Environmental Engineering Reference
In-Depth Information
=33
kg
−1
HHV MJ
:
8wt
ð
fractionC
Þ
+ 144
:
2wt
½
fractionH
2
ð
fractionO
2
=
8
Þ
ð
Eq
:
18
:
1
Þ
−
wt
kg
−1
) for
Biofuels are already on the market in the form of ethanol (HHV of 29.7 MJ
kg
−1
, as calculated for methyl linoleate)
for diesel engines. Usually, these components are used for blending (typically 2
gasoline engines and FAME (HHV of 41 MJ
10
vol.%) in oil-derived gasoline and diesel. Blending largely circumvents the need for
adjustments to existing car engines.
However, the engine performance of fuels is characterized by more than its HHV
value. Based on the properties of engines (sparked ignition vs. compressed spontane-
ous ignition), the organic structures need to meet certain requirements in terms of vol-
atility, stability, and boiling point trajectory. The performance of a fuel in the engine
has to be judged along the ten criteria of the ASTM standard (tinyurl.com/pogugy).
One of the indicators is the octane number (gasoline, octane number around 95)
or cetane number (diesel, cetane (C
16
H
34
) number around 45). For a chemical
perspective on the structure of biofuels, Table 18.1 gives an overview of the
elemental composition of traditional fuels versus their biomass counterparts (Petrus
and Noordermeer, 2006).
What is immediately noticed is that in classical fuels no oxygen is present. The
presence of oxygen in fuels in the form of alcohols and/or ethers lowers their energy
density but is reputed to improve the quality of the exhaust. Therefore, gasoline has to
be mixed with oxygenates such as ethanol, methyl tertiary butyl ether (MTBE), and
ethyl tertiary butyl ether (ETBE), in order to prevent knocking of the engine. Thus, the
message is that the complete performance of a biofuel compound has to be taken into
account and that simply mimicking the structure of fossil fuels is not necessarily desir-
able. For example, a number of favorable properties of bioethanol are that it is easily
produced through fermentation, has a reasonable HHV of 29.7 MJ
−
kg
−1
, is not very
toxic, and has a high octane number of 113. This has led to its massive use either in
blending or in flex cars. In Figure 18.1,
the reaction for fermentative ethanol
TABLE 18.1 Composition of gasoline and diesel compared with FAME, ethanol,
and carbohydrates in general
Gasoline
Diesel
FAME
Ethanol
Carbohydrate
Carbon chain
length
5 − 10
12 − 20
19
2
[5 − 6]
n
O/C molar
ratio
0
0
0.11
0.5
1
H/C molar
ratio
2
2
2
3
2
Polarity
Nonpolar
Nonpolar
Nonpolar
Polar
Polar
Structure
Branched/
aromatic/cyclic/
unsaturated
Linear/
saturated
Linear
C
2
H
5
OH Linear/
cyclic
polyalcohol
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