Abnormal adsorption and desorption of nitrogen at 77 K on coals: Study of causes and improved experimental method

摘要

Low-pressure nitrogen adsorption at 77 K is a widely-used technique for determining surface area, pore structure, and pore size distribution of coal. However, some high-rank coal samples show inaccurate results when investigated by the traditional experimental method due to non-negligible amount of helium being captured in the extremely narrow micropores ( < 0.7 nm) at 77 K, which affects measurement of the void volume. In this study, an improved experimental method, based on calibrating the free space value of the sample rather than using the measured value directly, was adopted to re-test the high-rank coal samples. The results showed that the adsorption/desorption isotherm of the high-rank coal samples was accurately obtained by the new experimental method, which was further used to calculate the Brunauer-Emmett-Teller specific surface area and the Barrett-Joyner-Halenda pore volume.