Spherical carbon beads with a uniform diameter of ca. 0.6–0.8 mm and high mechanical strength can be prepared by hydrothermal synthesis. To optimise the performance of these adsorbents for pulverised fuel post-combustion capture, the efficacy of potassium intercalation via a KOH treatment has been investigated, deliberately using nitrogen-free phenolic resin derived activated carbon (AC) beads so that the enhanced CO2 adsorption achieved by potassium intercalation could be delineated from any other effects. At 25 °C and CO2 partial pressure of 0.15 bar, the adsorption capacity of K-intercalated ACs nearly doubled from 0.79 mmol/g for the untreated carbons to 1.51 mmol/g whilst the effect on the morphology and mechanical strength is relatively small. It was found that only slightly more than ca. 1 wt.% of K is required to give the maximum benefit from intercalation that increases the surface polarity and the affinity towards CO2. The notably increased CO2 uptake of the K-AC beads as a result of modest increase in adsorption heat (32–40 kJ/mol compared to 27 kJ/mol for the original AC), coupled with the fast adsorption kinetics, suggest that the overall energy penalty is potentially superior to strongly basic polyethyleneimine and other amine-based solid adsorbent systems for carbon capture.