Impact characteristics of particles onto a flat wall relevant to entrained-flow gasifiers

Entrained-flow (EF) gasification represents a versatile technology towards paths which include energy conversion and/or production of chemicals from different solid fuels. Most industrial EF gasifiers operate in slagging conditions to ensure the destruction of tars and oils and to obtain high degrees of conversion. The performance of slagging EF gasifiers may be critically affected by the fate of char/ash particles as they interact with the wall slag layer. Different patterns of micromechanical char–slag interaction establish, depending on the stickiness of the wall layer and of the impinging char particle. The recent literature has investigated and described particle–wall interactions in terms of a coefficient of restitution (the ratio between the rebound and the impact velocities), an important parameter in the context of multiphase flow modelling of the gasification chamber, as it critically affects the boundary condition for particle–wall collisions. This research group has recently investigated the behavior of coal char particles with different carbon content upon the impact onto a flat target in cold and hot conditions. The present study aims to characterize the impact-deposition-rebound dynamical patterns of biomass particles colliding with a planar surface using high speed imaging and particle tracking. Rebound characteristics were studied in cold and hot conditions in terms of coefficient of restitution, and the influence of the target material and surface, of impact velocity and of carbon conversion degree has been assessed. The test rig consisted of a vertical tube furnace, equipped with an alumina tube and a hot impact zone, fitted at the bottom of the upper furnace. The impact chamber was purposely designed and built-up so as to guarantee the desired temperature and to enable optical access to the impact zone. Batches of micron-sized particles (150–200 μm) were fed at the top of the alumina tube and entrained along the tube by a stream of nitrogen fed at the top of the furnace. When the particles left the tube, they impacted on a target plate located in the impact chamber. The particle impact velocity was calculated as the sum of the gas velocity in the tube and the particle terminal velocity, and it was experimentally verified by particle tracking at the exit of the tube with a high-speed video camera. Particle velocities before and after the impact were determined experimentally, hence the restitution coefficient was calculated. Impact tests in cold conditions highlighted that the restitution coefficient for char and ash particles is smaller than 1, suggesting that some plastic deformation occurs even at ambient conditions. Furthermore, results highlighted a pronounced drop of the coefficients of restitution at high temperature. This feature can be explained by considering the change of mechanical properties of the particles as the temperature increases, and the concurrent onset of the char/slag transition at large carbon burn-off.