Experimental Research On The Reflux And Back Mixing Processes In CFB-FGD Tower

Semi-dry Flue Gas Desulphurization technology is a new kind of desulphurization technology which has following characteristics: simple technics, low investment, small floor space, low operation cost, being suitable to the improvement of exiting units, and so on. It is fit for the situation of our country and is used by more and more small-and-medium-sized units.CFB—FGD is a kind of Semi-dry FGD that is based on circulating fluidized bed technology. The research on CFB—FGD indicated that the desulphurization efficiency could be increased because of the internal circulation of the desulphurizing sorbents. The internal circulation of desulphurizing sorbents mostly formed at the top of the tower. The top separator, such as louver separator, makes part of the desulphurizing sorbents fall down along the inner wall, increases the particle concentration in the tower. It forms turbulent flow in the tower, increases the residence time of the desulphurizing sorbents. It also exacerbates the friction among the particles, renews the surface of the particles continuously and consequently increases the utilization rate of the desulphurizing sorbents.The process and regularity of the desulphurizing sorbents falling down from the top and back mixing into the main flow may affect the desulphurization efficiency. It is simulated and researched by experiments in this thesis. In order to examine the particle circumfluence separately, the particles were put in not within main flow but at the top of the tower. 3-Demension Particle Dynamics Analyzer(3-D PDA) was used to measure gas-solid two phase flow characteristics, mainly about the 2-demension velocity and the concentration distribution of particles. The variable flow characteristics were examined under different main flow velocities and particle refluence flux.The axial velocity, radial velocity and concentration distribution of particles were gotton through the data analysis:1. In the upper part of the tower, the flow can be divided into three regions: particle refluence region, back mixing acceleration region and main flow region. In the particle refluence region, the axial velocity of particles is opposite to gas; in the back mixing acceleration region, the axial velocity of particles turns and has a big gradient along the radial direction; in the main flow region, the flow is similar to parallel flow. In the lower part of the tower, the axial velocity of particles is along the main flow except the region near the inner wall. There is none particle near the Venturi tube because of the consumption along the falling course.2. In the upper part of the tower, the radial velocity points to the middle of the tower. It increases at first, and then decreases from the wall to the middle. The main back mixing region is where there is a big radial velocity. In the lower part of the tower, the radial velocity is so turbulent that there is no regularity.3. The particle concentration decreases from the top to the bottom in the tower. It decreases from the wall to the middle, it is same to the normal fast fluidized bed.4. Maximal falling length is defined in this paper. Under the same particle flux, it assumes an exponential decreasing type as the gas velocity increases. Under the same gas velocity, it assumes a linear increasing type as the particle flux increases, that means, it is directly proportional to the particle flux.5. Back mixing velocity is defined in this paper. Under this experiment condition, it relates to gas velocity, particle flux, diffusion coefficient and bed structure. It is directly proportional with particle flux.