Enhancement of CH4/N2 separation capacity of coal-based porous carbons via hydrothermal coupled KOH activation

The separation of CH4 and N2 is an important technology for the effective utilization of low-concentration CBM. To improve CH4/N2 separation properties of coal-based porous carbons, Shaanxi long-flame coal was hydrothermally treated followed by KOH activation for the preparation of porous carbons. The impact of hydrothermal treatment on the pore structure and CH4/N2 separation properties of porous carbons was investigated. The results show that hydrothermal decomposes the macromolecular structure of coal, which enlarges the pore size of the raw coal and facilitates the KOH penetrate deeper into the coal particles. Meanwhile, more release of methylene/methyl groups and in-stiu growth of carbon nanotubes develop the pore structure during the activation process of treated coal. The pore structure of porous carbons can be precisely controlled by adjusting the water-coal ratio in the hydrothermal process. Among them, C-TC1.25 shows a 22% increase in CH4 adsorption capacity over C-RC, which is mainly due to a significant increase in the micropore volume at 0.5–1.0 nm and generation of carbon nanotubes as gas transport channels. At 298 K and 101 kPa, C-TC1.25 exhibits a CH4 adsorption capacity reaching 1.72 mmol/g, which surpasses that of most known adsorbents, while the IAST selectivity is as high as 5.5. The breakthrough experiment confirms that the porous carbons effectively enable separation of CH4/N2 mixed gas. Kinetic studies show the CH4 adsorption process on C-TC1.25 conform to the pseudo-first-order model and exhibit surface-attached physical adsorption.