Single step in situ activation process for activated carbon synthesis from coconut shells

Biomass-based activated carbon is used in filtration and purification due to its high porosity and absorptivity. This paper reports a scalable single-step self-sustained process to synthesize activated carbon from coconut shells. The process uses mixtures of air and steam as the activation agent (referred to as the activator) in a counter-current packed bed system. The ratio of air to steam in the activator and the strain rate of the activator (a parameter derived from the activator flow velocity) are the key controlling parameters of activation. For a fixed ratio of air to steam in the activator, the extent of activation (quantified by the iodine value of the product) is found to initially increase with an increase in strain rate and starts to decrease beyond a certain critical value. For a given air-steam ratio, this critical value called as extinction strain rate is found to be linked to a fundamental characteristic of the gas-phase flame formed around fuel particles due to the release of volatiles. It is observed that activation increases with an increase in strain rate as long as the gas-phase flame engulfs the fuel particle. Beyond the extinction strain rate (found to be around 250 +/- 10 s(-1)), the engulfing flame is extinguished and activation starts to fall; this is inferred to be due to the surface oxidation of char due to the absence of gas-phase flame, leading to a lack of pore formation and hence a decrease in activation. For activator flow rates within the extinction strain rate, various regimes for AC synthesis are identified based on the degree of activation and yield. Maximum activation of 850 mg/g iodine value at 8% yield is obtained at an air to steam ratio of 2 and an activator strain rate of 244 s(-1)