Abstract: Apparatus and a method are disclosed for reducing sulfate contained within a flowable material to hydrogen sulfide by the use of two stages of reaction. The flowable material is deoxygenated and then introduced into a first reactor which contains a sulfate reducing bacteria such as a Desulfovibrio or Desulfotomoculum. Hydrogen sulfide is removed from the first reactor and the remaining portion of the flowable material is subjected to a separating action to produce a supernatant component and a sludge component. The sludge component is reintroduced to the first reactor which is generally devoid of packing or support and the supernatant is introduced into a second reactor which is preferably in the nature of a supported film reactor. Mixed products including carbonate may be removed from the first reactor. The second reactor also contains anaerobic sulfate reducing bacteria. Hydrogen sulfide, low sulfate water and carbonates may be withdrawn from the second reactor.

Abstract: Systems for desulfurization of a flue gas containing fly ash including formation of a dry powder in a spray dryer absorber resulting from reaction of an alkaline aqueous feed and the flue gas. The dry powder is collected and a specifically selected fraction is recycled to a feed preparation subsystem for inclusion in the aqueous feed. Selective recycle is based on physical or chemical properties and, for example, large diameter and dense particles provide enhanced reagent utilization compared to smaller and less dense particles.