The SO2 conversion rate of single absorption plants is lower than double absorption, and many single absorption plants installed tail gas scrubbers to reduce this SO2 content rather than convert the plant to double absorption. Scrubbing the tail gas to acceptable SO2 emission values can be done with seawater, caustic, lime slurry, hydrogen peroxide or ammonia. After exiting the absorber tower, the gas enters a scrubber tower where ammonia (NH3) is injected and contacts the SO2. Ammonium Sulphite and Bi-sulphite are formed and some of these salts are entrained in the exit air.
These salts are soluble, but can be emitted in quite large volumes, e.g. 2,000 – 15,000 mg/m3 and are generally < 3 microns in size, with a large amount < 1 mircron.
Prevent blockage of fibre bed by salts solidification.
Occasional risk of weak acid mist emission