Anaerobic digestion

A biogas plant is the production of biogas by anaerobic digestion of biomass. In agricultural biogas plants mostly animal excrements (slurry, solid manure ) and energy crops are used as a substrate. In non- agricultural installations material is used in the compost bin. As a by-product referred to as digestate fertilizer is produced. Most biogas plants, the resulting gas on site in a cogeneration (CHP) is used to generate electricity and heat generation.

• 3.1 Phase 1: hydrolysis
• 3.2 Phase 2: acidogenesis or acidification
• 3.3 Phase 3: acetogenesis or vinegar -forming phase
• 3.4 Phase 4: methanogenesis or methane forming phase

• 4.1 Wet and dry fermentation
• 4.2 Batch and continuous fermentation
• 4.3 Single and multistage systems

• 5.1 biogas
• 5.2 biomethane
• 5.3 digestate

• 6.1 General technical and market development
• 6.2 Development in Germany 6.2.1 Compensation in Germany
• 6.2.2 Tax treatment in Germany

• 8.1 advantages
• 8.2 disadvantages

Principle of a biogas plant

In a biogas plant the anaerobic (without oxygen) microbial degradation ( digestion ) of the substrate used is carried out. This usually consists of easily biodegradable biomass such as manure, energy crops ( mainly maize, corn and grass silage), agricultural by-products or bio-waste. Straw and wood containing mainly cellulose and lignocellulose, are difficult or not degradable under anaerobic conditions and are therefore not used.

Different types of micro-organisms use the complex composition, biomass (mainly carbohydrates, fats and proteins) as a nutrient and energy suppliers. Unlike the aerobic ( with oxygen) decomposition (eg composting), the organisms may use only a small part of the energy contained in the anaerobic digestion but. The anaerobically non- usable energy is located in the "waste product" methane. As a result, the specific rates of conversion of substrate with respect to the biomass, are considerably higher. The microorganisms must therefore carry relatively large amounts of substrate in order to meet its energy demand. Main products of anaerobic degradation are the energy-rich methane (CH4 ) and carbon dioxide (CO2). Since both are gaseous, they separate from the fermentation substrate and form the main components of biogas. CO2 is not oxidized, but can be supplied together with the high-energy CH4 in suitable CHP combustion.

Substrates for biogas production

→ Main article: substrate ( biogas plant )

The raw material used to produce biogas is usually referred to as a substrate or feedstock. In theory, is any kind of biomass under anaerobic conditions (fermentation ) is degraded. Due to the respective chemical composition (carbohydrates, fats, proteins, and so on ), there are conversion via mass different amounts of biogas methane with various proportions. This partly explains the shown in the table different methane contents when using different substrates. The table is to be noted that the yield is based on a ton of fresh mass. Are grown for use as a substrate plants, the expected yield of fresh matter per hectare is to be included. In practice, decide the shopping and mounting costs, the conditions established by the Renewable Energy Sources Act (EEG ) the remuneration and bonuses, and the suitability of the biogas plant on the substrate used.

Whereabouts of the substrate

A portion of the substrate serves as a nutrient for the microorganisms build cell mass of cell division ( anabolism). The required energy is obtained from the fermentation of the substrate. Since the energy gain compared to the aerobic respiration takes place is small, comparatively large masses substrate must be implemented per produced cell mass.

For degradable substrates, a large part of the dry matter is converted into biogas. It therefore remains an aqueous mixture of hard -degradable organic material, such as lignin and cellulose, as well as inorganic materials such as sand or other mineral substances, the so-called digestate back. This is most often used as an agricultural fertilizer, as it is still all trace elements contained in the substrate, almost all the nitrogen, phosphorus, and - depending on the procedure of the biogas plant - also contains almost all of the sulfur.

Microbial processes

The anaerobic degradation of biomass is based on the formation of fermentation gases such as landfill gas, sewage gas, marsh gas and biogas. Many different types of micro-organisms are involved. Occur and proportions of the types are dependent on the type of substrate, the pH, the temperature and the flow of the fermentation. Because of the diverse metabolic capabilities of these microorganisms community almost all the organic substances can be degraded. Only fibrous fractions of cellulose and woody portions from lignocellulose are enzymatically degradable difficult. Prerequisite for the formation of methane is a sufficient amount of water in the fermentation substrate.

