Environmental Engineering Reference
In-Depth Information
Table 2.6
The representative organisms and hydrogen production by each process. (Kotay and
Das
2008
)
Process
Representative organism
Maximum reported rate
(mmol H
2
/Lh)
Direct biophotolysis
Chlamy domonas reinhardii
0.07
Indirect biophotolysis
Anabaena variabilis
0.36
Photo fermentation
Rhodobacter spheroides
0.16
Dark fermentation
Enterobacter cloacae DM 11
75.60
Clostridium sp. Strain No.2
64.50
Two-stage fermentation
Enterobacter cloace
DM 11+Rhodobacter sphaeroides OU 001
51.20
Mixed microbial flora + Rhodobacter
sphaeroides OU 001
47.20
The first step of this system involves fermentation of biomass to acetate, hy-
drogen and CO
2
using a thermophilic dark reactor. In the second step the ac-
etate is used in an isolated photo bioreactor for conversion into CO
2
and H
2
.
This arrangement can result in reaching the worth nearer to the theoretical de-
termined manufacture of about 12 moles of hydrogen gas (Nath and Das
2003
).
The reactions are:
(a) Step I
+ → +
dark fermentation
[
]
C H
O
2H O
2CH COOH
2CO
+
4H
123
6
12
6
2
3
2
2
(b) Step II
Photo fermentation
[
]
2CH COOH
+→+
4H O
8H
4CO
123
3
2
2
2
Table
2.6
shows all the processes along with organisms used and the amount of
hydrogen gas produced by each process. The table clearly shows that two stage
methods produce the maximum amount of hydrogen while direct photolysis process
produces the least amount of bio hydrogen (Kotay and Das
2008
).
6
Economic Aspects of Biofuels Around the Globe
There is a huge market of motor vehicles and fuel industry. The current scenario of
fossil fuel availability and prices is not very encouraging to completely depend on
it, so the possible and reliable alternative is biofuels and they are massively growing
in the market day by day. In developed countries like USA there is already a satu-
ration of vehicle ownership which is expected to increase more at current rate. In
developing countries the vehicle ownership is growing very rapidly and is expected
to surpass the total number of vehicles by 2030. There is a worldwide estimation of
about 982 million vehicles which is expected to rise to 2.6 million by 2050. Once
people purchase a vehicle they use it for convenience in travel which means that
