Page 373 - Advances in Eco-Fuels for a Sustainable Environment

328 Advances in Eco-Fuels for a Sustainable Environment

much higher than the corresponding triglycerides. An important point to note is that
the lubricity of a substance is not a direct materialistic property and cannot be mea-
sured like other properties. Tests performed will evaluate the lubricant’s performance
for only a specific system, and it varies for other systems.

11.3.3.9 Carbon residue

Carbon residue, regardless whether it arises from biodiesel or petroleum ultralow sul-
fur diesel (ULSD), can cause engine damage and degradation. It can cause fuel injec-
tor fouling and cylinder scoring within an engine, leading to a decreased performance
or engine failure. Although biodiesel is known to leave fewer deposits than ULSD,
they are a widely recognized problem when burning any carbon-based fuel in an inter-
nal combustion engine.
Two main types of deposition mechanisms (for diesel and can be thought analo-
gous to biodiesel) are recognized: decomposition of hydrocarbons to elemental carbon
and hydrogen and polymerization of hydrocarbon species into poly-nuclear aromatic
hydrocarbons (PAHs) that grow into carbonaceous deposits. One major factor deter-
mining whether the hydrocarbons are decomposed or polymerized is the presence or
absence of a metal catalyst. If a metal catalyst is present, hydrocarbons are typically
decomposed into carbon residue, but in the absence of a catalyst (or thermal deposi-
tion), polymerization into carbon residue is the dominating mechanism.
The liquid phase thermal autoxidation at low temperature (<350°C) and gas phase
pyrolysis at high temperature (>450°C) are the two main regions in the thermal sta-
bility of jet fuels. Autoxidation causes a stage of hydroperoxide formation, and the
resulting deposits tend to have large amounts of oxygen and settle out as spheres
because they are insoluble with the bulk fuel. Pyrolysis mechanics are less understood,
but are believed to form aromatic deposits through the following steps:

Normal alkanes ! Alkenes ! Cycloalkanes=Cycloalkenes ! Alkylbenzenes
! PAHs ! Deposits

Vegetable oil used as a fuel without transesterification into biodiesel will lead to more
carbon deposits as the saturation level decreases in it. The more unsaturation in bio-
diesel, the greater the number of carbon deposits formed than a saturated fuel. The
percentage of carbon reside in AMC, Karanja, and Jatropha are 0.025%, 0.07%,
and 0.21%, respectively.
In the case of Jatropha biodiesel, the degree of unsaturation was found to be 77.2%.
For Karanja biodiesel, it was found to be 73.93%. For AMC biodiesel, the degree of
saturation was found to be 66.6%. Even the kinematic viscosity of Jatropha biodiesel
is less than that of Karanja; the carbon residue follows the reverse order. This shows
that the degree of saturation is the overriding factor of carbon formation and more
important than the viscosity of the fuel. Overall, carbon deposition from biodiesel
is much lower compared to that of petroleum diesel, potentially due to the lack of aro-
matics in biodiesel making the formation of large aromatic residue structures difficult.