Limestone slurry

SO2 absorbed in tower with NaOH—Na2S02 recycle solution. CaOH or CaCO added externally to precipitate CaSO, regenerate NaOH make-up NaOH or Na2C02 added. Process attempts to eliminate scaling/plugging problems of limestone slurry scmbbing. 

Limestone slurry scmbs flue gas. SO2 absorbed, reacted to CaSO. Further ak-oxidized to CaSO settled/removed as sludge. Lower cost and simpler than other processes. Disadvantages abrasive/corrosive, plugging and scaling, poor dewatering of... 

Single vessel used to absorb SO2 with limestone slurry and oxidize product to gypsum. 

Wet-Throwaway Processes. By 1978, three wet-throwaway systems were in commercial operation lime scmbbing, limestone slurry scmbbing, and dual alkah (1). Time/limestone wet scmbbing (Fig. 11) remains the most common post-combustion control technique appHed to utiHty boilers (67). The waste product from the scmbbers can either be sent to a landfill or be upgraded by oxidation to become saleable gypsum. 

Figure 14-6 Typical batch settling curve for a limestone slurry.

Desox A flue-gas desulfurization process in which limestone slurry absorbs the sulfur dioxide, forming calcium sulfite. This is then oxidized to saleable gypsum ... 

Throwaway processes generally remove sulfur dioxide by absorption into a lime or limestone slurry or a clear solution. Figures 3-5 show general diagrams for these processes (22,23,24, 25). 

In wet scrubbing of SOp from boiler flue gas by limestone slurry, the concentration of dissolved species in the scrubbing liquor that can react with incoming SOp gas is very low, about one to two m-mole/1. This is far below the SOp make-per-pass in the scrubber, typically about 10 m-mole of SOp absorbed per liter of liquor for one pass through the scrubber. Therefore, the SOp absorption rate is largely dependent upon the slow rate of limestone dissolution into the liquor passing through the scrubber. 

A chemical model has been developed that predicts equilibria for wet scrubbing of S02 by limestone slurry, with magnesia additive to enhance S02 removal. The model is implemented by an equilibrium computer program. 

Uchida, S., Moriguchi, H., Maejima, H., Koide, K. and Kageyama, S. Can. J. Chem. Eng. 56 (1978) 690. Absorption of sulphur dioxide into limestone slurry in a stirred tank reactor. 

Data of pressure drop in the flow of a 60% limestone slurry of density 1.607 g/ml were taken by Thomas [Ind. Eng. Chem. 55, 18-29 (1963)]. They were converted into data of wall shear stress xw = DAP/4L against the shear rate 8V/D and are plotted on the figure for three line sizes. 

Reactions between gases and liquids may involve solids also, either as reactants or as catalysts. Table 17.9 lists a number of examples. The lime/limestone slurry process is the predominant one for removal of S02 from power plant flue gases. In this case it is known that the rate of the reaction is controlled by the rate of mass transfer through the gas film. 

In wet FGD processes, either DA or limestone slurry, the combined effects of calcium and magnesium actually determine the limestone dissolution rate. Sjoberg s results(fi) indicated that Ca2+ can inhibit the CaCO dissolution rate much more effectively than Mg2+ by the same surface adsorption phenomenon. The combined effects of Ca2+ and Mg2+ can be described as competitive adsorption, and the limestone surface will act as an ion-exchanger. The fraction of surface occupied by adsorbed Ca2+ and Mg2+ can be expressed as ... 

Chan, P.K. and G.T. Rochelle, Modeling of S02 Removal by Limestone Slurry Scrubbing Effects of Chlorides, EPA/EPRI Symposium on Flue Gas Desulfurization, New Orleans, 1983. 

S02 emissions from sulfuric acid plants are controlled in spray towers. Effluent gases contain less than 0.5 percent S02. The S02 emissions have to be controlled (or recovered as elemental sulfur by, for example, the Claus process). An approach is to absorb the S02 in a lime (or limestone) slurry (promoted by small amounts of carboxylic acids, such as adipic acid). Flow is in parallel downward. The product calcium salt is sent to a landfill or sold as a by-product. Limestone is pulverized to 80 to 90 percent through 200 mesh. Slurry concentrations of 5 to 40 percent have been used in pilot plants. 

The Use of Limestone Slurry Scrubbing to Remove Sulfur Dioxide from Power Plant Flue Gases ... 

The solid-liquid limestone slurry enters the scrubber at 50 C the liquid portion of the slurry flows at a rate of 15.2 kg liquid/kg inlet gas and the solid-to-liquid ratio in the slurry is 1 9 by weight. The liquid is saturated with CaCOs and CaS03. The cleaned flue gas meets the EPA standard on SO2 emissions it leaves the scrubber saturated with water at 53X, containing the CO2 generated in the scrubbing but none of the entering fly ash. Sulfur dioxide is the only constituent of the flue gas that is absorbed in the scrubber. The cleaned flue gas is then reheated to 80°C, blended with the cleaned flue-gas stream from the other train, and sent to the stack where it is released to the atmosphere. 

A limestone slurry of density 1.693 g/mL is pumped through a 4-in. (152 mm) line at the rate of 4ft/sec (1.22m/sec). The pressure drop (psi/mile) will be calculated. The slurry behavior is represented by... 

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