Environmental Engineering Reference
In-Depth Information
Table 17.1 Composition of biogas from dedicated AD plants, sewage treatment
plants and landfill, and natural gas
Parameter
Farm-scale
AD plant
Centralised
AD plant
Landfill
Sewage
treatment
plant
Natural
gas
CH
4
(vol %)
55-60
60-70
35-65
60-65
81-89
Other hydrocarbons
(vol %)
0
0
0
0
3.5-9.4
H
2
(vol %)
0
0
0-3
0
—
CO
2
(vol %)
35-40
30-40
25-45
35-40
0.67-1.00
N
2
(vol %)
1-2
2-6
1-17
1-2
0.28-14.00
<
<
<
O
2
(vol %)
1
0.5-1.6
1-3
0.05-0.70 0
<
<
<
H
2
S (ppm)
25-30
0-2000
30-500
0.5-6800 0-2.9
<
NH
3
(ppm)
100
100
5
1-7
0
≈
≈
≈
<
Halogenated
0.01
0.25
0.3-225
0-2
—
compounds (mg/m
3
)
<
<
Siloxanes (mg/m
3
)
0.03-
0.2
0.08-
0.5
0.3-36
1-400
—
<
<
<
<
<
<
Wobbe index
24-33
24-33
20-25
25-30
44-55
Lower heating value
(MJ/Nm
3
)
19.7-21.5
21.5-25.1
10.7-23.3 21.5-23.3
31-40
: 1 kWh/Nm
3
= 3.6 MJ/Nm
3
.
Source: adapted with permission from Rasi
Note
et al
. (2007); and Petersson and
Wellinger (2009).
maintenance cost for reciprocating engines (four-stroke spark ignition and
diesel engines) increases with increasing H
2
S concentration in the biogas.
H
2
S scrubbing is a must if concentrations are higher than 1000 ppmv
(Chambers and Potter, 2002). In addition, biogas may contain more than
500 different contaminants such as halogenated hydrocarbons, higher
hydrocarbons and aromatic compounds (Rasi et al., 2007). Landfill and
sewage gas can also contain siloxanes, which may cause severe problems in
downstream utilisation (Rasi et al., 2007). Trace amounts of hydrogen (H
2
),
N
2
, CO, saturated or halogenated carbohydrates and O
2
are occasionally
present in the biogas. Usually, the mixed gas is saturated with water vapour.
Depending on the biogas source and its end use, these contaminants have to
be removed.
Biogas can be utilised in several ways, either as raw biogas or upgraded
biomethane. The challenge with the use of biogas as a fuel is the need for gas
cleaning to meet the quality requirements for the utilisation equipment. Use
of untreated biogas may increase the maintenance requirements of the
equipment fuelled by the biogas and can reduce its life span. As a
prerequisite for effective biogas utilisation, biogas has to be cooled, drained
and dried immediately after production to reduce condensation and, almost
always, it has to be cleaned of H
2
S.
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