Monthly Archives: June 2015

Biogas – a way of storing renewable electricity?

ATBEST logo resized

 

Many of the technological barriers to producing renewable electricity have now been overcome, with methods to harness wind, solar and tidal power now established, but storage methods are required in order to improve the utilisation of this energy. In this blog, Nikoletta Giantsiou, ATBEST researcher at University Duisburg-Essen, discusses a method of using biogas to do so. 

Anaerobic digestion (AD) is a well – established biological process which results in organic pollution reduction and bioenergy production. During the process organic wastes are degraded with simultaneous energy production in the form of biogas. Biogas is a mixture of mostly methane (CH4) and carbon dioxide (CO2). Usually, biogas consists of 50 -70% CH4 and 30-50% CO2. Carbon dioxide in the produced biogas is inert as a fuel and dilutes its’ energy content.  The low energy density of biogas renders biogas an inefficient energy carrier for long – distance transportation and energy storage. The low CH4 content limits the application of biogas. Currently, the most common utilization route is for electricity production, often combined with utilization of the excess heat (CHP units). In worst cases, biogas is burned through a flare.

Figure 1: Biogas reactor with emergency flare

Figure 1: Biogas reactor with emergency flare

Alternatively, after the removal of CO2, biogas can be upgraded to natural gas quality. CH4 content higher than 90% can increase the heating value giving more utilization possibilities as a renewable energy source. Upgraded biogas (biomethane) can be used as vehicle fuel, or it can be injected into the existing natural gas grid transporting biomethanefrom rural areas, where typically biogas plants are located, to urban areas where consumer density is higher.This widens up the opportunities in distant energy consumption locations.

The most common methods of biogas upgrading include water scrubbing, pressure swing adsorption, polyglycol adsorption, membrane separation, cryogenic separation and chemical treatment.

Figure 2: Membrane separation (http://www.heatandpowerltd.co.uk)

Figure 2: Membrane separation (http://www.heatandpowerltd.co.uk)

Generally, these methods are performed outside the anaerobic reactor and require investments in external equipment.Therefore, the cost is relatively high.The main disadvantage is that small amounts of CH4 are also removed, which can increase greenhouse gas emissions.

To circumvent these disadvantages, biological conversion of CO2 to CH4 for biogas upgrading can be achieved. In anaerobic digesters, hydrogen can be converted to CH4 by the action of hydrogenotrophic methanogens according to the following Equation:

CO2 + 4 H2 → CH4 + 2 H2O       ∆H = −165.0 kJ/mol

This is the Sabatier reaction in which microorganisms can bind CO2 with H2 and convert them to CH4. Carbon dioxide is already consisted in biogas.

Figure 3: Sabatier reaction

Figure 3: Sabatier reaction

Nevertheless, in order for the whole process to be considered as renewable, the H2 needed for the biogas upgrading should also be provided by renewable sources.  Wind and bioenergy are two of the most promising renewable energy sources.

The power production from wind mills vary due to weather conditions. Electricity demand vary as well.In times of overproduction a high percentage of the electricity from wind is judged to be a surplus. The potential is not fully utilized. Water electrolysis is an attractive way of exploiting excess electricity from wind mills, in order to produce hydrogen.

Electrolysis is an electrochemical process which takes place by conducting electricity via electrodes through water. The two electrodes, the anode and the cathode, have a positive and negative charge, respectively. The charge difference results in an ionization of the water molecules into hydrogen and oxygen ions. At the negatively charged electrode, the cathode, the positively charged hydrogen ions (H+) gather. At the positively charged electrode, the anode, the negatively charged oxygen ions (O2-) gather.

Figure 4: Electrolysis of wind production (http://www.hydrogennet.dk)

Figure 4: Electrolysis of wind production (http://www.hydrogennet.dk)

 

The produced hydrogen can then be stored in the existing gas network or used for fuel cell vehicles. However, hydrogen is a very light gas and contains much lower volumetric energy content compared to CH4. Storage costs of hydrogen are consequently high. As an alternative, hydrogen may be used for upgrading biogas to natural gas quality and thus making it storable in the gas network.

With our research project we intent to use the excess energy of renewable sources such as wind mills in order to produce high CH4 content biogas. That way, in times of energy oversupply the excessive energy could be converted into a storable gas.

