Activated carbon has been used since the First World War to protect infantry against chemical gas attack.
The base material can be coal, wood or even peat, but the best quality carbon uses coconut shells. The material is chemically or thermally treated, usually with superheated steam, to expose the internal pore structure. This increases the surface area by over 100 times. A teaspoonful of carbon can have the surface area of a football pitch.
The vast surface area and tortuous route through the pore structure encourages molecular to surface collisions. “Van der Waal” forces cause molecules to attract and remain within the carbon structure, this is known as “adsorption”.
A carbon block filter is typically in pellet or granular form in beds, arranged so that air can pass through. The time the air is in contact with the carbon block is known as “dwell” or “contact”
time. Typical dwell times range from 0.1 seconds to 1
second or more in large “deep bed” adsorbers.
When the bed is “saturated” with contaminant, then
“breakthrough” is experienced, with emissions rising. This
means the bed is exhausted and no further adsorption can