Biogas processing

from Wikipedia, the free encyclopedia

Under biogas processing method which are understood, biogas is cleaned so that it can then be fed to an energetic or material use. The crude biogas is used in biogas plants produced and contains a mixture of different gases. Also, sewage gas and landfill gas must be processed prior to recovery.

Goals and Variants

Composition of biogas
Fluctuation range average
methane 45-70% 60%
carbon dioxide 25-55% 35%
Steam 0-10% 3.1%
nitrogen 0.01-5% 1 %
oxygen 0.01-2% 0.3%
hydrogen 0-1% <1%
ammonia 0.01-2.5 mg / m³ 0.7 mg / m³
Hydrogen sulfide 10-30,000 mg / m³ 500 mg / m³
Biogas and natural gas pipelines.

A distinction can be made between the basic processing of raw biogas, which is necessary, for example, for utilization in a biogas combined heat and power plant, and the more complex processing of natural gas quality ( biomethane or bio natural gas). The adjacent table shows the composition of raw biogas after basic processing and biomethane. The proportions of the raw biogas can vary greatly depending on the substrate, the system concept and other factors. The nature of the biomethane is adapted to the corresponding natural gas qualities.

Biogas is mostly used directly at the biogas plant in a combined heat and power unit (CHP). For this purpose, desulphurisation and drying are necessary in the course of the basic treatment in order to avoid corrosion in the CHP. In order to feed biogas into the natural gas network or to use it as a motor vehicle fuel, more extensive processing is necessary. In this case too, drying and desulphurisation take place. In addition, carbon dioxide is separated off and the processed biogas is conditioned into biomethane, which then has properties comparable to natural gas. The advantage of upgrading biogas to natural gas quality is that the biomethane is routed through the natural gas network and z. B. can be converted into electricity in a CHP plant where the resulting heat is needed. For example, electricity can be generated by a CHP at a swimming pool, which has a high heat requirement all year round. Private gas customers also have the option of choosing gas supply contracts that contain a certain proportion of biomethane.

Procedural steps

The complete processing of raw biogas to biomethane usually comprises four process steps: biogas desulphurisation, gas drying, separation of methane and carbon dioxide and conditioning before it is fed into the natural gas network . These steps often build on one another, but can also be combined in some cases.

Desulfurization

In the case of low levels of hydrogen sulphide (H 2 S) in the raw gas, as is to be expected in the fermentation of energy crops in particular, sulphide precipitation is usually sufficient to roughly remove the H 2 S. With biogas from protein-rich substrates or with sewage gas, significantly higher H can be achieved 2 S concentrations occur. In such cases or with high gas volume flows, biogas washers are used. In some cases, the desulphurisation is also carried out in parallel with the separation of the gases contained in the raw biogas. This is often followed by fine desulphurization, with the gas being passed through several activated carbon filters connected in series .

The usual methods of desulphurisation of biogas include

  • Cleaning after gas production by desulphurisation filter: Here the gas is passed through filter material containing iron ( lawn iron stone , steel wool ). The filter material must be replaced when saturated or regenerated by heating.
  • Cleaning by adding atmospheric oxygen (biological desulphurisation, gas space desulfurization): The H 2 S is converted by bacteria that grow on surfaces in the gas space with the oxygen added in the fermentation space to form elemental sulfur and water. The sulfur is deposited and can accumulate in the system. This process is often used in practice and is usually sufficient to ensure that the maximum H 2 S concentrations recommended for CHPs are not exceeded. However, when it is subsequently processed into biomethane, nitrogen and oxygen residues in the added air can be problematic. An advantage of the method is that the sulfur mainly in the digestate is thus increasing its fertilizer value. Instead of entering the gas space, air or another oxidizing agent can also be added directly to the reactor liquid (Linde patent).
  • Caustic wash: The biogas is in a packing - column in countercurrent washed with brine. The lye must then be disposed of. In addition to reducing the sulfur content, this method also reduces the CO 2 concentration in the biogas. With lye washing with biological desulfurization (Paques patent), half of the lye is regenerated in a second, aerobic reactor. In addition to a sulfur-free waste water stream that is reduced compared to normal lye washing, an elemental sulfur sludge is produced.
  • Addition of iron ions : With high protein proportions in the starting substrate, the H 2 S concentrations can exceed 20,000 ppm . This exceeds the capacity of available filter types. The addition of iron ions helps to prevent the formation of hydrogen sulfide in the digester. Iron has a high affinity for sulfur and combines with it to form insoluble iron sulfide (FeS). The iron sulfide remains in the digestate as a solid .
  • Irreversible adsorption on activated carbon : The activated carbon as a filter medium is partially iodized to increase its loading capacity. This method is only suitable for very low H 2 S concentrations, e.g. B. as final cleaning.
  • Return of partially desulphurized biogas to the reactor liquid: This process improves the expulsion of the H 2 S that is still dissolved in the liquid . Since hydrogen sulfide is toxic and, if the concentration is too high, it inhibits the biological processes in the fermenter of a biogas plant, this process also serves to ensure process stability.

Drying

During the drying process , water vapor is removed from the raw gas. It must be removed from the biogas before it is used, so that natural gas quality is maintained. The fuel gas is also dried before it is used in biogas combined heat and power plants. In this way, the formation of water pockets through condensation (precipitation) and the corrosion of biogas engines and pipe systems can be avoided.

