Lime/limestone scrubber modifications: investigation of thiosulfate as an oxidation/scaling inhibitor. Part I, final report (general results)

In view of the conflicting reports in the literature regarding the effects of thiosulfate in FGD scrubber-type systems, this investigation was undertaken with the objectives of determining whether or not thiosulfate additive inhibits oxidation and scaling in lime/limestone FGD scrubber systems and, if so, the range of conditions under which it can be utilized. The results of the experimental laboratory batch scrubber study, in which the effects of magnesium, chloride, fly ash, and oxygen concentration in the flue gas also were investigated, indicated sodium thiosulfate, added in concentrations corresponding to about 20 pounds or more per ton of lime, can be an effective sulfite oxidation inhibitor. Including all conditions investigated, average oxidations with thiosulfate additions were down to 40 percent of the uninhibited value for pure lime scrubbing and to 50 percent of the uninhibited value when magnesium was present in the lime. In individual experiments, much stronger inhibition was observed - to as little as 1.4 percent of the uninhibited level. Lowering the oxidation level reduces the rate of sulfate formation and thus the potential for gypsum scaling. Experimental results and the results of analysis of variance of the data also indicate that: thiosulfate additions strongly inhibit gypsum saturation;more » thiosulfate additions can slightly enhance SO/sub 2/ absorption efficiencies; among the variables investigated magnesium (promoter) and thiosulfate (inhibitor) exert major effects on oxidation, while only minor effects are associated with chloride and fly ash in the ranges investigated.« less