Biofuels in Perspective

158 Biofuels
9.3.1.2 Hydrolysis
Hydrolysis of triglycerides is leading to the formation of free fatty acids, mono- and
diglycerides and glycerol.
Distinction should be made between chemical hydrolysis especially at high temperature
and in the presence of water and the enzymatic hydrolysis due to lipase activity present in
the raw material. Lipases can be deactivated by sterilizing as in the palm oil production.
Especially during cooking and frying, hydrolysis is occurring readily. During these processes
there is a combination of oxidation and hydrolysis while also glycerol is converted
into the highly reactive acrolein (CH2 = CH-CHO).
A combination of these reactions and oxidative dimerization (see Section 9.3.1.3) is
causing a multiplicity of chemical reactions in which undesirable compounds can be
formed and which are not easy to remove from biodiesel (Guesta et al., 1993).
The quality of cooking and frying oils is expressed by the polar content. While the polar
content of freshly refined oils is low (0.4–6.4 mg/100 g) successive frying gives rise to
a multiplication by 100–1000. More unsaturation results in a higher polar content. The
maximum amount of the polar content in the EU is set at 25 % in edible oil, otherwise the
oil has to be discarded (Bastida, 2001).
9.3.1.3 Thermolytic Formation of Dimers and Polymers
Upon heating the triglycerides at high termperature (higher than 180 ◦ C) in the absence of
oxygen, the saturated fatty acids are converted to alkanes, alkenes, carbonyl compounds,
CO and CO2. The unsaturated fatty acids are forming linear dimers and dehydrodimers
and polycyclic compounds. Especially the Diels-Alder reaction, which is occurring after
conjugation due to isomerization of the non-conjugated systems, is leading to a dimer which
can react further to trimers and polymers containing cyclohexene rings. Extensive heating
is causing dehydrogenation with formation of aromatic rings leading to polyaromatic
hydrocarbons (Nawar, 1984).
If the heating is performed in the presence of oxygen such as frying and cooking,
dimerization is observed involving oxidation reactions leading to a variety of reactions such
as ether linkage formation between unsaturated fatty acids and substitution of hydrogen by
hydroxy functions.
9.3.1.4 Contamination of Waste Oils
Crude vegetable oils and animal fats can be contaminated during or after the production by
proteins, carbohydrates, anorganic residues, packaging materials, cleaning agents, mineral
oils and lubrificants residues and dirt. The contaminants are preferably removed before
biodiesel production by a combination of technologies mainly involving filtration.
However, these treatments are causing additional costs and should be taken into consideration
for an economical production of biofuels in comparison with more refined feedstocks.
Therefore it is important to know in advance the kind and the level of undesirable
products.