An apparatus and process for removing acidic gases from flue gases produced by, for example, utility and industrial facilities. The acidic gases are removed as the flue gas flows upward through a contact zone within a passage, where the flue gas is contacted with an ammonium sulfate-containing scrubbing solution to absorb the acidic gases from the flue gas. The scrubbing solution and absorbed acidic gases therein are then accumulated, and ammonia and an oxygen-containing gas are injected into the accumulated scrubbing solution to react the absorbed acidic gases and produce ammonium sulfate. An acid solution is flowed across the passage above the contact zone of the passage, and the scrubbed flue gas is flowed upward through the acid solution to remove unreacted ammonia from the scrubbed flue gas. The acid solution is then removed from the passage after the acid solution has been contacted by the scrubbed flue gas.