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Developing the anaerobic digestion process through technology integration
Mälardalen University, School of Business, Society and Engineering. Mälardalen University, School of Business, Society and Engineering, Future Energy Center. (MERO)ORCID iD: 0000-0002-8268-1967
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Process optimization is needed for the development and expansion of the biogas industry and to meet the ever growing demand for methane. This thesis explores process technologies for the development of the anaerobic digestion process and includes pre-treatments, studies on the effects of different mixing modes and evaluation of a water treatment technology.

Two pre-treatments were evaluated, mechanical and electroporation, for treatment of ley crop silage. Mechanical treatment included two milling machines designed for recycling of paper, Grubben deflaker and Krima disperser, and showed an increased biogas production of 59 % and 43 % respectively as well as a positive energy balance and economic results.. Electroporation increased the biogas production with 16 %, however, development is needed to increase its energy efficiency.

Digester mixing has an effect on the digestion result. The performed review and experiments show that the mixing demand increases with organic loading. Excessive mixing during process start up, instabilities and shock loads leads to increased volatile fatty acid concentrations and process inhibition. Reduction of mixing reduces the effects of process instabilities and periodical mixing with mixing breaks has been shown to be beneficial for biogas production.

A high temperature membrane filtration unit was evaluated at 70 °C, 90 °C and 110 °C to determine separation efficiencies, permeation speed when treating process water at a biogas plant.  Improved separation can increase the capacity of the substrate pre-processing and reduce process related problems. The results show a total solids separation of 60 %, and an increasing filtration speed with temperature with fluxes of between 113 and 464 L/ h m2. The substrate pre-processing could theoretically handle up to 29 % more substrate as a result.

Integration of these technologies in a biogas plant show that the pre-treatments studied exhibits a good performance when integrated and that mixing reduction has the potential to lower the process electricity demand by 23 % in the performed case study. However, even though the membrane filtration unit shows promising results it would demand a relatively high energy consumption and lead to limited benefits to a process already at it maximum organic loading.

Place, publisher, year, edition, pages
Västerås: Mälardalen University , 2014.
Series
Mälardalen University Press Dissertations, ISSN 1651-4238 ; 165
National Category
Bioenergy
Research subject
Energy- and Environmental Engineering
Identifiers
URN: urn:nbn:se:mdh:diva-26081ISBN: 978-91-7485-166-3 (print)OAI: oai:DiVA.org:mdh-26081DiVA: diva2:753977
Public defence
2014-11-19, rum R2-025, Mälardalens högskola, Västerås, 09:00 (English)
Opponent
Supervisors
Available from: 2014-10-10 Created: 2014-10-09 Last updated: 2014-11-03Bibliographically approved
List of papers
1. Effects of mechanical pre-treatment on the biogas yield from ley crop silage
Open this publication in new window or tab >>Effects of mechanical pre-treatment on the biogas yield from ley crop silage
2012 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 97, 498-502 p.Article in journal (Refereed) Published
Abstract [en]

Previous studies on substrates for biogas production have shown that different types of pre-treatments make the material more accessible for microbial degradation by breaking down the complex structure of the organic material, thereby increasing their potential for gas production. In this paper, two different mechanical pre-treatment apparatus, i.e. a Grubben deflaker (Gd) and a Krima disperser (Kd), were tested in a full scale setup to evaluate their effects on ley crop silage. The treatments were investigated with regard to their effects on particle size, methane potential, capacity and energy balance. The results after 115 days of incubation in a batch assay show that methane production increased by 59% and 43% respectively after grinding with Gd and Kd. In both treatments, 90% of the ley crop was ground to particles of less than 2 mm and more than 50% of the sample was reduced to particles smaller than 0.125 mm. The energy balance was positive for Gd and around the break-even point for Kd. Analysis of the setup showed that Kd had almost twice the capacity of the Gd. If installed in the co-digestion biogas plant Vaxtkraft in Vasteras, Sweden, the Gd and Kd could increase annual biogas yields by 790 MW h and 585 MW h respectively. (c) 2012 Elsevier Ltd. All rights reserved.

