This invention relates to the removal of sulfur oxides from flue gases. More particularly, the invention is directed at a novel ammonia double alkali process for the extraction of sulfur oxides from furnace waste gases. This process of absorption and regeneration is usually cyclic. It comprises an ammonia-based extraction of sulfur oxides from a gas; calcium regeneration of the used scrubbing liquor with separation and disposal of the sulfur by-product, a portion of these solids being recycled to the regeneration stage; and recycle of the now regenerated liquor for further sulfur oxide extraction.
Sulfur dioxide (SO.sub.2) and sulfur trioxide (SO.sub.3) are air pollutants. Sulfur dioxide and to a lesser extent sulfur trioxide are present in stack gases resulting from the combustion of coal, oils and other fuels which contain sulfur, from pyrometallurgical operation involving sulfide ores, and from power generation, and various other chemical and petroleum operations.
Accordingly, many processes have been developed to remove sulfur oxides from atmospheric discharges of waste gases. Those processes finding effective use in commercial applications must remove sulfur oxides from waste gases with high process and equipment reliability, result in minimum added cost in equipment, material or energy, and produce a by-product which is easily and safely disposable. It should however be recognized that even ideal scrubbing processes and equipment represent a major added cost to industrial operations.
Presently, the most favored commercial processes for waste gas cleaning are aqueous systems containing lime or limestone. One class of these wet scrubbing processes is based on the reaction of lime or limestone with sulfur oxides to produce various sulfur salts of calcium, the insoluble calcium salts being removed from the system. U.S. Pat. Nos. 3,883,639, 3,980,756, and 4,024,220 illustrate these systems. The single scrubber circulating loop of this first class of conventional lime or limestone processes is seriously disadvantaged by scale formation. This scale causes numerous operating problems, uneconomical operating costs, and low process reliability. Moreover, sulfur oxide removal efficiencies of conventional single alkali scrubbing processes are often too low to meet the more strict waste gas standards now required of industry.
To avoid these disadvantages inherent in former single alkali processes, commercial operations are beginning to investigate and utilize double alkali processes wherein the former single stage process has been split into a number of intermediate steps designed to improve the reliability of operation, utilization of material, sulfur oxide removal efficiency, and handling properties of the solid waste. For example, whereas in conventional single alkali processes the absorption of sulfur oxide from flue gas and production of waste product occur primarily in a single reactive step, in double alkali processes sulfur absorption and waste production are separated through the use of an intermediate soluble alkali absorption stage. Oxide absorption and waste product production then occur in separate system components. Such separation accomplishes two important objectives. First, it permits flue gas scrubbing with a soluble alkali, thus the rate of the sulfur oxide absorption is limited by the rate of transfer from the flu gas to the scrubbing liquor. In former systems, the rate of lime/limestone dissolution was also an important factor limiting the rate of the scrubbing reaction. Therefore, double-alkali systems have the potential for higher oxide removal efficiencies than the former single alkali systems. Additionally, the use in double alkali systems of other than calcium containing scrubbing liquors minimizes calcium concentrations in the scrubber and piping so as better to prevent scaling and plugging in these critical areas. Finally, relegation of the lime/limestone reaction to a location more specifically designed for this chemical exchange increases the potential for high lime/limestone utilization in double-alkali processes.
There are a number of double-alkali processes described in the art. For example, sodium or other alkali metal-based processes are disclosed in N. Kaplan, "Introduction To Double-Alkali Flue Gas Desulfurization Technology", EPA Flue Gas Desulfurization Symposium, New Orleans, La., Mar. 8-11, 1976 and U.S. Pat. Nos. 3,775,532, 3,883,639, 3,944,649, 3,961,021, 3,965,242, 3,987,149, 3,989,796, and 3,989,797.
Ammonia-based sulfur scrubbing processes are also described in the art, for example in U.S. Pat. Nos. 1,740,342, 2,082,006, (Re. 21,631), 2,405,747, 3,579,296, 3,695,829, 3,843,789, 3,880,983 and 3,944,649. These processes employ heat, acidification and crystallization, alkaline earth metal oxides, or alkaline earth metal hydroxides to regenerate the used scrubbing liquors.
