Conversion methods and equipment for converting a calcium-based flue gas desulfurization (FGD) system to an ammonia-based FGD systems, including modifying a reagent system and absorber system of the calcium-based FGD system to be capable of, respectively, delivering an ammonia-based reagent to the absorber system rather than the calcium-based reagent, and modifying the absorber system to increase capacity of a reaction tank thereof.