A dry-contacting process is under development which would operate under net reducing conditions at temperatures of 370° to 540OC. Iron oxide is converted to the ferrous state, NO is reduced to Nz or NH3, and SOz is removed as ferrous sulfide or sulfate. Regeneration with air produces SO2 and reforms Fe20s.The reduction of SOz by CO and H2 was studied in fixed-bed reactors to determine the effects of temperature and of the other reactive components of flue gas (excepting fly ash) on the rate of reaction and the products formed. H2S and COS react with FeO to form FeS. Under readily attainable conditions, virtually complete removal of sulfur compounds was achieved for gas-phase residence times of about 1 to 10 ms. NO and Oz were also reduced. Conditions under which oxygen poisons the catalyst were determined.
DAVID T. CLAY and SCOTT LYNN Department of Chemical EngineeringUniversity of California Berkeley, Colifornia 94720
SCOPESulfur dioxide and nitric oxide are major air pollutants in the United States. Fossil-fueled power plants are major sources of both. Wet processes for SO2 removal are currently being developed commercially, but to date no process for simultaneous SO2 and NO removal has passed the laboratory scale. A dry removal process would be preferred, since this would avoid flue-gas reheating, the problems of contacting the gas with a liquid or slurry, and the crystallization phenomena usually encountered in flue-gas scrubbing. The objective of this study was to develop a dry process which not only achieves high NO and SO2 removals, but also converts NO to innocuous materials and SO2 to a salable product and is potentially economical to operate.The process flow sheet is shown in Figure 1. Iron oxide, supported on alumina, acts as a catalyst for the reduction of both SOz and NO by CO and H2. Simultaneously, it acts as an absorbent for sulfur compounds, converting them to FeS. The catalyst/absorbent is in the form of fine pellets (0.05 to 0.10 mm) and contacts the gas as a falling bed in concurrent flow at 370° to 540OC. The solid is regenerated with air at 675OC to produce a rich stream of SO2, suitable for conversion to H2SO4 or elemental sulfur. Fez03 is reformed in the particles. Experiments in fixed-bed reactors using mixtures of helium and one or more reactant gases were run to demonstrate the basic process chemistry, to estimate rates for preliminary design purposes, and to look for interfering reactions. A preliminary estimation of the capital and operating costs of the process was made on the basis of the results obtained.
CONCLUSIONS AND SIGNIFICANCEThe overall reactions listed in Table 1 were shown to proceed rapidly at 370" to 540°C in fixed-bed reactors using pellets of supported iron oxide 3.2 mm x 3.2 mm or particles 0.25 to 0.50 mm diameter. Iron oxide was reduced to the ferrous state, SO2 was reduced by both CO and H2, and the sulfur was absorbed ...