Ash deposition modeling using a visco-elastic approach

In pulverized fuel (PF) combustors and gasifiers, ash particles may deposit on heat exchangers reducing efficiency, performance and, in some cases, damaging boilers and gasifiers. In general, ash particle composition, size (fragmentation) and mechanical properties (hardness and stickiness) are affected by the history of each particle along their trajectory. In the present work, visco-elastic models are used to predict the Young Modulus according to time, temperature and ash composition. In commercial CFD programs, particle adhesion is usually modeled either as fully trapped or bouncing off according to a specific restitution coefficient usually given as input to the wall boundary surface. The authors apply the mechanical adhesion approach proposed by Johnson (JKR theory) which takes into account the surface energy, particle size and the hardness of both the particle and the impacted surface. The energy restitution coefficients are calculated as the remaining kinetic energy after elastic-plastic particle deformation and work of adhesion due to inertia impaction. To implement this approach, a specific Lagrangian particle tracking code was developed to track particle and predict their fate post-processing CFD flow data fields. Numerical results are validated against hollow glass particle and biomass ash experiments carried out in the entrained flow reactor in use at The Energy Research Center of The Netherlands (ECN). Hollow glass particle were used to mimic the high silica content ash obtained from co-firing biomass facilities. The final goal of this work is to present a comprehensive approach to mathematically describe deposition and deposit build-up by means of mechanical and rheological models for visco-elastic solids. Numerical results compare fairly well to available experimental data of the average particle dispersion.