The gas can be used to generate electrical energy, to operate vehicles or to feed into a gas supply network. For the utilization of biogas, the methane content is most important, since its combustion releases energy.
in m 3 per ton of
|Pressed sugar beet
Starting materials are biogenic materials like the following:
- fermentable biomass- containing waste such as sewage sludge , organic waste or leftovers
- Farm manure ( liquid manure , manure )
- Plants that have not previously been used and parts of plants (for example catch crops, plant remains and the like).
- specifically cultivated energy crops ( renewable raw materials )
Different raw materials result in different biogas yields and, depending on their composition, a gas with a variable methane content, as the adjacent table shows.
A large part of the raw materials, especially manure and plant residues, are in principle free of charge in agriculture, which is why this branch of the economy has the greatest potential for the production of biogas. The cultivation of energy crops has completely different effects:
The advantages of biogas can be weighed against the (possible) disadvantages of energy crops (" ecological balance ").
Biogas is created through the natural process of microbial degradation of organic substances under anoxic conditions. Microorganisms convert the carbohydrates, proteins and fats they contain into the main products methane and carbon dioxide . Anoxic conditions are necessary for this, i.e. the absence of oxygen.
The process consists of several stages, each of which is carried out by microorganisms of different metabolic types . Polymeric components of biomass, such as cellulose , lignin , proteins , are first converted to monomeric (low molecular weight) substances by microbial exoenzymes . Low molecular weight substances are broken down by fermenting microorganisms into alcohols , organic acids , carbon dioxide (CO 2 ) and hydrogen (H 2 ). The alcohols and organic acids are converted into acetic acid and hydrogen by acetogenic bacteria . In the last stage, methanogenic archaea form the end products methane (CH 4 ) and water from carbon dioxide, hydrogen and acetic acid .
The term biogas is used in summary for high-energy gases that are formed from biotic substances by microorganisms under anoxic conditions:
- Sewage gas : the gas produced when cleaning wastewater
- Digester gas : the gas only produced in the sewage sludge digestion
- Landfill gas : gas leaking from a landfill
The composition of biogas is very different because it depends on the composition of the substrate and the mode of operation of the digester. In Switzerland, biogas is produced exclusively from residual materials, e.g. B. via the Kompogas process .
Before biogas processing , the gas mixture consists of the main components methane (CH 4 ) and carbon dioxide (CO 2 ). In addition, it usually contains nitrogen (N 2 ), oxygen (O 2 ), hydrogen sulfide (H 2 S), hydrogen (H 2 ) and ammonia (NH 3 ).
The methane contained in around 60% of the water-saturated biogas is valuable. The higher its proportion, the more energetic the gas. The water vapor can not be used . Hydrogen sulphide and ammonia are the main problems in raw biogas . During biogas processing , they are removed before combustion in order to prevent human hazards, unpleasant odors and corrosion in engines, turbines and downstream components (including heat exchangers ). The CO 2 , which can be separated and recycled in certain applications, is also disruptive .
Climate and environmental protection
Methane is an important greenhouse gas . That is why testing the tightness of biogas plants and all associated components is an important contribution to climate protection.
Biogas plants are not completely tight; they must also remain accessible for maintenance work. This is why methane, which in the medium term has a 25 to 30 times stronger heating effect on the climate than CO 2 , can escape into the atmosphere when operating a biogas plant .
Biogas achieves its maximum efficiency and supply level when it is used to generate electricity and heat at the same time; in so-called combined heat and power (CHP) it has the best climate balance. When used in engines, biogas is i. d. Usually, however, due to the methane slip, more harmful to the climate than diesel. On the other hand, electricity generation without heat use or the purely thermal use of processed biogas in natural gas boilers are, as expected, not optimal, as the Agency for Renewable Energies found.
Biogas burns in a climate-neutral way, as the resulting CO 2 was previously bound by plants from the air. However, there are factors that can worsen the climate balance of biogas plants through the cultivation of energy crops:
The production of energy crops involves a high level of energy consumption. In contrast to waste recycling, a plant operated with maize silage consumes energy in all production steps: seed preparation, sowing, fertilizing, protection against pests (pesticide production and use), harvesting, transport, silage, fermentation with circulation and transport of the digestate back to the fields. The carbon footprint of energy crops can be improved if the energy required for production is covered by renewable energies, for example if the agricultural machinery used is also operated with fuels from energy crops or green electricity.
