Experiments On The Effect Of Geometry Structure On The Gas-solid Flow Pattern In A Circulating Fluidized Bed Riser

As a clean coal technology, Circulating Fluidized Bed (CFB) combustion technology has been spread rapidly in the world in recent years. At present, the research and development works are focused on the large capacity and high steam parameters CFB boilers. However, there are still some issues to be understood further, for example, the effects of the furnace structural feature points (such as the structure of the dense zone, the furnace exit, and secondary air nozzle) on the hydrodynamic characteristics in the furnace.In this work, measurements and analysis were carried out in a plexiglas rectangular cross-section CFB cold model with the cross-section of 350×480 mm2 and the height of 4.9 m, to investigate particles velocity field and particles concentration field under different kinds of operating conditions, by using a high-speed video camera combined with particle image velocimetry (PIV) technology and PC-6D reflective-type optical fiber concentration probe, with transparent glass bead as bed materials. The influences of the structural feature points on the gas-solid flow characteristics in the riser were investigated, as the structure of the dense zone, the structure of the transition zone and the furnace exit.Experimental results showed that the gas-solid flow characteristics in the dense zone was significantly affected by the structure of the dense zone. The radial distribution of particles flow velocity and particles concentration were basically symmetrical in the dense zone with the gradually expanding symmetrical structure. Whereas with the gradually expanding structure of back wall, along the direction of the front wall and the back wall, the distribution of particles flow velocity and particles concentration were non-centro-symmetric. There were more bubbles and particles flowing and back-mixing towards the gradually expanding back wall. Particles concentration near the back wall was significantly higher than that near the front wall. Particles flow velocity increased with the increase of primary velocity, while particles concentration decreased. With the increase of secondary air rate, the number of particles at the the bottom of the dense zone increased, and solids concentration increased accordingly.Particles flow distribution in transition region was changed by the projection section on the back wall. The deflection of particles flow direction was obvious near projection section, especially above and under the projection section, and particles lateral flow was more remarkable by the impact of projection section. In addition, projection section was dramatically eroded by a large number of gathered particles with continuous collision. The gas-solid two-phase flow structure was also significantly affected by the secondary air injection in transition region. Particles flow distribution was no longer the typical core-annulus structure, and particles of the increased lateral flow velocity flowed to the central zone with the secondary air jet. Particles concentration significantly reduced in the wall region, but with the increased concentration in the central area, leading to a obvious decrease for the radial nonuniformity of particles concentration. Particles concentration above the secondary air nozzle slightly changed with the increased secondary air rate.Particles flow structure of the exit region was influenced by the separator exit. In the left wall zone near the exit, particles mostly flowed to the exit of the back wall. There were still particles downflow existed near the front wall, like the typical mobility in the wall region. By the impact of the exit effect and wall friction, a region with high particles concentration formed beneath the exit, leading to the particle concentration near the back wall much larger than other regions. Because of the weaker outlet effect, the corner effect existed in rectangular cross-section riser, as well as part of the rebound particles by the roof, there were a high particles concentration region in the corner formed by the left wall and the front wall. The exit effect was weaken by the arrangement of the hanging screen to a certain extent, and meanwhile particles flow field was redistributed near the front wall by the influence of the hanging screen.