The degradation process is shown schematically in four successive single biochemical processes ( phases). In the most common system concepts constantly a substrate feed to the fermenter takes place, so that the four phases take place in parallel.

Phase 1: hydrolysis

Microorganisms can not record directly into the cell, the polymeric macromolecules (eg, carbohydrates, proteins). Therefore, different types of first exoenzymes, such as amylases, proteases and lipases are secreted. This hydrolyze the macromolecules in its soluble oligomers and monomers. Carbohydrates such as starch and hemicellulose are broken as in oligo-and monosaccharides ( multiple and simple sugars ). Proteins are degraded into peptides or amino acids. Fats can be hydrolyzed into its components, such as fatty acids and glycerol.

Phase 2: acidogenesis or acidification

The products of the hydrolysis are metabolized by acid-forming microorganisms. The products of acidogenesis are lower fatty and other carboxylic acids such as valeric, butyric, propionic acid, alcohols such as ethanol, as a degradation product of the proteins hydrogen sulfide ( H2S) and ammonia (NH3). And gives rise to acetic acid (acetate ), hydrogen and carbon dioxide, which serve as starting materials for the formation of methane.

Phase 3: acetogenesis or vinegar -forming phase

During the acetogenesis and lower fatty acids and lower alcohols by acetogenic microorganisms acetic acid (acetate ) are reacted.

Phase 4: methanogenesis or methane forming phase

In the past, only anaerobic running phase - of methanogenesis - the acetic acid is converted to methane by acetoclastic methanogens according to according to Equation 1. About 30% of the methane caused by Equation 2 from hydrogen and CO2, the intermediate products from the acetogenesis. In agricultural biogas plants this is done contrary to the previous textbook opinion at a higher loading rate and lower residence time and higher temperature ( utilizing hydrogen) mainly via the hydrogenotrophic pathway. It is less directly cleaved acetate into carbon dioxide and methane ( acetoklastischer, acetic acid cleaving way ), but first converted to hydrogen and carbon dioxide ( syntrophic acetate oxidation in Figure 3, Lace et al. 1999), then the hydrogenotrophic methanogenesis in biogas be implemented. The methane formation on the acetic acid cleavage takes place to a significant degree only at relatively low loading rate and longer residence time and low acetic acid contents instead ( Lebuhn et al, 2008a, 2009;. Bauer et al, 2008, 2009. ). Since methanogenesis an energy -donating ( exothermic ) process ( - DG ° ': reaction released energy), it can be the energy- consuming reactions of acetogenesis and the syntrophic acetate oxidation ( DG ° ': enable energy-consuming reaction).

Acetic acid divisive ( acetoklastisch ): ( 1) CH3COO - H → CH4 CO2 ( DG ° '= -35.9 kJ / mol)

Hydrogen utiwizing ( hydrogenotroph ): (2 ) CO2 4 H2 → CH4 2 H2O ( DG ° '= -131.0 kJ / mol)

The four phases can not strictly separate, as arise, for example, already in the acidogenesis acetic acid, hydrogen and methane. The methanogenesis, however, requires special metabolic abilities that are found only in methanogens. These microorganisms belong to the group of archaea ( class Methanobacteria, Methanococci and Methanomicrobia ) and are only distantly related to the bacteria that perform the other steps of degradation.

Plant operation

Little is known about the precise interaction of microorganisms. Therefore, it is difficult to adjust the various parameters ( type of substrate, amount of substrate, temperature, and so on Rührwerkseinstellungen ) optimally. Many measures are based on experience. In research projects characterizations of microbiological populations or communities be made to better understand relationships.

To maintain the fermentation process with the wet fermentation, a significant part of the waste heat from the biogas into electricity to maintain the digester temperature in the mesophilic target range of 30 to about 35 degrees Celsius is needed at low substrate concentrations. Plants with dry fermentation require a significantly smaller proportion of the heat produced. For the overall efficiency and profitability of a biogas plant the best use of waste heat ( heating buildings, timber and grain drying and so on ) is an important factor.

