Everything you need to know about biogas at a glance - for producers and investors

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What is biogas? (Definition, explanation of terms)

Biogas is a combustible, colourless and pungent (rotten egg) smelling gas that is produced when biomass of any kind rots or ferments. Both rotting and fermentation are natural processes in which the organic material is decomposed in the absence of air. The decomposition takes place in a technical plant for the production of biogas: a so-called biogas plant.

Biogas is therefore a fuel that can be burned to generate heat and/or electricity. And with this, the difference between renewable energies with biogas on the one hand and sun, water and wind on the other becomes immediately clear: biogas has to be burned in order to make the energy stored in it usable. Combustion as such is not without controversy from an ecological point of view, but more on that later.

Which biomass can be processed into biogas?

In such biogas plants, various raw materials are processed into biogas. These can be
        - organic waste on the one hand 
        - and renewable raw materials.
The raw materials are also called biomass. An alternative term is substrates.

These include, for example: Wood, so-called energy crops such as maize or maize silage (silage from the whole maize plant), rape or sunflowers, and plant waste, for example so-called green waste. Biogas can also be produced from organic waste, which is collected in organic waste bins all over the country. In addition, liquid manure and dung from cows, pigs or horses are also used as biomass for biogas production.

Which biomass is most commonly used for biogas production in Germany?
According to the German Maize Committee (Deutsches Maiskomitee e.V., DMK), which refers to data from the Agency for Renewable Resources (Fachagentur Nachwachsende Rohstoffe e.V., FNR), among others, about 46 per cent renewable raw materials (NawaRo), 49 per cent liquid manure and dung, 3 per cent municipal biowaste and 2 per cent residues from industry, trade and agriculture were used in German biogas plants in 2019.  

The DMK states that maize is the dominant crop among the renewable raw materials, and that its yield potential is unmatched by any other plant on favourable sites. However, on marginal sites, i.e. high altitudes and arid regions, maize loses its excellence and whole-plant cereal silage (GPS) can therefore bring economic advantages over maize. In grassland regions, on the other hand, grass silage is also gaining competitive strength due to low rental prices and the resulting low costs.

How is a biogas plant constructed and how does it work?

The biomass is usually collected in a so-called pre-pit (or collecting basin). These have come via various reception points (silos, slurry pits), to which sorting and cleaning systems are attached in some cases. It is important here that there is no mixing of animal excreta (slurry, manure) and biowaste in the preliminary pit.
This could reduce the efficiency of the system and would not comply with the requirements of animal disease and hygiene legislation. In addition, according to legal regulations, possible pathogens must be destroyed before the collected biomass is transported from the pre-pit to the biogas plant.
This is done with heat (thermal pre-treatment) or with pressure (pressure sterilisation).

The preparation of the biomass also varies: it can be shredded, ground, crushed or otherwise comminuted. Shredding is important for the success of the fermentation because it increases the surface area for the bacteria to attack.
Then the biomass is mixed, i.e. homogenised with liquid manure, liquid fermentation residues or process water.
The heart of a biogas plant is its so-called fermentation tank, which is called a fermenter in technical jargon. In so-called mini-biogas plants, such as those used on farms, it is 500 to 1,200 cubic metres in size, circular and has a reinforced concrete foundation. The digester provides a light- and oxygen-free (anaerobic) environment in which countless bacteria biodegrade the substrates. This means that the water contained in the biomass and the organic compounds in it decompose, for example carbohydrates (sugar, starch, hemicellulose, cellulose), proteins, fats and others. Wet and dry fermentation are common methods of operation.

On the one hand
       - fermented residues on the one hand and a gas mixture on the other,
       - and on the other hand a gas mixture: biogas.

Agitators are used, usually with so-called submersible motors, which are immersed in the biomass through the outer wall of the fermenter. The agitator has several tasks to fulfil: It stirs and mixes the biomass slowly and steadily so that the sought-after biogas is optimally released from the mass. At the same time, the agitator distributes the heat. This is because a uniform temperature of around 40 degrees Celsius (°C) is one of the best working conditions for the bacteria involved.