The nitrous oxide (also known as "laughing gas") produced by nitrogen fertilization during intensive agriculture must be included in the climate balance. The production of nitrous oxide is carried out by microbes, which form it from the oxygen in the air and excess nitrogen. Laughing gas has a greenhouse gas potential that is around 300 times greater than that of CO 2 . The change in land use must also be taken into account: When pastureland is plowed into corn fields, the humus contained there releases CO 2 and other greenhouse gases through contact with the air .
The cultivation of maize is ecologically controversial. Maize ( Zea mays ) is a grass of tropical origin. Cultivation is carried out in such a way that frost is avoided, so sowing takes place late in the year, the plants grow well in May / June and the harvest begins at the end of September. For most of the year, the fields planted with maize are exposed and are eroded by wind and rain. This can lead to the entry of pesticides and fertilizers into nearby waters, but also into the groundwater. The cultivation of maize in Europe is not possible without these additives. This poses a problem as it can lead to both eutrophication and silting up of the waters. Likewise, large quantities of dust can be blown from dry fields, which in turn affects soil fertility because important soil components are lost as a result; there is a long-term risk of desertification, which is particularly well known in the USA.
The large-scale cultivation of maize monocultures for the production of biogas has further ecological effects. Pasture land and wet meadows are converted into arable land and fallow land is reused. This has an impact on birds (e.g. lapwing , skylark , partridge ), insects and other animals, which lose feeding and breeding areas as a result.
In contrast to conventional farms with biogas plants, maize only plays a minor role as an energy crop for organic farmers. On the other hand, grass clover and residues such as liquid manure and manure are more important. Organic farming also offers ideas for conventionally working farms, for example the cultivation of catch crops and undersown crops or the simultaneous cultivation of several plants; In this way, conventional farms can also benefit from the experience of organic farms for their energy crops. Various pretreatment methods are also used to try to achieve the greatest possible biogas or methane yield from the substrate.
In 2014, biogas production in Germany corresponds to around 20% of natural gas imports from Russia. With the remaining potential, around another 10% can be replaced, taking technical and demographic developments into account, up to a total of 55%. In the EU, the current biogas production corresponds to around 6% of natural gas imports from Russia. With the remaining potential, about a further 26% can be replaced, taking technical and demographic developments into account, up to a total of about 125%.
Feeding into the natural gas network
After extensive biogas processing, it can be fed into the natural gas network . In addition to removing water, hydrogen sulfide (H 2 S) and carbon dioxide (CO 2 ), an adjustment to the calorific value of the natural gas in the respective gas network (conditioning) must take place. Because of the high technical effort involved, processing and feeding is currently only worthwhile for larger than average biogas plants. The first projects for this started in 2007. New developments such as the hollow fiber membrane from Evonik Industries in Essen enable biogas to be purified to a purity level of up to 99 percent and thus bring it to natural gas quality.
The following processing steps are necessary to achieve natural gas quality:
Desulphurisation: Desulphurisation is necessary to avoid corrosion. Sulfur is found in the form of hydrogen sulfide (H 2 S) in biogas, and when it is burned, aggressive acids, namely sulfuric acid (H 2 SO 3 ) and sulfuric acid (H 2 SO 4 ) , are formed in the presence of water vapor . Most of the time, the hydrogen sulfide content is low, but it can rise sharply with protein-rich substrates (grain, legumes, slaughterhouse waste and the like). There are various options for desulfurization, including biological, chemical and adsorptive processes. Several stages may be necessary, such as coarse or fine desulphurisation.
Drying: Since biogas is saturated with water vapor, considerable amounts of condensate would accumulate when untreated biogas were cooled, which can lead to corrosion in engines. In addition, the formation of water pockets should be avoided. Therefore the gas has to be dried. This is done by cooling the gas to temperatures below the dew point in a heat exchanger, the condensed water can be removed and the cooled gas is passed through a second heat exchanger and reheated to operating temperature. At the same time as drying, the readily water-soluble ammonia is removed.
CO 2 separation: Carbon dioxide (CO 2 ) cannot be oxidized and therefore does not contribute to the calorific value of the biogas. In order to achieve natural gas quality, the calorific value of the biogas must be adjusted to that of the natural gas. Since methane is the energy-supplying component of biogas, its proportion must be increased by removing CO 2 . The currently common methods of methane enrichment through CO 2 separation are gas scrubbing and pressure swing adsorption (adsorption process on activated carbon). In addition, other processes such as cryogenic gas separation (using low temperatures) or gas separation using membranes are being developed.
Conditioning: During conditioning, the biogas is adapted to the requirements in terms of dryness, pressure and calorific value. Natural gas has different calorific values depending on its origin, so the upper calorific value of the processed biogas must be adapted to the respective network.