More specifically, our objectives are:

- supply hydrogen to the organic waste feeding stream of the anaerobic digester with the goal to convert carbon dioxide from biogas into methane in situ.

- modify the anaerobic configuration so that H2 is used to upgrade the biogas efficiency, and to

- optimize H2 consumption by the hydrogenotrophic methanogens in anaerobic reactors.

Figure 5: Use of surplus electricity to biogas plants via hydrogen

Figure 5: Use of surplus electricity to biogas plants via hydrogen

With in situ biogas upgrading an amount of CO2 can be consumed, thereby, result in higher methane biogas content. This partial CO2removal can decrease the costs of upgrading biogas to natural gas quality. Finally, possible unconverted hydrogen mixed with methane, would improve the combustion properties of biogas as fuel. The storage cost of methane is lower compared to H2, due to its’ higher boiling point and higher volumetric energy density. Additionally, a number of countries already have natural gas infrastructure, which would make distribution of upgraded biogas feasible.

Overall, the expected outcome of this project is to implement an efficient process for converting the excessive renewable energy into CH4. This process will result in increased net CH4 production for biogas plants, decreased biogas upgrading costs and the possible use of biogas as an alternative to natural gas. The main benefit will be the use of the existing infrastructure system for storing electricity.

References:

  1. [Online]http://www.hydrogennet.dk/
  2. [Online]http://vav.griffel.net/filer/C_SGC2013-270.pdf
  3. A Single-Culture Bioprocess of Methanothermobacterthermautotrophicus to Upgrade Digester Biogas by CO2-to-CH4 Conversion with H2, Matthew R. MartinJeffrey J. Fornero, Rebecca StarkLaurens MetsLargus T. Angenent, Archaea, (2013), doi: 10.1155/2013/157529
  4. The Bio-Sabatier-Process and its potential for the electric industry, Dr. Ing. S. Schmuck, MSc.; Dipl.-Ing. B. Keser; Dr.-Ing. T. Mietzel, ICCE-2010

 

 

 

 

 

“Where ideas come to life” – visit to Linkӧping

ATBEST logo resized

“Där idéer blir verklighet” – “Where ideas come to life”. This motto of city Linkӧping has been particularly appropriate since the formation (1975) of the University of Linkӧping (LIU) that is one of the ATBEST collaborators (from 2013). Linkӧping itself is a calm, average-sized city (ca. 150,000 inhabitants and 42 km2) in the south of Sweden. It has many interesting places to see – such as the locks of Berg on the Gӧta Kanal or Gamla Linkӧping (Old Linkӧping; fig. 1 and 2).

Fig 1: Old Linkӧping

Fig 1: Old Linkӧping

Fig 2: Old Linkӧping

Fig 2: Old Linkӧping

Linkӧping, like the rest of the country, tries to be environmental-friendly and close to the nature. This is reflected in the fact that Swedish establishment to produce 50% of the whole energy from renewables by 2020 has already been reached. Of course, some of this green-energy is produced from the biogas. Sweden has 264 (year 2013) full-scale biogas plants, amongst which the most abundant (more than 50% of all digesters) are reactors operating with sewage sludge as a feedstock. All reactors produce 1,686 GWh/year (data from 2013). Biogas in Sweden is mostly used as a vehicle fuel (>50%; ca. 200 filling stations – year 2012) and for heating purposes (>30%; year 2012 and 2013). Due to very beneficial financial policy (i.e. no energy tax or financial support for manure-operating anaerobic digesters), there is a huge possibility for the development of energy production from biogas in Sweden. It was proposed that the energy production development will have been 1-3 TWh (in the worst scenario), 58 TWh, or 5-10 TWh (in the best scenario) by 2030. This is very promising forecast.

Fig. 3. Full-scale anaerobic digesters in Sweden (divided by type of the feedstock) (2013; data from: Statens energimyndighet and Energigas Sverige (2014))

Fig. 3. Full-scale anaerobic digesters in Sweden (divided by type of the feedstock) (2013; data from: Statens energimyndighet and Energigas Sverige (2014)) 

Recently, myself (Joanna Grebosz, QUB) and my supervisor – Professor Michael Larkin (QUB) have had a pleasure to visit Linkӧping city for a two days. We loved it from the first sight, especially because the city welcomed us with a very warm and sunny weather as well as very friendly people (most speaking fluent English everywhere – impressive!). During our first day of the visit, we met researchers (fig. 4 and 7) connected with Linkӧping University (Annika Bjorn, Luka Safaric, Bo Svensson, Carina Sundberg, Sepehr Shakeri Yekta, Magali Genero Marti, Eva-Maria Ekstrand and Anna Karlsson) and Scandinavian Biogas Fuels (Francesco Ometto) in order to discuss our projects and any possibilities of the collaboration in our research that brought the positive results and conclusions.

