Injection of calcium-based sorbents is currently being considered as a potential method of reducing SO 2 in existing coal-fired boilers. This study investigated the ability of small concentrations of additives to enhance sulfur capture with calcium-based sorbents. The interaction between the mineral ash (produced from coal combustion) and the sorbent-additive combination was also studied. Two furnace facilities and an X-ray diffractometer were used in the experimental study. The furnaces used included a 300 kW down-fired furnace capable of simulating time and temperature profiles for a variety of boilers, and a bench-scale 18 kW drop-tube furnace. Addition of known promoters with sorbents has effectively increased sulfur capture. Both alkali metals and chromium react with calcium to increase the accessibility of CaO sites by particle fragmentation, creation of large pores, and the presence of a liquid phase. In general, the chromium eliminated by ash interaction was small, while the ash effectively prevented a significant portion of sodium from contacting and reacting with the calcium sorbent.