Environmental Engineering Reference
In-Depth Information
Table 3.1 Overview of theoretical biogas yield and biogas composition of
carbohydrates, lipids and proteins
Substrate
Elementary formula used for
Buswell calculation
Carbohydrates 746 50 50 (CH
2
O)
n
a
Lipids 1390 72 28 C
16
H
32
O
2
b
Proteins 800 60 40 C
13
H
25
O
7
(N
3
S)
c
a
The elementary formula for soluble sugars (e.g. glucose) was used according to
VDI 4630. However, it would be more realistic to use the elementary formula for
starch/cellulose (C
6
H
10
O
5
)
n
, since most of the carbohydrates appear as chains
(subtracting one water molecule) in biomass. This would give 830Nl/kgVS.
However, since these theoretical values are never reached in reality, it makes little
difference.
b
Biogas yield
(Nl/kg VS)
CH
4
(%)
CO
2
(%)
The elementary formula of palmitic acid was used as an estimate for lipid
elementary formula.
c
A best estimate for proteins according to the amino acid composition is used.
However, the elementary formula of proteins can vary considerably.
Source: Adapted from VDI 4630.
COD balance
By measuring the COD content of a feedstock, the chemically oxidisable
material can be determined. This amount of energy is the absolute
maximum of energy that could be recovered by biogas. However, under
chemical conditions, some substances can be oxidised that are not accessible
under biological conditions and will therefore remain in the digestate. Using
continuous fermentation trials (see Section 3.4.5), the residual COD in the
effluent of a stable process can be measured and therefore the exact COD
degradation determined (corresponding to a specific OLR). For easily
available feedstocks such as high-strength wastewater and industrial by-
products, a COD degradation of 75-85% can be assumed. According to
Speece (1996), 1 kg of degraded COD will produce 350Nl of methane.
Consequently, for a COD degradation of 80%, the methane yield will be
280Nl methane/kg COD
Input
.
BMP test
A very practical way of determining the potential for energy recovery is to
measure the BMP (see Section 3.3.7). This gives a quite realistic result for
the expected methane potential. Although this approach is generally the best
option and much more accurate than the options already described, it has
some drawbacks. For example, the methane potential will change depending
on the OLR applied in a continuous fermentation process. Possible
inhibitors are diluted by the inoculum added in the BMP tests and the
synergistic or antagonistic effect of substrate mixtures can only be evaluated
Search WWH ::
Custom Search