Experimental Study On Segregation Of Solids And Jet Penetration In A Circulating Fluidized Bed

As a high effective and low pollution combustion technology, circulating fluidized bed (CFB) has promoted the development of clean coal power generation technology. In the processes of coal combustion and waste disposal, the solids generally have a wide size distribution and/or different solids densities. When the coarse particles are added into the fine particles, it is able to increase the solids holdup, enhance the heat and mass transfer rates and control the residence time of fine particles. Air classification that divides the total air (TA) into the primary air (PA) and second air (SA) has been adopted, which has advantage of reducing NOX emission, controling burning quality, reducing energy consumption of fans and controlling solids circulation rate (Gs). Currently the most common problem for secondary air in CFB is the lack of penetration. Particles used in previous studies on segregation of binary solids in CFB were mostly of Geldart B type with indistinctive difference in size, and no segregation was mentioned when the air was staging. For the penetration of SA jet, most authors did not study the penetration depth in dense region particularly, and less used inclined jet.The axial pressure drop of binary mixtures, and the distribution of coarse particles in the riser and the return line was measured in a 0.25m×0.25m×6.07m square circulating fluidized bed. The influence of gas velocities (U0), solids circulating rate (Gs) and initial coarse particles fraction (Xc0) on the coarse particles fraction distribution and the axial pressure drop profile were examined. Pressure drop decreases with the increasing of Gs and Xc0, increases as U0 decreases while the other operating conditions were consistent. It was found the pressure drop below the secondary air injection port increases while keeps constant or decreases slightly above the secondary air injection port when air was staging. The coarse particles fraction in the riser increases with increasing Xc0 and U0, decreases with air staging. Coarse particles fraction distribution became more uniformity with increasing Gs. Higher coarse particles fraction were found near the wall and less in the center, while intermediate at the corner.By using CO2 as tracer, the dispersion of SA jets has been investigated combined with the local solids concentration distribution. Secondary air was injected at the bottom of the riser. Particle concentration near the jet was measured by PV6D optical probe. A modified model for calculating the SA jet's penetration is developed. Particle concentration distributions in different conditions were compared. Jet penetration depth increases with the increase of jet velocity; at the same flow rate, jet penetration depth becomes smaller as the diameter increases; horizontal jet has the maximum jet penetration depth, while the angle increases, the smaller the penetration depth will turn up; increase of mainstream velocity and particle concentration will reduce the jet penetration depth.