A report by the German equivalent to the U.S. EPA predicts a trend toward wet scrubbing because of better elimination of sulfur dioxide, heavy metals, and other toxic substances (Mcllvaine 1989). In addition, spray drying and other dry processes have the disadvantage of increased residue production. The report does cite potential prob lems with wet scrubbers; however, water treatment and heavy metal precipitation and evaporation are promising solutions. Typically, a German APC system uses a packed tower to remove hydrogen chloride, sulfur dioxide, and condensed heavy metals and a high-efficiency ESP to remove dust.

Particulate and Heavy Metal Emissions

The emissions of particulates and heavy metals are best reduced by collecting them in one of two basic types of addon particulate control devices: fabric filters and ESPs (see Section 5.20). Heavy metals are captured because they are condensed out of flue gas onto the particles. These devices are designed to operate at temperatures lower than 450°F for flue gas leaving the boiler; some operate at temperatures as low as 250°F, which is beneficial for condensing and collecting acids, volatile metals, and organics. The state-of-the-art level for particulate emission is 0.010 g per dry cu ft.

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