Biogas

The biogas is the Gaz produced by the Fermentation of animal or vegetable organic matters in the absence of Oxygène.

This fermentation called also Méthanisation occurs naturally (in the marshes) or spontaneously in the Décharge S containing of the Déchet S organics, but one can also artificially cause it in Digesteur S (to treat muds of purification, waste organic agricultural Industrie ls or , etc).

Biogas is a mixture primarily made up of Méthane (typically 50 to 70%) and of carbonic gas, with variable quantities of Eau, hydrogen sulfide (H2S). One can find others made up coming from contaminations, in particular in biogases of discharges.

The energy of biogas comes only from the Méthane: biogas is thus the renewable form of the very current fossil energy which is the Natural gas which contains primarily methane but also butane, propane and other elements to him. One can also use the term Biométhane.

One distinguishes three beaches from production of biogas according to the temperature.

  • 15-25 °C : psychrophilous
  • 25-45 °C: mésophile
  • 45-65 °C : thermophilous

In fact the Digesteur S mésophiles are used (with 38 °C) in the moderate zones.

The recovery of the biogas produced by the discharges is all the more interesting as methane is a gas with Greenhouse effect much more powerful than the Carbon dioxide (CO2) produced by its combustion.

Sources of biogas

Biogas is the result of the Méthanisation or anaerobic digestion of fermentable waste. The most current sources of biogas come from voluntary or involuntary storages of organic matter:

  • discharges. Their content of biogas is more or less high according to the sealing of the mode of exploitation. In France, the recovery of the biogas of discharge is obligatory since years. Its energy valorization should be an obligation. The simple destruction in flare is only one makeshift. They is thus several thousands of m ³ /h of methane which could be recovered and used on the great discharges (1 m ³ methane = 1 liter of gasoline!)

  • the selective collection of putrescible waste allows a methanisation faster than discharges some by using specific bioréacteurs (digesters).

  • muds of the purification plants. Methanisation makes it possible to eliminate the organic compounds and makes it possible the station to be more or less autonomous in energy.

  • effluents of breedings. The regulation makes compulsory the equipment of storage of the effluents (liquid manure, manure) for a capacity higher than 4 months. This time of storage can be made profitable for the methanisation of the effluents. It is of the animal manure but also about other agricultural waste: residues of culture and ensilage, effluents of dairies, withdrawals of the markets, grasses etc

  • the effluents of agribusiness industries can also be methanized. The goal is mainly to avoid the too rich organic matter rejection, and can be accompanied by an energy valorization.

  • bottom of the lakes and marsh: biogas is produced there naturally by the organic sediments which if accumulate. The use of the biogas of the Lac Kivu was undertaken it more than 40 years and now developed there with large scales.

Greenhouse effect

Biogas is primarily made up of methane, Gaz with very important greenhouse effect. Its combustion produces carbon dioxide, also gas with greenhouse effect, but whose impact is largely less. A methane molecule (CH4) is equivalent to 22.5 carbon dioxide molecules (CO2) in term of greenhouse effect.

Advantage-uses

The advantages of the valorization of biogas as fuel are:

  • reduction of the emissions of Gas to greenhouse effect, as indicated above.
  • but that also makes it possible in substituent other exogenic energies (fossil and nuclear power) to be released from the incomes for the owner who saves on his energy expenditure and/or, more and more sells his energy.
  • Reduction in the carbon load of the vegetation wastes. Once digested, waste is less harmful for the environment; the risk of an organic pollution is largely reduced. Moreover, fermentation decreases the percentage of dry matter, making it possible to decrease volume with épandre.
The uses of biogas vary:
  • the most current method is to burn it in a gas engine or a small turbine, to produce electricity injected on the network (more than 4000 installations in Germany), and often of heat in Cogénération.
  • When an industry consuming fuel (thermal power station, cement factory, collective boiler room…) is located in the vicinity, biogas can be to him provided like fuel.
  • It is also often useful in the Serre S, to heat them and enrich them in.
  • It is used as fuel for vehicles GNV, in substitution for natural gas of the network which is fossil for him. It feeds from the captive fleets (like buses, garbage dumpsters) to see Biogaz carburizing, or even the individual vehicles (Suisse and Sweden).
  • It can also be injected on the natural gas network. It is the solution which offers the best energetic efficiency, if the network is rather close to the point of production. This solution runs up against the reserves of the gas groups, like was to it the renewable injection of electricity for EDF.
  • the methane contained in biogas can also be reformed to form renewable Hydrogène or biohydrogene.

In the last both cases, one extracts CO2, water, the sulfur compounds to obtain a gas made up with more than 96% of substitutable CH4 to fossil natural gas of the network. For the other applications, a gas containing methane 60% is largely sufficient, therefore to purify it would be an useless expenditure. One is then satisfied to remove the impurities which present problems of pollution, corrosion or odor, in particular the sulfur compounds.

To the Mali, pilot projects were carried out in distant zones and near largely illiterate populations, in order to see biogas up to what point could produce energy of domestic use from a durable point of view. The experiment showed that with the training of local craftsmen being able to deal with the production of the equipment necessary (gasometer, digester) and the training of the families to the maintenance of the equipment, biogas can be a viable alternative to the use of woody fuels for the cooking of the meals and improve the living conditions by other contributions in energy (refrigeration in particular). The pressure on the woody resources decreased and the produced compost was used in agriculture for the fertilization of the grounds. A financial support remains necessary for the installation of the system (equipment, installation, formation) Production and use of biogas in 4 communes of the area of Kayes ([[Mali])]

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