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The wet fermentation biogas process: Limitations and possibilities for efficiency improvements
Mälardalen University, School of Sustainable Development of Society and Technology. (MERO)ORCID iD: 0000-0002-8268-1967
2010 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The biogas process is known as an environmental friendly and sustainable way of producing energy and fuel but to be fully commercially competitive with other types of processes, efficiency improvements are needed. By doing a case study at the Växtkraft biogas plant in Västerås, Sweden, three specific limitations were identified and studied. Firstly, to improve the capacity of the plant, pre-treatments of the different substrates are needed to disintegrate the substrate and by doing so increasing the gas yield and the speed in which it is produced. Secondly, to improve the fermentation process itself more knowledge is needed around the mixing inside the digester. To be able to create an optimal and stable environment for the microorganisms the mixing is the key, because the mixing affects the mass transfer of all solids, nutrients, gases and other substances in the digester. Thirdly, the water treatment of the recirculated process water cannot reach the desired separation of dry matter (DM) and this is affecting the capacity of the plant negatively. The feed for the digester is produced by mixing the process water and the substrate to get a pumpable slurry with a DM content of 8-10 %. When there is too much DM in the process water to begin with, the mixing ratio between the substrate and the liquid changes, decreasing the amount of substrate that can be added to the mixture and later on fed to the digester.

 

The full biogas potential of most organic materials cannot be extracted during the relatively short retention time of most digesters because of their complex structures. The organic materials are broken down too slowly and the nutrients cannot become biologically available in that time span. This means that a lot of the bound energy in the organic material leaves the biogas plant with the liquid digestate. The efficiency of the process can be improved by pre-treating the material before digestion. Pre-treatment experiments to disintegrate ley crop silage using electroporation, a treatment using electrical fields, were conducted to study its effect on the biogas yield. The experiments resulted in up to twice the amount of biogas being produced from the pre-treated material compared to untreated material.

 

Numerical simulations of the mixing inside a digester were carried out to understand the effect that a gas lift mixing configuration has on the mass transfer in the system. The mixing dynamics were evaluated by testing five different flow rates of the injected gas and the effect that the liquid recirculation system has. The results indicate that there are large unmixed zones and that changing the gas flow rate only has a marginal effect on these areas. The simulation also suggests that the outlet of the liquid recirculation system is situated too close to the gas injectors, resulting in energy losses in form of diminished mixing of the digester.

 

Experiments to reduce the DM content of the recirculated process water were carried out using a ceramic ultrafiltration membrane. The flux through the membrane and the separation efficiency were investigated at different operation temperatures, 70°C, 90°C and 110°C. The results show that 59-63 % of the DM was separated in this temperature interval and that the flux/flow through the membrane increased with the temperature. These results correspond to a 29 % increase in the capacity to add new substrate. The energy required to heat the membrane, if heat recovery is used, is small in comparison to the increased methane yield.

 

In the best case scenario these above identified improvements could increase the methane yield by up to 40%.

Place, publisher, year, edition, pages
Västerås: Mälardalen University , 2010.
Series
Mälardalen University Press Licentiate Theses, ISSN 1651-9256 ; 125
Research subject
Energy- and Environmental Engineering
Identifiers
URN: urn:nbn:se:mdh:diva-10637ISBN: 978-91-86135-96-6 (print)OAI: oai:DiVA.org:mdh-10637DiVA: diva2:359473
Presentation
2010-12-14, Kappa, Mälardalens högskola, Västerås, 14:00 (English)
Opponent
Supervisors
Projects
Biogasopt
Available from: 2010-11-09 Created: 2010-10-28 Last updated: 2013-12-19Bibliographically approved
List of papers
1. ON MODELLING THE MIXING IN A DIGESTER FOR BIOGAS PRODUCTION
Open this publication in new window or tab >>ON MODELLING THE MIXING IN A DIGESTER FOR BIOGAS PRODUCTION
2009 (English)In: / [ed] I. Troch, F. Breitenecker, 2009Conference paper, Published paper (Other academic)
Abstract [en]

At the Vaxtkraft biogas plant the mixing is produced by pumping in biogas and releasing it at the bottom. The mixing inside the digester of a biogas plant is important for good biogas production and since it is complicated to study the mixing inside the digester while it is in operation, this study is based on numerical simulations using a computational fluid dynamic finite volume code. To study the mixing dynamics, five different flow rates of gas (air) injections ranging from 0.1 to 0.6kg/s were simulated. These gas flow rates produced an average liquid speed in the digester between 0.10 and 0.22 m/s. The liquid recirculation impact on the mixing was investigated through the simulation of a case where it is combined with the lowest gas injection flow rate. The results from the simulation suggest that the liquid outlet is situated too close to the gas injection, resulting in energy losses in form of diminished mixing of the digester. A complete redesign of the digester is needed to seriously overcome the mixing limitation.

Keyword
Biogas, Digester, Mixing, CFD
National Category
Energy Systems
Research subject
Energy- and Environmental Engineering
Identifiers
urn:nbn:se:mdh:diva-7630 (URN)
Conference
MATHMOD 09, 6th International Conference on Mathematical Modelling, Vienna, Austria, 2009
Projects
BioGasOpt
Available from: 2009-12-03 Created: 2009-12-03 Last updated: 2016-01-11Bibliographically approved
2. Pre-treatment of Substrate for Increased Biogas Production
Open this publication in new window or tab >>Pre-treatment of Substrate for Increased Biogas Production
Show others...
(English)Article in journal (Refereed) Submitted
Abstract [en]

The full biogas potential of most organic material cannot be exploited with today’s technology. The complex structures of the organic materials are broken down too slowly and the nutrients cannot become biologically available during the relatively short retention time of most digesters. This means that a lot of the bound energy in the organic material leaves the biogas plant with the liquid digestate. There is a possibility to increase the efficiency of the process by pre-treating the material before digestion. This paper explores a pre-treatment of ley crop silage using electrical fields, known as electroporation (EP). Different settings of the EP equipment were tested and the results were analysed using a batch digestion setup. The results from the experiments show that there is a possibility to double the biogas production at typical retention times of a CSTR. The energy balance of the EP equipment suggests that the pre-treatment yield is around 2-6 times larger than the energy input to the process, i.e. energy in the form of methane.  

Keyword
electroporation, pre-treatment, ley crop silage, anaerobic digestion, biogas, optimization
Research subject
Energy- and Environmental Engineering
Identifiers
urn:nbn:se:mdh:diva-10636 (URN)
Projects
Biogasopt
Available from: 2010-10-28 Created: 2010-10-28 Last updated: 2016-02-25
3. 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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