Modeling Mixing in Anaerobic Digesters with Computational Fluid Dynamics Validated by Experiments

Abstract

To keep biogas production competitive against alternatives in the energy sector, operating costs of biogas plants must be reduced. Up to 50 % of the energy consumption in biogas plants is contributed by mixing of biomass slurry. Thus, optimization of the mixing system is a promising approach to increase the overall efficiency of biogas plants. Investigations to find the optimal conditions of mixing are challenging in full‐scale biogas digesters. Two main strategies to study biomass mixing in laboratories are to build a scale‐down digester using an artificial substrate to mimic biomass, and to develop a computational fluid dynamics (CFD) model to simulate the mixing process. In this paper, a combined approach is carried out: a CFD model is presented and verified experimentally.

CFD Modelling of Biomass Mixing in Anaerobic Digesters of Biogas Plants

“…The absolute values were between 4 cm/s and 13 cm/s. The experimental data obtained using the acoustic velocimetry and the PIV spectroscopy were used to validate a computational model, which was recently developed to simulate the fluid dynamic in the laboratory digester on the basis of CFD calculations [15].…”

“…There are different possible meshing strategies, in the relation to the system to mesh: surface or volume. Here, the following four meshers were carefully chosen [15]: for the surface mesh, the Surface Remesher and the Automatic Surface Repair were selected. The latter one was used to identify and solve problems.…”

“…On the wall and bottom of the tank, no-slip conditions were set because more accurate, whereas the fluid surface on the top (transition to gas) was modelled with slip conditions. More details are reported in [15].…”

“…The modern StarCCM+ software, from CD-Adapco Siemens, is used to create three-dimensional geometries, to generate meshes, to solve the fluid dynamics by Ostwald-De Waele approach. Initially, the model was used to simulate the mixing in scaled-down laboratory digesters [7], [15] in order to validate the model. The laboratory digester was built in acryl glass to a scale of one to twelve and filled with an aqueous cellulose mixture, which was selected to represent the rheology of the substrate [16].…”

Einfluss der Rührwerksposition auf den Mischprozess in Biogasanlagen anhand eines Paddelrührwerks

“…Nach dem aktuellen Stand der Technik beschränken sich die Untersuchungen hinsichtlich des Förderverhaltens von Paddelrührwerken auf punktuellen Geschwindigkeitsmessungen, welche die Charakterisierung des Strömungs‐ und Mischprozessen nicht erlauben . Nur durch die detaillierte Beschreibung des Strömungsfeldes kann der Mischprozess verstanden und an das jeweilige Anlagenkonzept angepasst werden.…”