25 th German Flame Day, 14-15 September 2011, Karlsruhe, Germany CFD simulation of biomass grate furnaces with a comprehensive 3D packed bed model Ramin Mehrabian 1,2,  , Stefan Stangl 1 , Robert Scharler 1,2,3 , Ingwald Obernberger 1,2,3 , Alexander Weissinger 4 1 BIOENERGY 2020+ GmbH, Inffeldgasse 21b, 8010 Graz, Austria 2 Institute for Process and Particle Engineering, Graz University of Technology, Inffeldgasse 21b, A- 8010 Graz, Austria 3 BIOS BIOENERGIESYSTEME GmbH, Inffeldgasse 21b, A-8010 Graz, Austria 4 KWB - KRAFT UND WÄRME AUS BIOMASSE GmbH, Industriestrasse 235, A-8321 St. Margarethen/Raab, Austria Abstract A 3D CFD model for biomass packed bed combustion has been developed at BIOENERGY 2020+ in co-operation with BIOS BIOENERGIESYSTEME and KWB in a previous work [1]. It consists of an Euler-Granular model for hydrodynamics of gas-particle multiphase flow and a thermally thin particle model for combustion of biomass particles. In this paper, this model has been improved by the implementation of a layer model for thermally thick particles. The new packed bed model provides the advantages of considering the intra-particle species and temperature gradients and, accordingly, allows for parallel progress of the thermal conversion sub-processes. Moreover, the layer model considers a more realistic shape for biomass particles, e.g. cylinders. Enhanced models for pyrolysis and char oxidation are applied and char gasification reactions are included. Additionally, the products of char oxidation are CO and CO 2 , whereas the ratio between these species changes depending on the particle temperature. The simulation of a small-scale underfeed stoker furnace has been successfully performed by the application of the new packed bed combustion model. The positions of the drying, pyrolysis and char burnout zones in the fuel bed as well as the temperature distribution among the particles seem to be plausible and could be confirmed by observations. Furthermore, a good qualitative agreement concerning the flue gas temperatures measured by thermocouples at different positions in the combustion chamber and CO emissions measured at boiler outlet could be achieved. 1. Introduction and objectives CFD simulation techniques are an efficient tool for the design and optimisation of biomass grate furnaces. They have successfully been applied to predict the turbulent reactive flow in combustion chambers of furnaces [2-5]. However, at present there is a lack of reasonably accurate and computationally efficient simulation tools for packed bed biomass combustion, which directly integrate the packed bed modelling into the available models for the turbulent reactive flow. A combination of several sub-processes such as heat-up, drying, pyrolysis and char burnout represents the thermal conversion of solid biomass particles. Depending on the size and the physical properties of the biomass particles, temperature and species gradients may develop inside the particles causing intra-particle transport processes. The group of particles with gradients inside the particle and simultaneous progress of the different conversion stages is called thermally thick particles. The biomass particles in grate furnaces typically belong to this group. Several studies have been performed to describe the thermal conversion of a single thermally thick biomass particle [6-11]. In this work the packed bed is considered as an ensemble of representative particles, where each of these particles undergoes thermal conversion processes. The conversion of these particles is modelled by a thermally thick particle model. The layer model is able to describe the most essential characteristics of the thermal conversion of the thermally thick biomass particles, such as the intra-  Corresponding author: Tel: +43 316 873 9232; email: ramin.mehrabian@bioenergy2020.eu