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Flexible biogas plant operation – new concepts for stabilising bioenergy provision

The lack of flexibility with regard to peak demand for electricity – both for consumers and producers – is a well-known problem as far as the production of electricity from renewable resources is concerned. Biogas plants present a particular challenge due to the complex and relatively slow microbial processes involved. A research project called FLEXIZUCKER at the Universities of Ulm and Göttingen aims to make biogas production more flexible and hence the supply of renewable electricity more grid- and market compatible.

In 2016, the amount of electricity generated from biomass covered 8.6 percent of Germany’s gross electricity consumption (3.6 percent in Baden-Württemberg). The proportion of all renewable energies combined is around three times greater. However, developments in recent years have shown that biomass is increasingly used for energy production. The amount of electricity produced from biomass has increased 10-fold since 2000.1

Info box: Biogas plants6

A biogas plant is used for fermenting organic residues. This means that a broad range of different microorganisms decompose biomass in the absence of oxygen (anaerobic digestion). In addition to organic agricultural substrates such as liquid manure, biological waste and energy crops can also be used as substrates for generating biogas. Easily digestible biomass constituents such as carbohydrates and proteins are metabolised by the microorganisms. This anaerobic breakdown leads to the release of biogas. Substrate components such as lignin, cellulose and sand that are difficult to digest (digestate) contain nutrients such as nitrogen and phosphorus and can be used as agricultural fertilisers. Biogas quantity and composition varies with the chemical composition of the organic feedstock. The most important factor associated with biogas production is the amount of methane produced. The higher the quantity of methane, the higher the calorific value and thus the energy content of the biogas. This is of particular advantage if biogas is used for producing electricity (info box 3). Biogas consists primarily of methane (40 – 75 percent) and carbon dioxide (25 to 55 percent) as well as small quantities of problematic interfering gases such as hydrogen sulphide or ammonia. The aim is to obtain the highest possible quantities of biogas with the highest possible methane content.

The secure remuneration of electricity made from biomass up until 2020 as part of the German Renewable Energy Sources Act (EEG) has certainly played a decisive role in this development. However, since the publication of the latest EEG version in 2017, it has become clear that the funding of old biogas plants will come to an end.2,3 Competitive new concepts for producing electricity from biomass without government funding will be needed to prevent a massive dismantling of old biogas plants once funding has come to an end. 4

One of the major competitive advantages of biogas over electricity generated with wind power or photovoltaics could be the ability to generate electricity flexibly. The production of electricity from renewable resources is still subject to strong fluctuations on the part of suppliers. These fluctuations do not always correlate with fluctuations in demand, and sometimes even run counter to demand. This often leads to supply gaps, which need to be bridged with electricity from fossil resources. While the performance of wind power and photovoltaic (PV) plants depends directly on unpredictable climatic factors such as wind speed and solar radiation, the production of biogas can be controlled relatively well via the amount of substrate that is fed into biogas plants.

However, in practice, biogas production can only be adapted to demand in the long term. Short-term fluctuations are hard to counteract due to the sensitive microorganisms contained in the biogas reactors.5 Nevertheless, unlike PV and wind power plants, biogas has the decisive advantage that it can be produced continuously and at any time. Biogas can also be temporarily stored and turned into electricity at a later date. Thus, electricity produced from biomass makes an important contribution to closing the supply gaps that arise at peak times or when production bottlenecks occur.3

FLEXIZUCKER – flexible electricity production thanks to sugar beet

Group of researchers led by Prof. Dr. Marian Kazda at the University of Ulm standing in front of their laboratory-scale biogas plant. Kerstin Maurus, Wiebke Karad, Sharif Ahmed, Prof. Dr. Marian Kazda (from left to right). © Prof. Dr. Marian Kazda, Ulm University

In order to improve the flexibility of electricity production from biogas, researchers from the Universities of Ulm and Göttingen have joined forces in a research project aimed at developing new biogas plant feeding concepts. A project called FLEXIZUCKER, which has been running since last year, uses sugar beet silage to make electricity production flexible. The idea came about when the researchers realised in preliminary experiments involving maize as substrate that the methane content of biogas rose unnaturally immediately after sugar beet silage was added. “This effect has huge potential for the short-term and demand-oriented adaptation of biogas production,” says Prof. Dr. Marian Kazda from the University of Ulm. "The use of sophisticated sugar beet silage feeding management based on peak-time feeding has the potential to reliably cover the morning and evening gaps in renewable energy power supply.” This would make biogas plants more attractive again and enable them to self-finance once feed-in tariff remuneration ends. Kazda comments: “Problems faced by biogas production plants such as high investment and maintenance costs, relatively poor land area balances and expensive substrates cannot be ignored.” He further highlighted that expanding their key advantages, i.e. storage capacity and flexibility, is equally important.

