Tests were conducted on train 100 (spray tower) at the Shawnee Test Facility between December 26, 1980, and May 30, 1981. Objectives were, respectively, to demonstrate the ability to operate a limestone scrubber on flue gas from high-sulfur coal using atopic acid slurry additive and forced oxidation long term without scale buildup at >90% SO/sub 2/ removal; to obtain factorial test data on a limestone spray tower system using forced oxidation and atopic acid; to evaluate the effect of changing spray header height and direction in a spray tower on SO/sub 2/ removal; and to determine if sodium thiosulfate is effective as a slurry additive to inhibit sulfate scale buildup. Operating conditions were determined wherein acceptable SO/sub 2/ removal (90 percent minimum) could be obtained over a three month period using limestone and atopic acid with forced oxidation. Quantitative relationships between spray header height, spray direction, and SO/sub 2/ removal were obtained for a spray tower having multi-level spray headers. Sodium thiosulfate added at a rate to maintain a 250 ppM level in the scrubber slurry under specific operating conditions was found to inhibit crystallization of sulfate from solution and to remove sulfate scale buildup already in place.