Optimizing the performance of a large scale biogas plant by controlling stirring process: A case study

Highlights: • Five stirring scenarios inside large scale fermenters were studied and evaluated. • Stirring effect on biodegradable feedstock's homogeneity was examined. • Energy efficiency of large scale biogas plant was improved. • Very short and very long stirring periods should be avoided. • Optimum stirring time for this biogas plant: 3–5 min with pauses 25–30 min. -- Abstract: This work has investigated the effect of stirring on the performance of two anaerobic digesters for a period of six years. In each digester, two propellers and one hydro-mixer were installed. Five different stirring scenarios were tested and adopted for this large scale biogas plant. Agricultural residuals were used as feedstock for this biogas plant. The results showed that the optimization of the operating duration of the stirrers has led to an increase of the specific electricity yield and a reduction of the electricity consumed by the stirrers by 21.5% and 13.5%, respectively. Furthermore, the investigation found that the long turning off periods of the stirrers (45 min or longer) has led to a faster creation of sedimentation and lowering the biogas yield. The optimal operation durations were found to be 3–5 min with breaks of 25–30 min. The electricity yield and efficiency, as well as the homogeneity of the biodegradable feedstock in the fermenters, were used to evaluate the stirring scenarios. Moreover, a computational fluid dynamic model was used to assist in evaluating the stirring inside the fermenters at different total solid values and to examine the use of different mechanical stirring technologies on preventing the creation of dead zones.