Slurry lime/limestone

Originally, bubble cap plates had been used for absorption of pollutant gases such as sulfur dioxide. However, the solids in the slurries used as absorbents can more readily plug bubble caps. Typical absorbents used in current processes include, for example, conventional lime slurry lime-limestone slurries mixed sodium sulfite/ lime slurries and magnesium sulfite/bisulfite mixed with lime slurries. Conventional lime slurry towers may consist of a multilevel spray tower combined with a venturi scrubber. Venturi scrubbers will be discussed briefly below. Mixed sodium sulfite/lrme slurries may be contacted in a plate tower. Sieve plates might be used with larger than normal holes to help prevent plugging due to the solids in the slurries. 

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. 

Chemical scrubbing systems for SO/ absorption fall into two broad categories (a) Disposable systems and (b) regenerative systems. Typical of systems in use for a number of years are those that use an aqueous slurry of an insoluble caldum compound, which can be discarded after use. Disposable 02-removal systems use aqueous slurries of finely ground materials, such as lime, limestone or dolomite, to produce a mixture of insoluble sulfites and sulfates. On passing through the scrubber, S02 from the waste gas dissolves to form sulfurous acid S02 ... 

Costs of 10 alternative ECTs were calculated for three hypothetical and three actual coal-cleaning plants, and the cost of lime-limestone slurry coating was analyzed. 

Option 10. Lime-Limestone Slurry Coating of Waste Before Disposal. One major... 

A highly acidic Illinois Basin coal waste was mixed in plastic-lined 55-gal. barrels with wet slurries containing lime ranging from 0.17 to. LS wt% of the waste. In one case, 1.1% limestone slurry was mixed in after 0.3.3% lime had been used. These slurries were screened to remove excess water and placed in specially designed disposal boxes. Six boxes of each of the six lime/limestone/waste mixes were then placed in a pattern to await wet and dry weathering cycles. 

Option 10. Lime-Limestone Slurry Coating. Capital cost for this option was calculated to be 601 600 and essentially independent of plant throughput in the range here considered. The operating expense, other than materials, is also independent of throughput. Lime and limestone cost is a function of the tonnage and not the composition of waste. The data in the tables are based... 

This work shows that oxygen-free sulfite in lime/limestone slurries, exposed to sulfur dioxide, slowly decomposes under process conditions. In fact, auto-redox reactions of sulfur oxyacids can occur in all coal desulfurization systems, including coal-gasification systems and impurities present in commercial flue gas systems are capable of catalyzing the reaction under process conditions. Our experiments indicate that any large-scale coal utilization will depend on appropriate control of the autoredox reactions of sulfur species. 

Buffer Additives for Lime/Limestone Slurry Scrubbing... 

Buffer additives are attractive for enhancing SO2 removal and/or CaC03 utilization in lime/limestone slurry scrubbing processes for flue gas desulfurization. This work was sponsored by EPA to provide experimental data on commercial synthesis, gas/liquid mass transfer enhancement, and oxidative degradation of useful buffer additives. 

Lime/limestone slurry scrubbing is the dominant commercial technology for flue gas desulfurization 0.). SO2 is absorbed at 50-55°C and pH 5.5-6.0 in an aqueous slurry of excess CaC03 and product solids. The CaS03/CaS04 product is disposed of as solid waste. With greater than 500-1000 ppm SO2 in the flue gas, SO2 absorption is controlled by liquid-film mass transfer resistance because of the limited solubility of SO2 gas and alkaline solids. Additives that buffer between pH 3 and pH 5.5 enhance S02 absorption by providing dissolved alkaline species for reaction with SO2 (8). 

This paper summarizes the results of tests conducted from July 1978 through March 1981 at the EPA, 10-MW equivalent, lime/limestone wet-scrubbing FGD test facility, during which adipic acid as an additive was tested and shown to be a powerful scrubber additive for improving SO2 removal. The optimum concentration of adipic acid is only 700 to 1500 ppm at a scrubber inlet pH of 5.2 or higher. SO2 removal efficiencies in excess of 90 percent and reliable operation were demonstrated in four long term, limestone/adipic acid runs. Factorial tests were also conducted to characterize SO2 removal as a function of gas and slurry flow rates, pH, and adipic acid concentration. Intermediate duration optimization runs and favorable economics are also reported. 

The concentration of absorbing species in the scrubber solution is obviously an important consideration in scrubbing stack gas with lime-limestone slurry. Thus, the results of this study are useful both in understanding solution absorptivity and in pointing the way to possible improvement. 

By treatment with limestone slurry/lime water... 

If coal or oil is the fuel source, the FGD control technologies result in the generation of solid wastes. Wet lime/limestone scrubbers produce a slurry of ash, unreacted lime, calcium sulfate, and calcium sulfite. Dry scrubber systems produce a mixture of unreacted sorbent (e.g., lime, limestone, sodium carbonates, and calcium carbonates), sulfur salts, and fly ash. 

Lime slurry scrubbing Limestone slurry scrubbing Magnesium oxide scrubbing Sodium-base scrubbing... 

Sulfur dioxide Lime/limestone scrubbing Slurry of lime or limestone in water... 

Flue-gas desulfurization (FGD) is the best way to remove sulfur dioxide from fossil fuel power plants. FGD commonly uses either wet or dry scrubbers. In wet scrubbers, a slurry of limestone, CaC03, or hydrated lime Ca(OH)2 is sprayed... 

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