Mixing in biogas digesters: correlations between laboratory experiments on artificial substrate and simulations with computational fluid dynamics

Energy demand for mixing of biomass digesters is a crucial parameter in design and operation of biogas plants. Optimization of flow characteristics in the fermentation process is usually focused on the stirrers where their placement, shape and number, as well as their rotational speed and switching sequence are all important decision variables for overall energy efficiency planning. Digester stirring was analyzed in cylindrical transparent physical digester models located in the laboratories of Technische Hochschule Ingolstadt. Real biomass was substituted with chemical substrates based on cellulose exhibiting transparency, physico-chemical stability, ease of use and rheological behavior comparable to real biomass. The motion of the liquid within the tanks was investigated with pulsed laser PIV (Particle Image Velocimetry). The optical measurements of the flow velocity allow to measure flow fields in a non-invasive way. In order to gain more insight into the theoretical concepts of mixing and flow patterns within digesters the results of the laboratory experiments were verified with computer simulations using a commercial computational fluid dynamics (CFD) software tool StarCCM +.