practice the five problems just listed have dominated the design and all efforts have been devoted to overcoming these problems. Most of these problems are now solved or reduced to manageabl proportions by careful control of the process chemistry, good mechanical design, and careful operation. The design and operating practices are continuously being improved. But these scrubbers are still expensive and troublesome, and they generate large amounts of solid waste, hich are a disposal problem. Detailed descriptions of the design, chemistry, and operating experience of these scrubbers have been published Other Approaches During the period of development of the limestone scrubber, when its growing pains seemed unendurable(many believed that it would never work satisfactorily ) many other approaches to the problem were suggested and tested. As the technical difficulties with the limestone scrubber were worked out, it became the clear economical choice for scrubbing stack gas from the combustion of medium- or high-sulfur coal. The other processes are not being used now for new installations, and some of those installed 20 years ago are being convened to forced oxidation limestone scrubbers to save operating costs Other wet systems. Ca(OH)2(hydrated lime, quicklime) is an alternative to limestone in wet throwaway processes. (Throwaway processes are ones in which the reagent is used once and ther thrown away. Its use is similar to that of limestone, shown in Fig. 11. 6. Normally, Cao(lime, burned lime)is added to the oxidation tank and hydrates there to Ca(oh)z. It is more chemically reactive than limestone, mostly because it has a much higher surface area( Cao is prepared by eating limestone and driving off the COz. The result is a porous structure, as discussed. Typical urface areas are 15 m/g ) But to use Cao requires an extra process step to prepare it for insertion in the process shown in Fig. 11.6. In the early days of scrubber development this extra reactivity seemed necessary, but as the problems with wet limestone scrubbers have mostly been solved, the additional reactivity of lime has seemed less likely to repay its extra cost. The other reagents shown are all more expensive than limestone and would not be used in a wet, throwaway process where cheap limestone can be made to work In double alkali processes the scrubbing step is done with a sodium carbonate or sodium bicarbonate solution in the presence of a very low concentration of calcium. The solubility of sodium salts is much higher than that of calcium salts, so that in the scrubber all the salts are in solution and the liquid is practically free of solids The liquid is taken out of the scrubber, the alkali is regenerated with lime or limestone in a reaction tank. The main reaction in the scrubber is NaCO3+SO2→NaSO3+CO2 The overall reaction in the reaction tank is Na2SO3+ CaCO3+0.50+2H20> CaSO4 2H20+ Na2 CO3 (11.,19) which regenerates the sodium carbonate(or bicarbonate) in solution and precipitates the calcium as CaSo4 All the liquid from the reaction tank is sent to a thickener, where the calcium is removed, either as gypsum or as unreacted calcium carbonate (or as calcium hydroxide). Some dissolved sodium carbonate is lost in the solution in the moist solid waste stream so additional odium carbonate or bicarbonate is added in the thickener overflow tank, and the clear liquid from it is used as the scrubbing liquid The ease of operation and reliability of the double alkali systems allowed them to compete with the early limestone scrubbers. As the limestone scrubbers improved, the extra complexity(more chemicals to handle more vessels. pumps Alkaline reagent Exhaust gas with 99+% delivered to fumace of particles and=90% or to fue or both of SOz removed higher reagent cost of systems made them uncompetitive. 11.5.2.2 Dry systems. Furnace Particle control device The solids handling filter or ESP handling and disposal nteg Material being heated alkaline FIGURE 11.7 Flow diagram for SO2 control systems using dry solids addition11－10 practice the five problems just listed have dominated the design and all efforts have been devoted to overcoming these problems. Most of these problems are now solved or reduced to manageable proportions by careful control of the process chemistry, good mechanical design, and careful operation. The design and operating practices are continuously being improved. But these scrubbers are still expensive and troublesome, and they generate large amounts of solid waste, which are a disposal problem. Detailed descriptions of the design, chemistry, and operating experience of these scrubbers have been published. Other Approaches During the period of development of the limestone scrubber, when its growing pains seemed unendurable (many believed that it would never work satisfactorily), many other approaches to the problem were suggested and tested. As the technical difficulties with the limestone scrubber were worked out, it became the clear economical choice for scrubbing stack gas from the combustion of medium- or high-sulfur coal. The other processes are not being used now for new installations, and some of those installed 20 years ago are being convened to forced oxidation limestone scrubbers to save operating costs. Other wet systems. Ca(OH)2 (hydrated lime, quicklime) is an alternative to limestone in wet throwaway processes. (Throwaway processes are ones in which the reagent is used once and then thrown away.) Its use is similar to that of limestone, shown in Fig. 11.6. Normally, CaO (lime, burned lime) is added to the oxidation tank and hydrates there to Ca(OH)2. It is more chemically reactive than limestone, mostly because it has a much higher surface area. (CaO is prepared by heating limestone and driving off the CO2. The result is a porous structure, as discussed. Typical surface areas are 15 m2 /g.) But to use CaO requires an extra process step to prepare it for insertion in the process shown in Fig. 11.6. In the early days of scrubber development this extra reactivity seemed necessary, but as the problems with wet limestone scrubbers have mostly been solved, the additional reactivity of lime has seemed less likely to repay its extra cost. The other reagents shown are all more expensive than limestone and would not be used in a wet, throwaway process where cheap limestone can be made to work. In double alkali processes the scrubbing step is done with a sodium carbonate or sodium bicarbonate solution in the presence of a very low concentration of calcium. The solubility of sodium salts is much higher than that of calcium salts, so that in the scrubber all the salts are in solution and the liquid is practically free of solids. The liquid is taken out of the scrubber, the alkali is regenerated with lime or limestone in a reaction tank. The main reaction in the scrubber is Na2CO3 + SO2 → Na2SO3 + CO2 (11.18) The overall reaction in the reaction tank is Na2SO3 + CaCO3 + 0.502 + 2H20 → CaSO4 .2H20 + Na2CO3 (11. ,19) which regenerates the sodium carbonate (or bicarbonate) in solution and precipitates the calcium as CaSO4 .2H20. All the liquid from the reaction tank is sent to a thickener, where the calcium is removed, either as gypsum or as unreacted calcium carbonate (or as calcium hydroxide). Some dissolved sodium carbonate is lost in the solution in the moist solid waste stream, so additional sodium carbonate or bicarbonate is added in the thickener overflow tank, and the clear liquid from it is used as the scrubbing liquid. The ease of operation and reliability of the double alkali systems allowed them to compete with the early limestone scrubbers. As the limestone scrubbers improved, the extra complexity (more chemicals to handle, more vessels, pumps, lines, valves) and higher reagent cost of the double alkali systems made them uncompetitive. 11.5.2.2 Dry systems. The solids handling and wet sludge handling and disposal difficulties that are integral to wet throwaway processes