| abstract |
An ammonia-based double-alkali process for removing sulfur oxides from stack gases. In this process, usually continuous, stack gases are contacted with an aqueous wash liquid containing ammonium sulfite as a sulfur oxide reactive compound to form a spent absorbent liquor containing bisulfite, sulfite and sulfate salts of ammonia. A portion of the reactive compound is regenerated by reacting the spent scrubbing liquor with a source of calcium ions, usually limestone. During the regeneration, calcium sulfite solids are precipitated as a by-product, any gypsum solids precipitation during the process being tolerated for a time insufficient to degrade substantially process performance. The resultant calcium solids are separated from the regenerated liquor which is returned for further contact with the stack gases, a portion of these solids being recycled to the regeneration stage. The process is preferably regulated by monitoring and controlling of the calcium concentration of the regenerated liquor, upset conditions being corrected by the addition of make-up ammonia during regeneration of the liquor. |