## 2 Production of Activated Carbon
### Activation  
The activation of pyrolytic char or carbon black can be conducted through physical or chemical activation, as shown in Figure 2. Physical activation involves the treatment of pyrolytic char obtained from the carbonization process, which is subjected to gasification using CO\({}_{2}\) or steam at 800 \({}^{\circ}\)C-900 \({}^{\circ}\)C. CO\({}_{2}\) improves the pore structure and steam widens the micropores. Mostly, CO\({}_{2}\) is preferred over steam due to its ease of handling and cleanliness. On the other hand, the chemical activation process is a single-step process that uses chemicals such as NaOH, KOH, H\({}_{3}\)PO\({}_{4}\), and ZnCl\({}_{2}\), followed by pyrolysis, in the absence of air at 500 to 900 \({}^{\circ}\)C. The utilization of chemicals reduces the formation of tar and by-products during the pyrolysis process and also increases the yield and pore volume [28]. The physical or chemical activation process may be chosen depending on the type of raw material. However, the chemical activation process is more advantageous than the physical activation process, as the former produces a higher yield and pore volumes in a single-step process. while cutting down the time, energy, and temperature. However, care must be taken during the handling of chemicals such as ZnCl\({}_{2}\), KOH, and NaOH, as they are corrosive, toxic, unfriendly to the environment, and creates issues with the disposal of waste. For further improvement of activity and surface area, the pyrolytic char obtained during the gasification or hydrothermal process is subjected to heating at a high temperature [29]. There is another activation process, namely a microwave-assisted activation process, which is a blend of physical and chemical activation processes. It is a versatile technique that can produce higher-quality AC via either a one-step or a two-step activation process [30]. This process is an alternative to conservative approaches in the production of AC, due to its excellent features such as volumetric heating, and indirect contact between heated resources and the heating foundation. The factors which affect the microwave-assisted activation process are activation period, radiation strength, properties of raw materials, the interaction of chemicals with microwave, etc. This process produces AC with excellent pore volume (textural properties) and pore size distribution. The self-activation process is also one of the activation processes which decreases the gases emitted during pyrolysis and saves the cost of reagents used for the activation process, and also saves the environment from pollution. It is a green process that does not use chemicals, but the raw material is heated at high temperatures in the range between 900 \({}^{\circ}\)C and 1100 \({}^{\circ}\)C [31]. For waste tyre-derived AC, physical activation using steam is the commonly used method for the production of AC. It was reported that steam-activated carbons showed higher surface areas (1000 m\({}^{2}\)/g) than CO\({}_{2}\)-activated carbons (270-980 m\({}^{2}\)/g). Since the surface area is one of the key factors that are attributed to the adsorption features, physical activation using steam is the more regularly used method. Moreover, the steam activation was found to be two to three times faster than the CO\({}_{2}\) activation. This could be explained by the phenomenon that the water molecules are smaller than the CO\({}_{2}\) molecules and thus could diffuse rapidly into the pore matrix and develop micropores [32]. Two steps make up the overall process for producing AC from waste tyres. Firstly, thermal pyrolysis typically takes place at low temperatures in the range of 400 \({}^{\circ}\)C to 700 \({}^{\circ}\)C with nitrogen or helium to reduce the cross-linking of carbon atoms. Secondly, the activation process, which uses an activating gas (Steam or CO\({}_{2}\)) at temperatures between 800 and 1000 \({}^{\circ}\)C to generate a certain specific surface area and specific porosity.