Fractal analysis of CO2 and N2 adsorption data to assess textural changes during char gasification

Abstract

This study aims to show how fractal analysis applied to nitrogen and carbon dioxide adsorption isotherms helps to understand the textural changes during char activation. The materials under study were two series of activated carbons obtained by means of CO2 gasification of chars at 1123 K. The chars were obtained through carbonization at 1123 K of a high-volatile A bituminous coal that was previously oxidized in air at 543 and 473 K for periods of time from 0 up to 14 days. The fractal analysis of CO2 and N2 adsorption data corresponding to activated carbons is carried out by using the approaches of Neimark and Wang-Li. Fractal and other textural characteristics of the series of activated carbons are put side by side with those of the series of the corresponding chars. Pore network development through activation depends on the extent of the air preoxidation of the precursor coal as this step conditions the textural properties of the carbonized materials that also determine the behaviour during activation. Fractal and other textural characteristics indicate that coal preoxidation at 543 K leads to chars that are more stable during activation than those obtained from coals preoxidized at 473 K. The changes in fractal dimensions caused by gasification can be explained taking into account the classical textural properties, the effect of gasification and also the pore structure of the precursor char. The gasification process itself produces a decrease in fractal dimensions and the existence of non-accessible porosity in chars may contribute to an increase in fractal dimensions during activation. The prevalence of the smoothening effect of char gasification was found to be more important for the zones of the narrowest pores. Consequently, variation of fractal characteristics as a consequence of gasification depends on the technique employed for the analysis: CO2, N2 adsorption or mercury porosimetry.