Activated carbon filters : a few facts to sort out the truth

The second article in our series about air purification technologies concerns the air purifiers with activated carbon filter. Discover the truth about them.

What is an air purifier with activated carbon filter ?

Activated carbon filters are used to stop contaminants at a molecular level. They make it possible to purify the air from chemicals, odors and gas but they do not filter particles.

Activated carbon

Activated carbon is mainly composed of carbonaceous matter with porous structure. It can be produced from wood, nutshells, coconut husk, etc. Once activated (physically or chemically), the surface of the carbon can reach from 400 to 2 000 m².g-1. Then, the carbon can trap gas and water molecules. Its efficiency depends on humidity.

The pore size depends on activation method and the number of pores in the base material used. Wood can provide pores larger than 50 nm. The gas-adsorption requires 1-2 nm pores.

How do activated carbon filters work ?

Activated carbon

Activated carbon filters use a process called adsorption. Gas, chemicals and odors are trapped inside the pore structure of the carbon substrate. The air - purified from contaminants - comes out the filter.

Adsorption or absorption ?

There should be no confusion between absorption and adsorption. Absorption involves the diffusion of molecules of a substance into the bulk of liquid or solid to form a solution. Adsorption is a surface phenomenon.

Two types of carbon filters

There are 2 predominant types of carbon filters : granular loose-fill and bonded filters.

Activated carbon

Granular loose-fill filters consist of grains which can measure up to 3 mm long. They have a longer life span and are lighter than the bonded ones. In this form, the carbon can be found in combination with other substances. Some manufacturers use zeolite. Its pore structure is supposed to be better suited for the removal of formaldehyde and ammonia. However, there is no reliable scientific evidence to demonstrate these claims. In fact, zeolite is an inexpensive filler.

Bonded carbon filters consists of grains which measure from 10 to 50 µm. They are considered to be more manageable than granular loose-fill filters. IBR laboratories reseachers think that manufacturers use bonded carbon filters since they are cheaper. Concerning isoproponal filtration, bonded carbon filters are 26,8% less efficient that granular loose-fill filters.

Activated carbon

Their efficiency is dependent on various factors

The activated carbon efficiency can be limited by many factors :

  • The materials used to make activated carbon: marl, wood, coconut
  • The size of the grains and their density: the filter is less effective when the carbon is in powder form
  • The exchanging surface between the carbon and the air flow
  • The seal between the filter and the air duct
  • The pre-filters quality
  • The number of combined filter: the more filter there are, the more the activated carbon filter is protected against particles ; but a high number of filters reduces the air flow or increases the noise and the energy consumption ; the less filter there are, the faster the filter is clogged.
  • The air speed that passes through the filter: the faster it is, the less the activated carbon can purify the air.
  • The level of filter saturation by gas adsorption : gas molecules of pollutants can be re-emitted. Moreover, the saturation speed varies according to pollutants, it makes the calculation of its lifetime.
  • The reversibility of the physical phenomenon: the trapped molecules can be re-emitted if the conditions of temperature and humidity change.
  • Humidity : the activated carbon filter is useless in the case of high humidity and it would tend to adsorb the water molecules first.

The activated carbon filtration is a complex technology which is very difficult to control. It is dependent on various factors, so its efficency is often uncertain.