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For the development of the specific surface area and porous structure of activated carbon, the physical activation process is a favorable approach. Physical activation is achieved by carbonization in the presence of oxidizing gases such as steam, carbon dioxide (CO₂), or a mixture of these. This process resembles a partial gasification process which is generally carried out at temperatures between 700 and 1000 °C at atmospheric pressure. The furnace for the physical activation process is constructed with a gas flow unit, and a horizontal tube furnace or a fluidization system in a vertical tube are frequently selected [55, 56]. The oxygen in the activator gas molecule reacts with the carbon atom of biomass resulting in the generation of carbon monoxide (CO). The precise reaction pathways depend on the type of oxidizing gas and the activation temperature. Physical activation by steam and CO₂ during carbonization occurs through the endothermic reactions as shown in Eqs. (2.9) and (2.10).

(2.9)

(2.10)

As previously mentioned, the physical activation is similar to partial gasification because the oxygen atoms of the oxidizing gas molecules can react with the carbon atom inside the biochar during activation. From the steam activation in Eq. (2.9), the carbon atom reacts with water molecule yielding CO, which can increase the porosity and surface area of activated carbon. Other prominent points of physical activation are to provide less contamination of activated carbon than chemical activation [57]. Physical activation does not require any neutralization procedure for the removal of the oxidizing agent. So, this process is more environmentally friendly. However, the use of fresh biomass or cellulosic material tends to produce ash during the physical activation process, which results in low activated carbon yields [3].

The activation conditions are one of the important factors considered in promoting a high surface area and high product yield. The proper carbonization period is more influential on the final properties of activated carbon than the activation period. In the work of Rezma et al. [36], the biomass was first carbonized at 1000 °C and the resulting product was activated by CO₂ at 750–900 °C for 30 minutes. The activated carbon presented a very low surface area and product yield [36]. As shown by Grima‐Olmedo et al. [19], the surface area of carbonized eucalyptus was improved when the temperature was raised from 600 to 800 °C. The addition of CO₂ during carbonization strongly enhanced the surface area of the produced activated carbon [19].

Both micro‐ and mesopores can be generated through physical activation. For example, the micropores inside carbonized rubber wood sawdust could be changed into mesopores via steam activation, resulting in a high surface area of 1134 m² g⁻¹ [58]. The surface functional groups of physically activated carbons correspond to those produced by chemical activation [39]. The factors that affect the physical and chemical properties of activated carbon can be prioritized as: (i) carbonization and activation temperatures, (ii) type of feedstock, (iii) particle size of feedstock, (iv) heating rate, (v) gas flow rate, and (vi) activation time [28]. This information is significant for the design of the physical activation process.