Keyword
Mechanical pre-treatment, Biochemical methane potential, Biogas, Ley crop silage, Particle size
National Category
Engineering and Technology
Research subject
Energy- and Environmental Engineering
Identifiers
urn:nbn:se:mdh:diva-17725 (URN)10.1016/j.apenergy.2011.12.066 (DOI)000307196000057 ()2-s2.0-84862307407 (Scopus ID)
Conference
3rd International Conference on Applied Energy (ICAE), MAY 16-18, 2011, Perugia, ITALY
Projects
BioGasOpt
Available from: 2013-01-15 Created: 2013-01-15 Last updated: 2014-10-10Bibliographically approved
2. Evaluating the effects of electroporation pre-treatment on the biogas yield from ley crop silage.
Open this publication in new window or tab >>Evaluating the effects of electroporation pre-treatment on the biogas yield from ley crop silage.
Show others...
2014 (English)In: Applied Biochemistry and Biotechnology, ISSN 0273-2289, E-ISSN 1559-0291, Vol. 174, no 7, 2616-2625 p.Article in journal (Refereed) Published
Abstract [en]

Exploiting the full biogas potential of some types of biomass is challenging. The complex structures of lignocellulosic biomass are difficult to break down and thus require longer retention times for the nutrients to become biologically available. It is possible to increase the digestibility of the substrate by pre-treating the material before digestion. This paper explores a pre-treatment of ley crop silage that uses electrical fields, known as electroporation (EP). Different settings of the EP equipment were tested, and the results were analyzed using a batch digestion setup. The results show that it is possible to increase the biogas yield with 16 % by subjecting the substrates to 65 pulses at a field strength of 96 kV/cm corresponding to a total energy input of 259 Wh/kg volatile solid (VS). However, at 100 pulses, a lower field strength of 48 kV/cm and the same total energy input, no effects of the treatment were observed. The energy balance of the EP treatment suggests that the yield, in the form of methane, can be up to double the electrical energy input of the process.

Keyword
Electroporation, Pre-treatment, Ley crop silage, Anaerobic digestion, Pulsed electric field, Lignocellulose
National Category
Bioenergy
Research subject
Energy- and Environmental Engineering
Identifiers
urn:nbn:se:mdh:diva-26080 (URN)10.1007/s12010-014-1213-7 (DOI)000345331100023 ()2-s2.0-84916623709 (Scopus ID)
Available from: 2014-10-09 Created: 2014-10-09 Last updated: 2014-12-29Bibliographically approved
3. Effects of mixing on the result of anaerobic digestion: Review
Open this publication in new window or tab >>Effects of mixing on the result of anaerobic digestion: Review
2014 (English)In: Renewable & Sustainable Energy Reviews, ISSN 1364-0321, Vol. 40, 1030-1047 p.Article in journal (Refereed) Published
Abstract [en]

Mixing in an anaerobic digester keeps the solids in suspension and homogenizes the incoming feed with the active microbial community of the digester content. Experimental investigations have shown that the mixing mode and mixing intensity have direct effects on the biogas yield even though there are conflicting views on mixing design. This review analyzes and presents different methods to evaluate the mixing in a digester (chemical and radioactive tracers and laboratory analysis), tools for digester design (computational fluid dynamics and kinetic modeling) and current research on the effects of mixing on the anaerobic digestion process. Empirical data on experiments comparing different mixing regimes have been reviewed from both a technical and microbial standpoint with a focus both on full scale digesters and in lab-scale evaluations. Lower mixing intensity or uneven mixing in the anaerobic digestion process can be beneficial during the startup phase to allow for methanogenic biomass growth and alleviate process instability problems. Intermittent mixing has been shown to be able to yield a similar gas production as continuous mixing but with the possibility to reduce the maintenance and energy demands of the process. Problems often experienced with experimental design include the effect of mixing on the solids retention time, and measurement of steady state gas production because of startup instabilities. Further research should be aimed at studying the effects of mixing on a chemical and microbial level and on the different stages of anaerobic digestion (hydrolysis, acidogenesis, acetogenesis and methanogenesis). The focus should be on the effects of mixing on a multiple stage digestion process and also finding new methods to evaluate the effects of mixing in the one stage digestion process rather than evaluating a wider range of mixing modes, intensities and substrates.

