Scale-up of fermentation units in biogas plants by studying the mixing characteristics using computational fluid dynamics

The fermentation of biogenic raw materials to produce biogas can greatly contribute to a renewable energy supply of our society. The purpose of the presented research work is the investigation of the mixing behaviour and performance of the agitation systems in biogas plants and to support the scale-up process through carrying out Computational Fluid Dynamic (CFD) analysis.
CFD can be used to model and predict the actual fluid dynamics in a closed system. Thus it may help to plan optimal reactor geometries, position and combination of agitation systems as well as to minimize the energy requirements. Following this strategy, the energy efficiency of such facilities can be increased, reducing the net emission of greenhouse active CO2.
In this work, a two-stage fermentation system (main- and post fermenter) for biogas-production is investigated aiming for an optimal scale-up of the successfully applied technology. The system was designed by Biogest® GmbH and features a circular ground plan: the main fermenter is positioned outside the post fermenter and is imlemented as a circular ring (see Figure 1).

Figure 1: Basic principle of the biogas fermentation plant; 1: main fermenter,
2: post fermenter, 3: feed system, 4: central pumping station.

The installation of proper types of stirring units as well as their optimal positioning in the main fermenter is essential for the reliable and stable operation of the biogas production process. The task of the definition of the number of agitators as well as the positioning in the fermenter was worked out by using the methods and possibilities of numerical fluid dynamics.
The best up-scaled agitator setup was found to feature six mixing devices positioned evenly distributed on the perimeter of the main fermenter ring. Special attention was given to the discharge direction of the stirrers (orientation of the stirrer shafts) to achieve optimal mixing characteristics all over the fermenter volume. The mixing performance of the up-scaled geometry was benchmarked against the established system.
The results of this research work were successfully put into practice by Biogest® Energie- und Wassertechnik GmbH, the first technical-scale facility in scaled-up dimensions is in operation since December 2009.