Environmental Engineering Reference
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points where the biogas is supplied to a grid either as upgraded biogas
(green gas) or as electricity and heat. Storage of the digestate is included.
Excluded are emissions from the production and transport of (co)
substrates and the distribution and final use of upgraded biogas.
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Results. Figure 11.3 summarizes the emission results in different biogas
production chains.
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Emission from storage of substrates. The storage of manure is one of the
most important sources of methane emissions. Based on two sources, a
range of 20-45% of the total methane production has been found. This
is a large range, but it is clear that (long-term) storage of manure results
in very significant methane emissions. It should be noted that emissions
from storage also take place when the manure is stored for uses other
than biogas production. In fact, in combination with digestion, long-
term storage of manure is not practised as this has a negative impact on
gas potential. Instead of manure, the digestate is the main product that
will be stored over long periods, when land application is not allowed
(e.g. in winter).
Although the reported numbers vary significantly, it can be
concluded that emissions from the storage of co-substrates, such as
silage of various agricultural residues, are negligible. Taking a mix of
feedstocks into account (manure and co-substrates), the (short-term)
substrate storage at a biogas plant results in a relative emission of 0.2-
0.5% of methane production.
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Emissions from digestion. Two articles discuss emissions from digesters.
Besides leakages resulting from poor maintenance, both articles report
limited emissions, from 0.02 to 0.07% of the total methane production
for foil-covered digesters.
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Emissions from gas engines. Many of the studies report on emissions
from natural-gas-fueled engines. It is not clear from the studies to what
extent this represents the emissions of biogas-fueled engines, although
there is no reason to assume this differs very much. The reported
methane slip shows a wide range, from 0.5% up to 6.0%. The authors of
this chapter conclude that this is one of the most important sources of
methane emissions.
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Emissions from biogas upgrading. Very few quantitative data are
reported on methane losses in biogas upgrading. Equipment suppliers
give numbers relating to leakage, but limited independent measurements
are available. The sources based on measurement reports indicate
methane emissions up to 1.5% of the total methane production. From
the experience of the present authors, the market for biogas upgrading
technology is still developing and none of the available upgrading
technologies has yet become dominant. Since the leakage of methane
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