Air Pollution Control Systems for Boiler and Incinerators.Unique control problems_5

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Air Pollution Control Systems for Boiler and Incinerators.Unique control problems_5Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com 5-815-1/AFR 19-6 TMsevere pitting in stainless steels. This condition is fre- resistant to oxidizing acid environments, but arequently encountered in an incinerator which burns attached by acids under reducing conditions. Thelarge quantities of disposable polyvinyl chloride (PVC) equipment designer should select materials based onmaterials. individual case conditions including temperature, abra- b. Temperature. Corrosion rates generally increase sion, pH, etc.with increases in exhaust temperatures. This is due to 7-6. Auxiliary equipmentthe increased mobility of ions and increased reaction a. Gas transport.rates. However, in cases where the corrosion process (1) Ducts and stacks. Large boiler plant stacksis accelerated by the presence of oxygen, increasing the have a wind shield of reinforced concrete oracid temperature eventually boils out dissolved oxygen, of steel, with a separate inner flue orrapidly diminishing corrosion rate. This is the case with numerous flues of steel, acid-resistant brick,Monel, a nickel-copper alloy. and occasionally, stainless steel. The space c. Velocity. Often the corrosion resistance of an alloy between the inner flue and the outer winddepends on the existence of an adhering oxide layer on shield may be insulated with a mineral woolits surface. A high exhaust gas velocity can remove or wrapping. This is to prevent the condensa-erode the surface layer. Once removed, this layer can- tion of acid dew on the inside of the metalnot be renewed because the oxide film is washed away chimney, which occurs below dew pointas it forms. temperature, and also to prevent acid “smut” d. State of oxidation. Under reducing condition, from being blown out of the chimney. AcidMonel is very resistant to moderate sulfuric-acid con- smut is a term for ash particles contaminatedcentrations. Under oxidizing conditions, or in the pres- with acid. It is heavy and tends to fall out ofence of oxidizing ions, however, very rapid corrosion the gas plume soon after exiting from theoccurs. The reverse is true of stainless steels which are 7-7TM 5-815-1/AFR 19-6Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com stack. In smaller plants, stacks may be a pressure piping. Considerations must also be single wall steel construction with insulation made for weatherproofing against freezing and lagging on the outer surface. For wet conditions. scrubbing practice, chimneys for vapor-satu- (2) Pumps. Centrifugal pumps are used to rated gases containing corrosive substances supply the scrubbing liquid or recycled slurry may be made of rubber-lined steel, to the scrubber nozzles at the required fiberglass-reinforced resin or other volume flow rate and pressure. Where no corrosion-resistant material. With materials solids are present in the liquid, bare metal that have a limited maximum temperature, pumps, either iron or stainless steel provisions must be made to protect the stack construction, are used. In recycle systems from high temperatures because of loss of with solids in the liquid, special rubber-lined scrubbing liquid. Chimney or stack velocities or hard-iron alloy pumps are used to control are generally 30 ft/ sec to prevent re- erosion of the pump internals. These are entrainment of moisture from the stack wall generally belt driven to allow selection of the which would rain down around the plant. proper speed necessary for the design Sometimes cones are fitted at the top to give capacity and head. Solids content must still exit velocities as high as 75 ft/sec. The chief be controlled to limit the maximum slurry reason for high velocities is to eject the gases consistency to meet the scrubber and pump well away from the top of the stack to requirements. increase the effective height and to avoi ...