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Biological or chemical desulfurization
Spoilt for choice for biogas plant operators
gen sulfide, sulfide and their intermediates who stand in focus. These compounds
interfere with the biology and thus the
fermentation in the biogas plant. Sulfide
forms with trace elements such as cobalt,
nickel, molybdenum and selenium hardly
soluble compounds. So that these nutrients are no longer available for the methane bacteria, shortages may occur with
the consequence of a greatly reduced
methane formation. Intermediates accumulate and it comes to acidification - acidosis. Hydrogen sulfide is not only toxic
to the sensitive methanogens, it is also
harmful for equipment and engines.
Due to sulfuric acid there is acidic destruction on wood beam structures.
T
he biological desulfurization, partly
in combination with chemical methods is still widely used in biogas
plants. It seems to be simple and inexpensive: air is blown into the gas chamber and special wood beam structures or
networks are used as the sulfur bacteria
colonization surfaces. In the presence of
oxygen the bacteria convert hydrogen
sulfide to elemental sulfur, which is visible
as yellow deposits in the gas space. However, this method has also limitations
and risks in itself: sulfide or hydrogen sulfide are formed again from the elemental
sulfur.
When it comes to the desulfurization in
biogas plants, there are primarily hydro-
H2S
53%
Approximately
of claims
caused by hydrogen sulfide in a biogas
plant, happen in the CHP.
There is a chemical equilibrium between
sulfide and hydrogen sulfide in every
biogas plant. Changes in temperature
and pH can move it in favor of sulfide or
hydrogen sulfide. If biologically desulfurization is chosen over time considerable
sulfur deposits build up in the gas space.
These are usually classified as harmless.
However, since air is injected continuously
the sulfur in the deposits is further oxidized to sulfate - the salt of the sulfuric
acid. Sulfuric acid always means corrosion
of components. In particular, the transition zones of the liquid to the gas phase
are affected. Chunks break out of the debris and fall into the fermenter. Both the
sulfur as well as the sulfate are reduced to
unwanted sulfide and thus interfere with
the biology. It creates a cycle between hydrogen sulfide -> sulfur -> sulfide / sulfate -> hydrogen sulfide. Therefor no real
desulfurization takes place. The sulfur remains in the biogas plant. Thus biological
desulphurization makes little sense.
Who has recognized the limits of biological desulfurization, uses chemical
desulfurization processes. In general,
these are iron-based compounds, which
are applied. Methatec Detox S is chemical desulfurizing agent based on iron
hydroxide. Already in the liquid phase in
the fermenter, the sulfur compounds are
absorbed and precipitated as iron sulfide. This prevents the binding of trace
elements and the formation of hydrogen sulfide. Damage to components in
the fermenter and the CHP are avoided,
fermentation and methane production
run smoothly. Due to its very high active
substance content (50 percent iron), the
If the pH is measured in the sulfur deposits, it is always in the acidic range
and thus gives a clear indication for
the formation of unwanted sulfuric
acid.
use of Methatec Detox S Premium is very
effcient.
Questions are answered by Dr. Sabine
Rahn, Phone (+49) 251 682-2289. More
information is available under www.
terravis.info and www.biogasanlagen-fuettern.de.
i
At a glance:
Biological desulphurization
The biological desulfurization is used
to minimize the hydrogen sulfide content in biogas plants. By injecting air
into the fermenter aerobic bacteria
oxidize hydrogen sulfide to elemental
sulfur. However, the injected air will
also dilute the biogas formed and thus
reduces the concentration of methane.
If, for example during the gas analysis,
the oxygen content measured in the
biogas is 0.6 percent, this corresponds
to an air content of three percent in
the biogas.
Thus, the methane content is diluted
to 51 percent. However without air
injection it would be at 53.7 percent.
Thus in this example, the process of
biological desulphurization results in
a 2.7 percent reduction in methane
output. It is now known that using
biological desulfurization can reduce
the methane content even up to eight
percent.

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