Environmental Engineering Reference
In-Depth Information
Box 5.1 Proportion of CH
4
and CO
2
based on equation 5.1
H
2
O
CH
4
þ
CO
2
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
!
n = 17, a = 32, b =9
C
17
H
32
O
9
þ
4.5H
2
O
10.25CH
4
þ
6.75CO
2
→
380
þ
81
164
þ
297
→
461
461
→
285 kg VS
þ
61 kgwater
123 kg CH
4
þ
223 kgCO
2
[285 kg VS/t OFMSW]
→
200 kg VDS
dest
156 kg CO
2
[70% destruction]
Density of CH
4
= 16/(22.412m
n
3
/kg) = 0.714 kg/m
n
3
Density of CO
2
= 44/(22.412m
n
3
/kg)= 1.96 kg/m
n
3
Thus the proportion of gas by volume
þ
43 kgwater
86 kg CH
4
þ
→
120m
n
3
CH
4
80m
n
3
CO
2
= 200m
n
3
þ
→
biogas
Thus biogas contains approximately
60% CH
4
þ
40% CO
2
by volume
→
Methane production is 421 l/kgVS added
Maximum production of methane (100% destruction of VS) is 601 l/kg VS
added
1m
3
CH
4
≈
37.78MJ ; 1m
3
biogas @ 60% CH
4
= 22.7MJ/m
n
3
For the purposes of this example, it is estimated that the moisture content
of the OFMSW is 70%; 95% of the solids are volatile. Thus, one tonne of
OFMSW corresponds to 300 kgDS and 285 kg volatile solids (VS). A well-
designed digester aims at a minimum of 70% destruction of VS or
199.5 kg VS/t OFMSW. The number of molecules in a unit volume under
standard conditions is the same for all gases. This volume is 22.412m
3
/kg.
Methane has an energy value of 37.78MJ/m
n
3
. Box 5.1 shows a calculation
of the proportion of CH
4
and CO
2
based on equation 5.1. The biogas has
60% CH
4
and has an energy value of 22.7MJ/m
n
3
. A total of 200m
n
3
of
biogas is produced, which is equivalent to 4.5GJ/t OFMSW. The maximum
production of methane is calculated as 600 l/kg VS added. In this case, with
70% destruction of volatiles, the methane production equates to 421 l/kg VS
added.
In Box 5.2 the energy value of the feedstock is obtained using the modified
Dulong formula (equation 5.2) (Tchobanoglous et al., 1993). This would
suggest 406 l CH
4
is produced per kilogrammeVS added (good agreement
with a slight discrepancy with the value obtained in Box 5.1).
Box 5.3 presents a relationship between VS and COD. In a two-stage
process where hydrolysis is separated from methanogenisis, each kilo-
gramme of VS produces between 1.57 and 1.60 kg COD and each
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