Dual alkali system, limestone

Effects of Magnesium and Chloride Ions on Limestone Dual Alkali System Performance... 

It must be pointed out that the use of the term "active sodium" is simply one of convenience since it is only an indirect indication of the absorptive potential of the liquor. S02 is actually absorbed by or reacts with the sulfite or bicarbonate ions rather than the sodium ion. Also, even though the bisulfite cannot absorb any S02, it can be regenerated to sulfite (as will be discussed later) and, therefore, it is a potentially active species. The limestone dual alkali system operates at "active sodium" concentrations of 1.1 to 1.7 M. 

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. 

The total capital investment for a generalized 500 MW limestone dual alkali system is estimated at 51.7 million (1980 ), which is equivalent to 103.4/kW (3). This generalized system is assumed to be designed for a 95% SO2 removal efficiency when burning coal containing 3.5% sulfur. The estimated annual operating costs (raw materials, utilities, labor and maintenance, overhead and waste disposal) are estimated at 10.7 million (1980 ) or 3.1 mills/kWh. 

The estimated total capital investment for a generalized 500 MW limestone dual alkali system is 51.7 million, equivalent to 103.4/kW. The annual operating costs are equivalent to 3.1 mills/kWh. 

Valencia, J. A. Peirson, Jr., J. F. "Evaluation of the Limestone Dual Alkali System at the Scholz Steam Plant - Final Report,"EPA-600/7-81-141b, 1981. 

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. 

The subject of scrubber plugging has been the subject of intense study by investigators of conventional limestone and dual alkali scrubbing systems. 

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

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