Liquid nitrogen soaking lowers methane uptake and swelling across three coal ranks

A laboratory study of lignite, bituminous coal and anthracite found that a single liquid-nitrogen cold-soaking treatment reduced methane adsorption and the swelling deformation associated with methane uptake. Saturated methane adsorption capacity fell by 28.82% in treated lignite, 10.68% in treated bituminous coal and 11.60% in treated anthracite relative to the corresponding untreated samples. Methane-induced swelling was also smaller in every treated coal, with lignite recording the largest maximum reduction in volume strain at 13.66%.

Researchers compared untreated and cold-soaked samples using isothermal methane-adsorption experiments, swelling-deformation tests, Fourier transform infrared spectroscopy, nuclear magnetic resonance measurements and low-temperature nitrogen adsorption analysis. The three coal types were examined at different degrees of metamorphism. This experimental design linked the gas-adsorption and deformation responses to changes in pore structure and the chemical groups present on coal surfaces, rather than relying on adsorption measurements alone.

The pore-characterization results indicated that liquid-nitrogen cold soaking shifted the coals’ pore-size distributions toward larger pores and improved pore connectivity. At the same time, the treatment reduced the effective space available for methane adsorption. The study’s Pearson correlation analysis found that adsorption-pore specific surface area and BET specific surface area were strongly positively correlated with saturated adsorption capacity across coals of different metamorphic grades, with correlation coefficients greater than 0.71.

BJH total pore volume and average pore diameter were strongly negatively correlated with saturated adsorption capacity, with coefficients greater than 0.73 in magnitude. The authors describe the pore-network response as having two competing effects. Enhanced connectivity accelerated the initial migration of methane into the coal, whereas the smaller effective adsorption space slowed the subsequent deformation rate during gas uptake.

These reported mechanisms connect the larger, more connected pore network with the observed reduction in methane-induced swelling. The findings do not mean that all stages of methane migration were slower: the initial migration was accelerated, while the later deformation associated with continued uptake proceeded more slowly. Peak fitting of the Fourier transform infrared spectra showed that liquid-nitrogen treatment did not change the types of functional groups detected on the coal surfaces.

However, the contents of hydroxyl and carboxyl groups increased significantly. The researchers associated these groups with weaker coupling between coal and methane and report that they weakened the methane adsorption process. Together, the spectroscopic and pore-structure observations offer a microscopic explanation for the lower saturated adsorption capacities measured after cold soaking.

The size of the response differed among the coal types. Lignite showed the largest capacity reduction, while bituminous coal and anthracite experienced declines of similar, substantially smaller magnitudes. The reduction in swelling deformation also varied with coal type, although all three treated coals showed less swelling than their untreated counterparts.

The results therefore indicate that liquid-nitrogen cold soaking produced a consistent direction of change in the tested samples, but not an identical effect across coal ranks. The work is an experimental laboratory study, not a field trial, and its conclusions apply to the tested lignite, bituminous coal and anthracite samples under the reported conditions. The supplied findings do not establish whether repeated treatment would produce the same changes, how long the effects persist, or whether the process can reproduce these responses under underground temperature and pressure conditions.

The study also does not demonstrate consequences for methane recovery, mine safety, field-scale gas flow or operational performance in coal seams. Within those limits, the measurements connect surface chemistry, pore geometry, methane adsorption and mechanical deformation at the microscopic scale. The authors present the results as fundamental microscopic data for coal-reservoir research, providing a laboratory basis for examining how liquid-nitrogen treatment affects methane-adsorption behavior in different coal ranks.

Subject of Research: Effects of liquid-nitrogen cold soaking on coal pore structure, methane adsorption and swelling deformation

Article Title: Experimental study on microscopic mechanisms of coal deformation and methane adsorption under liquid nitrogen cold soaking

Article References: Liu, S., Wang, S., Li, X., Wang, D., & Fu, J. (2026). Experimental study on microscopic mechanisms of coal deformation and methane adsorption under liquid nitrogen cold soaking. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38314-6

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38314-6

Keywords: liquid nitrogen, cold soaking, coal deformation, methane adsorption, swelling strain, pore structure, pore connectivity, FTIR spectroscopy, NMR analysis, BET analysis, lignite, anthracite