Synthesis of AC from OFA consists of two steps. In the first step, 10 g of as received raw FA (Rabigh power plant, Saudi Arabia) was mixed with 200 mL of an acid mixture comprises of 20% H2SO4 , 40% HNO3 , and 40% H3PO4 (Panreac, Spain) in a round bottom flask. The resultant mixture was constantly heated for 4 h under total reflux conditions. Once the flask was cooled down to room temperature, treated ash was filtered from the acid solution to obtain the solid residue. The solid residue was washed several times with distilled water to remove the acid contents from carbon matrix. The treated ash cake was dried at 110 °C for 5 h and, then, stored in a closed cap glass bottles. The acid treatment removes the mineral contents and increases the percentage of carbon. The acid treated solid residue was further treated with CO2 as an activating agent in a horizontal tube. For this purpose, 6 g acid treated ash in the long horizontal stainless steel tube (ID = 1 cm) was heated to 990 °C at a rate of 50 °C/min in a furnace. The flow rate and inlet pressure of CO2 was fixed at 1L/min and two bar, respectively. After 1 hr of continuous operation, the CO2 supply and furnace was turned OFF and let the sample to cool down to room temperature. The AC was taken out from horizontal stainless steel tube and saved it in a desiccator for further application.