Numerical simulation of a biomass fired grate boiler

Computational fluid dynamic (CFD) analysis of the thermal flow in the combustion furnace of a biomass-fired grate boiler provides crucial insight into the boiler's performance. Quite a few factors play important roles in a general CFD analysis, such as grid, models, discretization scheme and so on. For a grate boiler, the modeling the interaction of the fuel bed and the gas phase above the bed is also essential. Much effort can be found in literature on developing bed models whose results are introduced into CFD simulations of freeboard as inlet conditions. This paper presents a CFD analysis of the largest biomass-fired grate boiler in Denmark. The focus of this paper is to study how significantly an accurate bed model can affect overall CFD results, i.e., how necessarily it is to develop an accurate bed model in terms of the reliability of CFD results. The ultimate purpose of the study is to obtain reliable CFD results that can be used for design, improvement and optimization of grate boilers. To achieve the purpose, first of all a structured mesh of good quality was generated to minimize numerical error, the dominant error in a general CFD analysis. Then two different approaches are used to treat the fuel bed to provide the inlet condition for the CFD simulations. The results indicate that the grid plays a very important role in the CFD analysis while the bed models (i.e., different profiles at bed top) play a less important role in CFD modeling of grate boiler. It could be attributed to the large amount of air streams of high speed in the freeboard, which affect significantly the mixing, flow and combustion patterns in the freeboard. As a result, the effect of different bed models might be limited only in the vicinity of the fuel bed.