Heat transfer investigations in a circulating fluidized bed combustor

The circulating fluidized bed (CFB) combustors are used for power generation from solid fuels due to their better performance, low emissions and fuel flexibility. The heat transfer mechanism inside the CFB combustor is a complex phenomenon and depends on operating conditions. The current research work involves investigations on bed-to-wall heat transfer in the riser column of a CFB combustor. The work is aimed to improve the understanding of the heat transfer mechanism under different operating conditions. The effect of dilute and dense phase operating conditions on the bed-to-wall heat transfer process is investigated using a cluster renewal mechanistic model. A cluster energy balance approach is proposed and the equations are solved using a numerical scheme to predict bed-to-wall heat transfer coefficient. The bed-to-wall heat transfer behaviour along the height of the riser column for different operating conditions is also investigated using a two-phase gas-solid thermal energy balance model. The riser exit geometry influences the bed hydrodynamics and heat transfer to surfaces in a CFB combustor. The effect of riser exit shape on the bed-to-wall heat transfer in the exit region as well as in the riser column is investigated using predictive hydrodynamic model for parametric analysis and axial mass balance model for axial heat transfer profile. The results from different models are in reasonable agreement with published experimental data and trends for similar operating conditions. The results and analysis from the different investigations reported in the current work along with appropriate interpretation, provides sufficient information for improving the fundamental understanding of the bed-to-wall heat transfer mechanism which will help in improving the design of CFB heat transfer surfaces.