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Mist Eliminators and Mist Eliminator Wash System
Mist eliminators (ME) remove entrained scrubbing liquids to minimize fouling of downstream equipment before the flue gas enters the stack and to minimize fallout from the stack. The ME is a momentum separation device that separates the high momentum liquid droplets from the gas as it passes through the tortuous path of the mist eliminator chevrons.
Mist from the absorber spray zone is collected by two stages of chevron type mist eliminators or a combination of chevron and tubular style mist eliminators. The first stage, which is closest to the spray headers, captures the larger particles while the second stage captures finer particles including the ME wash water droplets. The mist eliminators require washing to prevent solids which are formed by evaporation and slurry particle deposits from accumulating on the blades. An array of wash headers and wash nozzles are provided as part of the system. The top and bottom sides of the first stage are washed, but typically only the bottom side of the second stage is washed.
A variety of MEs and wash systems can be found on older generation wet FGD systems. Common old generation ME designs include a bulk entrainment separator followed by two levels of “teepee” style mist eliminators or two levels of early generation flat chevron style MEs. Figure 1 compares an old generation ME system to a typical modern set of MEs.
Older generation ME wash system designs tended to not effectively wash all surfaces of the MEs when compared to current designs. Fixed grid ME wash headers can be retrofitted in place of retractable lance-style wash systems or fixed grids that were ineffective in washing the ME surfaces.