The report gives results of an advanced test program on a prototype lime/limestone wet-scrubbing test facility for removing SO2 and particulates from coal-fired boiler flue gases. Major effort during the tests was concentrated on evaluating adipic acid as an additive for enhancing SO2 removal efficiency. Results show that adipic acid can substantially improve the performance of limestone spray towers or turbulent contacting absorbers. SO2 removals greater than 90% and reliable scrubber operation were demonstrated in four long-term (greater than 1 month) limestone runs. The optimum pH was determined to be about 5.1 with the additive, compared to 5.5-6.0 without. Because of the lower operating pH, limestone utilization was significantly improved, and tests showed lower limestone consumption and reduced waste solids production with the additive. Consequently, both capital and operating costs for an adipic acid enhanced limestone system are estimated to be 5% lower than for a conventional system. The improved limestone utilization also contributes to reliable scrubber operation, thus reducing maintenance costs. Mathematical models were derived which correlate the SO2 removal performance as a function of operating parameters such as gas and slurry flow rate, pH, and adipic acid concentration.