Biomedical Engineering Reference
In-Depth Information
Gasification of biomass also removes oxygen from the fuel to increase its
energy density. For example, a typical biomass has about 40% oxygen by
weight, but a fuel gas contains negligible amount of oxygen (
Table 1.4
). The
oxygen is removed from the biomass by either dehydration (
Eq. (1.1)
)ordecar-
boxylation (
Eq. (1.2)
) reactions. The latter reaction (
Eq. (1.2)
) while rejecting
the oxygen through CO
2
also rejects carbon and thereby increasing the H/C
ratio of the fuel. A positive benefit of the gasification product is that it emits
less GHG when combusted:
Dehydration:
C
m
H
n
O
q
-
C
m
H
n
2
2q
1
qH
2
OO
2
removal through H
2
O
(1.1)
Decarboxylation:
C
m
H
n
O
q
-
C
m
2
q
=
2
H
n
1
qCO
2
O
2
removal through CO
2
(1.2)
Hydrogen, when required in bulk for the production of ammonia, is pro-
duced from natural gas (mainly contains CH
4
) through steam reforming,
which produces syngas (a mixture of H
2
and CO). The CO in syngas is indi-
rectly hydrogenated by steam to produce methanol (CH
3
OH), an important
feedstock for a large number of chemicals. These processes, however, use
natural gas that is nonrenewable and is responsible for net addition of carbon
TABLE 1.4
Carbon-to-Hydrogen (C/H) Ratio of Different Fuels
C/H Mass
Ratio (
Oxygen
(%)
Energy Density
(MJ/kg)
b
)
a
Fuel
2
Anthracite
44
2.3
27.6
B
B
B
Bituminous coal
15
7.8
29
B
B
B
Lignite
10
11
9
B
B
B
Peat
10
35
7
B
B
B
Crude oil
9
42 (mineral oil)
B
Biomass/cedar
7.6
40
20
B
B
Gasoline
6
0
46.8
B
Natural gas (
CH
4
)
3
Negligible
56 (Liquefied natural gas)
B
Syngas (CO:
H
2
5
2
Negligible
24
1:3)
a
Probstein and Hicks (2006).
b
McKendry (2002).
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