**2.2 Production of AC through chemical activation**

Chemical activation, often known as wet oxidation, is usually suggested for organic precursor materials containing cellulose, such as wood, sawdust and phytomass. In this method, the precursors are activated at high temperatures in the presence of certain activating chemicals. In the first stage of activation, the raw material will be saturated or impregnated with oxidizing and highly dehydrated chemicals (activating agents) [7]. After impregnation, the suspension is dried and the remaining mixture will be heated for a given duration. Depending on the activating material and the properties of the final product, activation may require temperatures ranging from 400 to 900°C, when the cellulose is expected to break down and char. Ultimately, AC is obtained from the repeated washing of the resultant char.

Chemical activation agents, as mentioned earlier, are dehydrating agents that influence pyrolytic decomposition and inhibits the formation of bitumen, increase the carbon content and with subsequent changes in the thermal degradation of precursors result in the development of the porous structure of the carbon materials. These activating agents penetrate into the carbon structure creating porous network like structure in the AC, thereby increasing its surface area. It is to be mentioned that carbon particle size distribution, porous nature and surface area are the three important aspects that decides the final applications of the AC [8].

In physical activation, carbonization and activation phenomena occur in two separate furnaces while in the chemical activation these processes occur simultaneously in a single furnace. It is to be mentioned that the correct selection of the activation parameters like the amount of impregnation, weight ratio of the activating agents to dry precursors, temperature, final temperature of carbonization, carbonization time and activation atmosphere (space) are important to the quality and physical characteristics of the final AC produced by chemical activation. In fact,

chemical activation bestows more porous structure to the ACs than physical activation. Further, chemical activation is more economical as it requires a lesser processing temperature, time and yields higher carbon efficiency. Activated agents react with carbon matrices of the organic precursors and liberate various gases to form a porous structure. However, the need for repetitive washings to remove the unused activating agent from the final product at the end of activation process is one of the disadvantages of this method. In addition, toxic washings produced causes water pollution and therefore require secondary treatments. Different types of chemicals have different reactions with precursors and thus affect the features and nature of the ACs produced. **Figure 1** gives a rough scheme for producing AC by physical and chemical activation methods [3].
