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- Biogas technology in
agriculture - a round thing
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- Because of high
energy costs during the seventies, biogas plants became widely
used in both communal and industrial sewage treatment works,
where they not only helped produce energy but also performed
a variety of important functions in the pretreatment of highly
organically polluted sewage, hygienization and stabilization
of sewage sludge.
- Biogas technology
failed to have the same degree of success in the agricultural
field, however. On the one hand, this was due to the limited
rarige of operationally safe and well-tested systems available
and to negative expen- ence with cheap solutions in the past.
On the other, it was due to the relatively 10w pri<;e paid
by the power auth~rities for electricity fed into the public
network, along with the 10w level - or indeed total lack - of
account taken of such items as manure efficiency, soil improvement,
water protection and avoidance of gaseous emissions in agricultural
economics.
- Over the last
few years, there has been a drastic change in the perceived significance
of almost all these factors. As a result, it is actually the
ecological advantages of the biogas method which could make it
a vitally important factor for the continued existence of many
large- and medi um-scal e busi nesses:
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- The ,,fertilizer factory"
A biogas plant is not only a suppuer of energy. lt also helps
redaim valuable plant nutrients from liquid manure and slurry
in a biologically and ecologically viable form and to ,,manufacture"
valuable soil improvement material.
- Once well fermented, biogas
liquid manure can be applied all Year round - even to growing
crops - without risk of causticization. Optimized process control
ensures that nitrogen loss in ammonia form - resulting in unpleasant
smells and environmental pollution - is avoided. Thanks to the
anaerobic processing method, all the nitrogen content is in ammonium
form, which reduces the risk of erosion and ground water pol
lution to a minimum.
- After biological treatment in
the reactor, the organically bound nutrients are almost completely
mineral ized, i.e. transformed into a form in which theyare utilizable
by plants. The organic ,1liquid manure" has been replaced
by a fully-fledged ,,liquid fertilizer" with trace elements
and growth hormones.
- All that is actually extracted
frqm the liquid manure is carbon, hydrogen (CH4) and small quantities
of oxygen (C02). This means that around 80 to 90% of the biodegradable
organic substances are transformed in the reactor and the nutrients
they contain converted into a water-soluble form. All the plant
nutrients remain in the substrate and can be applied according
to plant growth, which greatly increases efficiency.
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- The digester
The technology of biogas production
is a complex one, since biological processes need to be optimized
taking individual structural and hydraulic requirements into
account. Perfect thermostatization, continuous blending, homogenization,
reduction and injection of the substrate are all vital preconditions.
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- The power station
- Biogas from liquid manure can
be used to provide hot water, electricity and automotive energy
without äny further processing (desulphurization). The energy
is provided by methane gas (CH4) which is produced by highly
special ized bacteria when organic material decomposes in an
oxygen-free atmosphere. During this process, the solar energy
stored by the plant in the form of an organic substance is bacterially
converted into a directly utilizable form.
- Biogas contains ca. 50 to 70%
methane, which corresponds to an energy content of 5 to 7 kW
per m³. In modern heat recovery generation sets (gas motor),
an efficiency factor of more than 90% is achieved in conversion
to hot water and electricity. (In comparison: modern calorific
and atomic power stations operate with an efficiency factor of
around 40%). Since there is no opportunity for transmission or
transport loss from high-voltage power cables, it is possible
to make virtually full use of theprimary energy available.During
combustion in heat recovery generation sets, the energy contained
in the methane gas is converted into electricity (or automotive
energy) and hot water at a ratio of 1:3. The waste gases mainly
cosist of C02 and water (catalytic mode). The C02 released is
a product of plant photosynthesis, is extracted from the air
during the - process of plant biomass production and is released
again when the biogas burns. This completes the C02 cycle so
that energy won from biogas does not contribute to the controversial
,,greenhouse effect".
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