This work is part of a systematic study on the energy barriers for the permeation of several molecules, like He, H2, CO, CO2, H2O, NH3, CH4 etc, through nanoporous single layer graphene, having pores with different shape, size, and type. In the present work, we focus on the permeation of CO2 through graphene pores which are constructed when neighboring carbon atoms of the graphene layer are removed from the structure, and nitrogen atoms have replaced the carbon atoms in the boundary of the pore. The energy barriers for each different pore are calculated using 2 different ReaxFF potentials along a path which the molecule would ideally follow in order to pass from the one side of the membrane to the other through the pore. Using the calculated values of the energy barriers, we estimate permeances by employing the kinetic theory of gasses. We give estimates for the preferable sizes and structures of the pores for permeability and demonstrate the ability of nanoporous graphene for CO2 separation.