Environmental Engineering Reference
In-Depth Information
gas yields based on VS or COD rather than on fresh matter due to large
variations in water content.
The Buswell formula
If the basic elementary formula (C
n
H
a
O
b
) of the feedstock is known, a
maximum biogas yield and methane content can be calculated according to
the Buswell equation (equation 3.2) proposed by Buswell and Hatfield
(1936). Using this formula, the theoretical uptake of water and the biogas
amount and composition can be calculated. The results, however, only give
a maximal biogas potential, and will often be much too optimistic since
neither non-degradable material nor energy demand of the microbes are
considered. A detailed example of how the Buswell formula can be used is
described in Chapter 5 of this topic.
a
4
b
2
n
2
þ
a
8
b
4
n
2
a
8
þ
b
4
C
n
H
a
O
b
þ
n
H
2
O
!
CH
4
þ
CO
2
½
3
:
2
Feed analysis
The content of lipids, proteins and carbohydrates of a biogas substrate can
be determined using standardised animal feed analysis methods. Then,
according to these feed components, the theoretical biogas yields and
methane concentrations can be estimated (VDI 4630; Baserga, 1998;
Bischofsberger et al., 2005; Braun, 2007). An overview of the theoretical
biogas yields and biogas composition is given in Table 3.1. The data in the
table were calculated by inserting best estimates for the elementary
composition into the Buswell equation (equation 3.2).
The calculation based on feed analysis gives only a rough estimate of the
theoretical maximum biogas yield. However, real values will be much lower
due to non-degradable material and energy demand of the microbes.
According to VDI 4630, for fats and carbohydrates from animal origin,
about 85% of the maximum biogas potential can be reached in practice; for
fats and carbohydrates from plant origin, reaching 50-70% of the maximum
is realistic. VDI 4630 also states that the methane content of biogas from
proteins can reach up to 70%. In practice, a BMP test is often preferable to
estimations based on feed analysis since it gives better information. However,
in some cases, feed analysis data are already available so additional
information on the maximal biogas recovery potential can be gained.
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