2.3. Properties of biodiesel from different feedstock
Apart from the cost of the raw materials, another important
factor to consider is the properties of the biodiesel obtained from
various types of oil. The properties of biodiesel vary accordingly to
the fatty acid composition in the feedstock oil which is used to
produce biodiesel. The properties of biodiesel have to be
comparable or better than petroleum-derived diesel oil in order
to ensure that it can be used in diesel engine without any
modification. The properties include flash point, viscosity, cetane
number, cloud point, pour point, calorific value, acid value, ash
content and cold flow properties. Table 3 shows some of the
important physical and chemical properties of biodiesel produced from different oil sources as compared to petroleum-derived diesel
oil. Among all the properties listed in Table 3, cold flow properties
(pour point and cloud point) are among the most important
properties that are usually looked upon. Cold flow properties
basically concern the changes of biodiesel properties such as
crystallization, gelling or viscosity increase due to temperature
changes that might adversely affect the operability of the vehicles.
These properties are reflected by the values of cloud point and pour
point. These properties will be the deciding factor if the biodiesel
produced can be used in cold climate countries. This is important
because currently the largest demand of biodiesel is in the
European countries. The cold flow properties of biodiesel are
determined by the types of fatty acid in the feedstock oil. Higher
percentage of unsaturated fatty acid in the feedstock oil will result
in biodiesel having better cold flow properties. For example,
biodiesel from palm oil has poor cold flow properties because it
has high content of saturated fatty acid (about 50%). Meanwhile
biodiesel from rapeseed oil which has high content of unsaturated
fatty acid has the best cold flow properties [29].
Fatty acid composition in the oil is also an important factor
that determines the storage stability of biodiesel. Bouaid et al. [30]
has conducted a research for a duration of 30 months to study the
long storage stability of biodiesel by using the allylic position
equivalent (APE) and bis-allylic equivalent (BAPE) concept. In this
concept, the oxidative stability is obtained based on the relative
rates of oxidation of these positions in unsaturated fatty acid as
well as their amounts. They have reported that the oxidative
stability of the oil may be more strongly influenced by the
presence of small amounts of more highly unsaturated fatty
compounds than by increasing amounts thereof. Two important
factors affecting the degradation of biodiesel were also observed
in their study, which are water content and air exposure. However,
the effect of the presence of unsaturated fatty acid on the storage
stability of biodiesel can be avoided by taking proper precaution
during the storage such as limiting contact to oxygen and
exposure to light and moisture