Flue gas scrubbing slurries

Table I. Equilibria Present in Flue Gas Scrubbing Slurries for the Lime or Limestone Processes...

Calculation of Supersaturation Ratios of Flue Gas Scrubbing Slurries... 

The absorption of sulfur dioxide into flue gas scrubbing slurries is enhanced considerably because of the reactions of dissolved sulfur dioxide with various dissolved alkaline species. The alkaline species of interest are the following ions and ion-pairs OH-, SO", HCO-, CO--, MgS0°, MgC0°,... 

The most significant chemical equilibria present in flue gas scrubbing slurries are outlined. Expressions for temperature dependent equilibrium constants are presented that are suitable for the temperature ranges encountered in scrubbing applications. Expressions for activity coefficients of ions and ion-pairs are presented that are suitable for the ranges of ionii strengths encountered for this type of applications. 

Calcium carbonate (hmestone) slurry, in flue gas scrubbing, 11 717 Calcium channel blockers, 5 116-135 arylalkylamines and benzothiazepins, 5 116-117, 118-120t 1,4-dihydropyridines, 5 124-129t Calcium channels, 5 85-86... 

AG° is the standard Gibbs free energy for the reaction at 298°K R is the ideal gas constant and T is 298°K. Since the actual temperature of most slurries or solutions in flue gas scrubbing applications usually does not exceed 50°C, the value of the equilibrium constant can be determined at some temperature other than 298°K by using the van t Hoff equation... 

Flue-Gas (Stack-Gas) Scrubbing. Flue-gas scrubbing is a refiner s last chance to keep NOx and SOx out of the air. In wet flue-gas desulfurization, gas streams containing SOx react with an aqueous slurry containing calcium hydroxide Ca(OH)2 and calcium carbonate CaCOs. Reaction products include calcium sulfite (CaSOs) and calcium sulfate (CaS04>, which precipitate from the solution. 

Magnesium Oxide Slurry - 120 49 - E E flue gas scrubbing liquor for SO3 removal. 238 days, moderate aeration... 

Oxidation of potassium compounds to the sulfate form occurs in the Recovery Scrubber process licensed by Passamaquoddy Technology, which was originally developed for cement plant applications. The process uses waste cement kiln dust, which contains limestone, alkali, and calcium sulfate, in an aqueous slurry as a flue gas scrubbing reagent The first commercial appUcation of the process began operating at the Dragon Products Company cement plant in Thomaston, Maine in December 1990 (Morrison, 1991). 

Howden An early flue-gas desulfurization process using a lime or chalk slurry in wooden grid-packed towers. The calcium sulfate/sulfite waste product was intended for use in cement manufacture, but this was never commercialized. The key to the process was the use of a large excess of calcium sulfate in suspension in the scrubbing circuit, which minimized the deposition of scale on the equipment. The process was developed by Imperial Chemical Industries and James Howden Company in the 1930s and operated for several years at power stations at Fulham, London, and Tir John, South Wales, being finally abandoned during World War II. British Patents 420,539 433,039. 

The throw-away processes with aqueous slurries of lime or limestone as the scrubbing media are the most extensively installed processes. These processes create a waste sludge containing calcium sulfite, calcium sulfate, fly ash, unreacted alkali, and other minor dissolved species in the free water contained in the sludge. Since flue gas contains oxygen, some of the dissolved sulfur dioxide is oxidized, and calcium sulfate is formed. 

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. 

In the lime or limestone FGD process, SO2 is removed from the flue gas by wet scrubbing with a slurry of calcium oxide or calcium carbonate [3]. The waste solid product is disposed by ponding or landfill. The clear hquid product can be recycled. Many of the lime or limestone systems discharge scrubber waters to control dissolved solids levels. 

The EDV Wet Scrubbing System utilized proprietary nozzles to produce high-density water curtains through which the gas must pass. Each nozzle sprays water droplets that move in a cross-flow pattern relative to the flue gas. These cover the entire gas stream and uniformly flush the vessel s surfaces clean. The spray nozzles are nonclogging and are designed to handle highly concentrated slurries. 

A variation of the nonregenerable absorption is the spray dry process. Lime slurry is sprayed through an atomizing nozzle into a tower where it countercurrently contacts the flue gas. The sulfur dioxide is absorbed and water in the slurry evaporated as calcium sulfite—sulfate collects as a powder at the bottom of the tower. The process requires less capital investment, but is less efficient than regular scrubbing operations. 

Pure SO2 is injected to achieve the desired concentration in the flue gas. Regeneration of spent scrubbing liquor is performed in the four-tank-in-series reactor train with a total residence time of 80 minutes. Limestone is fed to the first reactor as 45% slurry. 

Spray drying has become increasingly important in recent years as an alternative to wet scrubbing for sulfur dioxide control. In the spray dryer the sulfur-containing flue gas is contacted with a fine mist of an aqueous solution or a slurry of an alkali (typically Ca(0H)2 or soda ash). The sulfur dioxide is then absorbed in the water droplets and neutralized by the alkali. Simultaneously, the thermal energy of the gas evaporates the water in the droplets to produce a dry powdered product. After leaving the spray dryer the dry products, including the fly ash, are removed with collection equipment such as fabric filters or electrostatic precipitators. 

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. 

If staging and countercurrent operation are essential, a slurry tray column and a rotating disk column can be applied [9]. In flue gas purification where a low pressure drop is essential, lime or CaCC>3 slurries are spray contacted with the gas phase. Sometimes it is meant to evaporate the water phase but also simple slurry scrubbing is envisaged. 

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. 

One of the early downflow units employed an oil-scrubbing tower instead of a Cottrell (237,350,358). A portion of the feed was used as scrubbing medium and was preheated as it stripped catalyst from the hot flue gas. The recovered slurry, containing 0.6 to 1.0 lb. of catalyst per gallon, was injected into the reactor. The oil scrubber was later abandoned and a second stage of cyclones was installed (358). 

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