These conventional soluble alkali processes, although displaying more efficient sulfur oxide removal from flue gases, are still disadvantaged by inferior sulfate removal during the precipitation and regeneration stage, sulfate being produced in scrubbing systems by oxidation of sulfur dioxide and sulfites. Failure to remove sulfate contaminants during regeneration, a difficult task as calcium sulfate is more soluble than calcium sulfite in aqueous solutions over wide pH ranges, reduces the sulfur oxide absorption capability of the regenerated liquor by tying up ammonia values making them unavailable for further sulfur scrubbing. Moreover, sulfate concentration build-up can adversely affect alkali utilization during regeneration, contribute to gypsum scaling in the scrubber and other process equipment, and eventually lead to shut down of the chemical absorption-regeneration process. Therefore, avoidance of sulfate build-up is critical to any long term operation of these scrubbing systems. E.g., U.S. Pat. No. 3,579,296, column 3, lines 5-19.
Numerous attempts have been made in the art to avoid such destructive increases in liquor sulfate concentration. Most are techniques directed to encourage more effective removal of sulfate during regeneration usually as solid calcium sulfate or gypsum, gypsum being more soluble in aqueous solutions than calcium sulfite, the other major waste product. One such process is described in U.S. Pat. No. 2,082,006. There, the acidity of the lime/limestone regeneration solution is increased to prevent the precipitation of calcium sulfite and encourage calcium sulfate precipitation.
Another technique, described in U.S. Pat. No. 2,405,747, suggests removing a bleed stream from the regenerated liquor and reacting it with lime or limestone under conditions designed to precipitate gypsum from the partially regenerated liquor and thus prevent sulfate build-up in the liquor. Other bleed stream extractive techniques are disclosed in U.S. Pat. Nos. 2,086,379, 2,128,027, 3,695,829, 3,961,021, 3,965,242.
A third technique, described in U.S. Pat. Nos. 3,579,296, 3,873,532, 3,972,980, 3,980,756, provides a settling means following regeneration with a recycle of a portion of the settled solids to the regeneration stage to promote desupersaturation of the recausticized liquor relative to calcium sulfate and precipitation of gypsum. The recausticized liquor from this process must have the high calcium content consistent with gypsum precipitation. Therefore, it cannot be reused in the sulfur oxide extractive process without further treatment since high calcium concentrations in the scrubbing liquor contribute to scaling and reduce lime/limestone utilization efficiency. Accordingly, the regenerated liquor is usually treated with soda ash to reduce the calcium ion concentration to levels more acceptable to subsequent sulfur oxide scrubbing.
Another method disclosed, for example in U.S. Pat. No. 3,775,532, employs a two step regeneration process. First, the spent scrubbing solution containing both absorbed sulfite and sulfate ions is treated with limestone to precipitate and remove calcium sulfite. Then, the partially regenerated liquor is treated with lime to remove the sulfate as gypsum. Again calcium concentrations in the regenerated liquor are excessive and disadvantage later scrubbing and regeneration. Other multistage regenerations are disclosed in U.S. Pat. Nos. 3,944,649 and 3,987,149. In this regard it is noteworthy that the Environmental Protection Agency in EPA Report 600/7-77-050b has concluded that there is no viable approach which eanbles the use of limestone alone for regeneration of liquors having significant amounts of sulfate, even though there are considerable economic incentives for the substitution of limestone for lime in double alkali processes.
Therefore, while some of these techniques have been successful in limiting the concentration of sulfate in the regenerated liquor, all have required additional material, equipment, stages or bleed stream treatments to encourage the necessary gypsum precipitation. Moreover, failure subsequently to reduce those calcium levels necessary for gypsum precipitation from the process liquors contributes to scaling in the scrubber and disadvantages other process stages. Therefore other equipment, stages and treatment are necessary to remove that excess calcium ion inherent in regeneration schemes operated in the gypsum precipitation mode.