Methane emissions in biogas production - The Biogas: Science, Production and Applications - page 255

the closed buffer in the investigation was representative of an open buffer. In
practice, an open buffer would show a higher heat loss and therefore lower
average temperatures than for the situation under investigation. This would
likely reduce the production of methane in the digestate buffer. The value
presented was therefore a conservative value. It should also be mentioned
that the methane emissions from open storage systems depend on the
hydraulic retention time of the material in the digesters.
The above-mentioned result is representative for plants with long digester
retention times (longer than 100 days). In plants with the same feedstock
and a shorter retention time, the methane emissions during the digestate
storage phase would be higher, as less of the biomass carbon content would
have been converted to methane during the digestion stage.
The results presented by Liebetrau et al. (2011a, 2011b) on open digestate
tanks are not consistent because the emissions depend on many process
parameters (temperature, retention time, pressure, wind velocity, filling level
and so on). Results from ten digestate tanks show that some of the storage
tanks can produce substantial emissions, whereas others show very little by
way of emissions. Two types of digestate storage tanks were investigated -
open and covered. Methane emissions for open tanks ranged between 0.224
and 11.220% of the utilized methane. For covered tanks, methane
production (not emissions) amounted to 0.638-10.299% of the utilized
methane. The authors of this chapter believe that the similarity between
methane produced in a covered storage tank and methane emitted from an
open storage tank can be explained by coincidental occurrence and is not
directly connected to the process parameters mentioned above. Emissions
from properly covered tanks can be avoided to the same extent as in the
digester and for manure storage.
In the study by Amon et al. (2006), the lowest methane emissions were
measured from anaerobically digested slurry, as the fermentation process in
the digester had already degraded a considerable part of the organic dry
matter to methane and carbon dioxide. However, methane formation
continued during storage of the digested substrate. Methane produced after
the substrate has left the digester should be collected as well and used as
energy source.
Liebetrau et al. found the main emission sources to be the open digestate
tanks and the gas utilization system. The results for the open digestate tanks
do not represent the average emissions because they depend on too many
influencing factors (e.g. temperature, retention time, weather conditions,
etc.) and therefore can only represent a very short time span. All other
components play a minor role as long as they function properly and have no
leakages.
Burgstaler et al. (2011) reported losses up to 7% after a storage period of
one month. This storage period is shorter than the storage periods

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