As is well known, a by-product of steam or power generating facilities are combustion or flue gases which contain large amounts of sulfur. In an effort to remove this sulfur before it is discharged into the atmosphere, flue gas desulfurization (FGD) systems have been developed. These systems can be roughly grouped into either nonregenerable processes or regenerable processes.
A nonregenerable process is generally calcium based (i.e. lime or limestone). The reagent is added directly into the scrubber vessel and removed from the process as sulfur containing salts (CaSO.sub.3 .multidot.1/2H.sub.2 O, CaSO.sub.4.2H.sub.2 O, etc.). Nonregenerable processes typically produce a waste which requires ponding or treatment for acceptable sulfur disposal such as in a landfill. Alternatively, these processes can also recover sulfur from flue gas and convert it into marketable by-products.
On the other hand, a regenerable process (also known as a dual-alkali, double alkali, or magnesium oxide process) is usually sodium or magnesium based (NaOH, NaCO.sub.3, Mg(OH).sub.2, etc.). In this type of process, sulfur is removed from the flue gas by reaction with alkali species of sodium or magnesium. These reactions generally occur in the liquid phase and with much less energy than is usually required for nonregenerable processes due to the higher solubility of sodium and magnesium salts as compared to calcium based reagents. The active alkali in regenerable processes is often MgSO.sub.3 or Na.sub.2 SO.sub.3 and the reactions occurring for sulfur absorption or capture, in simplified form, are: EQU SO.sub.2 (g)+H.sub.2 O+MgSO.sub.3 (aq).rarw.==.fwdarw.Mg(HSO.sub.3).sub.2 (aq) (1)
or EQU SO.sub.2 (g)+H.sub.2 O+Na.sub.2 SO.sub.3 (aq).rarw.==.fwdarw.2NaHSO.sub.3 (aq) (2)
In such a regenerable process, the products of reactions (1) and (2) are converted back to sulfite (SO.sub.3.sup..dbd.) by the following regeneration reactions: EQU Mg(HSO.sub.3).sub.2 (aq)+Mg(OH).sub.2 .fwdarw.2MgSO.sub.3 (aq)+2H.sub.2 O (3a) ps EQU Mg(HSO.sub.3).sub.2 (aq)+Ca(OH).sub.2 .fwdarw.MgSO.sub.3 +CaSO.sub.3.1/2H.sub.2 O+.sup.3 /.sub.2 H.sub.2 O (3b)
or EQU 2NaHSO.sub.3 (aq)+CaCO.sub.3 (s).fwdarw.Na.sub.2 SO.sub.3 (aq)+CO.sub.2 (g)+CaSO.sub.3.1/2H.sub.2 O+1/2H.sub.2 O (4a) EQU 2NaHSO.sub.3 (aq)+Ca(OH).sub.2 .fwdarw.Na.sub.2 SO.sub.3 (aq)+.sup.3 /.sub.2 H.sub.2 O+CaSO.sub.3.1/2H.sub.2 O (4b)
Thus, the sulfur is captured yet the active alkali (MgSO.sub.3 or Na.sub.2 SO.sub.3) is regenerated in Reactions (3) and (4) for reuse in Reactions 1 and 2 so as to capture more sulfur.
In a magnesium oxide based regenerable system, the regeneration occurs in the absorber or scrubber vessel. In a sodium based "dual alkali" regenerable system, regeneration occurs in a tank or series of tanks outside the absorber vessel. For both such systems, whether sodium and magnesium based, the product of regeneration is a sludge which consists primarily of CaSO.sub.3.1/2H.sub.2 O, CaSO.sub.4.2H.sub.2 O, excess reagent, and minor species including magnesium, sodium and/or other calcium salts, fly ash, chloride ion, etc. This sludge also requires ponding or treatment for acceptable disposal such as in a landfill, as does the waste from nonregenerable processes.
As stated above, this invention pertains to a regenerable type of FGD system and it is an object of this invention to combine available FGD technologies with the production of a usable product so as to avoid landfill or ponding costs. A further object of this invention is to utilize either a regenerable dual alkali sodium based system or a magnesium oxide based system in such a manner as to produce gypsum as a usable end by-product. Still another object of this invention is to recover a wallboard quality gypsum or other commercial end product. Yet another object of this invention is to produce a product which is stable and, if such is desired, can be disposed of in a landfill if need be. These and other objects of this invention will become obvious upon further investigation.