Keyword
Anaerobic digestion, CFD modeling, Continuously stirred tank reactor, Intermittently mixed, Mixing, Tracer
National Category
Other Engineering and Technologies
Identifiers
urn:nbn:se:mdh:diva-25908 (URN)10.1016/j.rser.2014.07.182 (DOI)000345473600081 ()2-s2.0-84906834389 (Scopus ID)
Available from: 2014-09-12 Created: 2014-09-12 Last updated: 2015-01-19Bibliographically approved
4. The effects of different mixing intensities during anaerobic digestion of the organic fraction of municipal solid waste
Open this publication in new window or tab >>The effects of different mixing intensities during anaerobic digestion of the organic fraction of municipal solid waste
2014 (English)In: Waste Management, ISSN 0956-053X, E-ISSN 1879-2456, Vol. 34, no 8, 1391-1397 p.Article in journal (Refereed) Published
Abstract [en]

Mixing inside an anaerobic digester is often continuous and is not actively controlled. The selected mixing regime can however affect both gas production and the energy efficiency of the biogas plant. This study aims to evaluate these effects and compare three different mixing regimes, 150 RPM and 25 RPM continuous mixing and minimally intermittent mixing for both digestion of fresh substrate and post-digestion of the organic fraction of municipal solid waste. The results show that a lower mixing intensity leads to a higher biogas production rate and higher total biogas production in both cases. 25 RPM continuous mixing and minimally intermittent mixing resulted in similar biogas production after process stabilization, while 150 RPM continuous mixing resulted in lower production throughout the experiment. The lower gas production at 150 RPM could not be explained by the inhibition of volatile fatty acids. Cumulative biogas production until day 31 was 295. ±. 2.9, 317. ±. 1.9 and 304. ±. 2.8. N. ml/g VS added during digestion of fresh feed and 113. ±. 1.3, 134. ±. 1.1 and 130. ±. 2.3. N. ml/g VS added during post digestion for the 150 RPM, 25 RPM and minimally mixed intensities respectively. As well as increasing gas production, optimal mixing can improve the energy efficiency of the anaerobic digestion process.

Keyword
Anaerobic digestion, Continuous mixing, Continuously stirred tank reactor, Intermittent stirring, Mixing intensity, Municipal solid waste
National Category
Engineering and Technology Other Engineering and Technologies
Identifiers
urn:nbn:se:mdh:diva-25313 (URN)10.1016/j.wasman.2014.04.006 (DOI)000338601000006 ()2-s2.0-84901950694 (Scopus ID)
Available from: 2014-06-19 Created: 2014-06-19 Last updated: 2014-10-10Bibliographically approved
5. Membrane filtration of process water at elevated temperatures: a way to increase the capacity of a biogas plant
Open this publication in new window or tab >>Membrane filtration of process water at elevated temperatures: a way to increase the capacity of a biogas plant
2011 (English)In: Desalination, ISSN 0011-9164, E-ISSN 1873-4464, Vol. 267, no 2-3, 160-169 p.Article in journal (Refereed) Published
Abstract [en]

 Waste water from a biogas process is often recirculated and mixed with the incoming organic material to produce a feed for the digester. The dry matter (DM) content of the final mixture should be as high as possible to maximise the capacity of the plant without exceeding the capability of the pumps. This means that the DM content of the recirculated process water has a large impact on the amount of substrate that can be processed. Experiments to reduce the dry matter content of the recirculated process water were carried out using a ceramic ultrafiltration (UF) membrane. The influence on the flux through the membrane and the separation efficiency at different operation temperatures, 70°C, 90°C and 110°C, were investigated. Higher temperatures resulted in increased flux/flow through the membrane. The DM content was reduced from 4% to 1.6%, corresponding to a 29 % increase of new material that could be added to the process. The energy required to heat the membrane when using heat recovery is small compared to the energy of the methane produced from the additional added substrate. The lifespan of the membranes and uncertainties in the substrate DM content are showed to be important for the economic result.

Keyword
ultrafiltration, ceramic membrane, biogas, anaerobic digestion, digester, high temperature
National Category
Natural Sciences
Research subject
Energy- and Environmental Engineering
Identifiers
urn:nbn:se:mdh:diva-10635 (URN)10.1016/j.desal.2010.09.020 (DOI)000286851900005 ()2-s2.0-78650294233 (Scopus ID)
Projects
Biogasopt
Available from: 2010-10-28 Created: 2010-10-28 Last updated: 2014-10-10Bibliographically approved

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