Methane emissions in biogas production - The Biogas: Science, Production and Applications - page 254

Boilers and flares

No experimental data have been reported regarding boilers and flares in the
literature covered in this chapter.

11.3.2 Storage of digestate

Methane emissions during digestate storage are considered by Amon et al.
(2006), Woess-Gallasch et al. (2007a) and Liebetrau et al. (2011a, 2011b).
Among the different treatment options, AD of manure is reported to lead to
the highest methane reduction. Amon et al. (2006) report that 1242.6 g of
methane are emitted per m 3 of digested cattle manure. This corresponds to
14% of the total methane production.
In a report commissioned by the LandesEnergieVerein Steiermark in
Graz, Austria, Woess-Gallasch et al. (2007a) investigated possible emissions
of methane and other greenhouse gases from biogas plants. Specific
measurements were performed for the emissions from the digestate buffer
of a full-scale biogas plant in Paldau, Austria. Greenhouse gas emissions
from other sources were estimated on the basis of literature values and
applied in a life cycle assessment for different configurations.
The digestate buffer at the plant investigated had a closed container with
a concrete roof. The feedstock for the previous digestion step was a
combination of cattle and pig manure along with corn and grass silage. The
biogas production in this buffer and the actual methane concentration were
measured and considered to be representative for a similar plant with an
open digestate buffer. The production of biogas resulted from continuation
of the digestion process in the buffer, as the temperatures remained near the
optimal digestion temperature. Emissions in the summer months (between
May and October 2006) were determined, corresponding to a complete cycle
of filling up and withdrawal.
The total biogas production of the plant was 2365 million Nm 3 of biogas,
corresponding to an input of 9.900 tons of feedstock. The feedstock
comprised corn silage (27%), corn cobs (32%) grass silage (7%), pig
manure (31%) and cattle manure (3%). The plant consisted of two main
digesters and two secondary digesters, with residence times of 64 and 35 days
respectively. These values were considered representative.
The yearly biogas production corresponded to 270Nm 3 /ha on average.
The emission from the digestate buffer was determined to be 3.9Nm 3 /ha.
The methane content in the biogas was 63.8% and was clearly higher than
the average methane content of the biogas produced, which was 48.8%.
Taking this difference into account, the methane loss from the digestate
buffer was determined as 1.9% of the total production of methane.
It should be noted that this calculation was based on the assumption that

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