Comparison of low-field NMR and mercury intrusion porosimetry in characterizing pore size distributions of coals

In this study we investigated how traditional mercury intrusion porosimetry (MIP), constant-rate-controlled mercury porosimetry (CMP), Low-field NMR spectral analysis (LFNMR), and micro focus computerized tomography (μCT) compare in revealing the pore size distribution (PSD) characteristics of coals. The comparison was made using the same source samples throughout. Two limitations of mercury porosimetry are addressed. First, the high-pressure intrusion by mercury may either deform or destroy the coal sample and eventually induce suspect value of coal porosity, thus correction of pore structure compressibility must be made in analyzing lignite or other coals that with very open structure. Second, pore shielding effects can induce high uncertainty of MIP results, in particular when clusters of smaller pores occur in isolated domains in a continuous network of larger pores. This can result in temporary mercury entrapment during the extrusion process and result in inaccurate estimations of PSD. Another pore shielding effect is due to isolated clusters of large pores in a continuous network of smaller pores. Her mercury is prone to be trapped permanently. This effect can induce inaccurate estimations of the total pore volume. CMP is an effective method that can provide much more detailed PSD information of macropores, however it is deficient in analyzing mesopores of coals. After comparison with the results by μCT and other traditional methods, it was found that LFNMR is an efficient tool for nondestructively quantifying the PSD of coal.