Different partly quite different plant concepts are applied in practice. In particular, the composition of the substrate determines which concept is applied. But conditions by the Renewable Energy Sources Act, which determine the remuneration for the electricity fed, are relevant., Will affect the planning of a biogas plant requirements for disinfection and to avoid emissions.

Wet and dry fermentation

A distinguishing feature of biogas plants is operation as a wet or dry fermentation or anaerobic digestion. In the wet fermentation, a high water content makes the fermentation substrate, the mass can be stirred and flowing and is mixed during the fermentation. The dry fermentation or solid-state fermentation is carried out with stackable organic biomass. In contrast to the wet digestion here the digestate is not liquefied yet there is a constant mixing during fermentation. The choice of procedure depends mainly on the substrates from.

For the manure use only the wet digestion of the question, while structurally rich biomass often blocked the necessary for the wet digestion agitators. In the wet fermentation therefore the solid biomass has to be well crushed and kept pumpable liquid. In Germany the wet fermentation is predominant because most plants were built by farmers with livestock, which often use both energy crops and manure.

Domain of dry fermentation is the stackable biomass, such as is obtained in gardening and landscaping, and the fermentation of meadows or arable grass. Dry fermentation is used as a supplement or replacement for composting. However, the term " dry fermentation " is misleading, as the necessary for the fermentation microorganisms require a liquid medium for their survival and metabolism. Since 2004, a technology bonus of 2 cents / kWh was paid injected current for dry fermentation under the Renewable Energy Sources Act ( EEG). Therefore, in the following years, the importance of dry fermentation increased greatly in agriculture. For newly constructed facilities from 2009 the technology bonus does not apply because the process is now considered a well-established technique.

Batch and continuous fermentation

Most plants are operated with a continuous fermentation, in which the process at regular intervals, usually several times a day substrate is fed and biogas and digestate be removed. Advantageously, the automation and the relatively uniform gas production so that subsequent components such as gas purification, combined heat and power (CHP) and gas processing are also working continuously. In addition to the wet digestion (also wet fermentation ) can also dry digestion (also dry fermentation ) allow a continuous plant operation. If the content of dry matter, however, is very high or very fibrous substrate, such as organic waste, household waste and green waste, the batch fermentation is often used. Here, the biogas production is completed for each substrate and the batch digester emptied before the next batch is inserted. Through coordination of several fermenters also a quasi-continuous gas production is possible.

Single and multistage systems

The individual steps of microbial degradation have certain optima. Thus, the hydrolysis is running at a low optimum, a slightly acidic pH, and therefore a hydrolysis with subsequent stage methane is present in many systems. The methanogenesis preferably takes place in a slightly alkaline environment. Often, only one or several parallel fermenter is but without separation of the stages of decomposition. Usually is still a downstream storage container, which is hermetically sealed and, therefore, acts as a secondary fermenter.

Use of the products

Biogas

→ Main article: Biogas

At present ( 2012) is used directly at the biogas plant for decentralized coupled mainly in Germany biogas heat and power generation (combined heat and power) in combined heat and power ( CHP ); rarely the biogas is upgraded to biomethane. For the direct use, the gas mixture is dried, desulphurised and then supplied to a biogas motor which drives a generator. The so- produced electricity is fed into the grid. The heat contained in the exhaust gas and engine cooling water is recovered in a heat exchanger. Some of the heat is needed to heat the fermenter to grow because the microorganisms that decompose the biomass, best at temperatures of 30-37 ° C ( mesophilic ) or 50-60 ° C ( thermophilic). Excess heat from the engine, for example, for heating buildings, for drying the crop (cereals) or operation used by aquaculture facilities. Particularly economical and energy efficient is the system if the excess heat can be used or sold throughout the year.

