contained hydrophobic groups (C–H, C–C, and C=C) [48]. As the hydrocarbon groups in the oil‐rich biomass could be cracked during the carbonization process, it generated more aromaticity of the activated carbon. Therefore, not only the type of chemical activating agent but also the category of biomass feedstock influence the chemical properties of activated carbon.
2.3.2.2 Physical Activation
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).
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.
2.3.3 Hydrothermal Carbonization
Hydrothermal carbonization (HTC) is an environmentally friendly thermochemical process for biomass conversion. This process operates at temperatures of 180–300 °C under the pressure of 8–20 bar in water [59]. The thermal processing at high pressure in water destructs the biomass structure and the resulting carbon material is called a hydrochar. The HTC is proposed as a productive process for fine biomass particles because it presents a higher yield of the bio‐based carbon product compared to regular carbonization [60]. Moreover, carbonization under high vapor pressure of water can produce hydrochar with uniform particles.
According to the hydrothermal reaction of the lignocellulosic material, the cellulose and hemicellulose are hydrolyzed to obtain CO2 as a by‐product according to Eq. (2.11).
Carbon atoms do not appear in Eq. (2.11), which differs from the ordinary carbonization process shown in Eq. (2.1), as the transformation of biomass into hydrochar is essentially associated with three reactions: hydrolysis, dehydration, and decarboxylation [61]. However, the operating conditions of the HTC process are not able to destroy the lignin structure. Fourier‐transform infrared (FTIR) spectra showed the amount of lignin that was still present in the carbon material produced from the HTC process [62]. The HTC process is thus not appropriate for the conversion of lignin‐rich biomasses. Interestingly, the hydrothermal conversion of glucose (C6H12O6) or other monosaccharides as seen in Eq. (2.12) could directly affect the carbon yield at 200 °C and 20 bar [27].
Even though hydrochars usually have a lower surface area than biochars, hydrochars possess a 1–4 times higher oxygen content than the carbon materials derived from carbonization and pyrolysis processes [33]. The hydrochars prepared from pinewood, peanut shells, or bamboo showed several oxygen‐containing groups such as C–O, –OH, and C=O, which were absent in the biochar [30]. The superior oxygen content in hydrochar makes it advantageous for many industrial sections such as catalysts or catalyst support. Nevertheless, an increase in hydrothermal temperature decreased the amount of oxygen and, conversely, the carbon content increased [11, 38]. Hydrochar is frequently used as a starting material for the production of activated carbon. The oxidizing agent can react with the oxygenated groups on the hydrochar surface resulting in its excellent catalytic behavior [63]. Moreover, the surface of hydrochar could be modified for specific applications. For example, hydrochar treated with sulfonic groups was effective in the production of biofuels [64].
2.3.4 Graphene Preparation from Biomass
Graphite is an infinite three‐dimensional (3D) material comprising several stacked layers anchored by weak Van der Waals forces. Each stacked layer is a two‐dimensional