Important to know: A biogas plant itself generates heat during fermentation. However, this is usually not enough, especially when it is cold outside. Therefore, a very good insulation of the plant is just as recommendable as the use of the heat of a downstream combined heat and power plant (in short: CHP) as process heat in order to increase the efficiency of the plant.
A distinction is made between single-stage and two-stage biogas plants. The latter ferment the biomass in two stages, first in the fermentation stage, then in the secondary fermentation stage. The duration of the fermentation process (retention time) is usually prescribed by law; 100 days is a proven period of time for the fermentation process to be completely finished.

The biogas is stored until it is used again, either internally, i.e. directly in the gas storage tank, or externally. The internal gas storage tank is located in the bonnet of the fermenter. Its storage volume should be at least a quarter of the daily production, so that fluctuations in gas production can be easily compensated. In addition, the storage tank must be able to compensate for pressure changes inside. So-called low-pressure storage tanks with an overpressure range of no more than 30 mbar have proved their worth. These are gas-tight foil bonnets that are supplemented with a second foil as weather protection above the actual sealing foil. The upper film is a so-called supporting air roof, where air is blown into the intermediate space by means of a blower to keep the upper film in position.

Regarding the digestate, you need to know that - depending on the biomass used - it is rich in humus-forming substances and nutrients, including easily plant-available nitrogen, phosphorus, potassium, sulphur and trace elements. Uncontaminated digestate can be used in agriculture as a high-quality organic fertiliser and can largely replace or supplement mineral fertilisers. The fermentation residues are stored in the fermentation residue storage of the biogas plant until their final use. They must be covered gas-tight to prevent the emission of methane, also known as greenhouse gas, which is the main component of raw biogas (see next question).

Depending on the condition of the fermentation residues, they are then brought to the fields in slurry barrels (liquid) or with manure spreaders (solid). It is also possible to dry the fermentation residues using the heat that is released during biogas production. If the digestate, in whatever form, is spread within a farm, the return of the digestate to the farm is an important part of the process.

What is biogas made of?

The composition of biogas depends on the biomass that is processed. Initially, i.e. before biogas upgrading, the resulting biogas, which is also called raw biogas contains,
        - Methane (CH4)
        - and carbon dioxide (CO2)
        - Often, Nitrogen (N2)
        - Oxygen (O2),
        - Hydrogen sulphide (H2S),
        - Hydrogen (H2)  
        - and Ammonia (NH3) are also present

Methane is considered to be valuable in the water-saturated biogas, and its share can be up to 65 percent. The higher the proportion, the more energy-rich the biogas. The water vapour, on the other hand, is not usable. Interfering factors in raw biogas are, in particular, hydrogen sulphide and ammonia. Therefore, biogas upgrading takes place before the raw biogas is burnt.
The two compounds are specifically removed in a biogas upgrading plant (BGA).

The result: people are less endangered. Odour nuisance is reduced. Corrosion in engines, turbines and downstream components (including heat exchangers) is prevented. CO2 (up to 50 percent) in the raw biogas is also a nuisance. For this reason, it is separated and recycled in certain applications.

What can the biogas be used for?

The biogas can be
- The biogas can either be purified and refined in a biogas upgrading plant so that it reaches natural gas quality and can then either be fed into a so-called natural gas filling station or into the natural gas grid. Natural gas is (still) an important fossil fuel for industry in Germany: more than a third of the natural gas used in this country each year is consumed in the sector, mostly for heat generation.
This is because various manufacturing processes depend on so-called process heat, for example glass production, porcelain production and steel production. As you can see, biogas could replace some of the fossil natural gas and thus ease the situation on the gas market (more on this later under the advantages of biogas).

