Pore structure of Illinois No. 6 coal

The authors present data on the adsorption (BET) of CO2 and a set of aliphatic hydrocarbons on Illinois No. 6 coal from the Argonne Sample Bank. These data are inconsistent with the generally accepted structure of coal pore systems, that is, an interconnected network of bottle-necked pores. The coal surface areas obtained by using CO2, ethane, and cyclopropane, all of similar cross-sectional area, cannot be rationalized with a constricted pore model. To see whether adsorption on the coal surface might be responsible for the observed dependence of measured surface area on adsorbate size, a fractal analysis of the data was performed. The authors propose that most pores in this coal are closed. They cannot be reached by diffusion through the pore network but can only be reached by diffusion through solid coal. CO2 gives accurate total surface areas because it dissolves in and rapidly diffuses through coals, reaching all the pores. This surface area is irrevelant to materials that are slightly soluble or insoluble in coals for these can reach only a small portion of the pores. Thus, diffusion rate controls the pore size determined.