A simple method was developed to tune the porosity of coal-derived activated carbons, which provided a model adsorbent system to investigate the volumetric CO₂ adsorption performance. Specifically, the method involved the variation of the activation temperature in a K₂CO₃ induced chemical activation process which could yield activated carbons with defined microporous (< 2 nm, including ultra-microporous < 1 nm) and meso-micro-porous structures. CO₂ adsorption isotherms revealed that the microporous activated carbon has the highest measured CO₂ adsorption capacity (6.0 mmol∙g^–1 at 0 °C and 4.1 mmol∙g^–1 at 25 °C), whilst ultra-microporous activated carbon with a high packing density exhibited the highest normalized capacity with respect to packing volume (1.8 mmol∙cm⁻³ at 0 °C and 1.3 mmol∙cm^–3 at 25 °C), which is significant. Both experimental correlation analysis and molecular dynamics simulation demonstrated that (i) volumetric CO₂ adsorption capacity is directly proportional to the ultra-micropore volume, and (ii) an increase in micropore sizes is beneficial to improve the volumetric capacity, but may lead a low CO₂ adsorption density and thus low pore space utilization efficiency. The adsorption experiments on the activated carbons established the criterion for designing CO₂ adsorbents with high volumetric adsorption capacity.