Renewable energy: Biogas fermenters

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Renewable energy covers approx. 22% of the daily energy consumption in Germany today. Approx. 30% of the energy from these renewable sources are provided by biogas.

The economic operation of a biogas facility is determined to a large extend by the design of its stirring devices. An efficient design of the stirrer blades needs to address the following issues

- Optimal mixing of the heterogeneous biomass,

- Highest pumping capacity possible in order to achieve a continuous motion in the entire fermenter, and

- Avoidance of agglomeration of substances at the stirrer, which impairs its performance.

An investigation and optimization of stirrer blades employing three-dimensional numerical simulations offers considerable savings in cost and time compared to experimental testing.

However, biomass exhibits a strong non-newtonian behavior. Unlike water, its local viscosity varies strongly depending on the local shearing imposed by the stirrer. Therefore it is inevitable to employ appropriate material models in the simulations of biosubstrates being agitated in fermenters. Figure 1 shows the large variations of the local viscosity of biosubstrates in the wake of a stirrer, which can change by almost one order of magnitude.

A main aspect of a blade optimization for stirrers is to improve mixing characteristics and pumping capacities at an unchanged level of electrical power supply. Besides the well-known analysis of surface pressure and velocity distributions at the blade, the axial velocity in the wake of the stirrer, see Figure 2,  also gives valuable information, which has been used to improve the blade design towards a higher pumping capacity.

For an assessment of the substrate mixing, vortices in the wake of the stirrer are analyzed, see Figure 3. By comparing different vortex systems for varying blade geometries an optimal stirrer configuration has been selected for a given power supply.