CFD Modelling of Biomass Mixing in Anaerobic Digesters of Biogas Plants

Abstract

Cut in greenhouse gas emissions, increment of energy from renewables and improvement in energy efficiency represent the three key targets for future energy systems. Among the available bioenergy technologies, biogas production via biodegradation and anaerobic digestion is a widely applied approach, not only to produce biofuels but also to manage industrial and domestic organic waste. Within the biogas production, a sufficient mixing of the organic mass is a crucial step to ensure high biogas yields by bacteria and enzymes. Measurements of the electric power consumption of biogas plants revealed that the electrical energy demand of the stirrer system has a high share of the total electricity consumption of a biogas plant. Investigations on real biogas digesters to optimize the mixing process are cost and time intensive. Therefore, laboratory prototypes and computational simulations represent promising alternatives to analyse and improve the efficiency of mixing systems. In this paper, a computational fluid dynamics (CFD) model is presented, which is applied to commercial stirring systems. The case of two propeller stirrers, located in diametrically opposite positions in a tank filled with ca. 1400 m3 of substrate is described in detail. For the simulation, the rheology of the fluid is adapted to a biomass with 12 wt % dry matter content and obeying the non-Newtonian generalized Ostwald-de Waele power law. The developed simulation procedure considers the rotation angle of each propeller and its height. A total of 441 mixing configurations are calculated and evaluated in terms of the technical benefit. The investigation reveals that locations of the rotors far away from the bottom and high rotational angles cause advantageous fluid dynamics.

Evaluation Criteria and Benefit Analysis of Mixing Process in Anaerobic Digesters of Biogas Plants

“…1(b)). Therefore, a total number of 441 different configurations are possible [11]. In the present study, the 441 configurations are investigated to establish the design, which delivers the optimum operation results.…”

“…On the wall and bottom surface layers of the tank no-slip conditions were adopted, whereas the fluid surface on the top (transition to gas) was modelled with slip conditions [23]. Details of the modelling procedure and CFD setting are presents in [10], [11], [17].…”

Numeric Simulation‐Based Analysis of the Mixing Process in Anaerobic Digesters of Biogas Plants

“…Concentric to the central support, eight regions are in the top half (called t ) of the fluid region and eight regions are in the bottom half (called b ). Recently, we used the same sequence of volumetric regions to illustrate results of the CFD model, which are complementary to the present study .…”

“…Dependency of global fluid velocity v G (a) and z ‐component v G ,Z , (b) with the possible rotation angle α of the propeller. The profiles refer to eight bottom (#_b, dotted lines) and eight top (#_t, continuous lines) circular regions .…”

“…Additional details about the mesh strategy and specific figures, which illustrate the different mesh resolutions and the sequence of locally refined meshes around propellers and tank wall, are presented in ref. .…”