Plug flow attracts continuous interest due to its advantages of low particle attrition, low pipeline wear and low energy consumption. A novel non-mechanical draft tube type feeder (DTF) has been proposed in our previous work for vertical plug conveying of coarse particles. Detailed particle motion behaviors and plug formation mechanisms in the draft tube type feeder are investigated by the combined approach of Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) in the present study. The CFD-DEM method based on local averaging of granular matter is used to deal with fluid cell sizes smaller than particle sizes under the specific conditions. The applicability of the model is verified by comparing the calculated results with the experimental results of vertical plug formation of mm glass beads in a draft tube type feeder, in terms of plug flow pattern, pressure drop and solid mass flow rate. Detailed analysis of the particle entrainment in feeder container, flow properties at riser inlet and natural plug formation in riser are then carried out. It is found that both gas and particle flows result in entrainment effect on particles, respectively, through fluid-particle and particle-particle interactions. The linear increase of solid mass flow rate with superficial gas velocity is closely related to the linear increase of particle vertical velocity at riser inlet. Plug formation is achieved in a natural way, which shows the features of self-organization phenomenon. Particle concentration is a key parameter in this self-organization process. This study provides some insights into the natural plug formation of coarse particles in a non-mechanical feeder. The model should be useful to study the gas-solid flow of coarse particles in a confined domain with complex geometry, where the fluid cell size may be smaller than the particle size. (C) 2018 Elsevier B.V. All rights reserved.