Heat transfer calculation methods in three-dimensional CFD model for pulverized coal-fired boilers

Abstract

Computational fluid dynamics (CFD) has been used extensively to study the combustion characteristics of coal-fired boilers. However, very limited of the boiler CFD studies were used to quantitatively predict the heat transfer distributions in practical boiler applications. This paper presents a systematic description on the heat transfer calculation methods in three-dimensional CFD model for coal-fired boilers. The primary objective is to provide a set of heat transfer submodels that can be easily employed to solve and analyze large scale practical boiler problems. All types of boiler heating surfaces are considered with particular focus on the heat transfer calculation of furnace water wall. It is implemented in the CFD model in terms of a thermal boundary condition (B.C.) describing the energy balance between the boiler’s fire side heat transfer and water side heat absorption. The physical significance of the associated B.C. parameters and their accurate prescription are discussed in detail in this paper. It is found that furnace wall heat transfer is largely affected by the wall ash deposit conditions such that the heat transfer coefficient of furnace wall can be determined based on the thermal properties of ash. Although ash thickness may vary dramatically over furnace wall and is extremely difficult to predict, its thermal conductance was found to only vary within a small range for normally operated coal-fired boilers. Establishing the connection between the thermal conductance of wall ash deposits and the heat transfer calculation of furnace wall bypasses the tremendous difficulty and uncertainty incurred in predicting the ash thickness, and thereby, greatly simplifies the wall heat transfer calculation process. The heat transfer calculation methods introduced in this study embody the key physics associated with boiler’s heat transfer process while making a reasonable balance between the level of model complexity and their applicability to practical boiler problems. In this paper they are illustrated and validated on a 330 MW tangentially-fired subcritical boiler. The results show that the predicted boiler heat transfer distributions are in close agreement with the boiler’s operating data.