The discrete element geometric model of the horizontally buried-pipe steam generator was set up. The effects of particle size (20 mm-80 mm) on performances of the horizontally buried-pipe steam generator using waste heat in a bioethanol steam reforming hydrogen production system was studied. When the particle size increases, the particle layer flatness decreases, the particle layer flow ununiformity increases. The volatility of the particle residence time distribution increases with the particle size increases, and the standard deviation of the particle residence time increases. When the particle size increases, the voidage of the particle system increases. So the particle thermal resistance in the steam generator increases with the particle size increases, the steam production of the generator decreases, and the system hydrogen production of decreases. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.