Heat transfer in ash deposits: A modelling tool-box

Abstract The objective of this paper is to review the present state-of-the-art knowledge on heat transfer to the surface of and inside ash deposits formed in solid fuel-fired utility boilers, and-based on the review-to propose models for calculation of heat transfer, e.g. in deposition models. Heat transfer will control the surface temperature of the deposit, thereby influencing the physical conditions at the deposit surface, e.g. if the surface is molten. The deposit surface conditions will affect the deposit build-up rate as well as the removal/shedding of deposits: molten deposit may lead to a more efficient particle capturing, but may also flow down the heat transfer surfaces. The heat transfer parameters of prime interest are the convective heat transfer coefficient h, the effective thermal conductivity of the deposit keff, and the surface emissivity e of the deposit. The convective heat transfer coefficient is a function of flow characteristics, and can be calculated using different correlation equations, while the other two parameters depend on the deposit properties, and can be calculated using different structure-based models. The thermal conductivity of porous ash deposits can be modelled using different models for packed beds. These models can be divided into two major groups, depending on the way they treat the radiation heat transfer, i.e. the unit cell models and the pseudo homogeneous models. Which model will be suitable for a particular application depends primarily on the deposit structure, i.e. whether deposit is particulate, partly sintered or completely fused. Simple calculations of heat transfer resistances for deposits have been performed, showing that major resistances are in the heat transfer to the deposit (by convection), and the heat transfer through the deposit (by conduction). Very few experimental data on the thermal conductivity of ash deposits, especially at high temperatures where radiation is important, are found in the literature. Although the structure of the deposit is essential for its thermal conductivity, most of the measurements were done on crushed samples. The results obtained using different models were compared with the experimental data published in Rezaei et al. [Rezaei, Gupta, Bryant, Hart, Liu, Bailey, et al. Thermal conductivity of coal ash and slags and models used. Fuel 2000;79:1697–1710.], measured on crushed coal ash samples. Although errors of the predictions were very high in most cases, two models were proposed as suitable for heat conductivity calculations, i.e. the Yagi and Kunii model for particulate deposits, and the Hadley model for sintered and fused deposits. This literature study showed the need for a wide range of experimental data, which would help in evaluating and improving the existing thermal conductivity models. Also, it is necessary to formulate a more accurate model for the thermal conductivity of solid mixtures, in which potentially important sources of errors can be identified.