Compression: For feeding into the natural gas network, depending on the type of network, mostly low to medium pressures of up to around 20 bar are necessary. Direct feed into the long-distance gas network with up to 80 bar is less common . If, after processing, the biogas has a lower pressure than the network, it must be compressed with the help of a compressor .
Further cleaning and processing steps: Landfill and sewage gases can contain siloxanes as well as halogenated and cyclic hydrocarbons . Siloxanes cause significantly increased engine wear. Halogen hydrocarbons lead to emissions of toxic compounds. Siloxanes and hydrocarbons can be removed from the biogas by means of gas scrubbing, gas drying or adsorption on activated carbon .
In addition to its own use in agriculture, biogas is also suitable as a contribution to an energy mix from renewable energies. This is because, on the one hand, it can be used as a base load, i.e. the biogas, unlike other renewable energy sources such as wind or sun, is continuously available, and on the other hand, biomass and biogas can be stored, which can contribute to the energy supply at peak times. This is why this bioenergy carrier is ideal for balancing out short-term fluctuations in the electricity supply from wind and solar energy . So far, most biogas plants have been operated continuously, more or less as a base load power plant. The following options are available for using the energy contained: Combined heat and power (CHP) on site: Biogas is used in a block -type thermal power station (CHP) to generate electricity and heat (CHP); all of the electricity is fed into the grid, and the waste heat, which makes up around 60 percent, can be used on site. Alternatively, the biogas can be fed into the supply network after appropriate processing.
Since the majority of biogas yields are generated through the sale of electricity, the heat consumer has a CHP unit, which produces electricity as the main product for feeding into the grid and ideally feeds heat into a local or district heating network. The bioenergy village of Jühnde is an example of a local heating network . So far, however, in most agricultural biogas plants, only a small part of the heat has been used due to the lack of heat demand on site, for example to heat the fermenter and residential and farm buildings.
An alternative is the transport of biogas in biogas pipelines via micro gas networks. The electricity and heat production can therefore take place at heat consumers.
Other types of use
- Main article: Biomethane
Biogas can be used as an almost CO 2 -neutral fuel in motor vehicle engines. Since upgrading to natural gas quality is necessary, most of the CO 2 must be removed. After separation, it can be used commercially, for example in the beverage industry. So-called biomethane or bio natural gas has to be compressed to 200 to 300 bar in order to be used in converted vehicles.
In Switzerland, trucks from Walter Schmid AG and the associated company Kompogas have been running on biogas since 1995; the first truck reached its millionth kilometer in the summer of 2010. From 2001 Migros Zurich also drove with Kompogas and since 2002 also McDonalds Switzerland .
So far, however, biogas has seldom been used in this way. In 2006 the first German biogas filling station was opened in Jameln (Wendland).
Because of the high electrical efficiency, the utilization of biogas in fuel cells could also be of interest in the future . The high price for the fuel cells, the complex gas purification and the short service life in practical tests have prevented a broader application of this technology.
While biogas has only reached the awareness of the European population in the last 10 years, in India biogas was already used for energy supply at the end of the 19th century. The economic spread of biogas use depends above all on world energy policy (e.g. during the oil glut from 1955 to 1972 and the oil crisis from 1972 to 1973) and the respective national legislation (e.g. the Renewable Energy Sources Act in Germany). Independently of this, small biogas plants were built in countries like India, South Korea and Malaysia for private energy supply, with over 40 million domestic plants most of which are in China.
From 1999 to 2010 the number of biogas plants grew from around 700 to 5905, which produce around 11% of electricity from renewable energies. At the end of 2011, around 7,200 biogas plants with an installed electrical system output of around 2,850 MW were in operation in Germany. This means that Germany's biogas farmers are replacing more than two nuclear power plants and supplying over five million households with electricity. Due to the uncertain political framework, the expansion has decreased significantly since 2012, by only 200 MW in 2013.
In 2013, a total of 7,720 biogas plants with a total electrical output of around 3,550 megawatts were installed in Germany, producing 27 million megawatt hours of electrical energy or 4.3% of German demand or an energy density of 2mw / m³. In addition to electrical energy, a further 13.5 million MWh of thermal energy were generated, which corresponds to 0.9% of Germany's annual demand. To supply these biogas plants, around 75% of which are owned by farmers, 1.268 million hectares of cultivation area were used, which corresponds to around 10.6% of the areas used as arable land in Germany.
