How is the structure of biogas




As Biogas a mixture of the main components methane and carbon dioxide is predominantly referred to, although other gases (e.g. hydrogen) are also of biological origin. The valuable part that is used energetically is methane. In addition, depending on the initial conditions, it contains small amounts of water vapor, hydrogen sulfide, ammonia, hydrogen (H.2), Nitrogen (dinitrogen N2) and traces of lower fatty acids and alcohols.

common knowledge

Biogas is produced during the anaerobic (oxygen-free) fermentation of organic material. Suitable starting materials for the technical production of biogas are:

  • fermentable, biomass-containing residues such as sewage sludge, biowaste or leftovers,
  • Farm manure (liquid manure, manure),
  • Plants or parts of plants not previously used (e.g. catch crops and grass clover in organic farming)
  • specifically cultivated energy crops (renewable raw materials).

Agriculture, with the last three options mentioned, has the greatest potential for the production of biogas. Except for the last possibility, these are basically free starting materials (apart from transport and other ancillary costs).

Depending on the origin, after

  • Sewage gas (from wastewater treatment plants), (in large wastewater treatment plants, the gas is used after cleaning in gas engines to generate electricity, with which, for example, the fans for the activated sludge basins are driven),
  • Landfill gas (from landfill) and
  • Biogas (made from plants in biogas plants)

differentiated.

In the case of the uncontrolled emergence through natural processes and the unused escape into the atmosphere - from water, manure, rice fields and animal sources such as the rumen of ruminants - the gas is called in general Digester gas or Swamp gas.

Biogas always contains undesirable components such as hydrogen sulphide, which can be withdrawn before technical use if necessary. In Germany, due to the legally guaranteed remuneration, biogas has so far mostly been used as fuel for combined heat and power plants to generate electricity or for heating purposes. In Sweden, where electricity generation from biogas is unprofitable due to lower electricity prices, the most widespread usage variant is upgrading to natural gas quality and feeding it into the gas network, as well as using it as fuel in gas vehicles (so-called fordonsgas). In Germany, too, people are thinking about feeding biogas into natural gas pipelines. [1]

composition

The information on the composition of biogas that can be found in the literature is very different. In general, the gas composition depends on various parameters, such as the composition of the substrate and the mode of operation of the digester. The following table shows reference values ​​for the most important gases contained according to the latest DVGW study.

Fluctuation rangeaverage
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 / m³0.7 mg / m³
Hydrogen sulfide 10-30,000 mg / m³500 mg / m³

Methane is valuable in biogas. The higher its proportion, the more energetic the gas. The carbon dioxide and water vapor cannot be used. The main problems in biogas are hydrogen sulfide and ammonia, which have to be removed before the combustion process in order to protect the gas engines from these chemically aggressive substances.

use

Europe

Biogas energy 2006 (GWh)[2]
Country total Landfill gas Sewage gas Other
Germany 22 370 6 670 4 300 11 400
UK 19 720 17 620 2 100 0
Italy 4 110 3 610 10 490
Spain 3 890 2 930 660 300
France 2 640 1 720 870 50
Netherlands 1 380 450 590 340
Austria 1 370 130 40 1 200
Denmark 1 100 170 270 660
Poland 1 090 320 770 10
Belgium 970 590 290 90
Greece 810 630 180 0
Finland 740 590 150 0
Czech Republic 700 300 360 40
Ireland 400 290 60 50
Sweden 390 130 250 10
Hungary 120 0 90 40
Portugal 110 0 0 110
Luxembourg 100 0 0 100
Slovenia 100 80 10 10
Slovakia 60 0 50 10
Estonia 10 10 0 0
Malta 0 0 0 0
EU (GWh) 62 200 36 250 11 050 14 900
Biogas 2006 (MWh / 1000 residents)[2]
Country MWh / 1000 Bw.
UK 327
Germany 271
Luxembourg 226
Denmark 202
Austria 166
Finland 141

Germany

The information on the use of biogas in Germany is not entirely uniform because of the inconsistent allocation and problems with data collection. Sometimes sewage gas is fed separately, sometimes not; the classification of wood gas remains unclear. According to the web link (BMU) listed below, there are now around 2700 biogas plants in Germany in 2005 with an installed electrical output of 665 MW. In 2005, 2,500 GWh of electricity were generated. This corresponds to 0.42% of the total electricity consumption of approx. 600,000 GWh in Germany. The source gives these figures for electricity generation from gaseous energy sources and thus also contains an unrecognized share of electricity from wood gasification plants. This avoids the emission of 2.5 million tons of carbon dioxide annually.

