belt filter, from which a semidry gypsum product leaves the system. The overflow from the first hydroclone is further treated to produce a liquid waste stream with a very low solids content, which goes to water treatment and disposal. The fresh water enters the system almost exclusively as wash water for the entrainment separators. The scrubber operates at or near the adiabatic saturation temperature of the entering flue gas, which is about 125 F In some installations the cleaned flue gas is reheated to about 175 F to restore plume buoyancy and prevent acid corrosion of the ducts and stack downstream of the reheater. Other installations discharge the gas at scrubber temperature and use corrosion-resistant materials to deal with the small amount of acid liquid that is not removed by the entrainment separators. This example shows that the liquid circulation rates ill these scrubbers are veryvery large. As a consequence, even though they remove most of the SO from the gas, the scrubbing slurry passes through them practically unchanged. Most of the hemical reactions take place in the effluent hold tank. (The slurry spends about 3 seconds pass in the scrubber and about 8 minutes between passes through the scrubber in the hold tank. Observations suggest that very little of the preceding 1% possible reaction occurs while the drops are falling; there is not enough time. This results in the ph of the drops declining as they fall, educing their absorptive capac The development problems with limestone scrubbers. In the 1970s and early 1980s the electric utility industry suffered through the very painful development period of limestone scrubbers. By now the major problems have largely been solved, and these devices are reasonably reliable and useful if designed and operated properly. The major development problems were these 1. Corrosion: The exhaust gases from coal combustion contain small amounts of many chemical s, e.g., chlorides. In an acid environment these proved much more corrosive to metals, including stainless steels, than the designers of the first systems had anticipated 2. Solids deposition, scaling and plugging: Calcium sulfate and its near chemical relatives are slightly soluble in water and can precipitate on solid surfaces to form hard, durable scales that are very difficult to remove. These are the"boiler scales"that collect in teapots and hot water heaters The scales formed in valves, pumps, control instruments, and generally anywhere that their effect could cause the most trouble 3. Entrainment separator plugging The spray nozzles shown in Fig. 11.6 do not produce totally uniform drops; some of the drops are small enough to be carried along with the gas and must be removed from the gas in the entrainment separator. If they are not removed, they will plug and corrode the ductwork downstream of the scrubber. The early entrainment separators were plugged by the solids contained in those small drops 4. Poor reagent utilization: The product sulfates and sulfites can precipitate on the surface of the limestone particles, thus blocking their access to the scrubbing solution ed a high percentage of the limestone to pass unreacted into the solid waste product, raising reagent and waste disposal costs. 5. Poor solid-liquid separation: Caso3 0.5H20 tends to form crystals that are small, flat plates These are very good at trapping and retaining water. If the solid product has too many of these it will have the consistency of toothpaste and not be acceptable for landfills. CaSo4 2H20 forms larger, rounder crystals that are much easier to settle and filter. Flocculating agents added to the thickener improve this separation The solution to these problems has been found by careful attention to engineering and chemical detail. The rate of liquid rejection to waste water(Fig 11. 5) is chosen to control the chloride content of the circulating liquid. It is kept low enough to protect the very expensive materials it contacts. (The most widely used metal for lining the surfaces of the modules is alloy C-276, 55% Ni, 17% Mo, 16% Cr, 6% Fe, 4%W. It costs roughly 15 times as much as ordinary steels ) The solids deposition was caused by local supersaturation with gypsum. Enough gypsum is kept in the circulating slurry to prevent that supersaturation, vastly reducing the scale deposition. The original entrainment separators were of the woven wire variety, which plugged easily. The chevron type shown in Fig. 11.6 is much easier to keep clean. All the fresh water entering the system comes in as entrainment separator wash water, which is applied as strong jets for a few minutes of each hour The liquid holding tanks were made larger, thus allowing more time for the reagent to dissolve This additional time plus more vigorous application of oxidation air resulted in convening = 95% of the captured sulfur to gypsum, which forms large, easily filtered crystals. Some plants produce a gypsum waste stream clean enough and dry enough that wallboard manufacturers will purchase it, thus converting the plant's waste disposal cost to a by-product sale In principle these systems are designed by the same methods as in Examples 11. 1 and 11. 