Fig. 4. LIU, QUB and SBF researchers

Fig. 4. LIU, QUB and SBF researchers

Fig. 5. LIU campus

Fig. 5. LIU campus

 

Fig. 6. Linkӧping river

Fig. 6. Linkӧping river

Fig. 7. LIU, QUB and SBF researchers at the main square in Linkӧping

Fig. 7. LIU, QUB and SBF researchers at the main square in Linkӧping

Linkӧping University (Tema M laboratory) is focused mostly on the research of the trace elements in terms of biogas process optimization. Researchers are also interested in the optimal use of Swedish paper mill wastes, hydrolysis optimization (i.e. the use of enzymes) and the rheology (crucial role of trace elements – see Luka’s blog piece) in the anaerobic digestion process. They believe that the optimized trace elements addition might result in higher methane production even by 15-20% (ca. 100 GWh). LIU has an impressive research facilities that we were able to see on the second day of our stay in Linkӧping during the walking lab-tour guided by Annika Bjӧrn (fig. 8, 9 and 10). She explained the lab-work they’re doing and showed some of the lab-scale digesters they’re running at the moment. Also, LIU works closely with Scandinavian Biogas Fuels (SBF) which focuses on the best proportion of algae in the reactors in the co-digestion of the other feedstock at different temperatures. SBF is testing algae as a feedstock (in the co-digestion) in their 5 L mesophilic and thermophilic CSTR reactors (fig. 10).

Fig. 8. Oxygen-replacing machine at LIU

Fig. 8. Oxygen-replacing machine at LIU

Fig. 9. Anaerobic chamber at LIU

Fig. 9. Anaerobic chamber at LIU

 

Fig. 10. Annika Bjӧrn (LIU). In the background - CSTR thermophilic and mesophilic lab-scale anaerobic digesters (property of SBF)

Fig. 10. Annika Bjӧrn (LIU). In the background – CSTR thermophilic and mesophilic lab-scale anaerobic digesters (property of SBF)

The QUB co-operation with the researchers from LIU and SBF would be very beneficial for broadening the knowledge about the microbial community of anaerobic digesters – because that is the issue I’m dealing with. My project is connected with the genomic optimization of the hydrolysis step of anaerobic digestion process. Thus, I’m using metagenomic (454-pyrosequencing) and molecular biology (PCR, qPCR, RT-qPCR) techniques in order to analyze microbial content (and microbial population shifts as a result of changes in different AD parameters) in the reactors.

The visit in Linkӧping was very productive in terms of networking and planning the future co-operation between researchers from QUB, LIU and SBF. I’m really looking forward for the secondment in Linkӧping!

 

Acknowledgements:

I’d like to thank all of the researchers we’ve met in Linkӧping for very valuable discussions and kind help.

References:

1.  Statens energimyndighet, Energigas Sverige (2014) Produktion och användning av biogas och rötrester år 2013. Statens energimyndighet, Energigas Sverige [Online] http://www.energimyndigheten.se/Global/Statistik/officiell%20statistik/Produktion%20och%20anv%C3%A4ndning%20biogas%202013.pdf

2. Persson T, Baxter D (ed.) (2014) Task 37. Biogas Country Overview (country reports). IEA Bioenergy [Online] http://www.biogasportalen.se/BliProducentAvBiogas/MerLitteratur/~/media/Files/www_biogasportalen_se/BliProducent/Rapporter/Countryreportsummary2013.ashx

3. Dahlgren S (2013) Realiserbar biogaspotential I Sverige år 2030 genom rӧtning och fӧrgasning. WSP [Online] http://www.biogasportalen.se/BiogasISverigeOchVarlden/~/media/Files/www_energigas_se/Publikationer/Rapporter/BiogaspotentialSverige2030.ashx

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