Effects Of Returning Gas-solid Flow And Secondary Air On The Characteristics Of Hydrodynamics And Combustion Of Circulating Fluidized Bed

As a low-cost and effective clean combustion technology, circulating fluidized bed (CFB) combustion technology has been developed greatly. Due to the complexity of gas-solid flow field in the bed, both the effects of return gas-solid flow and secondary air (SA) injection on the hydrodynamics of riser are not fully understood, especially when the riser cross-section is square or rectangular. By the experiments on a square cold circulating fluidized bed model and the numerical simulation, detailed study has been performed on the return gas-solid flow and the SA injection as well as the hydrodynamics in the riser. Furthermore, some optimization combustion tests were done on a 440t/h CFB boiler to evaluate the effects of return gas-solid flow and SA distribution on the coal combustion in the bed.By pressure drop analysis, local solids concentration and particle velocity distribution measurement and jet's dispersion tracing, the hydrodynamics of the square riser has been researched on a cold CFB model. When the CFB system was operated without air-staging, the thesis focused on the heterogeneous gas-solid distribution in the bed. The back feeding particles penetrate the gas-solid stream in the bed and get to front wall, which leads to a relatively denser region near the front wall in the bottom bed. More work has been done on the effects of gas velocity (U0) and solids circulation rate (Gs) on the asymmetry of solids concentration and particle velocity distribution in the riser. Adding some coarse particles into the bed material makes the lateral distribution of solids more symmetrical. With constant gas velocity, the solids circulation rate was taken as a variable to research the flow properties in the bed with different solids inlet height. Similar work was performed when the air-staging was adopted.Four SA injection modes have been investigated in the paper, i.e. traditional wall SA, corner SA, internal SA with a horizontal SA duct and internal SA with a vertical SA duct. A denser bottom zone is formed when the primary air was reduced and some up-rising solids deflected to the lower bed by the cut-off effects of SA jets. SA jets has limited influenced region especially when the SA nozzles are far away from the distributor. When the Gs is beyond the saturation carrying capacity of gas with air-staging, the pressure drop at the bottom bed doesn't change any more and S shaped pressure drop profile along the riser is formed, no matter what SA mode is used. When the coarse particle is added into the CFB system, higher coarse particle fraction is found in the bottom bed for the case of air-staging, while the coarse particle fraction in the external circulating particles decreases.Using CO2 as tracer, the dispersion of loop-seal fluidizing air and SA jets have investigated combined with the local solids concentration distribution. A modified model for calculating the SA jet's penetration is developed when the core-annulus gas-solid flow field is considered. The predicted value by the model has good agreement with the experimental results.The dynamic behavior of gas-solid flow in the experimental CFB setup is predicted based on the theory of Euler-Euler gas-solid two-phase flow and the kinetic theory of granular flows. The simulation includes the operation case with air-staging and without air-staging. The SA modes include Wall SA, Corner SA and Internal SA. The simulation results are in agreement with the experimental results qualitatively.Some optimizing tests were performed on a 440t/h CFB boiler to get more information about the effects of return gas-solid flow and SA injection on the flow properties and coal combustion in a CFB boiler. Some operating parameters have been chosen for combustion optimization, e.g. the loading rate, coal characteristics, oxygen component in the flue gas, SA rate, SA distribution along the riser and distribution between the front wall and the rear wall. Smaller SA nozzles were adopted to enhance the SA jet's penetration for increasing the oxygen concentration in the central region of furnace, which is beneficial for the char burn-out.