Acids Mercury Dioxin and Furan Emissions

Scrubbers, followed by an efficient particulate control device, are the state-of-the-art equipment for controlling emissions of acids such as hydrogen chloride, sulfur dioxide, and sulfuric acid. Scrubbers generally use impaction, condensation, and acid-base reactions to capture acid gases in flue gas. Since greater removal efficiencies usually accompany greater condensation, devices that lower gas temperatures and thus increase condensation can enhance scrubber effectiveness. The lower temperatures also allow mercury, dioxins, and furans to condense so that they can subsequently be captured by a particulate device.

Three types of scrubbers are in use: wet scrubbers, spray-dry scrubbers, and dry injection scrubbers. The first two scrubbers are condensers, while dry injection scrubbers require a separate condenser (either a humidifier or a heat exchanger). In all cases, temperature and, for dry and spray-dry scrubbers, the amount of lime (an alkaline substance that neutralizes acids) are the key factors affecting scrubber effectiveness. In general, to maximize emission control, the scrubber should be adequately sized, operate at temperatures below 270°F, and allow flue gas circulation through the scrubber for at least 10-15 sec.

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