Diffusion of gases in American coals

Abstract

A gas solid chromatography technique coupled with moment analysis has been used to measure effective diffusivities for methane and nitrogen in eight different coals. The pore structure of these same coals has been studied previously using nitrogen adsorption/condensation, carbon dioxide adsorption, mercury porosimetry, helium density, and low-field n.m.r. spin-lattice relaxation measurements. The measured effective diffusivities (303 K) ranged from 1.82 × 10 −7cm 2s −1 to 1.11 × 10 −4cm 2s −1 for nitrogen and from 2.93 × 10 −7cm 2s −1 to 3.70 × 10 −5cm 2s −1 for methane. The diffusivity was found to be a strong function of particle size, indicating that the unipore diffusion model was not appropriate. Plots of inverse diffusivity versus inverse particle size squared were linear, which allowed the extraction of a micropore diffusion group, D x R 2 x , where D x is the effective micropore diffusivity and R x is the characteristic diffusion length. For all eight coals, the magnitude of D x R 2 x was found to vary with the micropore porosity to a power of 2.2. Only three coals, corresponding to the three coals with the highest N 2 surface areas, showed significant adsorption of methane and nitrogen. A particle size effect was observed for adsorption. The pore structure of coal is complex and the bidisperse pore model proposed is clearly not intended as a realistic physical description. However, the model does include some qualitative features of coal pore structure (i.e., multimodal) in a fairly simple mathematical formulation. The fact that analysing diffusion data using this model results in quite reasonable values of the micropore tortuosity factor for eight coals ( τ = θ x −1.2) and describes the observed particle size effect represents a significant improvement compared with the previously used unipore model.