Methane emissions in biogas production - The Biogas: Science, Production and Applications - page 248

Table 11.2 Reported emissions from different AD components

Min. average loss
of methane (%)

Max. average loss
of methane (%) AD components Configuration

Silage storage — 0.00065 0.00065

Feeding systems Screw conveyor 0.0079 0.0079

Dosing feeder 0.00029 0.16

Substrate storage
tank

0.005 0.311

Mixing tank 0.013 0.288

Digester Foil cover 0.006 0.0244

Gas-tight cover 0 0

Concrete roof 0 0

Digestate storage tank Open 0.224 11.22

Covered 0.638 10.299

Gas utilization CHP 0.44 2.43

Upgrading 1.5 1.5

emissions of 1800 gCH 4 /ton manure for manure storage. With the same
assumptions as above, this corresponds to 20% of the total methane
produced. It is noted that the long-term storage of 80 days refers to storage
at farms and actually is outside the scope of this study. However, this is
included here to give some insight into the magnitude of storage emissions.
A typical storage period at a digestion plant is around one week, and
corresponding emissions will therefore be around 10% of the values

mentioned above.
Liebetrau et al. (2011a, 2011b) analyzed emissions from ten biogas plants
with different types of components, capacity, process conditions and so on.
The study focused on five major plant components: silage storage, feeding
systems, digester (foil cover, gas-tight cover), digestate storage tanks (sealed,
not gas-tight covered and open) and gas utilization (CHP or upgrading).
The average methane emissions are summarized in Table 11.2.
Focusing on emissions during silage storage, the average loss of methane
at the ten biogas plants was shown to be extremely low, amounting to only
0.00065% of methane produced. This is considerably lower than the value
reported by Zwart et al. (2006), who mentioned that methane emissions
during long-term storage of substrate are 3100 gCH 4 /ton substrate. This
depends not only on the biogas yield of the energy crops, but also on process
conditions such as the storage temperature and storage period. Since the
data of Liebetrau et al. are based on actual measurements while those of
Zwart et al. are based on an assumption that could be invalid, it can be
safely concluded that emissions from storage of co-substrates are negligible.
Zwart et al. (2006) also note that 95% of methane emissions during

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