- Or, and this use is much more common in practice, the biogas is burnt directly on site (i.e. directly connected to the biogas plant) in a combined heat and power plant. This usually uses an Otto engine, optimised for gas operation. The engine's task is to burn the biogas produced and to generate electricity directly via a connected electricity generator.
Heat is produced as a by-product.
Both electricity and heat can be used as an end product, either for own consumption or sold by the buyer or its customers. While the electricity is fed into the company's own grid or into the public grid, the utilisation of the heat is more complicated
The reason: considerable energy losses occur when heat is transported. Therefore, the heat is only suitable for use in a local heating network. The following applies: the better the heat can be used close to the biogas plant, the greater the degree of efficiency. Thus, large heat consumers such as swimming pools, hotels or shopping centres near the biogas plant are advantageous.

Important: In some cases, the CHP heat is fed back into the biogas plant to support the fermentation process (see above).
CHP gas engines are operated in so-called lean-burn mode with turbocharging so that as little nitrogen oxide as possible is emitted. The CHP is largely fully automatic. But: A CHP is quite noisy.
Therefore, the location of the CHP unit must meet noise protection requirements. Well-insulated containers with integrated air supply have proven their worth. As an alternative to the classic gas Otto engine, ignition jet or Stirling engines as well as micro gas turbines and fuel cells are used, albeit less frequently.
The latter are currently not very widespread because of the comparatively high investment in fuel cell technology.In the future, we will have to supply ourselves completely with renewable energy. In addition to the renewable energy sources of sun, water and wind, biogas will also play a role in the renewable energy mix.
If you are thinking about entering the biogas business as a producer or investor, you will find the answers to all the important questions about biogas in this article. With the knowledge at hand, you will make the right decision, we promise!

Why is biogas a renewable energy source?

Compared to fossil natural gas, biogas is produced from biomass. This consists of renewable plants and manure and slurry from animal husbandry.
This is why biogas is counted as a renewable energy source.

What role has biogas played in the German energy mix so far?

According to Statista.de, around 9,700 biogas plants were in operation in Germany at the end of 2020. The Federal Environment Agency (UBA) writes here that 50.4 billion kilowatt hours (kWh) of electricity were provided from biomass in Germany in 2021. Compared to 2020 (50.9 billion kWh), this was about 1 percent less electricity from biomass.

According to the report, the following were the main sources of electricity generation from biomass
        - Biogas (28.5 billion kWh),
        - solid biomass (11.4 billion kWh) and the biogenic
        - and the biogenic share of waste (5.6 billion kWh).

Overall, according to the UBA, electricity generation from biomass has been at about the same level over the last 5 years.
The installed capacity had increased by about one percent to 10,431 megawatts (MW) in 2021. Compared to 2016, the increase in installed capacity is a good 20 percent. However, the expansion of biomass plant capacity in recent years has primarily served to make electricity generation more flexible. This so-called "overbuilding" has hardly led to an increase in the amount of electricity generated annually in recent years, but it ensures that renewable electricity can be provided more flexibly according to demand (for example, in times of low wind and photovoltaic electricity generation).

According to BDEW, the majority of biogas plants in Germany use the biogas produced directly on site. According to the German energy and water industry association, this means that the biogas is converted into electricity and heat directly at the point of origin by means of a combined heat and power (CHP) process in a cogeneration plant.
Typically, internal combustion engines (gas engines) are used for this. The electricity generated is usually fed into the public grid and remunerated in accordance with the Renewable Energy Sources Act (EEG). The heat is used on site, fed into a local heating network or used to heat the biogas plant.

What is the basic potential of biogas?

The German Association of Energy and Water Industries (BDEW) has summarised the potential of biogas in its energy info "Gas kann grün: Die Potentiale von Biogas/Biomethan. Status quo, facts and development" (as of 2019) summarised it thus:

        - Biogas could make an important contribution to the energy transition as a flexible renewable energy. According to the report, it provides secure power and can be stored over long periods of time.
        - The potential of biogas has not yet been exhausted. The new diversity of biomass as a raw material offers water-saving possibilities for expansion. A cost degression is possible by bundling the feed-in and thus opening up other utilisation paths.
        - According to the BDEW, up to 10.3 billion m³ of biogas could be fed into the German gas grid annually by 2030, which would correspond to 100 terawatt hours (TWh).

