Activation of biomass-derived charcoal with supercritical water

A charcoal prepared by pyrolyzing stones of awara, a nut from a common Guyanese palm tree, was physically activated with water according to two different procedures. Either superheated steam (SHS) at 600, 700 or 800 °C and atmospheric pressure, or supercritical water (SCW) at 600 °C and 250 or 350 bar, was used. The main characteristics of the resultant activated carbons were measured and compared; for that purpose, gasification rates, activation energy, pore texture parameters (surface area, micro- and meso-pore volumes, average micropore width), and surface chemistry were investigated. It was found that, despite the corresponding lower temperature activation, SCW leads to a faster development of the microporosity at lower burn-off than in the case of conventional SHS. Nevertheless, the micropores produced by SCW activation are not narrower. Higher reaction times are required with SCW for obtaining a given burn-off, so a better control of porosity development may be achieved through the use of supercritical activation. Finally, not only the gasification kinetics is influenced by the state of the water: at similar surface areas, more oxygenated functional groups are always created by SCW activation.