Reliability Assessment and Methodological Refinement of Liquid Nitrogen Adsorption in Medium- and High-Rank Coals

Low-temperature liquid nitrogen adsorption is a widely applied technique for characterizing coal pore structures, yet its validity for high-rank coals remains problematic due to inconsistent experimental outcomes. This study investigates the causes of result inaccuracies through systematic experiments on six Qianxi coal samples with varying metamorphic grades. By analyzing adsorption–desorption curves under controlled conditions (e.g., degassing temperature, time, and sample mass), we identify three critical factors influencing validity: (1) coal metamorphism-driven pore heterogeneity, where high-rank coals exhibit unique negative adsorption pressure responses and significant adsorption–desorption hysteresis (separation coefficient K > 10% versus <1% in middle-rank coals); (2) enhanced microporosity and gas adsorption capacity in high-rank coals, which distort nitrogen replacement efficiency; and (3) volatile-induced pore throat blockage during testing. To address these issues, we propose optimized protocols: 1 g sample mass, 300 °C degassing for 12 h, and preassessment of pore surface area to guide parameter selection. Crucially, liquid nitrogen adsorption shows limited applicability for high-rank coals with underdeveloped pore networks, suggesting the need for complementary characterization methods. These findings provide actionable criteria to enhance the reliability of pore structure analysis in heterogeneous coal systems.