2. In l1-911－9 belt filter, from which a semidry gypsum product leaves the system. The overflow from the first hydroclone is further treated to produce a liquid waste stream with a very low solids content, which goes to water treatment and disposal. The fresh water enters the system almost exclusively as wash water for the entrainment separators. The scrubber operates at or near the adiabatic saturation temperature of the entering flue gas, which is about 125。 F In some installations the cleaned flue gas is reheated to about 175。 F to restore plume buoyancy and prevent acid corrosion of the ducts and stack downstream of the reheater. Other installations discharge the gas at scrubber temperature and use corrosion-resistant materials to deal with the small amount of acid liquid that is not removed by the entrainment separators.This example shows that the liquid circulation rates ill these scrubbers are very very large. As a consequence, even though they remove most of the SO2 from the gas, the scrubbing slurry passes through them practically unchanged. Most of the chemical reactions take place in the effluent hold tank. (The slurry spends about 3 seconds per pass in the scrubber and about 8 minutes between passes through the scrubber in the hold tank.) Observations suggest that very little of the preceding 1% possible reaction occurs while the drops are falling; there is not enough time. This results in the pH of the drops declining as they fall, reducing their absorptive capacity). The development problems with limestone scrubbers. In the 1970s and early 1980s the electric utility industry suffered through the very painful development period of limestone scrubbers. By now the major problems have largely been solved, and these devices are reasonably reliable and useful if designed and operated properly. The major development problems were these: 1. Corrosion: The exhaust gases from coal combustion contain small amounts of many chemicals, e.g., chlorides. In an acid environment these proved much more corrosive to metals, including stainless steels, than the designers of the first systems had anticipated. 2. Solids deposition, scaling and plugging: Calcium sulfate and its near chemical relatives are slightly soluble in water and can precipitate on solid surfaces to form hard, durable scales that are very difficult to remove. These are the "boiler scales" that collect in teapots and hot water heaters. The scales formed in valves, pumps, control instruments, and generally anywhere that their effect could cause the most trouble. 3. Entrainment separator plugging: The spray nozzles shown in Fig. 11.6 do not produce totally uniform drops; some of the drops are small enough to be carried along with the gas and must be removed from the gas in the entrainment separator. If they are not removed, they will plug and corrode the ductwork downstream of the scrubber. The early entrainment separators were plugged by the solids contained in those small drops. 4. Poor reagent utilization: The product sulfates and sulfites can precipitate on the surface of the limestone particles, thus blocking their access to the scrubbing solution. This caused a high percentage of the limestone to pass unreacted into the solid waste product, raising reagent and waste disposal costs. 5. Poor solid-liquid separation: CaSO3. 0.5H20 tends to form crystals that are small, flat plates. These are very good at trapping and retaining water. If the solid product has too many of these it will have the consistency of toothpaste and not be acceptable for landfills. CaSO4 .2H20 forms larger, rounder crystals that are much easier to settle and filter. Flocculating agents added to the thickener improve this separation. The solution to these problems has been found by careful attention to engineering and chemical detail. The rate of liquid rejection to waste water (Fig. 11.5) is chosen to control the chloride content of the circulating liquid. It is kept low enough to protect the very expensive materials it contacts. (The most widely used metal for lining the surfaces of the modules is alloy C-276, 55% Ni, 17% Mo, 16% Cr, 6% Fe, 4% W. It costs roughly 15 times as much as ordinary steels.). The solids deposition was caused by local supersaturation with gypsum. Enough gypsum is kept in the circulating slurry to prevent that supersaturation, vastly reducing the scale deposition. The original entrainment separators were of the woven wire variety, which plugged easily. The chevron type shown in Fig. 11.6 is much easier to keep clean. All the fresh water entering the system comes in as entrainment separator wash water, which is applied as strong jets for a few minutes of each hour. The liquid holding tanks were made larger, thus allowing more time for the reagent to dissolve. This additional time plus more vigorous application of oxidation air resulted in convening ≈ 95% of the captured sulfur to gypsum, which forms large, easily filtered crystals. Some plants produce a gypsum waste stream clean enough and dry enough that wallboard manufacturers will purchase it, thus converting the plant's waste disposal cost to a by-product sale. In principle these systems are designed by the same methods as in Examples 11.1 and 11.2. In