The first part of the three-year project has already been completed. The researchers now have a fairly accurate picture of how much sugar beet silage is needed to achieve a specific increase in biogas methane content. A group of researchers led by Prof. Dr. Rolf Daniel at the University of Göttingen will now focus on identifying the microbiological processes behind the rapid rise, and why this rise occurs immediately after the addition of sugar beet silage to the organic waste in the fermenter. “I think it is just like in humans. When we eat glucose, our blood sugar level immediately rises too. Maybe a similar mechanism occurs in biogas plants when easily accessible sugar from sugar beet silage becomes available,” suggests Kazda. The analyses of the Göttingen researchers will provide further details. In the third year of the ongoing project, the researchers will also carry out economic analyses to ensure that the new feeding concept will be economically viable.

Infobox 2: Which processes are carried out in a biogas plant?

The conversion of biomass into biogas and fermentation residues in biogas reactors is down to a variety of microorganisms whose metabolic activity depends on external conditions such as temperature, biomass water content and substrate pH. Practice has shown that the microorganisms of biogas plants prefer stable parameters. This is why the most important parameters are monitored 24/7.

Biomass digestion is divided into four phases:

1) Hydrolysis: During hydrolysis, polymeric macromolecules (carbohydrates, fats, proteins) are broken down into their short-chain monomers (e.g. monosaccharides, amino acids and fatty acids) by enzymes derived from certain microorganisms.

2) Acidogenesis: Acidifying microorganisms break down the hydrolysis products into short-chain organic acids and alcohols (e.g. butyric and propionic acid), hydrogen sulphide and ammonia.

3) Acetogenesis: Certain acetate bacteria convert the acidogenesis products into acetic acid and hydrogen; the pH decreases.

4) Methanogenesis: This final phase, which is the most important one, involves producing methane (CH4) and carbon dioxide (CO2) from acetic acid using methanogenic archaea. At the same time, elementary hydrogen (4H2) and CO2 are turned into methane and water.

Bioeconomy in practice – grid-compatible electricity and biogas from biomass

Last but not least, the researchers are hoping to make a small contribution to the bioeconomy in Baden-Württemberg. Kazda is convinced that biogas will not only play an important role in the flexible generation of clean electricity in the future but he believes that, in view of the current diesel scandal, the trend towards alternative fuels in the automotive sector will also continue growing. Biomethane would be a green and clean alternative to fossil natural gas. "Our goal is to design something akin to a recipe, a recommendation for plant operators, which will provide them with information on how much sugar beet silage is needed to achieve a certain biogas production capacity and quality,” says Kazda. If the researchers manage to make biogas production highly flexible, it would be possible in practice to obtain a reliable supply of electricity and fuel on demand from local biomass. This would be the "bioeconomy at its best".

Conversion of biomass into electricity and local heat. The fermenter is shown in the centre of the schematic. Substrate (left) flows into the fermenter. Biogas produced in the fermenter is transported to a block heat and power plant where it is converted into electricity and heat. A second biogas stream enters a gas purification module which turns biomass into biomethane that can be fed into the natural gas grid. The heat from the block heat and power plant can be used for heating houses while the electricity can be fed into the power grid. The fermentation residues will be used as fertilisers in the agricultural sector where some of the biogas plant substrate is once again produced. © Viola Hoffmann

Info box 3: Utilisation phases of biogas?

Biogas that is currently produced is suitable for two utilisation routes. One route involves the direct generation of electrical power via the combustion of biogas in a combined heat and power plant (CHP). CHPs combine electricity and heat from a single fuel type and thus achieve efficiency levels of up to 95% (depending on the combustion engine used). The electricity generated can be fed into the power grid and is remunerated as stipulated in the German Combined Heat and Power (CHP) Act. The waste heat resulting from the combustion process is partly returned into the biogas process to heat the fermenters. The rest can be fed into the local heating network.

Another interesting, though somewhat more costly, utilisation route involves converting biogas into biomethane and subsequently feeding it into the natural gas network. Biomethane can then be used as fuel for natural gas vehicles. However, due to the complexity of biogas plant technology, the production of biomethane is currently only profitable for large biogas plants.


1 https://www.bundesverband-bioenergie.de/themen/strom

2 https://www.erneuerbareenergien.de/biomasse-strom-auch-nach-2020/150/437/97063/

3 http://www.energieatlas-bw.de/biomasse/hintergrundinformationen/energieerzeugung-aus-biomasse

4 http://www.fnr-server.de/ftp/pdf/berichte/22400815.pdf

5 https://mediathek.fnr.de/media/downloadable/files/samples/s/c/schriftenreihe_band_32_web_neu.pdf

6 https://mediathek.fnr.de/leitfaden-biogas.html






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