Regenerative limestone slurry process for flue gas desulfurization

Abstract

The feasibility of the regenerative limestone slurry process was demonstrated on a bench scale. CaCOB and H2S were recovered from waste sludge from the limestone slurry flue gas desulfurization process. CaC03 would be recycled to the scrubber. H2S would be converted to elemental sulfur in a Clause plant. At 950-980 "C in the presence of coke or coal, 95% of the Cas04 and Cas03 in the dry sludge was reduced to Cas. The rate-determining reaction was the reduction of Cas04 to Cas. The estimated rate constant for this reaction a t 850-980 "C wash (min-l) = 1.7 X 10l3At 50 "C, carbonation of an aqueous slurry of the resulting CaS gave more than 98% conversion to CaC03 in 35 min. The rate constant for the carbonation at 30-55 "C was h (min-') = 8.2 X 10 e-2s/RT. The overall efficiency of the regeneration was 91-95%. The utilization of successively regenerated limestone was 70% compared to 80% utilization for fresh limestone.The limestone slurry throwaway process is considered the most advanced for control of SO2 emissions from power plants ( 1 , 2 ) . Solid waste disposal is still a serious unsolved problem with this process ( 3 ) . A regenerative process to recover CaC03 for reuse and to produce S in a salable form offers a number of advantages over the limestone throwaway process:The reuse of CaC03 reduces raw material and transportation costs.Because the limestone is recycled, only a small amount of makeup limestone is needed. Therefore, capital and operating costs for grinding are reduced significantly.Solid waste disposal costs are almost eliminated.* Holding ponds for waste slurry and regenerated CaC03can be used to maintain a constant production rate in the regeneration plant. There are three significant advantages here: Since CaCOz is inexpensive, inventory costs will be low; Ca salts can be stored in solid form, which is more convenient and less costly than storage in the liquid or gaseous forms as in other recovery processes; and if it were advantageous or if the regeneration plant were shut down for any reason, some waste sludge could still be produced for disposal.By-product S would be produced for sale.A thermodynamically favorable route ( 4 ) for recovering CaC03 for reuse is to convert the S-bearing sludge components to CaS by reductive roasting and then to carbonate the CaS in aqueous slurry using COn from the roasting. The H2S obtained is an attractive intermediate for the production of elemental S.The technical feasibility of reductive roasting followed by carbonation has been demonstrated on a bench scale. The chemistry and kinetics of the main reactions have been studied. The efficiency of the various steps in the process and the effect of regeneration on CaC03 reactivity have been estimated. ExperimentalMaterials. Materials used were metallurgical coke composed of 1.5% volatile matter, 90% fixed C, 8.5% ash, and 0.74% S; analytical grade CaC03 and CaS04; and CaS03, freshly prepared by saturating an aqueous slurry of CaO with N2 containing 3% SOz. The sample was filtered and dried under an N2 blank...