Page 355 - Advances in Eco-Fuels for a Sustainable Environment

310 Advances in Eco-Fuels for a Sustainable Environment

in 2015; it is projected to reach 700 quadrillion kilojoules in 2030 [4]. Over-
dependency and burning of fossil fuels leads to increases in CO 2 emissions and global
warming [5]. Ever-increasing energy demand, depletion of fossil fuels, and air pollu-
tion problems have created the awareness and attention toward ecofriendly and renew-
able fuels [6].
Recently, biodiesel has gained attention as a promising alternative to fossil fuels to
meet future energy demands [7]. Biodiesel is biodegradable and can be directly usable
in a diesel engine without any modifications in the form of blending with diesel. Sev-
eral studies on biodiesel-fueled diesel engines have been performed, showing that they
emit fewer toxic emissions than diesel [8]. Right now worldwide, more than 360 feed-
stocks have been found to produce biodiesel. Nonedible feedstocks had no competi-
tion with edible oils and are considered wastes, which gained noteworthy attention due
to the low cost, positive energy balance, and environmental impacts compared to food
feedstock-based biodiesels [9].
Biodiesel is a mono-alkyl ester produced from triglycerides of plant and animal
feedstocks via the alcoholysis process known as transesterification [10]. Trans-
esterification is a conversion process of fatty acids into alcoholic esters in the presence
of acid or an alkaline catalyst [11]. The conversion process takes place in three steps in
which triglycerides are converted into di, mono, and finally esters with glycerol as a
byproduct [12, 13]. Stoichiometrically, three moles of alcohol are required to convert
one mole of fatty acid into esters. However, more alcohols are required to forward the
reaction because transesterification is a slow and reversible process. Generally, the
homogeneous or heterogeneous catalyst is used to accelerate the reaction [14].
Biodiesel physicochemical properties differ from one source to another due to their
different fatty acid composition, which affects the fuel properties [15, 16]. Moreover,
the biodiesel properties depend on the alcohol used, pretreatment, production, and
posttreatment processes. International standards, namely ASTM D6751 and
EN14214, have been set up to assess the quality of biodiesel to resolve these issues.
This standard specifies the quality requirement and test methods to determine the
physicochemical properties of biodiesel and its blends [17]. The key parameters to
utilize biodiesel in a diesel engine are acid value, iodine value, saponification value,
density, kinematic viscosity, oxidation stability, calorific value, cetane number, and
cloud and pour point [10]. The fatty acid composition, the number of double bonds,
and the chain length of biodiesel are major influences on the biodiesel properties. The
density and kinematic viscosity properties of the biodiesel influence the atomization
and vaporization characteristics of the fuel during combustion [18].
Biodiesel with high fatty acid content increases the cetane number and pour point
temperature and decreases the kinematic viscosity [19]. Unsaturated fatty acid content
of biodiesel improves the cold flow behavior, calorific value, and density [20].The
cetane number of biodiesel is directly correlated with the calorific and density value
of the fuel. Biodiesel has a higher cetane number due to its higher oxygen content com-
pared to diesel, which improves engine performance and combustion characteristics
[21]. Even though biodiesel has many advantages, the higher unsaturated fatty acid
content of biodiesel is the major drawback that decreases the oxidation stability
[22]. The poor oxidation stability of biodiesel deposits the gums over the period of stor-
age, which clogs the fuel filter and corrodes and damages the various engine parts [23].