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When they are placed after a scrubber, particulate control devices also collect heavy metals and other pollutants that have condensed out of flue gas onto particle surfaces. Placing a scrubber first helps lower the temperature of gases entering a fabric filter. However, wet scrubbers cannot precede fabric filters because the wet particles in flue

FIG. 10.9.15 Schematic of Commerce waste-to-energy plant in southern California. (Reprinted, with permission, from Commerce Refuse-to-Energy Authority.)

gases clog the filters. Thus, facilities with wet scrubbers place their scrubbers after the particulate control device.

The smallest particles are the most potentially damaging when inhaled into the lungs, and dioxins, furans, acid gases, and heavy metals are adsorbed in the largest quan tities on these smaller particles. Thus, a state-of-the-art par-ticulate control device should achieve even lower emission levels for particulates below 2 microns in diameter.

Since many heavy metals condense at temperatures of 450°F (230°C), both ESPs and fabric filters collect heavy

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