Conversion Technologies 87
O
O
O OHO
RI
RII
RIII
R 1
O CH 2
CH 2
CH
++
Catalyst O
O O
O
R 1
R 1
R 1
O
O
HO
HO
CH 2
CH 2
CH
HO
O
RI
RII
RIII
Figure 3.10Scheme of transesterification. Biodiesel is synthesised in a chemical reaction of
transesterification of an oily feedstock, triglyceride with a short-chain alcohol, usually methanol in the
presence of a catalyst. Transesterification yields alkyl esters of fatty acids (biodiesel) and a by-product,
glycerol. RI,II,IIIalkyl chains of fatty acid (usually C14–C22), and R 1 alkyl group of an alcohol (usually
methyl or ethyl).
ester (biodiesel) and glycerol. The reaction scheme is presented in Figure 3.10. The resul-
tant product, biodiesel, has comparable characteristics with fossil diesel and all major
parameters, such as higher heating value (HHV) (39–41 MJ kg−^1 ), flash point, cetane
number and kinematic viscosity, are similar to its fossil alternative [29].
Efficient transesterification requires a catalyst and/or high temperature and pressure
toconvertsubstratestoproducts.Typeofthiscatalystisfundamentalasitaffectsboth
the type of feedstock that can be used and up- and down-stream biodiesel processing.
There are several methods of producing biodiesel: alkali, acid, enzymatic and super-
critical alcohol [30]. All of these methods have their advantages and drawbacks and
currently the predominant method of industrial biodiesel production is utilisation of a
two-step acid–alkali catalysed process. The first step of the conversion is used to upgrade
oily feedstocks with high content of free fatty acids (esterification), and the second step
results in the high efficiency of the transesterification of triglycerides to biodiesel. After
the reaction is complete, both products (biodiesel and glycerol) are separated in set-
tling tank or centrifuge due to significant density difference between both products
(880 kg m−^3 esters of fatty acids; 1050 kg m−^3 of glycerol) and purified. Purification pro-
cess is composed of repetitive heating and washing steps according to the following
scheme: catalyst neutralization, deodorisation and pigment removal [30]. Production of
biodiesel through a process of transesterification is a mature technology already applied
by the industry worldwide.
Hydroprocessing– Hydroprocessing is a thermochemical method widely established
in the petroleum industry that has been adapted to the conversion of renewable oily
feedstocks to hydrocarbons [31]. One of the major advantages of hydroprocessing is a
range of products that can be synthesised depending on reaction conditions that include
all types of hydrocarbon-based fuels like gasoline, jet fuel and diesel. The overall goal
of the reaction is to crack the alkyl chain of fatty acid chain into smaller fragments,
saturate all unsaturated bond with hydrogen and remove oxygen atoms from structure.
The resultant fuel is chemically indistinguishable from current fossil fuels and therefore
100% compatible with all fuel infrastructures. Major drawbacks of hydroprocessing is
itshugerequirementforhydrogenthatiscurrentlyproducedalmostexclusivelyfrom
non-renewable sources mainly from steam reforming of natural gas and high energy
consumption during the process. One of the major challenges to make hydroprocessing
a truly sustainable technology is to develop methods of renewable hydrogen production
that could supplement this promising technology with renewable hydrogen.
