DEM/CFD modeling of the fuel conversion in a pellet stove

Abstract

This study presents an approach to simulate wood pellet combustion in domestic heating systems. Until now the challenge of such simulations lies in the description of the movement and conversion of the solid fuel while interacting with the surrounding gas phase. To tackle this problem the discrete element method (DEM), a method which allows simultaneous tracking of all individual reacting particles, was combined with computational fluid dynamics (CFD). The pellet shape is represented by polyhedrals to approximate their cylindrical geometry. Pellet length size distribution has been taken into account. Particles are allowed to shrink during the combustion process. Heating, drying, pyrolysis and char combustion are accounted for by a three-dimensional resolution of the particle. Radiative transfer among pellets is calculated based on a surface triangulation of each particle. The typical batchwise feeding of the fuel in a pellet stove has been taken into account in the simulations. Furthermore, experimental results from a 13kW domestic wood pellet stove are compared with simulations. The simulations reveal the details of the unsteady release of water vapor, pyrolysis gases and combustion products. The non-uniform geometrical distribution of the pellets on the burner grate resulting from non-symmetric fuel feeding leads to a deflection of the gas flame above the pellet bed to one side of the combustion chamber. This is also reflected in the measurements. The trend of the gas phase temperature above the pellet bed as well as the mean CO emissions as a function of stoichiometry are well represented by the simulations, however, further room for improvement is left.