Activated biocarbons with a porous structure and nitrogen functionalities are synthesized from the prolific waste biomass, Arundo donax, and an organic material, chitosan, by a simple one step chemical activation with ZnCl2. The nitrogen functional groups in the porous activated biocarbons are significantly increased by the addition of chitosan in the synthesis mixture. The textural properties and N content can easily be tuned by varying the amount of activating agent and the carbonization temperature. The optimized biocarbon sample was obtained at a fairly low carbonization temperature of 500 °C by employing an impregnation mass ratio of ZnCl2 to Arundo donax/chitosan of 3. This optimized material exhibits the best specific surface area of 1863 m2 g-1, pore volume of 1.0 cm3 g-1 and nitrogen content of 5.4 wt%. The presence of nitrogen functionalities on the materials provides a negatively charged surface which is critical for the adsorption of CO2 molecules which are acidic in nature. The sample shows remarkable adsorption capacities of 18.2 mmol g-1 at 0 °C/30 bar and 13.1 mmol g-1 at 25 °C/30 bar under the conditions corresponding to pre-combustion CO2 capture from flue gas streams. High CO2 adsorption values of 3.6 mmol g-1 at 0 °C/1 bar and 2.1 mmol g-1 at 25 °C/1 bar are also observed under post-combustion CO2 capture conditions of the material. The N-doped activated biocarbon shows an exceptionally high value of isoelectric heat of adsorption (32.2 kJ mol-1), indicating that the surface polarity generated by N dopant plays a crucial role in enhancing interactions between CO2 and porous N-doped activated biocarbons.