Thermodynamics and surface structure of coals

We propose that most pores in coals are closed. They cannot be reached by diffusion through a pore network but can only be reached by diffusion through solid coal. Co{sub 2} gives accurate total surface areas because it dissolves in and rapidly diffuses through solid coals, reaching all the pores. This surface area is irrelevant to materials which are only slightly soluble or insoluble in coals for these can only reach a small portion of the pores. Thus, the diffusion rate controls the determined pore size. For the large molecules involved in direct-liquefaction and most organic reactions, the effective surface areas of these coals are very small, only a few m{sup 2}/g. The consequences of this structure model for coal processing and reaction are significant. These coals cannot be considered high (hundreds of m{sup 2}/g) surface area materials which are easily accessible. They are low surface area solids only slightly permeable to hydrocarbons. The advantages of fine grinding are obvious. Using solvents which will cause coal to become rubbery rather than glassy will enhance diffusion and thus reactivity.