Research On The Effect Of Furnace Structural Features On The Hydrodynamics Of CFB Boiler

As a high effitient and clear coal combustion technology, the circulating fluidiszed bed technology has developed quickly in recent years. At present, the research works are focused on improving the capacity and high steam parameters of CFB boilers.During this largh-scale process, understanding the effects of furnace structure arrangement on gas-solid flow properties has became one of the key technology. By sponsor of the Key Projects in the National Science&Technology Pillar Program during the Eleventh Five-Year Plan Period, in this paper, the experiment and numerical simulation research work on the effects of furnace strcutual feature points on the hydrodynamic characteristics were carried out and this paper also provides reference for the design of600MW supercritical CFB boiler at last.In this paper, experiment measurements and analysis were carried out in a plexiglas rectangular cross-section CFB cold model with the cross-section of350×480mm2and the height of4.9m. A high-speed camera combined with the particle image velocimetry (PIV) technology and PC-6D reflective-type optical fiber concentration probe were used to investigate the particles velocity and concentration field distribution under different kinds of operating conditions. The transparent glass bead was used as bed materials. The influences of the structural feature points (as the structure of the dense zone, the structure of the transition zone, the furnace exit position and the screen arrangement) on the gas-solid flow characteristics in the riser were investigated.The experiment results showed that:the gas-solid flow pattern in the dense zone was significantly affected by the structure of the dense region. Particles movement was dominated by bubble wake and inclined to the rear wall on the influence of expanding cross-section of rear wall, and the radial distribution of solid concentration was asymmetry, the partical local volumn concentration near the rear wall was higher than that near the front wall. The gas-solid flow pattern in the transition region was typical core-anuular distribution, and the projection section in transition region decreased the flow area and increased the particle lateral velocity.In the dilute region the furnace hydrodynamics was controlled by the furnace extit effect, the particles local velocity and volumn concentration reached the maxim near the exit poisiton. The size of the furnace exit affected the solid radial velocity distinctly and the influence on axial velocity distribution was limited. The superheater screen arrangement increased the wall boundary control in the dilute region, and the particles movement in the area was controlled by the gas flow and the constrain of the screen wall. High particle concentration areas and solid back-flow were more easily formed near the screen.At last, under the support of the Fluent computationl software, this paper simulated the hydrodynamic characteristics of the CFB cold-model bed and6kinds of600MW supercritical CFB boilers with different structural features using Two-Fluid Model (TFM) combined with the Kinetic Theory of Granular Flow (KTGF). Concidering the defects of the tradial drag coefficient correlations, a new based on EMMS model drag coefficient computational method was adapted. According to the simulation results, the influence of furnace outlet arrangement, the superheater screen and the dense region structure on the gas-solid flow pattern were investigated.The simulaition results show that:along the axial height of the furnace, dense reigon existed stably under the height of the pant-legs and the solids volumn concentration and velocity radial distribution in this area were typical "W" and "M" shape; The gas-solid two phase flow pattern in the up diliute region were controlled by the outlets arrangement.When the outlets arrangement were compact mode, the fluctuation of the furnace outlet rate increased apparently, but when increasing the number of the oulets, the fluctuation decreased; after the addition of the super-heater screens, the solid concentration in the whole dilute region and the flunctuation of the outlet rate increased correspondingly. Considering the penetration of the secondary air, when single furnace bottom mode was adapted, the solids mixing and uniforimity condition in the bottom region was worse than that in the pant-legs bottom structure..