Methane emissions in biogas production - The Biogas: Science, Production and Applications - page 249

manure storage can be avoided by feeding the manure as soon as possible to
the digester. This is not the case for stored co-substrates.
In this chapter, emissions have been converted to a percentage of the
methane that is actually utilized. These relative emissions from storage of
substrates, however, depend on the feedstock mix that is applied at the AD
plant. For manure, methane emissions from storage are significant, but
potential biogas production is low, leading to high relative emissions. For
other co-substrates, the opposite is true. Since the specific biogas production
from co-substrates is generally much higher than for manure, the relative
emissions from the combined feedstock are much lower than for manure
alone.
This can be explained by assuming a feedstock mixture of 50%manure and
50%maize. The emissions from the short-term storage of the manure will be
10% of 1800-4000 gCH 4 /m 3 manure, with a biogas production of 21m 3 /
m 3 manure. Emissions from the storage of maize are negligible, but biogas
production is 180m 3 /ton. The combined emission is 180-400 gCH 4 for
201m 3 of biogas produced. With 60% CH 4 in the biogas, this corresponds to
an emission of 0.2-0.5% of the methane production. For a feedstock mix
with 50% manure and 50% maize, the relative emissions from feedstock
storage can be estimated at 0.2-0.5% of methane available for utilization.

11.2.2 Digester

In the analysis of greenhouse gas emissions from ten biogas plants within the
agricultural sector, Liebetrau et al. (2011a, 2011b) reported on emissions
from digesters with different types of covers. Besides the detected leakages,
the general emissions from the digesters with supposedly gas-tight covers
were quite low. Leakages generally resulted from poorly maintained service
and inspection ports and a poor design and/or quality of lead-through pipe
work. No emissions were detected after taking care of these types of leakages.
Digesters with a concrete roof did not show detectable emissions at all. For
digesters with a flexible foil cover, emissions were occasionally reported. The
maximum emissions amounted to 0.0244% of the utilized methane. All
emission sources were traced to the connection between the foil and the
digester. No emissions were found through the membrane material itself.
In a report commissioned by the Swiss Bundesamt fu¨ r Energie (BFE),
Bu ¨ eler (2011a, 2011b) reported experimental data for emissions due to
diffusion or leakage through membranes fixed on top of a digester. This
work dealt specifically with possible emissions from the gas buffers
commonly applied in biogas plants, consisting of a flexible ethylene
propylene diene monomer M-class rubber (EPDM) membrane, normally
fixed to the top of the digester; such gas buffers are applied either as a single-
or double-membrane version. In the single-membrane version, the volume

￿ ￿ ￿ ￿ ￿ ￿