Petroleum coke was pretreated by HNO3 before urea modification and KOH activation to synthesize N-doped porous carbons. The as-synthesized samples were carefully characterized by various techniques. This series of samples demonstrate high CO, uptake at 1 bar up to 4.13 mmol/g at 25 degrees C and 6.24 mmol/g at 0 degrees C, respectively. In addition, these sorbents possess high CO2/N-2 selectivity, stable reusability, moderate heat of CO2 adsorption, and high dynamic CO2 capture capacity under simulated flue gas conditions. Further comparison shows that sorbents synthesized by HNO3 pretreatment possess higher nitrogen content and narrow microporosity than the control sample without HNO3 pretreatment but a lower CO2 uptake. Additional investigations show that in addition to nitrogen content and narrow micropores, the size of narrow micropores, the narrow micropore size distributions, surface acidity, and the content of specific N and O species on the surface of the N-doped porous carbons also play major roles in determining the CO2 capture capacity under ambient condition. This work is intended to provide useful information and to inspire ways to develop new carbonaceous sorbents for removing CO2 from combustion flue gas.