Biogas plants: flexible production of renewable energy from biomass
Biogas technology uses environmentally friendly natural processes
Objectively, biogas is considered a flammable byproduct of the anaerobic fermentation of organic matter. Biogas is one of the most flexible elements in the mix of renewable energies. It can be stored in the form of heat or electricity so that the energy can be used when it is needed. This flexibility makes biogas an ideal supplement to solar or wind energy, which both fluctuate in reliability and are difficult to regulate.
Vogelsang provides reliable components for the economic production of biogas. From pump technology to maceration, disintegration technology and solid matter feeding systems, Vogelsang offers a comprehensive portfolio of biogas technology for the entire range of fermentation. This is why we see ourselves as partners in the biogas sector. Together with our customers, we are continuously developing our biogas technology, adapting it to ongoing changes in the underlying conditions to deliver solutions for the efficient production of biogas. An important aspect of this is the individual analysis of every single biogas plant. This is the only way to find and fully realise potential for optimisation.
International biogas market
Today in Ireland there are over 50 biogas plants that mainly recycle agricultural and food waste. Across Europe there are over 17,000 with approximately 10,000 of these biogas plants in Germany providing power to the population. The average size of a biogas plant differs from country to country based on the type of biomass used. This means that in some countries, small biogas plants are particularly encouraged. In Ireland, biogas plants are typically 500kW. However, larger industrial plants are operating up to several megawatts (MW). The input materials range from recycled renewable materials to household organic waste, food waste and sludge.
Applications on biogas plants
A biogas plant is constructed so that it provides optimal conditions for biogas production: The biomass is kept in large containers ("digesters") where it ferments in the absence of light and oxygen. This anaerobic fermentation takes place in four stages, during which each specialised bacterium converts the biomass step by step, producing the biogas. These “steps (also called “phases”) are as follows:
- First step: Hydrolysis (digestion of raw materials)
- Second step: Acidogenesis (acidification)
- Third step: Acetogenesis (formation of acetic acid)
- Fourth step: Methanogenesis (formation of methane)
The biogas itself consists mainly of methane and carbon dioxide, but also of numerous other gases such as hydrogen sulfide, nitrogen, oxygen and ammonia. At the end of the fermentation process, you are left with the "digestate" fermentation residue, which can be used in agriculture as fertiliser. This completes the natural cycle; with no other usable by-product, biogas is highly sustainable.
Biogas: the concrete cow
Considering the process steps described above, there is great similarity to the digestive system of a cow - which is why a biogas plant is also known as a concrete cow. In both the biogas plant and the digestive systems of ruminants, bacteria convert biomass into energy. The process in modern biogas plants known as "substrate preparation" is the equivalent of the cow chewing. Hydrolysis takes place in the cow's first stomach (the rumen). The acidification and formation of acetic acid then takes place in the omasum and abomasum, while the energy is generated (i.e. absorbed into the cow's body) in the small and large intestines, as well as the appendix. Another striking similarity is in the solid input material (substrate). Like their cows, many farmers feed their biogas plants with renewable materials such as corn silage and grass silage.
The elements of a biogas plant
Important core elements of every biogas plant are as follows:
- Substrate / solid matter feeding
- Digester tank
- Mixers for mixing in the digesters
- Gas storage
- Fermentation residue storage for the remaining fermented material
- Biogas utilisation is generally used for a cogeneration unit or more rarely for gas processing and storage in the network
A cogeneration unit is a gas engine that is coupled with a generator that generates electricity (power) from the biogas energy. Mixers are necessary, because only they can create optimum conditions for the generation of biogas when the digester is mixed well and evenly. The goal of solid matter feeding is to introduce the solid biomass into the digester. Ideally, this is done with low energy consumption and trouble-free operation, regardless of the type of biomass.
The solid matter that can be used in a biogas plant depends on several factors. Basically, the organic content should be very dense, since only this can be converted into biogas. The retention time must also match the solid matter used. Retention time refers to the average time that the biomass spends in the digester until it flows out. If this time is too short, then the bacteria does not have enough time to break down most of the solid matter as needed. As a result, the energy contained in biomass is not fully utilised. From a technological perspective, this is not a problem, but it does impact the cost-effective operation of the biogas plant. High energy input materials also do not automatically provide the most economic yield. It is also necessary to consider the costs of the input material and the actual energy that it can provide. Finally, official permission must be requested for the selected biomass.
Wet fermentation and dry fermentation - the two fermentation methods
Biogas plants are often categorised according to different aspects. For example, a distinction is made between wet fermentation and dry fermentation. During wet fermentation, the solid matter is mixed with a liquid. The resulting organic suspension is usually flowable and is moved using pumps. During dry fermentation, stackable biomass is packed into a "box digester" or "garage digester" and then sprayed with a liquid (the "percolate"). Percolate leaking out of the bottom is collected and re-applied to the top of the biomass. This facilitates the fermentation process and therefore enables biogas production. This type of plant is not so widely used and is primarily used for the fermenting of organic waste such as green cuttings or food waste from households.