For Lower Saxony alone, the potential is estimated at at least 1,500 systems, which together with 2,600 GWh could cover at least 5 percent of the total electricity consumption in Lower Saxony.

Due to the independence from wind or solar radiation, the biomass and thus also biogas makes a meaningful contribution to taking a supplement in the energy mix of renewable energy sources.

The first public biogas filling station in Germany was opened in June 2006 in Jameln im Wendland.

Switzerland

A Swiss biogas plant manufacturer and operator gains biogas from the fermentation of biowaste. This company operates 22 of 83 gas filling stations in Switzerland. At many gas filling stations, a mixture of Kompogas and natural gas is sold under the name "natural gas". In May 2007 Kompogas' share in the Zurich area was around 40%, while in French-speaking Switzerland it was 0%. Throughout Switzerland, the share of biogas in "natural gas" should be at least 10%. In Switzerland, Kompogas is often used when referring to biogas. Since there are currently no legally prescribed subsidies (feed-in tariffs for electricity generated from biogas) throughout Switzerland, the potential of biogas production in Switzerland, in particular in agriculture, is still relatively little exploited. The potential of agriculturally operated biogas plants in Switzerland is around 700 plants by 2020. Co-substrate exports to EU countries are currently considered to be economically problematic. Biogas plant operators in Switzerland are very often inferior in price to the co-substrate buyers from EU countries.

Manufacturing

Biogas from energy crops

Analogous to the use of wood in biomass cogeneration plants, more and more plants are being cultivated specifically for decay in biogas plants, i.e. for the production of biogas. In principle, this can be any agricultural crop or grass. Currently (2004) the use of maize, grain (fields) and grass (meadow) is most widespread.

To estimate the use for power generation with average efficiency:

1 ha of maize = approx. 2 kW electr. Continuous output
1 ha of grain = approx. 1.5 kW
1 ha of grass = approx. 1 kW
Manure from 1 cow = approx. 0.15 kW

Example: With the manure from 4 cows or 32 pigs, or with the yield of 6,000 square meters of silage maize, enough biogas could be produced to supply a four-person household with electrical energy.

However, the decisive factor for the overall efficiency of land use is the crop sequence. If a crop with a high energy yield is the only economically viable crop in the year of cultivation, this may be less efficient overall than the cultivation of several economically usable crops (i.e. as food, feed or biogas raw material) over the year, each with crops lower energy yield.

Microbial processes

The biogas production takes place in a biogas plant. Various types of anaerobic microorganisms are involved in the controlled process of biogas formation, the proportions of which are influenced by the starting materials, pH value, temperature and digestion process. Due to the adaptability of these microorganisms to the process conditions, almost all organic substances can be broken down by putrefaction. Only higher proportions of wood can be used poorly due to the microbiologically difficult to decompose lignin. A prerequisite for successful methane formation is a water content in the starting substrate of at least 50%.

According to the current state of knowledge, a distinction is made between four parallel or sequential and interlocking individual biochemical processes that enable the anaerobic degradation of biogenic substances:

 

