The paper discusses the fundamental processes in sulfur dioxide (SO2) capture by calcium-based adsorbents for upper furnace, duct, and electrostatic precipitator (ESP) reaction sites. It examines the reactions in light of controlling mechanisms, effect of sorbent physical properties, and important process variables. Upper furnace reactivity is limited to 900-1200 C by rate and equilibrium constraints, respectively. Sulfation is a function of in-situ sorbent characteristics of porosity, particle size, and surface area. Conversion of the sorbent is ultimately limited by the formation of the calcium sulfate (CaSO4) product layer. The in-duct reaction is accomplished through sorbent scavenging in the flue gas stream by a water spray. The scavenging efficiency of the sorbent by the water droplets limits the process, while reaction is controlled by the dissolution rate of the sorbent. The E-SOX process in a modified ESP simulates a short time spray dryer through injection of a calcium hydroxide--Ca(OH)2--slurry. The Ca(OH)2 undergoes aqueous phase reaction to remove SO2. Evaporation of the droplets prior to the ESP field conditions the flue gas for more efficient particulate matter collection by lowering the gas resistivity.