Adsorption is a promising method for CO2 separation. The surface physical and chemical properties of adsorbent play a critical role during the adsorption process. Biochar, as the by-product from biomass fast pyrolysis for bio-oil, is renewable resources with good adsorption property. In this work, the adsorption behavior of biochar and its correlation with physical porosity and surface chemistry were investigated. A novel method for biochar modification by high temperature CO2-ammonia mixture is proposed and compared with the conventional CO2 activation and ammonia treatment. The results show that CO2-ammonia modification combined the advantages of both CO2 activation and ammonia treatment. The surface area of CO2-ammonia modified chars is increased significantly up to about 627.15 m(2)/g, and the ammonification (in the presence of ammonia) can introduce N-containing groups into the biochars (the N content up to 3.91 wt.% in CO2-ammonia modified biochar). The breakthrough curve of CO2 adsorption is fitted with deactivation model from the viewpoint of adsorption dynamics. The CO2 adsorption capacity of biochar is proportional to the micropore volume at lower temperature (20 degrees C); however, it is dependent on the N content of biochar for CO2 adsorption at higher temperature (120 degrees C). (C) 2014 Elsevier B.V. All rights reserved.