Pilot plant tests have been made to evaluate the effectiveness of raw, hydrated and calcined limestones and dolomites injected into the flue gases to reduce sulfur dioxide concentration. The effects of additive fineness, temperature of injection and injection rate were also investigated for selected additives. The influence of limestone and dolomite on fly ash strength and on coal ash slag viscosity and fusion temperatures has been determined to evaluate the effect on ash deposition on furnace walls and in tube banks. The influence of additives on the ash collection properties as determined by bulk and in situ resistivity measurements has also been determined. Hydrates were somewhat more effective than the raw stones and effectiveness did not decrease when they were injected into lower gas temperatures. The hydrates as a class had greater specific surface areas than the raw limestone prepared in the normal manner. This could be one factor affecting their effectiveness, and tests with the raw additives indicated that increased fineness improved performance. (Author)