temperature, then not all the drops will evaporate, the resulting particles will be wet and sticky (plugging the filter), and the dryer and downstream equipment will suffer severe corrosion. The legend lists a test condition of"20 F approach temperature, "indicating that the amount of water fed in the slurry was limited to keep the gas temperature 20 F above its adiabatic saturation temperature. Test data show that the collection efficiency improves as one approaches saturation, presumably because much of the reaction takes place before the droplet is completely converted to a solid and a close temperature approach keeps the drops wet longer. In addition, at high relative Water adsorption increases the collection efficiency after the droplets evaporate, borg aperature Z. humidities the particles of Cal OH)2 will adsorb one or two molecular layers of water, thus grea increasing their reactivity compared with that of totally dry Ca( OH)2 at the same ten spray dryer and in the baghouse. Operators carefully monitor their approach to the adiabatic saturation temperature: A close approach gives the best removal efficiency; too close an approach produces a sticky cake and corrosion The high solids recycle rate shown is needed to get good reagent utilization. With once-through solids use the utilization is poor. In some cases the recycle material is first ground to break the particles open and provide better access to the unreacted materials in the centers of the particles Regenerative systems. In these some kind of absorbent or adsorbent is used to capture SO2 from the flue gas. Then in some separate device or set of devices the adsorbent or absorbent is regenerated to produce a flow of relatively pure SOz or HSO4. These systems were under intense study and development when it appeared that the problems with wet limestone scrubbers were insoluble. As those problems were solved, interest in regenerative systems waned. Recently, work has begun on regenerative processes that will simultaneously capture both SOz and NOx. These systems have not yet advanced to commercial scale, but they may have a major role in future air pollution control Tomorrows limestone control devices. The forced-oxidation limestone scrubber is a great technological accomplishment. It does a difficult task with high efficiency and reliability at a high but not impossible cost. However, industry would like a simpler, cheaper system. The manufacturers of forced-oxidation limestone scrubbers have shown in pilot plants that one can operate them at gas velocities up to 18 ft/s, if one can make the entrainment separators work well enough to capture the small drops that are carried along with the gas stream at that velocity. With this higher velocity one can use a much smaller scrubber with a large cost saving Industry continues to try to develop dry limestone-based processes. As discussed above the low reactivity of limestone makes these difficult. However, if one recycles most of the captured particles through the boiler or through an intermediate gas-solid contact vessel and humidifies the gas almost to adiabatic saturation, then one can get satisfactory SOz capture and satisfactory reagent utilization with these devices. Whether they will be more economical than the forced-oxidation limestone scrubber remains to be seen 11.5 Alternatives to"Burn and then scrub When the electric power industry first faced regulations requiring it to reduce the emissions of SO2 from power plants, it decided for the most part to leave the power plant alone and to scrub the gas leaving the power plant. This approach is still the most common, using either wet limestone scrubbers or lime spray dryers. But the ind\ar dust Air Amendments of 1990 to reduce ver entirely abandoned the investigation of alternative approaches. With strong pressure from emissions of acid rain precursors, the electric power industry has renewed interest in these other possibilities Change to a lower Sulfur Content Fuel If the management of a power plant can replace a high-sulfur coal with a low-sulfur coal,it reduces the so2 emissions quickly, simply, and without having to install expensive So2 control devices or to deal with their solid effluent (Switching coals can cause some problems in the plant, which was presumably designed for the coal originally used, but such problems are generally manageable. Many power plants that burned high-sulfur eastern coals switched to lower-sulfur coals from the Rocky Mountain states. This decision was a boon to the economies of Wyoming and Montana and a blow to the economies of the midwestern and eastern coal-producing states This approach has been vigorously attacked, mostly on the grounds of job losses, by the midwestern and eastern coal miners and their elected representatives it is a continuing politi11－12 temperature, then not all the drops will evaporate, the resulting particles will be wet and sticky (plugging the filter), and the dryer and downstream equipment will suffer severe corrosion. The legend lists a test condition of"20。 F approach temperature," indicating that the amount of water fed in the slurry was limited to keep the gas temperature 20。 F above its adiabatic saturation temperature. Test data show that the collection efficiency improves as one approaches saturation, presumably because much of the reaction takes place before the droplet is completely converted to a solid and a close temperature approach keeps the drops wet longer. In addition, at high relative humidities the particles of Ca(OH)2 will adsorb one or two molecular layers of water, thus greatly increasing their reactivity compared with that of totally dry Ca(OH)2 at the same temperature. Water adsorption increases the collection efficiency after the droplets evaporate, both inside the spray dryer and in the baghouse. Operators carefully monitor their approach to the adiabatic saturation temperature: A close approach gives the best removal efficiency; too close an approach produces a sticky cake and corrosion. The high solids recycle rate shown is needed to get good reagent utilization. With once-through solids use the utilization is poor. In some cases the recycle material is first ground to break the particles open and provide better access to the unreacted materials in the centers of the particles. Regenerative systems. In these some kind of absorbent or adsorbent is used to capture SO2 from the flue gas. Then in some separate device or set of devices the adsorbent or absorbent is regenerated to produce a flow of relatively pure SO2 or H2SO4. These systems were under intense study and development when it appeared that the problems with wet limestone scrubbers were insoluble. As those problems were solved, interest in regenerative systems waned. Recently, work has begun on regenerative processes that will simultaneously capture both SO2 and NOx. These systems have not yet advanced to commercial scale, but they may have a major role in future air pollution control. Tomorrow's limestone control devices. The forced-oxidation limestone scrubber is a great technological accomplishment. It does a difficult task with high efficiency and reliability at a high but not impossible cost. However, industry would like a simpler, cheaper system. The manufacturers of forced-oxidation limestone scrubbers have shown in pilot plants that one can operate them at gas velocities up to 18 ft/s, if one can make the entrainment separators work well enough to capture the small drops that are carried along with the gas stream at that velocity. With this higher velocity one can use a much smaller scrubber with a large cost saving. Industry continues to try to develop dry limestone-based processes. As discussed above the low reactivity of limestone makes these difficult. However, if one recycles most of the captured particles through the boiler or through an intermediate gas-solid contact vessel and humidifies the gas almost to adiabatic saturation, then one can get satisfactory SO2 capture and satisfactory reagent utilization with these devices. Whether they will be more economical than the forced-oxidation limestone scrubber remains to be seen. 11.5 Alternatives to "Burn and Then Scrub" When the electric power industry first faced regulations requiring it to reduce the emissions of SO2 from power plants, it decided for the most part to leave the power plant alone and to scrub the gas leaving the power plant. This approach is still the most common, using either wet limestone scrubbers or lime spray dryers. But the industry never entirely abandoned the investigation of alternative approaches. With strong pressure from the Clean Air Amendments of 1990 to reduce emissions of acid rain precursors, the electric power industry has renewed interest in these other possibilities. Change to a Lower Sulfur Content Fuel If the management of a power plant can replace a high-sulfur coal with a low-sulfur coal, it reduces the SO2 emissions quickly, simply, and without having to install expensive SO2 control devices or to deal with their solid effluent. (Switching coals can cause some problems in the plant, which was presumably designed for the coal originally used, but such problems are generally manageable.) Many power plants that burned high-sulfur eastern coals switched to lower-sulfur coals from the Rocky Mountain states. This decision was a boon to the economies of Wyoming and Montana and a blow to the economies of the midwestern and eastern coal-producing states. This approach has been vigorously attacked, mostly on the grounds of job losses, by the midwestern and eastern coal miners and their elected representatives; it is a continuing political struggle