Dissipation of translational energy during non-spherical particle wall collisions as related to entrained coal gasifiers

A fundamental research project has been undertaken by researchers at NETL in the US DOE and Pennsylvania State University to better understand fouling and slagging in coal gasifiers. To predict the partitioning of diverse coal particles into different product streams the aerodynamic behavior and carbon conversion process are tracked and simulated individually. DPM-Eulerian models allow the researcher to track the trajectories of coal and char particles through the gasifier and their impact with the wall. For these computational models, the coefficient of restitution, which is the ratio of the rebound velocity to the impact velocity, is used to characterize particle wall impact behavior. This value is proposed to be used to predict the probability for particle adhesion to the slag layer. Towards that end, cold flow experiments were initially conducted by using an educator to eject high density polyethylene particles onto a flat horizontal steel plate to measure the coefficient of restitution. Meanwhile, high speed video was used to characterize velocity before and after collision with the surface. Based on the results of high polyethylene particles impacting a steel plate, the maximum coefficient of restitution was attained with a rebound angle at 90° while tapering off at higher and lower angles of rebound. Moreover, rotating non-spherical particles were observed in part due to particle-particle interactions and turbulent gas flow eddies. However, the size and sphericity was evaluated as the influence the magnitude of this effect. Such rotational motion can lead to significant energy dissipation upon particle wall collision resulting in a higher damping force in the normal component of motion. To investigate the causes for the wide distribution of the coefficient of restitution values, drop experiments using a vibrating pneumatic feeder were performed to compare the rotational motion determined for both extruded polyethylene and non-spherical coke particles to spherical polystyrene. The coke is a coal derived char produce under slow heating conditions and was used to simulate the behavior and physical properties of coal prior to carbon conversion. Polyethylene is a thermoplastic polymer and was used to simulate the behavior and properties of coal during and after carbon conversion under the thermodynamic conditions of a gasifier. For the most part, the sphericity of char can range from 0.47 (mixed porous type char) to 0.82 (Crassisphere type char) and varies according to the particle size distribution of the original coal. However, the physics, and thus validity of particle wall impact models such as the JKR-Hertz, DMT, and Maugis Models hinges on the geometry of the ideal sphere. The overall objective of this work is to resolve the physics as it relates to the geometry of the particle in order to better adapt the appropriate particle wall impact model to model char and slag interaction in an entrained flow gasifier.