The report gives results of an investigation of the fundamental rate processes that govern sulfur capture in power plant ducts during humidification of flue gases. The specific application was the reactivation of partially sulfated calcium-based sorbents from in-furnace injection. The results suggest that physical contacting between the spray water and the sorbent particles is necessary to achieve significant reaction rates. Several ways to promote such contacting were investigated, and a general approach to contacting was proposed. These hypothetical predictions were tested in a subscale rig using laser-based measurements. The reactivity of slurry drops was investigated in a dilute-phase reactor. The results indicate that calcium availability (i.e., dissolution into the liquid) and droplet lifetime were the principal constraints on sulfur capture.