Examination of the Influence of Particle Parameters on Contact Heat Transfer on a Single Hearth Furnace Floor Using DEM

  • Hilse, Nikoline (Institute of Energy Plant Technology, RUB)
  • Scherer, Viktor (Institute of Energy Plant Technology, RUB)

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Multiple hearth furnaces are used in industry for the thermal treatment of solids. Typical examples are the calcination of minerals such as magnesite, dolomite or kaolin but also the torrefaction of biomass. The furnaces consist of multiple circular floors stacked above each other, on which the granular material is agitated by rotating rabble arms. These rabble arms are equipped with blades which move the bulk material in radial direction, either to the periphery or the centre of the floor. From there, the particles fall to the next subsequent floor. Radially installed burners are providing heat for the thermal treatment. The main heat transfer mechanisms are radiation and convection from the flame and contact heat transfer from the heated furnace floors. In the current study, a model of a single hearth furnace floor operated in batch mode is used, to evaluate contact heat transfer from the floor to the particle bulk. The numerical model consists of a circular bottom plate with a diameter of 0.55 m and three rotating blades. The blades have a total length of 0.06 m and the both ends of the blades are hemispheres with a diameter of 0.01 m. The particle bulk consists of spherical particles. In the simulation, the cold particles are set on the heated bottom plate at 100°C. The aim of this work is to examine the influence of particle parameters such as coefficient of friction and particle size on the contact heat transfer. First results show that heating rates for thermally thick particles are larger for the steady state operation compared to the agitated simulation cases. It can be seen that for 0.02 m particles with properties similar to those of a polymer after 1000 seconds, a temperature difference of 12.5 K has developed. For thermally thin particles based on a material with a high thermal conductivity, with 0.01 m diameter, a delta of 2 K is observed after 350 seconds. The main reason for temperature differences between steady state and agitated case is the formation of piles of particles in front of the mixing blades when the rabble arm rotates. The piles increase the effective thermal resistance for heat transfer from the hot plate to the particle bulk. As pile formation mainly depends on particle properties such as particle size and friction coefficients, this study is focused on the variation of these parameters to evaluate their effect on the variation of these parameters to evaluate their effect on the bulk heating rate by contact heat transfer.