Biogas plants and bio-methane plants – it's what comes out that matters
Another distinction is biogas plants versus bio-methane plants. Both plants produce biogas. In biogas plants (the most common type in Germany), this biogas is processed a little and then converted directly into electricity and heat in a nearby cogeneration unit. Bio-methane plants process the biogas so thoroughly that – like natural gas – it consists almost entirely of methane. This bio-methane can therefore be fed directly into the natural gas network where it can be transported and burned where the energy is needed.
Waste plants: sensible use of organic waste
Another common classification is based on the biomass used. It distinguishes between cofermentation plants or waste plants and renewable material plants. The term "cofermentation plant" goes back to the early days of the biogas boom in Germany. Farmers built systems in which they wanted to generate and use the energy that was still in the slurry in the form of biogas. They soon realised that they could produce much more biogas if they also added cosubstrates to the slurry, such as uneaten food, grains, or organic waste material. This gave rise to the term cofermentation plant for biogas plants that ferment industrial residues and waste materials as well as agricultural waste such as slurry and dung. As time went on, the proportion of these agriculturally based biogas plants that ferment waste materials decreased sharply. Instead, many industrial plants were built for the management of municipal, commercial and industrial waste. During this development, the term "waste plant" came to be used for biogas plants where organic waste is fermented.
Renewable material plants - what are they and what makes them different
The term renewable material plant came from a period when the government decided to promote the fermentation of renewable materials, such as energy crops in particular. In addition to agricultural waste, these biogas plants may only ferment renewable raw materials. Agricultural waste is usually dung and slurry. Typical renewable materials (energy crops) are:
- Corn (maize) and grass silage,
- GPS (whole crop silage)
- Crops such as potatoes and turnips, etc. provided they are/were not intended for food production.
When all these points are taken into account, suitable solid matter feeding technology that can feed the biomass cost effectively must be installed in the biogas plant. Liquid waste from agriculture, such as manure and glycerin or residues from grease traps can usually be pumped directly into the digester using pumps. Because this often means the waste contains foreign matter such as stones, metal parts and impurities (e.g. wood, belts, etc.), we recommend the use of macerators and foreign matter separators to reduce the material to a manageable level.
When using input materials that are subject to hygiene regulations, a deep cleaning is required to ensure the destruction of any parasites, viruses or bacteria contained in the material that may pose a threat to the environment. This applies especially to substrates of animal origin such as abattoir waste and food waste. The most common procedure entails heating the input material up to at least 70°C for one hour, and then macerating it to a size of max. 12 mm. Proven systems for this process are contact-based macerators combined with positive displacement pumps. Vogelsang positive displacement pumps often pump viscous and abrasive waste safely, while a 12 mm honeycomb sieve in the RotaCut macerator ensures that all elements are reduced to the maximum permissible size. Special options ensure that no input material gets past the macerator untreated.
Feeding process for non-flowable biomass
Biogas plants are mainly fed with non-flowable solid matter (also called substrates). The feeding processes can be roughly divided into two categories: Dry solid matter feeding and liquid feeding. For dry solid matter feeding, the input materials are put directly into the digester. Feed screws are the most commonly used method, conveying the solid matter upwards and onto the organic suspension. The main advantage of this technology is the relatively low investment costs. However, the solid matter must be stirred by the mixers in the digester, which is only possible at very high energy levels. Also, this technique is not fully suited for long-fibred substrates (e.g. car grass, straw, or agricultural waste such as dung).
For liquid feeding, the biomass is first mixed with a liquid suspension, the digestate or so-called recirculate from the digester. This process is also called mashing. The resulting feed mash is then conveyed to the fermentation space (this usually means pumped). The advantages of this procedure are that the biomass arrives in the digester in liquid form and mixing requires significantly less energy consumption with a digester content that mixes well with the organic suspension. If the solid matter is also processed simultaneously, the reduced energy requirements are also accompanied by increased gas generation. This also significantly reduces the widespread problem represented by the formation of floating layers.
Alternatively, there are liquid feeding systems that, in addition to renewable materials, can also feed organic waste with extremely different properties. Many biogas plants rely on such systems. This is either because they also want to use crops such as potatoes or beets in addition to classic renewable materials, or because the biomass used has extremely fluctuating properties (e.g. food waste or domestic organic waste). Other biogas plants react very flexibly because they buy up the biomass currently available on the market at favorable terms. Systems such as PreMix are designed for these kinds of biogas plants. They can feed many different types of biomass reliably, easily and well-mashed into the digester, meeting a basic requirement for the economic operation of these biogas plants.