Environmental Engineering Reference
In-Depth Information
Table 8.1
Comparison between actual hydrogen yield (Y
exp
) and the estimated hydrogen yield
(Y
est
)
Temperature
(ºC)
Yield of acetate or butyrate
(mol product/mol glucose)
Hydrogen yield
(mol H
2
/mol glucose)
Ratio of
Y
exp /
Y
est
Y
HAC
Y
HBU
Y
exp
Y
est
37 ± 2
0.44
0.35
0.11
1.58
0.07
55 ± 2
0.53
0.50
2.73
2.06
1.33
80 ± 2
0.63
0.49
3.43
2.24
1.53
Estimated by (2x Y
HAC
+2x Y
HBU
)
Y
HAC
acetate yield,
Y
HBU
butyrate yield
Thermophilic reactor
(
)
C H
O
→+
+
2.73nH 0.53nCH COOH
0.50nC H COOH
6
10
5
2
3
n
+
2.6nCO
+
others
(8.5)
37
2
Hyper-thermophilic reactor
(
)
C H
O
→+
+
3.43nH 0.63nCH COOH
0.49nC H COOH
6
10
5
2
3
n
+
3.01nCO
+
others
(8.6)
3
7
2
From Eqs. 8.4, 8.5 and 8.6, it is clear that fermentative characteristics in each reac-
tor (including yields of hydrogen and intermediate products) are different at differ-
ent fermentation temperatures. Stoichiometrically, 2 mol of hydrogen is generated
concomitant with 1 mol of acetate or butyrate (Eqs. 8.2 and 8.3).
Table
8.1
compares actual hydrogen yields (Y
exp
) with the estimated hydrogen
yield (Y
est
). The results show that the ratio of experimentally obtained hydrogen to
those estimated was 0.07, 1.33 and 1.35 under mesophilic, thermophilic and hyper-
thermophilic temperatures respectively.
This indicates that most acetate and butyrate yields were produced during hydro-
gen production under thermophilic and hyper-thermophilic temperatures. On the
other hand, most acetate and butyrate yields were produced during methane and low
hydrogen production under mesophilic temperatures.
8.3.6
Effect of Temperature on the Net Energy Gain
The net energy gain is the total energy produced from hydrogen generation minus
any energy required to raise the influent volume from ambient to fermentation tem-
perature. Estimates of net energy gain under three different temperatures for differ-
ent influent cellulose concentrations, calculated based on the H
2
yield obtained in
this study, are presented in Fig.
8.8
.
