Proposals for positioning local flow speed sensors around the circumference of a vertical biogas digester to monitor the mixing process are provided. The installation locations are selected based on a high correlation between flow velocity at the point of installation and a corresponding global mixing parameter. This approach is demonstrated for three different types of agitators.

Abstract

Mixing quality in agricultural biogas plants is crucial for nutrition supply to the microorganisms and needs to be ensured by an energy-efficient mixing strategy. Various approaches have been proposed to determine the mixing quality in biogas digesters. Among them are magnetic-inductive or force sensors brought into vertical digesters through the concrete wall. This article presents an approach to locating these sensors effectively around the circumference of the digester to avoid positioning in dead zones and maximize information quality regarding flow and mixing behavior. Particle image velocimetry is used to measure flow fields in a transparent model digester. Finally, optimum measurement locations considering three widely used agitator geometries are suggested.