Biomethane

→ Main article: biomethane

In several projects, the biogas is now cleaned in processing plants to natural gas quality and biomethane ( bio natural gas ) is fed into the natural gas grid. Thus, the economics of biogas plants in sites with no heat collector can be improved. The biomethane example, can be converted into electricity in CHP plants, which are directly continuous heat consumers, such as indoor swimming pools, built. This waste heat is almost completely deductible. Upgraded biogas can also be used as fuel for natural gas-powered vehicles. The plant technology for upgrading biogas to biomethane and fed into the natural gas network is currently still quite expensive and therefore economically viable only for " large systems " (from 1.5 MWe ).

Digestate

→ Main article: digestate

The digestate from biogas plants are largely used as agricultural fertilizer. They are chemically far less aggressive towards the plants as raw manure, nitrogen availability is higher and the smell less intense. The digestate wet fermentation ( " biogas slurry ") is a slurry- like substance. In the dry fermentation not compost but stackable digestate which can be used as fertilizer and also accounts for about half of the initial quantity produced. The amount of digestate can be reduced by an aerobic post-treatment even further. In addition, the load is reduced by disease germs ( decontamination ), as well as exposure to hydrogen sulfide compounds by this treatment. A further amount of combustion for reduction and / or energy is possible.

Development of biogas plants

General technical and market development

The Italian physicist Alessandro Volta is one of the first who studied biogas. Already in 1789 he has a combustible gas collected, which was created in the sediment of Lake Como. Many well-known scientists, including Faraday ( which it identified as a hydrocarbon ), Davy and Dalton have the experiments of Volta understood. Avogadro discovered the chemical formula for methane ( CH4). This easy-to -sustaining gas was very popular in the 19th century in physical and chemical experimental lectures.

End of the 19th century it was discovered that wastewater can be clarified by anaerobic digestion. In 1906, wastewater treatment systems have been installed in the Ruhr area with heated digesters. The aim was then (as later ) not really the biogas production, but the reduction of waste. It was not until about 1922 biogas was collected and fed into the urban gas network. Some sewage treatment plants earned so much that they could cover their operating costs. Until 1937, some German cities had converted their fleet to biogas. The waste disposal in the city of Zurich drove biogas to 1973.

First attempts to produce biogas not only from sewage, were first made ​​in the late 30s and in the 50s with solid manure and later with manure. It emerged at that time about 50 plants. Because of the time ever cheaper oil has set these tests again.

In the energy crisis of 1973, the biogas technology was currently playing. But the further development was slowed again by falling oil prices.

Due to the large amount of agricultural waste and manure, the Netherlands, Switzerland Kompogas, and Sweden have the most experience with biogas. In these countries, CHP are used less frequently. Here, the biogas is upgraded to biomethane. In the Netherlands and in Switzerland it is fed into the natural gas grid. In Sweden it is used for motor vehicles.

Although biogas plants has moved in the last 10 years in the consciousness of the European population, the end of the 19th century biogas was used for the energy supply in India already. The economic dissemination of biogas use depends primarily on the world energy policy ( eg during the oil glut of 1955 to 1972 and the oil crisis of 1972-73 ) and the respective national laws (for example, the Renewable Energy Sources Act in Germany ). Regardless of small biogas plants have always been built in countries such as India, South Korea, Taiwan and Malaysia to private energy supply, with about 40 million household plants, most are in China.

Here first the biogas was recovered from dung and manure and used mainly for cooking and heating. It applies here as an important factor against deforestation. The power generation is of lesser interest. In particular, the decentralized use and small plants are locally to improve the quality of human life. Leaders are here India, Botswana and China.

Development in Germany

The cultivation of renewable raw materials for biogas use in Germany has increased from 400,000 ha in 2007 to 530,000 ha in 2009.

The number of plants and the installed capacity has risen sharply in recent years. A relatively high increase can be identified with the applicable since 2004, first amendment of the Renewable Energies Act (EEG). Fraud, the number of plants in 2004, before the amendment in 2010, they were in 2005 already 2690 installations in Germany. In 2007, this number is further increased to 3711. This development can be explained by the increase in the remuneration of the kWh generated by biogas plants. The electric power increased from 247 MW in 2004, 665 MW in 2005 up to 1270 MW in 2007. Since the performance of newly installed systems increases, the overall performance is increasing faster than the number of installations. Because many biogas plants give off a large portion of the waste heat waste to the environment, there is still further potential, for example through the establishment of district heating networks or other waste into biomethane.

