Hot syngas filtration in the freeboard of a fluidized bed gasifier: Development of a CFD model

Abstract

An Eulerian–Lagrangian CFD model was described and validated against experimental findings, able to simulate the fluid dynamic behavior of gas and solid particles inside a fluidized bed freeboard where filtration candles are inserted, with the aim to integrate the steam gasification of biomass and the hot gas cleaning system into one reactor vessel. Fluidization tests with a cold model consisting of a bed of sand particles containing also a fraction of Geldart group A/C fine powder were carried out batchwise in a 10-cm ID column, at different operating conditions, to quantify solid ejection into the freeboard zone. A Particle Elutriation Model (PEM) was proposed to set the boundary condition at the bed surface (freeboard inlet) for fine particles. This model accounted for elutriation and attrition based on the assumption that the generation of fines by attrition is a nonlinear function of time and depends on the percentage of agglomerated fines. By fitting the experimental data with PEM equations, the elutriation rate constants and attrition rates were evaluated at various particle diameters, and it could be observed that the entrainment rate at low air velocities was affected by interparticle adhesion forces. The PEM was then interfaced with the CFD open source software MFIX in order to simulate transport of particles in the freeboard, the deposition of fine particles on the filter candle and thus the pressure drop profile as a function of time due to cake formation on the candle surface. A k-ε model was used for turbulence. A Discrete Random Walk (DRW) model was implemented to consider the particles turbulent dispersion. 2D simulations of the freeboard were carried out at different static bed height (10 and 20cm), superficial gas velocity (11 and 16cm/s) and mass of fines initially charged inside the bed. The numerical predictions were compared with experimental results obtained when a filtration candle is inserted in the freeboard of a bubbling fluidized bed system with similar dimensions and operated at the same conditions as those simulated by the model. In order to match experimental and numerical results, it was necessary to assume that clusters of very fine particles (with a diameter smaller than 20μm) were transported in the freeboard. The results of the numerical model were in fair agreement with the experimental results.