A small pilot plant that burns pulverized coal was used to evaluate a dry limestone injection process. More than 400 tests were performed with over 100 different limestones or dolomitic-type additives to evaluate their effectiveness for reducing sulfur dioxide emission under a variety of test conditions. Some of the major variables studied include temperature at the point of additive injection, residence time of the additive in the reactive zone, additive/sulfur ratio, additive surface area and chemical form of the additive. The effects of the additives on ash deposition and fly ash resistivity were also studied. The process is considered only as a stopgap measure when applied as the sole means for removing SO2 from flue gas. Under certain conditions additives can produce or aggravate furnace-wall slagging and ash deposition problems. Electrical resistivity of reacted additive-fly ash products is increased, which will decrease performance of electrostatic precipitators.