Environmental Engineering Reference
In-Depth Information
various organic acids are also produced. These compounds can subsequently be
converted to hydrogen by a process denoted as photo fermentation.
Direct photolysis
. In this approach, the process takes advantage of the pho-
tosynthetic capability of algae and cyanobacteria to split water into O
2
and H
2
via direct absorption of light and transfer of electrons to two groups of enzymes
that participate in biological hydrogen metabolism: hydrogenases and nitrogenases
[129].
Indirect photolysis
. Alternatively, biohydrogen can be prepared through the use
of some microorganisms (algae) that can directly produce hydrogen under cer-
tain conditions [126, 127, 129]. Most specifically, sealed cultures of green algae
become anaerobic in the light under deprivation of sulfur nutrients and sponta-
neously induce the “hydrogenase pathway” to photosynthetically produce hydrogen
[127]. Substantial rates of hydrogen production were obtained over 60 h in the
light although the hydrogen production leveled off reaching a point (after 100 h)
in which the algae go back to the normal photosynthetic pathway in order to restore
the consumption of internal starch and proteins that takes place in the course of the
hydrogen production [132].
8.3 Engine Performance of Biofuels
8.3.1 Diesel Engines Performance Using Biodiesel
Short- and long-term performance tests in diesel engines using biodiesel (mainly,
ethyl and methyl esters from fats or vegetable oils including soybean, rapeseed and
sunflower oils) have revealed an increase in the volumetric brake specific fuel con-
sumption, due to the lower volumetric calorific value. Engine power and torque
differ slightly or remain unchanged, while smoke emissions of biodiesel are much
lower compared to diesel fuel [133-137]. Some of these properties of biodiesel
compared to diesel fuel are summarised in Table 8.4.
To improve the combustion properties and cold-weather behavior, several investi-
gations have recommended the use of biodiesel blended with diesel fuel in different
percentages [139-142]. The heating value of biodiesel mixtures becomes higher
than that of biodiesel due to the lower heating values and stoichiometric air/fuel
ratios of biodiesel compared to diesel fuel [143]. A wide range of diesel engines
of different sizes and types has been tested. Direct injection, turbocharged, and
Table 8.4
Fuel Specifications of biodiesel and mineral diesel fuel [138]
Property
Diesel fuel (EN-590)
Biodiesel (EN 14214)
Density at 15
◦
C (kg/m
3
)
820
−
860
860
−
900
Kinematic viscosity at 40
◦
C(mm
2
/s)
2
−
4.5
3.5
−
5
Flash point (
◦
C)
> 55
> 120
Cetane number (CN)
> 46
> 51
Gross heating value, GHV (MJ/kg)
45
−
46
30
−
42
