This paper presents a numerical study of horizontal pneumatic conveying by the combined approach of computational fluid dynamics (CFD) and discrete element method (DEM), with special reference to the use of periodic boundary condition (PBC) for computational efficiency. A new iterative method is proposed to generally control the solid flowrate simulated in a CFD-DEM model with PBC. The characteristics of the flows in the start-up section are analyzed in detail and compared with those in the well-developed flow section for different flow regimes. On this basis, two semi-theoretical correlations are formulated to respectively predict the start-up section length and the relation between particle number and solid flowrate. The applicability of PBC to the CFD-DEM modeling of pneumatic conveying, as well as the two correlations, is verified by comparing the measured and calculated results under different conditions. (C) 2013 Elsevier Ltd. All rights reserved.