A coarse-grain collisional model has been developed to study molecular nitrogen internal energy excitation and dissociation behind shock waves and in nozzle flows. The starting point is the rovibrational collisional model recently developed at NASA Ames Research Center consisting in reaction rate coefficients obtained from ab initio calculations. The master equation for this model has been already coupled with a one-dimensional inviscid flow solver for the investigation of nonequilibrium effects behind shock waves by assuming a uniform distribution of energy levels in energy bins. In the present work, we propose a coarse-grain model based on a Boltzmann distribution of energy levels within a bin. Applications are presented both for normal shock waves and nozzle flows. Governing equations are solved by means of Finite volume method and, for shock waves, dissipation effects are also taken into account. Computational results are compared against those obtained by means of multi-temperature models and a vibrational collisional model already developed from the same ab initio database.