In 2009, in Germany 4671 biogas plants in operation, producing a total of approximately 11 % of electricity from renewable energy sources. End of 2011, 7,100 biogas plants with an installed capacity of around 2,800 MW in operation, according to the rated output of two large nuclear power plants. With the EEG 2012 and the new compensation structure of the additional construction of biogas plants in 2012 was significantly lower than in the previous year. The Biogas Association specifies the investment portfolio at 31 December 2012 to 7515 with an installed capacity of 3,352 MW.

With the current 2009-2011 2nd amendment to the EEG a liquid manure bonus was introduced, which is intended to promote smaller systems with a high proportion of manure. In Germany, only an estimated 15% of the manure from the livestock be used for energy. With the use of this potential biogas technology could strengthen their contribution to climate protection on.

The EEG 2012 no longer provides for bonuses.

Compensation in Germany

In Germany, the feeding of electricity from renewable sources into the power grid is controlled by the Renewable Energy Sources Act ( EEG). The transmission system operators must remove the power generated at defined prices, these costs but pass it on to the end customer. Between the transmission system operators to compensate for the additional costs for the mandatory purchase of electricity takes place, so that the burden of end users nationwide equal. The level of remuneration in accordance with EEG amendment in 2009 is listed simplified in the table below. If the biogas is only thermally processed, the biogas plant operator receives no feed-in tariff. For landfill and sewage gas its own minimum rates of remuneration and bonuses are determined in the EEG.

The level of remuneration is from the year of commissioning, guaranteed for 20 years. An inflation adjustment does not take place. For new installations, a reduction of the tariff of 1% per year applies. For a plant that was commissioned in 2009, so the rates A system that will be operational in 2010 are valid for 20 years after EEG 2009., Gets 20 years for 99% of these sets and so on.

The RRM bonus is granted if only plants or plant parts are used in the biogas plant, which occur in agriculture, forestry, horticulture or landscape management and have no purpose other than the recovery in the biogas plant. In addition, manure can be used in a renewable resource system. At least 30% share in the manure substrate (anytime ) also a slurry bonus is granted to the 500 kWe is 1 cent / kWh el for stations up to 150 kWe 4 cents in plants.

The height of the CHP bonus is, depending on the system design, variable. It depends on the one of the power to heat ratio ( SKZ ) that is calculated by dividing the electrical through the thermal efficiency of the cogeneration plant. Multiplying SKZ and the amount ( kWhth ) actually used and useful CHP waste heat results in the amount of electricity ( kWh el ) for which the bonus is actually granted. A high electrical efficiency and a large amount of actually used heat thus ensure a high bonus. Eligible heat utilization concepts are defined by the EEG 2009.

The technology bonus is granted for use of innovative technologies in the biogas plant, if a heat recovery takes place or certain electrical efficiencies are achieved. These may be, for example, the use of a Stirling engine, an ORC turbine, a Kalina process, a fuel cell or a gas turbine. In addition, the bonus is in preparation of the biogas to natural gas quality for injection into the gas grid, and specific method for the digestion of biowaste.

For systems up to 500 kWe an emission reduction bonus is granted (increasing the base salary by 1.0 c / kWh for plants after BimSchG in compliance with the corresponding formaldehyde limit values ​​for emission minimization requirements of the TA Luft ) of 1 cent / kWh, if certain thresholds are met.

Significant new features compared to the EEG 2004, the abolition of the technology bonus for dry fermentation, the increase in the base salary of small plants and Nawaro bonus, the introduction of a manure - landscape management and emission reduction bonus and structured promotion of gas supply different capacities as well as numerous detailed regulations.

Since the introduction of the EEG 2012, the remuneration structure has been redefined. There are no bonuses, only a basic remuneration and compensation for use classes. There are also within the scope of direct marketing a market premium and a Flex Award.

