Gas mixing and flow dynamics in circulating fluidized beds with secondary air injection

In circulating fluidized bed (CFB) boilers, secondary air (SA) injection is obtained by splitting the total fluidization air into primary and secondary streams, the latter being injected usually laterally at a certain height above the distributor plate. The main motivation of the SA injection lies in its effectiveness in controlling the nitrogen oxide formation, especially in combustion of highly volatile fuels. Despite its common utilization, information on the effects of SA injection on riser hydrodynamics is scarce.; This thesis investigates the flow dynamics and gas mixing in CFB risers with SA injection. In the experimental part of this thesis, axial static pressure measurements are carried out to find the axial solids distribution along the riser and tracer gas experiments are performed to investigate the effects of the SA on gas mixing. Experiments are performed in a laboratory scale (0.23m ID, 7.6 m high) Plexiglas CFB riser with sand particles. Three different types of SA injectors are used in the experiments, radial, mixed and tangential injectors. In the numerical part of this study, a two-dimensional gas-solid flow model is proposed to describe the gas-solid hydrodynamics in the riser section of a CFB with SA injection. The model uses an Eulerain-Eulerian (two-fluid) approach based on the kinetic theory of model of Hrenya and Sinclair (1997) for solids phase closures and is implemented using FLUENT V4.5, the commercial computational fluid dynamics (CFD) package.; The axial pressure measurements show that the SA injection significantly alters the axial solids distribution in the riser compared to the case without SA injection at the same superficial gas velocity and solids circulation rate. The tangential secondary air injector results in the highest increase in the overall solids hold up in the riser compared to other two modes of injections.; The mean square displacement model which fundamentally depends on classical Taylor's theory of turbulence dispersion is used to calculate the radial dispersion coefficients under SA injection conditions. The tracer gas measurements and the calculated dispersion coefficients indicate that SA injection can be used as a promising toot to enhance the observed poor radial gas mixing in CFB risers. Significant improvement is obtained in radial gas dispersion especially when radial SA injector is used. This is attributed to the large scale unsteady gas motion caused by the two impinging SA jets. The effects of the design of the SA injector on the radial and gas backmixing are also shown and discussed.; The CFD model is tested against the experimental data of Ersoy (1998) for the simulation of a laboratory scale CFB riser with SA injection. The model results for the axial solids velocity and the pressure drop along the riser are compared to experimental measurements with FCC particles and reasonable agreement is reached between the measured and computed parameters. The model has also been able to capture some of the measured and observed characteristic aspects of the SA injection successfully.