Tubular drag conveyors are an excellent means of transporting powdery and granular materials in many industries. The operating principle of such a conveyor is to transport bulk material using discs attached to a chain or cable moving inside a tube. The key advantages of tubular conveyors include simple design, quiet operation, easy route geometry configuration, and dust and gas tightness. The developed mathematical models incorporate certain simplified assumptions that prove insufficient for granular and coarse materials. This is due to the complex interactions between the conveyor components and the conveyed material. The key parameter, which is omitted in theoretical models, is the grain composition of the material being conveyed. The particle size, combined with the clearance between the disc and the pipe, determines the so-called gap effect, which significantly impacts the conveyor's energy consumption. The gap is the clearance between the inner diameter of the pipe and the outer diameter of the disc. During transport, particles of the bulk material enter the gap, causing a significant increase in resistance to motion. These interactions are influenced by many factors, including random ones, making this issue challenging to solve theoretically and experimentally. The aim of the research in this paper was to investigate the effect of grain composition on the power consumption of a conveyor drive. Coal fines specimens with different particle-size distributions were tested. The results demonstrated a strong correlation between the power consumption and the grain composition of the transported material. The experimental results were compared with the theoretical model and correction factors accounting for the gap effect were determined.