A method for continuously removing mercury from a supply of combustion gas is provided. The method includes introducing SO.sub.3 into the supply of combustion gas to oxidize at least a portion of elemental mercury within the supply of combustion gas into ionic mercury. A turbulent flow of combustion gas is produced to suspend particulate matter including fly ash contained in the supply of combustion gas. A substantial portion of the mercury is absorbed within the particulate matter. The supply of combustion gas is filtered to remove the particulate matter from the mercury. The introduction of SO.sub.3 into the combustion gas is controlled based at least partially on at least one of an oxidation rate of mercury and an absorption rate of mercury.