The arrangement of emission control devices other than the devices for nitrogen oxides is usually standard: a scrubber and condenser, followed by a particulate collector, followed by an induction fan that sucks flue gases up to the stack. Two reasons for this arrangement are:
Fabric filters cannot operate at the high temperatures at which gases exit the boiler without risk of fire. Thus, placing the scrubber between the boiler and the fabric filter or ESP permits cooling and often humidification that prevent fire. Cooling the gases also plays a role in reducing acid gas, mercury, and dioxin emissions. Dioxins and heavy metals are trapped more effectively by particulate control devices when they are first condensed out of the flue gas and adsorbed onto the surface of particulate matter, as happens in a scrubber-condenser system.
An alternate arrangement, common in European plants, involves an ESP followed by a wet scrubber. The ESP is not damaged by high temperatures, and the wet scrubber cools and condenses gases and captures particulates.
The location of control devices for nitrogen oxides depends on the type of technology used. These devices can be in the furnaces or the boiler as well as at the back end of the plant.
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