Flue Gas Treatment

Hot gas filtration can remove only solid particles from a flue gas stream - pollutant gases or vapours will pass right through even the most efficient filter. However, use of the dry scrubbing technique enables vapour phase capture such as acid gas absorption or VOC removal as well as particulate capture.

Dry gas scrubbing involves the addition of a sorbent powder to the flue gas before the gas reaches the hot gas filter. In the filter the sorbent particles are collected on the ceramic filter elements and form a temporary fixed reactive layer. When the flue gas passes through this layer components of the gas react with the sorbent powder and are retained within it. Online reverse pulse cleaning of the filter removes the accumulated solids along with the reacted vapour phase pollutants.

Acid gases are common flue gas pollutants and these can be very effectively removed by dry gas scrubbing using the addition of powdered hydrated lime, Ca(OH)_2, to the gas upstream of the ceramic hot gas filter. Lime reacts well with HCl, HF and H₂SO₄ and is claimed to remove a proportion of any NOx that is present. NaHCO₃, sodium bicarbonate,is an alternative sorbent.

Caldo has considerable experience in the use of dry scrubbing sorbents for acid gas removal in conjunction with high temperature ceramic filters and we supply a range of powder feeders. These metering devices use a small screw feeder to dispense the chosen sorbent into an entrainment eductor from where the powder is blown into the process. They are available with several ranges of powder feed rate and incorporate a variable speed gear box, which allows the feed rate to be more finely adjusted within the range of the particular unit. The powder can be conveyed to the process several meters away, allowing the feed unit to be sited in a location convenient for charging with dry scrubbing sorbent powder.

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