To increase the biogas potential, BDEW proposes the following steps:
        - Optimise biogas plants in terms of processes, flexibility and biomass procurement.
        - Converting on-site electricity generation plants as widely as possible to biomethane feed-in. Sample calculations here would show potentials of up to 26 percent of gas sales in various regions
        - Use and development of further biomass feedstocks
        - Systemic connection of biogas upgrading and power-to-gas

BDEW also notes that the prerequisite for exploiting biogas potential is a significantly improved market. According to the industry association, no further expansion is foreseeable with the current framework conditions for biogas in the sales markets for electricity generation, heat and fuel. Market incentives, stabilisation of the regulatory framework and plant optimisation would together provide the potential for increasing decarbonisation in gas.

What are the advantages and disadvantages of biogas?

These are 6 advantages of biogas:
        - Biogas is considered CO2-neutral because when it is burned, only the amount of CO2 is released that the plants that have been fermented in the biogas plant have absorbed up to that point. According to this calculation, biogas does not emit any extra CO2. However, the production and use of biogas is not completely free of CO2 emissions. Greenhouse gases are emitted during the construction and operation of the plant, the storage and spreading of the fermentation residues, the harvest, the use of fertiliser and the driving of the tractors. Nevertheless, the bottom line is that electricity from biogas plants emits significantly less CO2 than electricity from fossil fuels.
          
For comparison:
        - Lignite-fired power plants cause over 1,000 grams of CO2 equivalent per kilowatt hour of electricity generated,
        - Biogas plants produce less than 250 grams of CO2 equivalent per kWh. If the heat generated by the biogas plant is also used, the balance is even more positive.
        - Biogas plants as an additional source of income for farmers
A biogas plant is an attractive source of income for farmers. Under the Renewable Energy Sources Act (EEG) of 2000, they receive a fixed feed-in tariff for the electricity generated from biogas, although they do not necessarily have to feed in the gas produced, but may also consume it themselves. And they can use the fermentation residues as fertiliser on their fields.

        - Biogas complements renewables independently of weather conditions
Unlike wind and solar energy, biogas can be produced independently of the weather and can also be stored quite well. It can therefore cover the base load and compensate for fluctuations in the grid.
        - Biogas is energy "Made in Germany
          Since biogas is "Made in Germany", long transport routes are avoided, added value and jobs are kept in the rural region, and at the same time a contribution is made to Germany's energy independence.
        - Biogas promotes the circular economy (cradle to cradle)
          With biogas, we recycle biological waste that we produce. This reduces the burden on the environment and makes fossil fuels less necessary.
        - With biogas we also obtain a fermentation substrate as fertiliser.
          Fermented manure smells less than normal manure and the smell also subsides more quickly. Plants absorb the fermentation residues better than normal manure, which improves the yield on the fields. In addition, the farmer saves on the cost of artificial fertilisers and avoids environmental pollution as a result of their use.

And these are the disadvantages of biogas
        - Energy crops tie up arable land.
          So-called energy crops are often cultivated as biomass for biogas production. This requires land, which increases the land competition resulting from the interest in land of the three sectors of settlement and transport, agriculture and renewable energy production.
        - Odour nuisance
          Many people who live near a biogas plant find its "exhaust gases" a nuisance.
        - Biogas production can pollute the climate
          In addition to biomethane, ammonia, hydrogen sulphide and other problematic substances can be produced during biogas production. If these get into the environment, they pollute the soil, groundwater and atmosphere.
        - Biomass transports cause CO2 emissions
         Large biogas plants need a lot of biomass, which is not always available locally. If biomass and slurry have to be transported over long distances by lorry, CO2 is emitted during the transport.