CO2 activation of chars: Effect of burn-off on texture and fractal properties

• Activated carbon series of different burn-off are studied.
• Textural properties and fractal analysis of mercury porosimetry data are considered.
• Fractal dimensions evolution is correlated with pore generation mechanisms.
• Fractal analysis helps to follow textural evolution during gasification.

Series of activated carbons obtained by char gasification with CO2 with burn-off interval from 0 to 79% were obtained and characterized in order to evaluate the mechanisms that determine pore development during the activation step. This work complements previous studies related to textural development during the steps involved in the activated carbon manufacturing processes: coal oxidation, coal pyrolysis and char gasification. The textural development is highly conditioned not only by the degree of burn-off of the char, but the properties of the precursor chars and coals are also of extreme importance. Raw data collected from mercury porosimetry and helium pycnometry allow us to determine conventional textural parameters as well as to carry out a more thorough study of the pore volume generation as the activation progresses. In addition, data obtained from the mercury porosimetry determinations also constitute inputs for fractal determinations following the methods proposed by Friesen–Mikula, Zhang–Li and Neimark. The values of the average fractal dimensions pass through a minimum as the degree of burn-off increases. Additionally fractal profiles and regional surface fractal dimensions were calculated.