The problem of reducing carbon dioxide emissions from flue gas, particularly from flue gas originating from coal-firing CFB systems, is currently an important challenge. Many centers around the world have tested post-combustion CO₂ capture systems. One of these systems, operated using DR-VPSA adsorption technology (dual-reflux vacuum pressure swing adsorption), was tested under the Strategic Project in Poland. The flue gas in this study originated from a supercritical CFB boiler (460 MWe). An important problem involved in capturing CO₂ from flue gas is the occurrence of SO₂ and NO_x. These substances have a negative effect on the CO₂ adsorption process. In this study, commercial impregnated activated carbon was used to remove SO₂ and NOx from CFB flue gas in the pre-treatment section during the tests of a pilot CO₂ capture unit in a large-scale CFB boiler at the Lagisza Power Plant (Poland). The spent activated carbon was analyzed using several different methods (N₂ adsorption–desorption isotherms, SEM-EDX, XRD, FTIR, and TG) to evaluate the efficiency of the operation and life span of the adsorbent used in the SO₂ and NO_x removal unit. The results demonstrate that using commercial impregnated activated carbon in the pre-treatment section ensures sufficient flue gas purification and the removal of sulfur oxides but remains insufficient for nitrogen oxides.