Method and system for so.sub.2 and so.sub.3 control by dry

Abstract

A method and system for removing SO.sub.3 and SO.sub.2 a flue gas produced by the burning of a fossil fuel. Particulates are removed from the flue gas to produce partially cleaned flue gas. A sufficient amount of calcium-based, sodium based or magnesium-based dry sorbent, preferably having a particle size larger than 1.0-2.0 microns, is injected into the flue gas to react with and remove substantially all of the SO.sub.3 from the partially cleaned flue gas to produce a substantially SO.sub.3 -free flue gas containing reacted and unreacted dry sorbent. The substantially SO.sub.3 -free flue gas is conveyed to wet scrubber means for removing SO.sub.2 to produce cleaned flue gas. In a first embodiment, sufficient dry sorbent is provided to remove desired amounts of SO.sub.3 and SO.sub.2. In a second embodiment, a make-up reagent provided to the wet scrubber means wets the dry sorbent, removes both the reacted and unreacted dry sorbent from the substantially SO.sub.3 -free flue gas, and makes the unreacted dry sorbent available as a reagent for SO.sub.2 removal, the make-up reagent and the unreacted dry sorbent reacting with and removing the SO.sub.2 from the substantially SO.sub.3 -free flue gas to produce cleaned flue gas.