Alkali, dual, limestone process

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,... 

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. 

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 limestone dual alkali technology consists of four distinct operations SO2 absorption, absorbent regeneration, waste solids dewatering, and raw materials storage and feed preparation. A typical process flow diagram is shown in Figure 1. 

The presence of sodium sulfate and sodium chloride is principally the result of secondary absorption reactions. Sodium sulfate is formed by the oxidation of sodium sulfite via reaction with oxygen absorbed from the flue gas. Oxidation also occurs in other parts of the system where process solutions are exposed to air however, the amount of oxidation is small relative to the oxidation which occurs in the absorber. At steady state, the sulfate must leave the system either as calcium sulfate or as a purge of sodium sulfate at the rate at which it is being formed in the system. Although a practical limit for the level of oxidation that can be tolerated by the limestone dual alkali system has not yet been established, it appears that oxidation rates equivalent to 15 to 20% of the S02 removed might be accommodated without intentional purges of sodium sulfate. 

Thus, the limestone dual alkali technology appears to be technically and economically feasible. However, further testing is needed to reinforce such conclusions and to develop sufficient process information needed for full-scale commercialization purposes. 

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. 

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. 

The Dowa Dual Alkali Process uses a solution of basic aluminum sulfate to absorb SO2, air injection to oxidize sulfite to sulfate, and limestone to precipitate the resulting excess sulfate in the form of gypsum. 

The pH of the process solution is maintained in the range of 3.0 to 3.5. Because of the low pH and the complete oxidation realized, all of the limestone reacts to precipitate gypsum. Unlike convaitional limestone/lime or the concratiated mode dual alkali processes, there is no calcium suliite produced. The low pH also affects the rate of absoiption of SO2 and an efRcioit contactor must be used to obtain high levels of SO2 removal. 

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... 

Dowa Also called Dowa Dual Alkali A flue-gas desulfurization process in which the sulfur dioxide is absorbed in a basic aluminum sulfate solution. The product solution is oxidized with air and reacted with limestone to produce gypsum. Developed by the Dowa Mining Company, Japan, in the early 1980s. In 1986, the process was in use in nine plants in Japan and the United States. 

The Dowa process is claimed to offer sevoal advantages in comparison with both conventional limestone/lime and sodium dual alkali systems (Nolan and Seaward, 1983). These include... 

The Dowa process is another dual alkali system. It uses basic aluminum sulfate solution for SO2 removal and low-cost limestone to regenerate the aluminum sulfate. A gypsum by-product is produced. The process is common in Japan. 

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