Limestone dual alkali

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

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,... 

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. 

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. 

In all likelihood, a properly working and adequately sized filter and associated pumps and piping would have produced a waste cake of the desired concentration, or even exceed it as was the case in the previous pilot plant tests in 1977 when the filter cake typically contained 55 to 65% solids. Limestone dual alkali pilot... 

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. 

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. 

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. 

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. 

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