Acta Univ. Agric. Silvic. Mendelianae Brun. 2013, 61(5), 1269-1278 | DOI: 10.11118/actaun201361051269

Determination of mixing quality in biogas plant digesters using tracer tests and computational fluid dynamics

Luděk Kamarád1, Stefan Pohn2, Günther Bochmann1, Michael Harasek2
1 Dep. IFA-Tulln, Institute for Environmental Biotechnology, University of Natural Resources and Life Sciences, Vienna, Konrad Lorenz Straße 20, A-3430 Tulln, Austria
2 Institute of Chemical Engineering, Vienna University of Technology, Getreidemarkt 9/166, A-1060 Vienna, Austria

The total electricity demand of investigated biogas plants (BGP) makes up 7-8 % of the total electricity produced. Nearly 40 % of this energy is consumed just for mixing in digesters and the energy demand for mixing in some biogas plants can be even higher. Therefore, optimal mixing in anaerobic digesters is a basic condition for efficient plant operation and biogas production. The use of problematic substrates (e.g. grass silage or other fibrous substrates), installation of unsuitable mixing systems or inconvenient mixing intervals may lead to mixing problems. Knowledge about mixing in biogas digesters is still insufficient, so the objective of this study was to fill the information gaps in the literature by determining the minimal retention time of substrates fed into anaerobic digesters and to describe substrate distribution and washing out rates from investigated digesters. Two full-scale biogas plant digesters (2000 m3 and 1500 m3) using different mixing systems and substrates were investigated. To characterize the substrate distribution, lithium hydroxide monohydrate solutions were used for tracer tests at concentrations of 47.1 mg Li+ / kg TS and 46.6 mg Li+ / kg TS in digester. The tracer concentration in the digester effluents was measured during two hydraulic retention times and compared. Although the tracer was detected in the digester effluent at nearly the same time in both cases, the tracer tests showed very different distribution curves. The tracer concentration in effluent B grew much slower than in effluent A and no significant short circuiting streams were detected. Although the data calculated by computational fluid dynamics methods (CFD) showed a very good agreement with the full scale results, full comparison was not possible.

Keywords: mixing, digester, tracer tests, substrate distribution, CFD methods, biogas
Grants and funding:

This study was carried out in the frame of project AD-CFD, greatly funded by Klima- und Energiefonds and supported by the IFA Biogas Research and Consulting Group.

Received: October 31, 2012; Published: September 28, 2013  Show citation

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Kamarád, L., Pohn, S., Bochmann, G., & Harasek, M. (2013). Determination of mixing quality in biogas plant digesters using tracer tests and computational fluid dynamics. Acta Universitatis Agriculturae et Silviculturae Mendelianae Brunensis, 61(5), 1269-1278. doi: 10.11118/actaun201361051269
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