Textural characterization of chars using fractal analysis of N2 and CO2 adsorption

The application of fractal analysis to nitrogen and carbon dioxide adsorption isotherms is used together with other textural analysis techniques with the aim of obtaining a better knowledge of the phenomena leading to char formation. This is an important point since chars are key intermediate materials during the manufacturing process of active carbons. The materials under study were series of chars obtained from a high-volatile A bituminous coal oxidized in air at 543 and 473 K for different periods of time extending up to 42 days. The fractal characteristics of the chars were obtained from the nitrogen and carbon dioxide adsorption isotherms by using the methods proposed by Neimark, Wang and Li, and the fractal version of the Frenkel-Halsey-Hill method. The latter approach was found suitable for the high pressure interval of N-2 isotherms. Evolution of the fractal properties in the series of chars is compared with that corresponding to the series of precursor coals. Changes in pore network during carbonization depend on the severity of the air preoxidation of the coal. This happens since preoxidation conditions other properties and processes such as the texture of the precursor coals, the release of the volatile matter as well as the plastic behaviour of coals. These three points were found to be the key factors that determine the fractal characteristics of char. The role of the pore characteristics of the coal on the textural features of the obtained chars is boosted if coal preoxidation is carried out at the more elevated temperature, 543 K. The role that the phenomena occurring throughout the carbonization of oxidized coals have on the textural characteristics of the chars obtained is explained considering fractal features together with other textural properties. The results of this work are compared with previous fractal studies based on mercury porosimetry data and conventional textural techniques.