Tax treatment in Germany

The tax treatment of biogas plants see BMF letter of 6 March 2006 IV C 2 - S 2236 - 10/05/IV B 7 - S 2734-4 / 05 release.

Development in Switzerland

Since 1 January 2009 in Switzerland is considered the most cost -covering feed (CRF ); related to this is an increased feed-in tariff ( feed-in tariff for electricity produced from biogas electricity) for renewable energy, which also includes biogas. The remuneration consists of a fixed purchase price and an additional so-called agricultural bonus, which will be granted if at least 80 % of the substrates consist of manure. The Swiss model of assistance to the sustainable development in the energy sector accelerate, especially since it promotes the manure -based, and thus sustainable biogas plants.

The Swiss support instrument for renewable energies ( CRF) contributes to the biomass utilization into account the fact that no land available for the cultivation of renewable raw materials. The law has so far caused no increase in agricultural biogas plants in the use of manure. The low attractiveness of green waste as a co- substrate for agricultural equipment and thus energetically untapped potential has prompted biogas companies to design new investment models. Combined with solid manure, food waste or organic waste from communities, new possibilities, without the raw materials to be transported over long distances to centralized systems offer. The simultaneous possibility of manure processing represents a novel concept for generating renewable energy

Security

Since biogas plants in large quantities of flammable gases are produced and processed, the operating safety of great importance. Incorrect operation of the biogas plant, with construction defects and material damage, there is the possibility of a deflagration or detonation, such as, inter alia, significantly in three accidents in biogas plants in 2007 was ( in Riedlingen, Walzbachtal and Deiderode ). Consequence of heavy can thus be related environmental damage, especially by the introduction of fermentation substrates or digestate waters, as was the case with biogas accidents in Barßel ​​in Bassum. In individual cases, harmful gases may be released to a significant extent, eg Hydrogen sulfide were killed in an accident near Zeven in 2005, in which four people.

Reviews

Biogas plants are plants next to hydropower plants, solar systems, biomass ( power ) and wind turbines important producers of electricity and heat from renewable sources. Depending on the substrate type and plant design, biogas systems have advantages and disadvantages:

Benefits

• Renewable energy sources ( renewable, locally available raw materials) as well as fossil fuel savings
• Use of previously unused plants and plant parts ( cover crops, crop residues)
• High energy yields per cultivated area in comparison with other bioenergy (biodiesel, BTL)
• CO2 emissions is almost neutral, but you have the growing and harvesting process and take into account the fertilization with
• Decentralised power generation can reduce transport route to the final consumer
• Through continuous baseload power generation, alternatively could provide but also control energy, thus good addition to electricity from wind power and solar power plants
• Improved quality of the digestate fertilizer compared to raw slurry: decreased intensity of odor and corrosivity when applying
• Better plant availability of nutrients

Disadvantages

• Negative environmental impacts of intensive agriculture ( loss of biodiversity ).
• Regional competition for land between crops for food, feed and energy crop production possible.
• Significant increase in rental prices for agricultural land, high cost factor for the farmers, an indirect result: rising food prices
• Resulting in the biogas plant gases can cause an explosion if not used properly, asphyxiation or poisoning. Protein-rich substrates can lead to relatively high levels of the highly toxic hydrogen sulphide in the biogas. Appropriate safety regulations must be observed.
• Methane has a mass per 25 -fold higher greenhouse effect than carbon dioxide. Therefore, emissions are to be avoided at biogas plants through proper operation.
• For the spreading of digestate enough space must be available. However, the surfaces of the substrate cultivation are usually sufficient for this purpose.
• During the winter months, no manure and digestate may not be applied. During this time, the digestate must - as well as unfermented manure from livestock - are stored. Appropriate storage capacity shall be verified and are therefore already set up a rule for the construction of a biogas plant.
• It must be prevented that liquid manure from animals which have been treated with antibiotics, at high concentration, enters the digester.
• Due to the increased cultivation of maize monocultures increase the Schwarzwildbesätze, resulting in more game damage.