POTENTIALITY OF NMR SPECTROSCOPY FOR STUDYING OF PORE STRUCTURE IN FOSSIL COALS

The results of measurements of diffusion parameters for water and methane diffusion in the porous structure of gas- and water-saturated fossil coals are presented. The NMR spectroscopy is an effective tool for the measurement of porous structure of sorbents. However, the presence of two phases in the pore volume requires a more careful separation of these phases while studying media with pore size distribution within broad ranging. Measurements by a spin-echo spectrometer with a constant magnetic field gradient were made for a set of water-saturated samples: silica gels having 1.4; 6.5, 26.0; 38.0 nm mean pore size and also for coals – anthracite and D-rank coal. The pore size dependence of Ds for silica gels has been obtained. With the pore size of d = 1.4 nm the ‘motionally averaging’ mode is realized when the exponential attenuation of the spin-echo signal occurs under the partially restricted geometry. The increase of the value of the self-diffusion coefficient (which is an effective coefficient either for vapor or liquid phase of water) up to D s = 2.3⋅10 –5 m 2 /s is observed for coals [1]. Depending on pore size, it is necessary to use methods developed for these purposes both for liquid, and gas phases. Obtained values of Ds for methane are in accordance with the conception of the transition from volume diffusion to the Knudsen regime with a decrease of the methane pressure in a porous space. The largest scales of monofractals for pores of water-saturated specimens of anthracite and D-rank coal were obtained (Lint = 5.1 μm and 9.6 μm) on the base of coal matter fractal structure conceptions. They are close to pore sizes calculated by measured values T2 on the assumption of their spherical shape. The activation energy of methane desorption was obtained by comparison of an NMR pulse spectrometer with NMR autodyne method data, as well from methane desorption experiment. The duration of such desorption depends on concentration of gas in closed pores and on the solid state diffusion process. The NMR method gives a possibility to measure a transport diffusion coefficient which is an effective coefficient Deff determining the methane transport in a pore system and includes both transport mechanisms: methane filtration in the coal pore space and the methane from close pores by solid state diffusion.