The mixing and segregation characteristics of different types of solid particles in a downer reactor are of fundamental importance in the pyrolysis of low-rank coals where sand particles are used as the heat carrier. However, relevant fundamental studies are seldom reported. This work presents a computational investigation of the hydrodynamic behavior of binary gas-solid flows in a downer using an Eulerian-Eulerian method with a modified gas-solid drag force model, taking the effect of mesoscale particle clustering structures into consideration. Three-dimensional CFD simulations were carried out and the results obtained were compared with the experimental data available in literature. It was shown that a reasonable agreement with experimental data can be obtained with the proposed drag modification. The segregation/mixing behavior in the downer was then analyzed with two popular methods, which results in the conclusion that the solid particles in the downer reactor can mix reasonably well, therefore offering theoretical support to the idea of coal pyrolysis using sand particles as the heat carrier.