Dual alkali process

Several power plants have been equipped with dual alkali processes. These are throw-away processes with two liquid loops. In one common process, the scrubbing liquid is a clear solution of sodium sulfite. The absorption of sulfur dioxide converts the sodium sulfite to sodium bisulfite. In the regeneration loop, an alkali such as lime slurry is added the sodium sulfite solution is regenerated and a mixture of calcium sulfite and calcium sulfate is precipitated. The slurry is... 

In the dual or double alkali process, an alkali salt that is considerably more soluble in water than limestone is used. The alkak salt is then regenerated using a second alkali, CaC03. There are several alkalies used in the absorber the most common are magnesium sulfite, sodium sulfite, and ammonium sulfite. A typical process using magnesium sulfite suAbsorption... 

Chang, J. C. S. and N. Kaplan, "Pilot Evaluation of Limestone Regenerated Dual Alkali Process," In Proceedings Eighth Symposium on Flue Gas Desulfurization, New Orleans, LA, November 1983, Volume 1, EPA-600/9-84-017a (NTIS PB84-223031), July 1984. 

In the sodium-based dual alkali process, the acid gases are absorbed by a solution of sodium salts at a pH range of 5-8. The solution is regenerated outside the scrubber with lime or limestone to produce a solid waste containing calcium sulfate and calcium sulfite. Some sodium salts are lost with the waste and must be made up by the addition of NaOH or Na2C03. The principal chemical reactions are as follows ... 

The principal advantage of a dual alkali process applied to the MPF is the substitution of a relatively inexpensive filtering step for the expensive drying step in the single alkali system. 

Application of the sodium-based dual alkali process to high chloride gas streams is of interest not only to the Army but also to other operators of combustion systems using conventional fuels such as coal where toxic chloride-containing wastes are co-fired for destruction purposes. 

Valencia, J.A. The limestone dual alkali process for flue gas desulfurization. In Flue Gas Desulfurization Hudson, Wells, Eds. American Chemical Society Washington, DC, 1982 325-347. 

The Limestone Dual Alkali Process for Flue Gas Desulfurization... 

The limestone dual alkali process developed by Thyssen-CEA Environmental Systems, Inc., and Arthur D. Little,... 

Thyssen-CEA Environmental Systems, Inc. (initially Combustion Equipment Associates, Inc.) and Arthur D. Little, Inc., have developed, over the past few years, a dual alkali process for removing SO2 from flue gas generated in coal-fired utility boilers This process is based on the absorption of SO2 in an alkaline sodium solution, followed by regeneration of the absorbing solution by reaction with a second alkali, calcium. These reactions generate insoluble calcium-sulfur salts which are discharged from the system as a moist cake. 

The dual alkali process has been tested extensively at laboratory, pilot plant, and prototype levels using lime (calcium hydroxide) as the source of calcium for the regeneration reactions (1). The performance of the process in these test programs prompted the federal Environmental Protection Agency (EPA) to select it for a full-scale demonstration plant. This lime-based dual alkali system has been installed on a 300 MW boiler at Louisville Gas and Electric s Cane Run Station. The system is currently undergoing a one-year test program. 

Figure 1. Dual alkali process flow diagram.

Four chapters address alternatives to throwaway slurry scrubbing. The development of the limestone dual alkali process is reviewed. Two chapters present results related to dry scrubbing with nahcolite or lime. A conceptual design and economics are given for MgO scrubbing using a spray dryer. 

The milk of lime-sulfite process was widely used for early power station flue gas desulfurisation projects. It had a lower capital cost than the limestone-sulfite process and gave high absorption and reagent efficiencies. Subsequently, the problem of disposal of the calcium sulfite sludge led to three variants based on lime to be adopted — the gypsum process, the dual alkali process and the maglime process. 

This is an elegant variant of the dual alkali process, which exploits the fact that many magnesium salts are soluble. 

Lime solution is employed by some operators since caustic soda is costly. However, the pipelines and spray nozzles tend to get choked due to formation of insoluble calcium salts. A variant of this method is the use of the dual alkali process wherein NaOH solution is circulated in the system and lime is used to regenerate NaOH from the spent scrubbing liquor. Other systems of proprietary design are also offered which are low in capital costs and utility consumption. 

In the dual-alkali process, a recycled alkaline solution of sodium salts is the scrubbing liquid. The scrubber effluent is treated with slaked lime to precipitate insoluble calcium sulfite and calcium sulfate, while regenerating the alkalinity of the solution. Then, the calcium salts are thickened and filtered from the recycled solution. This system requires only a small make-up of sodium alkali and produces a smaller amount of solids for disposal than a simple lime scrubbing system. 

In the double alkali (or dual alkali) process for flue gas desulfurization, die gas is contacted with a solution of soluble alkali, such as sodium sulfite or sodium hydroxide, which absorbs the SO2. The resulting solution is then reacted with a second alkaline material (normally lime or limestone) to precipitate the absorbed SO2 as insoluble calcium sulfite and regenerate the absorbent solution. Several alkali combinations are possible however, this discussion is limited to the sodium/calcium case. 

---