Environmental Engineering Reference
In-Depth Information
in the exhaust [169]. Nitrogen oxides (NO
x
) can be very important in polluted
air. Regionally high NO
2
concentrations can cause severe air quality deterioration.
Practically all anthropogenic NO
x
emissions enter the atmosphere as NO generated
from the combustion of fossil fuels in internal combustion engines. Like carbon
monoxide, NO binds to hemoglobin and reduces the oxygen transport efficiency.
However, the concentration of NO normally is much lower than that of carbon
monoxide so that the effect on hemoglobin is less significant [170]. In general,
reported results have revealed an increase in NO
x
emissions for biodiesel [137, 157,
171-173]. Dorado et al. tested a diesel direct injection Perkins engine fueled with
waste olive oil methyl esters at several steady-state operating conditions. Results
revealed that the use of biodiesel resulted in lower emissions of CO, CO
2
, NO,
and SO
2
, with an increase in NO
2
emissions [138]. The efficiency of the combus-
tion remained constant using both biodiesel and diesel fuel [137]. Recent reports
pointed out the formation of higher quantities of NO
x
can be correlated to the higher
temperatures and quantities of oxygen expected in the combustion chamber due to
the improved biodiesel combustion [174, 175]. However, the lower sulfur content
present in biodiesel may allow the use of designer control technologies (e.g. cata-
lysts) to the abatement of NO
x
emissions that cannot be otherwise employed with
conventional diesel.
The fact of the improved combustion process is partially believed to be a result
of the advanced injection derived from the optimum physical properties of biodiesel
(viscosity, density, compressibility, sound velocity) [175], in good agreement with
a variety of reports [152, 153, 159]. Monyem and Van Gerpen [162] found a good
correlation between the start of injection and NO
x
, independently of the fuel used,
which suggests this is the only reason for NO
x
increase. Another plausible expla-
nation points to the electronic improvements in the injection pump when biodiesel
is used instead of diesel fuel [176]. Thus, it seems that the main reasons for the
increase of NO
x
emissions using biodiesel as fuel are injection-related.
The acid rain, caused by the deposition on the earth's surface of acids (e.g.
SO
2
), can be mostly attributed to industrial operations emissions and fossil fuel
combustion. As a result of its widespread distribution and effects, it is an air pol-
lutant that may pose a threat to the global atmosphere [170]. Nevertheless, since
biodiesel is sulfur-free, less sulfate emissions and particulate reduction are reported
in the exhaust and thus it may contribute to reduce the problem of acid rain due to
transportation fuels [174].
8.3.2 Spark Ignition Engines Performance Using Bioethanol
Bioethanol is an oxygenated fuel containing 35% oxygen that exhibits a higher
octane number (108), lower cetane number (less than 10), broader flammability
limits, higher flame speeds and higher heats of vaporization than gasoline [51, 177].
These properties lead to a higher compression ratio, shorter burn time and leaner
burn engine, enhancing the theoretical efficiency over gasoline [178]. The autoigni-
tion temperature and flash point of ethanol are higher than those of gasoline, which
