CPFD Simulation And Experimental Investigation Of Gas-solids Flow Characteristics In A Large Scale Circulating Fluidized Bed With Annular Combustion Chamber

As a clean coal combustion technology with bright prospect, supercritical circulating fluidized bed (CFB) technology becomes an important development trend for coal-fired power plants. With the increase in the scale of CFB boiler, conventional CFB boilers with straight shaft structures will meet serious challenges, i.e. the penetration depth of secondary air and the arrangement of heating surface. The "pant-leg" design is one of the options to improve the secondary air penetration depth. However, the lateral bed-inventory between two legs will damage the dynamic performance of a CFB. In allusion to these problems, annular combustion chamber was proposed to improve the secondary air penetration and provide adequate space to arrange the heat transfer surface.In this thesis, intensive investigation on gas-solids flow characteristics and non-uniformity distribution of gas-solids flow among different outlets was conducted based on CPFD simulation and experiment investigation. The objective of the research is to improve the understanding of the gas-solids flow hydrodynamics behaviour in the annular combustion chamber and to optimize the geometrical structure of the chamber.The gas-solids flows in a CFB systems is complex and includes different flow regimes including a dense flow regime in the bottom region, a dilute regime in the top region and strong swirling regime in cyclone separators. These make the CFD simulation is difficult in a CFB system. In this thesis, computational particle fluid dynamic (CPFD) simulation with Wen-Yu/Ergun drag model is used to characterize the gas-solids flow in the annular combustion chamber. Based on experiment results, an improved CPFD simulation model was proposed and applied for the simulation of a full-loop of gas-solids flow in a large scale CFB with six cyclone separators. The results show that the solids concentration is higher in the near wall region and lower in the centre region. However, the distribution of solids axial velocity is in contrast to solids concentration. The maximum deviation of solids flux at different outlets is 23.09%.Pressure analysis and CPFD simulation were combined to investigate the hydrodynamics characteristics in the bottom region of the annular combustion chamber. Different flow regimes, including single bubble regime, exploding bubble regime and turbulent fluidization regime, were identified based on the simulation and measurement results. The comparation results of pressure power spectra show that the gas-solids flow in the annular combustion chamber was proved similar to that in a conventional CFB riser.To optimize the chamber structure, the effect of different structure parameters on gas-solids flow characteristics and non-uniformity distribution of gas-solids flow among different outlets were investigated. The basic structure of annular combustion chamber was confirmed by the investigation of key parameters, i.e. the combustion chamber height and length-width of cross-section of combustion chamber, the height of top region, the arrangement of outlets and height-width ratio of outlets. The results show that different structure parameters have no significant effect on the axial distribution of solids concentration. The non-uniformity of solids concentration is a key factor which affects the non-uniformity of gas-solids flow among different outlets.Based on the improved CPFD model, the gas-solids flow in a full-scale 660 MW supercritical CFB boiler with annular combustion chamber was investigated and solids concentration shows similar distribution to that of the cold test rig. In the cross-section, the radial distributions of solids concentration and solids axial velocity are approximate linearization. However, in the cold test rig, the radial distributions are parabolic. Compared with the maximum deviation (23.09%) of solids flux at different outlets in the cold test rig, better uniformity exists in the 660 MW supercritical CFB boiler with the maximum deviation (6.29%).In summary, CPFD simulation and experimental measurement are applied in the investigation of of gas-solids flow in a circulating fluidized bed with annular combustion chamber and multi-cyclones. As a new type combustion chamber, the characteristics of solids distribution in the annular combustion chamber were revealed and a series of suggestion to obtain better uniformity of gas-solids flow was presented. The results can provide theoretical and technical support for the commercial design of annular combustion chamber.