Process for controlling ammonia slip in the reduction of sulfur dioxide emission

A process for removing sulfur dioxide from flue gases, the method comprising the steps of:

• delivering flue gases containing sulfur dioxide to a contactor region of an absorber;
• circulating a scrubbing solution containing ammonium sulfate from a vessel to the absorber, the scrubbing solution contacting the flue gases and absorbing sulfur dioxide from the flue gases;
• accumulating the scrubbing solution containing the sulfur dioxide in the vessel, the vessel having a bottom wall and a side wall that defines a cross-section of the vessel;
• agitating the scrubbing solution in the vessel with an agitation means;
• introducing a mixture of air and an aqueous ammonia solution into the vessel through a single injection means to react with the sulfur dioxide to produce ammonium sulfate, the aqueous ammonia solution having a weight ratio of water to ammonia of about 7:1 to about 3:1, the aqueous ammonia solution and the air being mixed at an ammonia:air weight ratio of about 1 to about 5 prior to being introduced into the vessel at a distance of about three meters above the bottom wall of the vessel and dispersed evenly across the cross-section of the vessel at the bottom wall of the vessel through the single injection means, the single injection means comprising a plurality of conduits that extend across the cross-section of the vessel, the conduits injecting the mixture above an outlet through which the scrubbing solution is recirculated to the contactor region;
• and
• recirculating the scrubbing solution to the contactor region;
• wherein the mixture of air and the aqueous ammonia solution is intermixed with the scrubbing solution within the vessel and prior to the scrubbing solution within the vessel being recirculated to the contactor region so as to eliminate portions of scrubbing solution within the vessel having a higher pH and a higher ammonium sulfite level than the remainder of the scrubbing solution within the vessel.