Mathematical modeling and experimental study of gas/solid flows and combustion in the vortexing fluidized bed combustor (VFBC) freeboard

A new solid fuel combustion technique, known as Vortexing Fluidized Bed Combustion (VFBC), was recently developed to increase the combustion intensity, calcium utilization, and turndown capability, and to reduce the freeboard height, elutriation of fines, and emission of pollutants for small- and medium-scale boiler applications.; A mathematical model of gas/particle flows and combustion in VFBC freeboard was developed. A modified Algebraic Stress turbulent Model (ASM) was proposed for the non-isotropic turbulent swirling gas flow. Trajectory-Statistical particle Model (TSM) was newly developed to deal with particle cloud suspension. The detailed formulations were given in cylindrical coordinates for axisymmetric flows. An associated computer program was written, debugged and tested. The effects of chamber Swirl number and geometry of exhaust pipe on isothermal gas flow were discussed. The predicted results were in good agreement with the experimental data. The gas combustion calculation showed a strong tangential secondary injection should be used for reacting flow in order to maintain a desired hydrodynamics characteristic in the VFBC freeboard. A parametric study of particle dynamic behavior in the VFBC freeboard was conducted. The results showed that the particle flow pattern is strongly affected by the particle momentum transfer number, Froude number, gas velocity field, and particle initial radial locations. The particle average velocities, bulk density distribution, particle velocity, and size spectra in the VFBC freeboard were also calculated and discussed.; Four VFBC cold test models (4", 7", 10" and 18" in diameters) were designed, fabricated, and tested. The measurements of gas flow field, particle mass flux, size distribution and elutriation rate were conducted. The test results showed that the tangential secondary injection, combined with a specially designed exhaust pipe, can largely prolong the particle residence time and significantly reduce the fines elutriation. A stable layer of suspended particles in the freeboard produced by the swirling motion was also observed under certain operation conditions, which can be used to improve the VFBC performances in combustion and pollutant control.