Detailed assessment of mesh sensitivity for CFD simulation of coal combustion in a tangential-firing boiler

Computational fluid dynamics (CFD) has become an essential tool for optimizing the design and diagnosing the operation of a boiler. However, the validity of the results depends on the degree of numerical diffusion as well as the reliability of the submodels. This study aims to assess mesh sensitivity in the reacting two-phase flow of pulverized coal in a common tangential-firing boiler. Three mesh versions were constructed for the boiler with the number of cells ranging between 1.2 million and 5.4 million, corresponding to 0.0114 -0.0022 m³ per cell in the burner zone. The velocity distribution was found to be highly sensitive compared to temperature, heat flux, and NO concentration. By contrast, the use of key performance parameters such as total wall heat absorption, exit NOx concentration, and carbon conversion, was not appropriate criteria for the mesh sensitivity test. These parameters were determined by integration over the entire surface or volume, which made them sensitive to the overall reaction stoichiometry instead of the mesh fineness. It suggests that the use of a coarse mesh could be acceptable in evaluating the key performance parameters influenced by major operation variables, such as air distribution and fuel properties. However, sufficient mesh fineness is necessary for studies requiring accurate prediction of detailed flow patterns such as the evaluation of burner tilting/yawing or ash deposition on the wall.