CO 2 surface area of coals: The 25-year paradox

Abstract

Coals are highly porous materials with most of their surface area enclosed in pores of molecular dimensions. Even though coal porosity plays a key role in practically all aspects of its utilization, we still do not know with any certainty the magnitude of coal surface area and its pore size distribution. Surface areas of coals measured by the conventional BETN 2 method are very low, primarily because of the activated diffusion phenomenon. In order to overcome this drawback, the use of CO 2 adsorption for measuring surface areas of coals was proposed about 25 years ago. In the 1970's and 1980's, there was a growing realization that CO 2 over-estimates the surface area because it induces swelling in coals. The present status of our understanding of CO 2-induced swelling in coals and its implications towards surface area and nature of porosity in coal are discussed. While CO 2 gives higher surface area, at CO 2 pressures <1 atm and for short “equilibrium” time the contribution of swelling to total surface area is small. Because of the uncertainties in the values of CO 2-measured monolayer capacity and the cross-sectional area of an adsorbed molecule in pores of molecular dimensions, surface areas of coals are meaningless and, therefore, should not be reported.