Heat transfer in circulating fluidized bed boilers - do we know sufficiently?

Artikel i vetenskaplig tidskrift, 1997

A considerable amount of work, mostly measurements, has been dedicated to heat transfer in circulating fluidized bed combustors. The results show that the radiative constituent of heat transfer varies between limits depending on the optical thickness of a particle boundary layer, and that the convective constituent can be related to the average cross-sectional suspension density. Based on this information reasonable estimates of heat transfer coefficients can be made. Experimental data from boiler conditions and ambient conditions are compared introducing a compensation for thermal radiation and changes in the properties of the gas with temperature. A good agreement is achieved between the two sets of data, except for cross-sectional average suspension densities below about 5 kg/m3. Locally there is a variation in heat transfer across the membrane-tube wall, and the heat transfer to the tube crest can be twice that to the adjacent fin. Heat transfer models are reviewed and applied to a hot (boiler) case and a cold (laboratory) case. The deviation of computed heat transfer coefficients from the measured ones was larger than the scatter in the measured data. The differences in the results from the models are primarily explained by uncertainties in the fluid-dynamic model parameters. For refined estimates and optimisation, knowledge of the detailed behaviour of the fluid-dynamics of the combustor is necessary. In this field the thickness of the a gas-gap between wall and particle suspension, the cover factor of the time-resolved fluctuating particle concentration, and the in-flow of particles towards the wall are insufficiently known and further work is necessary. In addition, further work is needed concerning the local heat transfer to membrane-tube walls