Biogas is dehumidified by cooling the gas in the ground or using compressor cooling . If the water vapor falls below the dew point temperature , the water condenses. The condensate is collected and discharged at the lowest points of the mostly underground biogas pipeline (condensate separator). When cooling by refrigeration machines , the water (condensate) accumulates in the cold registers and can be collected and drained there. Since ammonia is readily soluble in water, it can be removed from the gas during drying and discharged with the condensate .

Separation of gases

The following processes are available for separating carbon dioxide and methane : pressurized water scrubbing , cryogenic processes, membrane separation processes , but also pressure swing adsorption and other processes for separating carbon dioxide by absorption (such as amine scrubbing or the Selexol process ).

The scrubbing solution binds carbon dioxide, hydrogen sulfide and water vapor from the biogas under pressure and at low temperatures. The gases are desorbed again by relaxing and heating the washing solution. Biological separation of sulfur by means of bio- scrubbers or bio- diesel bed reactors is also possible. For example, amine scrubbing leads to a very high product gas quality, but requires the use of process heat at a high temperature level. In most processes, the raw gas has to be roughly freed of suspended matter, water droplets and hydrogen sulfide.

The pressurized gas scrubber separates the methane from the carbon dioxide and concentrates the methane to 95–99% by volume as required. It is then dried by cooling to the dew point at the respective application pressure.

Another common process for carbon dioxide separation with integrated desulfurization and dehumidification is the biogas amplifier process. This organic-physical process works with an organic washing solution that is regenerated in the process. There is therefore no process water for disposal. The scrubbing solution binds carbon dioxide, hydrogen sulfide and water vapor from the biogas under pressure and at low temperatures. The gases are desorbed again by relaxing and heating the washing solution.

Conditioning

During conditioning, the processed biogas is adjusted to the quality parameters for natural gas in terms of dryness, pressure and calorific value Wobbe index before it is fed into the gas network . The quality and nature of the methane to be fed in has been regulated by the German Association for Gas and Water (DVGW). Since there are gas networks in Germany for natural gas of various qualities (H-gas and L-gas), the quality requirements for biomethane can be different. Conditioning also includes odorization .

Spread and Economics

Biogas and biomethane as a link in the sector coupling.

Because of the investments required, biogas processing is currently considered to be economically viable from a capacity of around 250 to 500 m 3 of biomethane per hour. This would correspond to a system capacity of around 1 to 2 megawatts of electrical output (MW el ) with direct biogas power generation in the CHP. In particular, biogas plants, which cannot dissipate the waste heat from converting biogas into electricity on site, are suitable for gas processing and feed-in. The basic remuneration according to the Renewable Energy Sources Act (EEG) is also paid for electricity from biomethane . In addition, the law granted a technology bonus for gas feed-in ( Annex 1 EEG 2009) and a combined heat and power bonus (CHP bonus, Annex 3 EEG 2009). Together with the income from the more complete sale of heat, the complex gas processing becomes economical. In Germany in 2010 there were around 33 plants for biomethane production with capacities between 148 m 3 and 5000 m 3 per hour.

The declared goal of the federal government is that by 2020 around 6 billion m 3 of biomethane will be produced in Germany per year. This corresponds to the capacity of around 1200 to 1800 biomethane plants.

Since January 2005, the ARA Region Lucerne has been the first sewage treatment plant in Switzerland to feed its sewage gas, processed according to natural gas parameters, into the natural gas network as biofuel . In Switzerland, more large biogas processing plants are being built in the Bern region. A prototype from the German Biomass Research Center (DBFZ) for the large-scale production of synthetic biogas from wood chips is in operation in Austria . In Sweden, too, processing and feeding is already being carried out.

Individual evidence

  1. DVGW
  2. VDI 3896: 2014-06 (draft) emission reduction; Upgrading biogas to natural gas quality. Beuth Verlag, Berlin, p. 15.
  3. DVGW, 2008: DVGW worksheet G 260: Gas quality
  4. "Biogas Use in Rural Areas - The Contribution of Various Plant Concepts to Regional Value Creation and Their Environmental Performance, Study by the Institute for International and European Environmental Policy", 42 pages ( pdf  ( page no longer available , search in web archivesInfo: The link was automatically created as marked defective. Please check the link according to the instructions and then remove this notice. ).@1@ 2Template: Toter Link / www.sachsen-anhalt.de  
  5. World's first large-scale production of biomethane from wood chips

literature

  • Fachagentur Nachwachsende Rohstoffe eV (Ed.), 2007: Feeding biogas into the natural gas network. Study, 3rd edition, 199 pages. ( pdf )
  • Institute for Solar Energy Supply Technology Association at the University of Kassel e. V. (Ed.), 2008: Biogas upgrading to biomethane. 6. Hanauer Dialog, proceedings. ( pdf )
  • Fraunhofer UMSICHT market survey on biogas processing and feed-in (pdf; 1.96 MB)
  • P. Hunziker et al. (2005): Feeding biogas into the natural gas network. Pioneering role of the ARA Lucerne region. GWA 4/2005: 1-8.
  • M. Faulstich et al. (2006): Energetic Use of Biomass: Potentials - Developments - Chances. Waste Days Baden-Württemberg, Stuttgart, September 26 and 27, 2006
  • A. Lühring / U. Walter (2007) Central electricity generation from biogas after passing through a separate gas network (PDF file; 1.23 MB)
  • dena - Deutsche Energie-Agentur GmbH (ed.), (Berlin 6/2009): Biogas partners - feed in together. Biogas feed-in in Germany - market, technology, players
  • German Association for Water Management, Sewage and Waste (DWA): Leaflet 361 "Preparation of Biogas", (October 2011), ISBN 978-3-942964-06-7

Web links