A total of seven tests were conducted at the Otter Tail Power Company, Hoot Lake Station to determine the effectiveness of limestone injection in controlling flue gas sulfur dioxide (SO/sub 2/) emissions. The unit tested, Unit 2, is a 53-MW lignite-fired boiler with tangential burners. The key variables for the tests were limestone injection rate, injection location, and burner firing configuration (low-NO/sub x/ versus conventional firing). The observed effects of these variables are discussed briefly. Sulfur dioxide removal efficiencies were found to increase with calcium-to-sulfur mole ratio (expressed as the ratio of moles of calcium carbonate in limestone injected to moles of sulfur in the lignite). The range of calcium-to-sulfur mole ratios tested varied from 1.6:1 to 5.0:1. The resulting SO/sub 2/ removal efficiencies varied from 21 percent at the lowest calcium-to-sulfur mole ratio to 47 percent at the highest mole ratio tested. The resulting limestone utilization values ranged from 9 to 15 percent, with the lowest value being observed when injecting limestone at the highest mole ratio. Limestone utilization is defined as the percentage of limestone that actually reacts with sulfur dioxide to form calcium sulfite or sulfate solids. Choice of injection location was not found to significantly affect SO/sub 2/ removal efficiencies. Six of the tests were conducted in a low-NO/sub x/ firing, or overfire air mode. In a seventh test, limestone was injected through the burners while operating in a conventional firing mode. However, variations of firing mode did not appear to affect SO/sub 2/ removal performance. Apparently, the large particle size of the limestone leaving the pulverizer during burner injection had a greater influence in limiting results than did the burner firing mode.