Dry Scrubbing

Dry scrubbing is a technique for removing vapour components of  gas flow using a hot gas filter. Generally filtration can remove only solid pollutants - unwanted materials that are present as gases or vapours will pass right through even the most efficient filter. If, however, the unwanted material can be reacted with a solid then it can be removed along with the solid. This technique is known as dry scrubbing and the solid material is called the sorbent.

Dry scrubbing reactions between gases or vapours and sorbents are often quite slow. Generally the sorbent is blown into the gas stream and there is not much sorbent compared with the total amount of waste gas. This means that a long residence time is needed to ensure that the molecules of pollutant actually come into contact and react with the sorbent particles. When using a filter for dry scrubbing, the medium collects a fixed bed of the sorbent particles on its surface. The fixed bed of sorbent defines relatively narrow and tortuous paths for the gas to pass between the particles. When the gas passes through these tortuous paths it is forced into contact with the sorbent particles and reacts much more quickly and efficiently.

Acid gases are a common gas phase pollutant and these can be very effectively removed by dry scrubbing using the addition of powdered lime (actually slaked lime, Ca(OH)₂) to the gas upstream of the ceramic 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 filters and we supply a range of powder dosing units. 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 depending on the selection of fixed speed gear box. They also 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 powder.

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