Steam for activation can be used to activate almost all raw materials. A variety of methods have been developed but all of these share the same basic principle of carbonisation and initial oxidation followed by an activation step with steam.
Steam activation process: oxidation, carbonization and activation
The initial oxidation and carbonization, done at temperatures up to 500°C, is a highly exothermic process where the temperature is strictly controlled. The oxidation stage starts at 150°C increasing slowly up to 500°C at the carbonization stage. The product made during this carbonization step is virtually free of volatiles and already contains some pores that are, however, too small or restricted for it to be useful as an adsorbent. The creation of the internal surface is done during the activation step with steam at temperatures over 1,000°C. Since the overall reaction (converting carbon to carbon dioxide) is exothermic, the process generates an excess of energy that can be used to produce steam or energy.
|C(graphite, solid) + H2O(g) → CO(g) + H2(g)||130||Endothermic|
|C(graphite, solid) + CO2(g) → 2 CO(g)||171||Endothermic|
|C(graphite, solid) + ½ O2(g) → CO(g)||-111||Exothermic|
|C(graphite, solid) + O2(g) → CO2(g)||-393||Exothermic|
|CO(g) + ½ O2(g) → CO2(g)||-283||Exothermic|
|H2(g) + ½ O2(g) → H2O(g)||-241||Exothermic|
A number of different types of kilns and furnaces can be used for carbonisation/activation and include rotary (fired directly or indirectly), multi-heart furnaces and fluidized bed reactors.