Flow velocities and flow profiles in a thoroughly mixed biogas fermenter

Abstract

The mixing of fermentation substrate using mechanical stirrers is considered one of the most important processes in biogas-production. However, this is associated with high energy use, resulting in high costs for the operator of the biogas facility. Lately, the improvement of the stirrers as well as the stirring-management has been attracting more attention. Numerous scientific studies attempt to examine and specify the rheology, biology and flow velocities in the fermenter using simulations and experiments on the laboratory scale. Nevertheless, the results still show considerable discrepancies between laboratory and practical outcomes. Studies on the practical scale were carried out at the research biogas facility of the University of Hohenheim in order to fill the gap between those research disciplines. The flow velocities at various spots in the fermenter were determined using a magnet-induced measuring system comparing different dry matter contents and viscosities of the fermentation substrate. The results show that a dry content increase from 7.74 to 10.75% during the experimental period is followed by a decrease in flow velocity from 8.71 to 63.77 cm · s-1 down to 0.05 to 37.36 cm · s-1. Therefore, the average flow velocities on the same stirrer setting were reduced due to the increase in dry matter content by an average of 70%. Complementary experiments proved that an increase of the dynamic viscosity can diminish the stirring induced circulation at the bottom and the surface to metrologically undetectable levels. This paper supports the thesis that the reduction of the flow velocities can cause “dead zones” in the fermenter.

Keywords

• biogas fermenter, flow velocity, mixing quality, mixing