  1. During the hydrolysis the biopolymers are broken down into monomeric building blocks or other soluble degradation products. Fats are converted into fatty acids, carbohydrates, such as B. Polysaccharides, broken down into mono- or oligosaccharides and proteins, into peptides or amino acids. These reactions are catalyzed by facultative anaerobic microorganisms, which hydrolyze the reactants by means of excreted exoenzyme. This reaction step is rate-limiting due to the complexity of the starting material.
  2. As part of the Acidogenesis (also generally referred to as fermentation) - which takes place at the same time as hydrolysis - the monomeric interducts are converted into lower fatty / carboxylic acids, such as B. butyric, propionic and acetic acid, on the other hand in lower alcohols, such as. B. ethanol implemented. In this conversion step, the facultative anaerobic microorganisms gain energy for the first time. In this reaction, up to 20% of the total amount of acetic acid is formed.
  3. During the Acetogenesis the lower fatty and carboxylic acids as well as the lower alcohols are converted to acetic acid (anion acetate) by acetogenic microorganisms.
  4. In the last, obligatory anaerobic phase - the Methanogenesis - the acetic acid is converted into methane by corresponding acetoclastic methane formers according to equation 1. According to Equation 2, around 30% of the methane is formed from hydrogen and CO2.
Equation 1:
Equation 2:

What remains is a mixture of poorly biodegradable organic material, for example lignin and inorganic substances such as sand or other mineral substances, the so-called digestate. It can be used as a fertilizer because it still contains all trace elements and also almost all of the nitrogen in the substrate in bioavailable form (NH4 + at neutral pH).

technology

The biogas production takes place in a biogas plant. The interaction of the microorganisms is not sufficiently well known. It is difficult to find control parameters for a regulated process designed for maximum methane yield (these are mostly based on experience). Research projects to explain the exact course and characterization of the microbiological populations or communities will soon be able to provide information about the exact course.

To maintain the digestion process, with low substrate concentrations and thus large amounts of water, around half of the waste heat from electricity production with biogas is required to maintain the temperature of the biogas plant. Dry fermentations, which process solid, stackable substrates without adding water, require a maximum of 10% of the heat produced. The remaining heat can be used for other heating purposes. The optimal use of waste heat and temperature control in the process are therefore decisive for the overall efficiency of such a system.

Cleaning and processing

As already indicated in the previous section (composition of biogas), the contamination by hydrogen sulfide and ammonia has a negative effect on the use of biogas. It is therefore almost always necessary to carry out cleaning and reprocessing. There are essentially four process steps.

Desulfurization

There are different possibilities for this. If necessary, several stages are necessary, such as coarse or fine desulphurisation.

  1. Purification after gas production by desulphurisation filters. Here the gas is passed through ferrous filter material (lawn iron ore, steel wool). The filter material must be replaced or regenerated (lawn iron ore) when it is saturated.
  2. Cleaning in the gas space by adding oxygen. The H2S (hydrogen sulfide) is converted into elemental sulfur. The sulfur is deposited in the gas space. This is the most common and cheapest method so far, but has the disadvantage that elemental sulfur accumulates in the system. The process can only neutralize a limited amount of hydrogen sulfide. In addition, this process is only possible if the biogas is used on site in a CHP or burner. When purifying to natural gas quality, the remaining air fractions can hardly be removed, since air and methane behave similarly in many ways.
  3. An improved method is to add air or other oxidizing agent directly to the reactor liquid (Linde patent). The same process takes place as with gas space desulphurisation.
  4. Lye washing: The biogas is washed with lye in countercurrent in a packed column. The lye must then be disposed of. The caustic wash also depletes CO2 in the biogas.
  5. Biological desulphurisation: Similar to caustic washing, half of the caustic is regenerated in a second aerobic reactor. The result is a sulfur-free wastewater stream and elemental sulfur sludge that is reduced compared to lye washing. (Paques patent)
  6. With high protein contents in the starting substrate, the hydrogen sulfide concentrations can already exceed 20,000 ppm. Every filter is overwhelmed here. The addition of iron ions helps to prevent the formation of hydrogen sulfide in the digester because of the high affinity for iron. The iron combines with sulfur to form insoluble iron sulfide (FeS). The iron sulphide remains in the liquid manure as a solid.
  7. Irreversible adsorption on activated carbon. The activated carbon is partially iodized to increase the loading capacity. This method is only suitable for very low H2S concentrations, e.g. as final cleaning.
  8. Return of partially desulphurized biogas to the reactor liquid. This causes the H which is still dissolved in the liquid to be driven out2S improved.

