CFB reactors , CFD and particle / turbulence interactions

The design of furnaces and boilers for solid fuels is supported increasingly by powerful softwares and ever faster computers. A round-robin over the most important CFD (computational fluid mechanics) models and softwares currently available shows, however, that numerical procedures for particle/turbulence interactions are not (yet) very advanced. Particle/fluid interphase slip is considered, but turbulent velocity fluctuations are largely ignored. Particle trajectories can be calculated with reasonable accuracy, but realistic calculations involving gas/solid mass and heat transfer and gas/solid chemistry are impossible. Here, the influence of particle/turbulence interactions on circulating fluidised bed (CFB) combustor riser chemistry is discussed, based on multi-phase mixing by turbulent dispersion of solid particles in a dilute suspension. After a short discussion on CFB hydrodynamics modelling, commercial CFD codes are analysed. Thereafter, an approach for taking into account the effect of particle/turbulence interactions and gas/solid mass transfer on gas/solid conversion is given, illustrated by a typical example.