The report discusses physical and chemical processes and reaction mechanisms for lime spray drying and dry injection of sodium compounds in dry flue gas desulfurization (FGD) processes. It includes: chemical reactions, physical changes, proposed reaction mechanisms and mathematical models, process parameters affecting reactions and their rates, and data needed to verify proposed reaction mechanisms and models. Published technical papers were the primary reference sources. The report focuses on coal-fired boiler dry FGD applications. Lime spray drying reactions are primarily gas/liquid-phase reactions, with SO2 removal depending on moisture in the lime slurry droplet. Initially, the moisture content is high, and the reaction rate is controlled by diffusion of SO2 to the droplet surface; most SO2 removal occurs during this phase. As evaporation reduces the moisture, the dissolution of Ca(OH)2 into ions limits the SO2 removal rate. Later, the precipitation of CaSO3.1/2 H2O onto the surface of the lime particles retards diffusion of SO2 to the unreacted sorbent. Injecting sodium compound powders into flue gas removes SO2 via gas/solid reactions. First, NaHCO3 is thermally decomposed to Na2CO3 (small pores in the sorbent particles increase the particles' surface area and reactivity). Then the SO2 reacts with Na2CO3 to form Na2SO3, starting at the particle surface.