The inhibiting effect of the hydrogen sulphide dissolved in the liquid manure on methane formation should be noted here. If only the desulphurisation of the gas is taken into account and the desulphurisation of the reactor liquid is neglected, methane formation is reduced to zero due to the poisoning of the methane-producing bacteria.

compression

The compression of biogas is usually necessary when biogas is to be fed into the natural gas network after it has been processed. Above all, however, for use as fuel, strong compression to over 200 bar is necessary in order to obtain sufficient energy densities. Such high pressures can only be achieved with multi-stage compression.

Drying

Biogas is dehumidified by cooling the gas in the ground or by using compressor cooling. If the water vapor falls below the dew point, the water condenses (changes from the gaseous to the liquid phase). Then the water can be collected and drained off at the lowest points of the mostly underground biogas pipeline. When cooling by refrigeration machines, the water in the biogas falls out of the cold registers and can be collected and diverted there.

CO2-Separation

The processing of the biogas includes, in addition to the processes already described for desulphurization and for reducing the ammonia content, above all the reduction of CO2- and O2-Proportion. The currently common methods of methane enrichment by CO2- Separation is gas scrubbing such as pressurized water scrubbing (absorption process with water or special detergents) and pressure swing adsorption (adsorption process on activated carbon). As early as the 1980s, two sewage treatment plants in Germany separated the CO2 successfully operated for years in sewage gas through absorption agents such as monoethanolamine solution (amine scrubbing), in order to then feed them into the natural gas network. [3]In addition, other processes such as cryogenic gas separation (using low temperatures) or gas separation using a membrane are being developed for general use in the biogas sector.

See also

  • Future technology
  • Renewable energy
  • Bioethanol (ethanol fuel)
  • Biodiesel

literature

  • Biogas: electricity from liquid manure and biomass. Planning, technology, funding, returns. Top agrar, the magazine for modern agriculture. Landwirtschaftsverlag, o.O. 2000, ISBN 3-7843-3075-4
  • Martin Stroh, Saarbrücken: "From farmer to energy supplier. The future of the energy manager" (250 pages) free book about the development of the biogas industry
  • Fachagentur Nachwachsende Rohstoffe e.V. (FNR): various studies and publications on the subject of biogas and other bioenergy sources at http://www.bio-energie.de under the heading "Literature"
  • Heinz Schulz, Barbara Eder: Biogas practice. Basics, planning, plant construction, examples. Eco book, o.O. 2005, ISBN 3-922964-59-1
The book conveys the basics of biogas formation and production and deals with the plant technology (tanks, agitators, gas storage) and the associated operating equipment. A separate chapter is dedicated to the co-fermentation of organic residues (e.g. from food processing), which can improve the profitability of biogas plants.With an overview of advice centers and a detailed list of suppliers.
  • Bavarian State Ministry for the Environment, Health and Consumer Protection (StMUGV): Biogas Handbook Bavaria. Munich, November 15, 2004. [1]
The free brochure (50 pages) contains the basics and techniques for producing biogas as well as information on licensing procedures. The 500-page volume of material can be downloaded from the web address [2].
  • Agency for Renewable Resources e.V .: Frank Hofmann, André Plättner, Sönke Lulies, Dr Frank Scholwin, Dr Stefan Klinski, Klaus Diesel: Feeding biogas into the natural gas network; Leipzig 2006; ISBN 3-00-018346-9 [3]
  • Bernward Janzing: Not eligible - A nationwide energy supply is not possible with biodiesel. - Biogas versus biodiesel. In: taz May 20, 2006, p. 11
  • Analysis and evaluation of the use of biomass. A study on behalf of the BGW and DVGW (Wuppertal Institute, 2005). The study is available on the web (see links below): Volume 1- Overall Findings and Conclusions. Volume 2 presentation of the biogas potential for feeding into the existing natural gas network. Volume 3 Technologies and Costs of Biogas Upgrading to Natural Gas Substitutes.

swell

  1. spiegel.de: Natural gas from the corn field
  2. ab Biogas barometer 2007 - EurObserv’ER Systèmes solaires - le journal des énergies renouvelables n ° 179, p. 51-61, 5/2007
  3. http://www.idowa.de/aktuell/nachricht/nachricht/nac/2168636/red/66.html

Category: Biomass