Dynamic micromapping of CO2 sorption in coal

Area of Science:

• Geochemistry
• Materials Science
• Chemical Engineering

Background:

• Coal is a complex porous medium with significant potential for carbon dioxide (CO2) sequestration.
• Understanding CO2 sorption mechanisms in coal is crucial for effective carbon capture and storage (CCS) technologies.

Purpose of the Study:

• To investigate the kinetics and mechanisms of subcritical CO2 sorption in various coal types using advanced scattering techniques.
• To determine the density of sorbed CO2 within coal's nanoporous structure.
• To elucidate the influence of coal composition, particularly mineral matter, on CO2 sorption behavior.

Main Methods:

• Application of X-ray and neutron small-angle scattering (SAXS, SANS, and USANS) techniques.
• Analysis of CO2 sorption across a range of coal pore sizes (nano to meso).
• In-situ measurements under varying temperature and pressure conditions.

Main Results:

• SAXS, SANS, and USANS provided pore-size-specific insights into CO2 sorption kinetics.
• Observed densification of sorbed CO2 by a factor of up to five compared to free CO2.
• Sorption behavior varied significantly between coal types, influenced by mineral matter content.
• Mineral matter accelerated sorption kinetics but reduced overall CO2 absorption capacity per unit volume in purely organic matrices.
• Smaller pores were preferentially filled with CO2.
• Apparent diffusion coefficients ranged from 5x10(-7) cm2/min to over 10(-4) cm2/min.

Conclusions:

• Small-angle scattering is a powerful tool for characterizing fluid-porous media interactions in coal.
• CO2 sorption in coal is a complex process influenced by pore structure, CO2 density, and coal composition.
• Mineral matter plays a dual role, enhancing sorption speed while potentially limiting capacity in certain coal matrices.