Towards Lattice-Boltzmann modelling of unconfined gas mixing in anaerobic digestion

Davide Dapelo,Robin Trunk,Mathias J Krause,John Bridgeman

doi:10.1016/j.compfluid.2018.12.008

Davide Dapelo, Robin Trunk + Show 2 more

Open Access

https://doi.org/10.1016/j.compfluid.2018.12.008

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Abstract

A novel Lattice-Boltzmann model to simulate gas mixing in anaerobic digestion is developed and described. For the first time, Euler–Lagrange multiphase, non-Newtonian and turbulence modelling are applied jontly with a novel hybrid boundary condition. The model is validated in a laboratory-scale framework and flow patterns are assessed through Particle Imaging Velocimetry (PIV) and innovative Positron-Emission Particle Tracking (PEPT). The model is shown to reproduce the experimental flow patterns with fidelity in both qualitative and quantitative terms.The model opens up a new approach to computational modelling of the complex multiphase flow in anaerobic digesters and offers specific advantages, such as computational efficiency, over an analogous Euler-Lagrange finite-volume computational fluid dynamics approach.

Highlights

Sludge is usually treated via anaerobic digestion, in which it is mixed with anaerobic bacteria at 22–41 °C
The aim of the work reported in this paper is to demonstrate the first step towards the simulation of flow patterns inside an anaerobic digester using the Lattice-Boltzmann framework and, in particular, the first-ever LB model for anaerobic digestion is described and subsequently validated against experimental laboratory data
The presence of a solid phase alters the liquid phase rheology depending on the total solids content (TS) and the temperature [1], and gives rise to a wide series of non-Newtonian phenomena such as shear thinning and thixotropy [13]

Summary

Introduction

Sludge is usually treated via anaerobic digestion, in which it is mixed with anaerobic bacteria at 22–41 °C. As bacteria degrade sludge to more stable compounds, a methane-rich biogas is produced. This can be harnessed as a renewable energy source, usually through combined heat and power technology. Mixing is crucial for stable process operation and accounts for a large portion of a digester’s energy consumption (17–73%, Owen [29]). It is clear that mixing design and operation should be optimised to achieve a better balance between input mixing energy and output biogas yield. Kress et al [21] shown experimentally that it is possible to halve input mixing energy without impacting nutrient distribution, and without impacting biogas yield

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