It is assumed that the production of bio natural gas can be expanded to 12 billion m³ of biomethane annually by 2020. This would correspond to a five-fold increase in the capacities of 2007. The Renewable Energy Sources Act (EEG) ensures that the feed-in tariff is higher than that of conventional electricity and guaranteed for 20 years. For the use of the heat, the system operator also receives a bonus for combined heat and power generation (CHP bonus) , which is also specified in the EEG . The use of heat is promoted through high energy prices and financial incentives and the Renewable Energies Heat Act, which has been in force since January 2009.
Since 2007, gas suppliers in Germany have been increasingly offering nationwide deliveries of pure biogas or admixtures to fossil natural gas for end customers. Nationwide, gas customers can opt for at least one, but sometimes up to ten gas tariffs with biogas admixture.
Flexibilized biogas plants are of particular importance for the electricity market, as they can offer an available compensation potential of around 16,000 MW in total. Within a few minutes, this capacity could be reduced in the event of oversupply in the network or increased in the event of increasing demand. For comparison: the capacity of the German lignite power plants is put at around 18,000 MW by the Federal Network Agency. However, due to their technical inertia, these large fossil power plants could only provide a few thousand megawatts for short-term balancing of solar and wind power.
Frequently there is talk of a " cornification " of the landscape. In fact, the proportion of maize acreage rose from 9.3% in 1993 to 14.9% (2013). However, this must also be seen against the background of the EU's common agricultural policy. As part of the set set-aside , agricultural holdings had to set aside up to 15% of the land in order to limit agricultural overproduction . In 2000 closures were reduced to 10%, in 2005 to 5% and abolished in 2009. As early as the early 1990s, however, renewable raw materials were allowed to be grown on the set-aside areas. With an increase of around 5%, the cultivation of maize for use in biogas plants has hardly any impact on the cultivation of crops in Germany. With regard to the area distribution, cultivation is at a moderate level.
In Switzerland , pure biogas is usually referred to as Kompogas . At many Swiss gas filling stations, a mixture of Kompogas and natural gas is sold under the name “natural gas”. In 2010 there were 119 natural gas filling stations in Switzerland where natural gas with a biogas content of at least 10% is offered. These are mainly located in the west and north of the country.
The cost-covering feed-in tariff (KEV) has been in effect in Switzerland since January 1, 2009 ; This is associated with an increased feed-in tariff (feed-in tariff for electricity generated from biogas) for renewable energies, which also includes biogas. The remuneration consists of a fixed purchase price and an additional so-called agricultural bonus, which is granted if at least 80% of the substrates consist of farm manure. The Swiss funding model is intended to promote sustainable development in the energy sector, since it promotes in particular the manure-based and therefore most sustainable biogas plants.
When it comes to biomass utilization, the Swiss funding instrument for renewable energies (KEV) takes into account the fact that there are no areas available for the cultivation of renewable raw materials. So far, the law has not resulted in a substantial increase in agricultural biogas plants in the area of the use of liquid manure. The low attractiveness of green waste as a co-substrate for agricultural systems and the resulting untapped energy potential has led biogas companies to design new system models. Combined with solid manure, leftover food or organic waste from municipalities, there are new possibilities without having to transport the raw materials over long distances to central plants. The possibility of refining liquid manure at the same time represents a new concept for generating renewable energy.
The pioneer for the Swiss Kompogas was the building contractor Walter Schmid , who was interested in energy efficiency . After studying specialist literature, he made his first attempts on the balcony at home and at the end of the 1980s was convinced that he could use the gas from organic waste. With the support of the federal government and the canton, it commissioned the first test plant in Rümlang near Zurich in 1991, which was the first Kompogas plant to go into normal operation in 1992. The Kompogas company built further systems worldwide and Schmid was awarded the solar prize in 2003. In 2011, the Kompogas Group was completely taken over by the department of the Axpo Group called axpo neue energien as Axpo Kompogas AG .
France represents a potentially large biogas market that is also served by German plant manufacturers. The country is characterized by a productive agriculture with various rich substrates as well as a stable support system for the generation of electricity and heat from biogas and for the feed-in of biomethane. In summer 2013 there were around 90 agricultural biogas plants. The expansion plan for agricultural systems (“EMAA Plan”) announced in April 2013 with a target value of 1,000 systems by 2020 signals an accelerated market development.
In Sweden, electricity generation from biogas is currently still unprofitable due to lower electricity prices (approx. 10 euro cents / kWh). The majority of the biogas (53%) is used to generate heat. In contrast to other European countries, such as Germany, upgrading to natural gas quality ( biomethane ) and using it as fuel in gas vehicles is a widespread variant with 26%.
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