Estimation of size and shape of pores in moist coal utilizing sorbed water as a molecular probe

This work has been undertaken aiming to estimate the size of pores in moist coat on the basis of the nuclear magnetic resonance relaxation characteristics of water sorbed in the pores as the molecular probe. Raw Yallourn brown coal (water content; 1.46 kg-water/kg-daf coal) and Beulah Zap lignite (0.53) were employed as the raw materials and were partially or completely dried at 303 K. Samples with known water contents were subjected to H-1 NMR analysts taking a Carr-Purcel/Meiboom-Gill (CPMG) pulse sequence. The results of the analysis revealed that the initial amplitude of the CPMG signal corresponds quantitatively to the total amount of 'mobile' proton. It was found that there are three components with different transverse relaxation times (T-2), that are attributed to free water, pore water (freezable bound water and nonfreezable water), and mobile coal hydroxyls. The relaxation characteristics of the water components were further analyzed on the basis of a theory that relates the dimension of pore to the relaxation time of pore liquid, and finally the following conclusions were reached: (i) pores filled with the pore, water are slitlike rather than cylindrical in shape; (ii) the dimensions of pores are about 3 and 2 nm for the raw Yallourn and Beulah Zap, respectively; (iii) the pore dimension decreases with the content of pore water roughly in a linear manner.