Activated Carbon Manufacture: Steam Activation
The use of steam for activation can be applied to virtually all raw materials. Steam activation is the most widely used process to activate carbonaceous materials. Steam activated carbons are produced in a two-stage process.
First, the raw material in the form of lumps, pre-sized material, briquettes or extrudates is carbonized by heating in an low oxygen atmosphere so that dehydration and devolatilization of the raw material occurs. Carbonization reduces the volatile content of the source material to under 20%. A coke or charcoal (depending on the raw material) is produced which has pores that are either small or too restricted to be used as an adsorbent.
A variety of methods have been developed but all of these share the same basic principle of initial carbonization at 500-600 degrees C，followed by activation with steam at 800-1,100 degrees C.
Since the overall reaction (converting carbon to carbon dioxide) is exothermic it is possible to utilize this energy and have a self-sustaining process：
- C + H2O (steam) —> CO + H2 (-31 Kcal)
- CO + ½ O2 —> CO2 (+67 Kcal)
- H2 + ½ O2 —> H2O (steam) (+58 Kcal)
- C + O2 —> CO2 (+94 Kcal)
A number of different types of kilns and furnaces can be used for carbonization/activation and include rotary (fired directly or indirectly), vertical multi-hearth furnaces, fluidized bed reactors and vertical single throat retorts. Each manufacturer has their own preference.
As an example, production of activated carbon using a vertical retort is described below.
Raw material is introduced through a hopper on top of the retort and falls under gravity through a central duct towards the activation zone. As the raw material moves slowly down the retort the temperature increases to 800-1000 degrees C and full carbonization takes place.
The second stage, which can take place later in the same kiln, is activation which enlarges the pore structure, increases the internal surface area and makes it more accessible. The carbonized product is activated with steam at very high temperatures. The chemical reaction between the carbon and steam takes place at the internal surface of the carbon, removing carbon from the pore walls and thereby enlarging the pores.
The activation zone, at the bottom of the retort, covers only a small part of the total area available and it is here that steam activation takes place. Air is bled into the furnace to convert the product gases, CO and H2 into CO2 and steam which, because of the exothermic nature of this reaction, reheats the firebricks on the downside of the retort, enabling the process to be self-supporting.
Every 15 minutes or so, the steam injection point is alternated to utilize the “in situ” heating provided by the product gas combustion. The degree of activation (or quality) of the product is determined by the residence time in the activation zone.