Modeling of the heat transfer in large-scale fluidized bed furnaces

Paper in proceeding, 2012

A 3-dimensional model for the heat transfer in the furnace of a fluidized bed boiler is presented. The model, which is part of a comprehensive modeling work for large-scale CFB boilers, describes separately the convective and radiative heat transfer mechanisms at different heat extraction surfaces in the furnace (waterwalls, wing walls and division walls). The focus of the paper is on the heat transfer in the furnace, but the heat balance closure at a unit level including the return leg is also treated. Modeled data are compared to measurements from CFB boilers at two different scales: in the Chalmers 12 MWth research boiler and in a large scale boiler of about 300 MWth. Modeled and measured data generally show a good agreement. Since convective heat extraction depends strongly on the local properties of the solids flow, 3D modeling of the convective heat transfer requires other expressions than those found in literature (which are typically. based on cross-sectional averaged solids concentration). In regions with low solids concentration (upper part of furnace), the radiative heat transfer is significantly influenced from regions several meters away from the heat extraction surface. Thus, in the description of the radiative heat transfer, optical factors accounting for the absorption in the gas-solids suspension are used. The model results reveal the importance of the exchange of radiative heat between the upflowing core and the downflowing wall layers. In addition, the importance of the fluid dynamics (wall layer flow properties, local solids flow properties such as the backflow effect and corner effects,) on the heat transfer is discussed with help of the model presented.