Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 187

Commercial production of F-T fuels is well established in South Africa, Qatar, and Malaysia.
By 2012, global production is scheduled to be more than 30,000 barrels per day (Bartis and
Van Bibber, 2011). Due to the high temperatures and pressures required for the F-T process,
the refining process is energy-intensive and releases higher quantities of carbon dioxide (CO 2 )
during manufacture than petroleum refining. Consequently, although this fuel is the most com-
mercially viable alternative fuel, it requires capture and sequestration of the carbon emissions
in order to be considered a sustainable fuel (Bartis and Van Bibber, 2011; Kinsel, 2010). With
sequestration, the environmental impact of F-T fuels is comparable to that of petroleum (Bartis
and Van Bibber, 2011). However, the capture/sequestration steps can add substantially to the
investment cost for such fuels. Nevertheless, F-T fuels are the most commercially ready solution
for current energy needs. For the longer term, the CO 2 emissions of petroleum-based F-T fuels
render it less attractive than biofuels, which have the potential to be CO 2 -negative. In this case,
substituting biofuels for petroleum-based fuel would reduce the level of greenhouse gases in the
atmosphere.
While the molecules in petroleum-derived fuel are primarily composed of pure hydrocarbons,
vegetable oil and biodiesel are comprised of hydrogen, carbon and oxygen. The long hydrocarbon
chains that are typical of paraffins are present in biodiesel as a component of a fatty acid, but
they are coupled at one end of the chain to other functional groups. The molecular composition is
R -(CH 2 ) n -COOH, where R indicates an alkyl group, which looks like a hydrocarbon chain with
a missing hydrogen at one end of the chain. This formula describes an ester, which is obtained
through the condensation of an alcohol and a fatty acid. Most often, the alcohol used is methanol,
because it is inexpensive and provides good fuel properties. Ethyl esters, based on ethanol, are
common in areas like Brazil in which ethanol is abundant. Since the ethanol in Brazil is largely
derived from sugar cane, this type of biodiesel is completely based on renewable sources. In
contrast, methanol is usually derived from natural gas. Butanol or propanol can also be used to
form butyl and propyl esters, respectively, but this is much more expensive and less common.
When the ester has only a single fatty acid, the biodiesel blend is comprised of mono-alkyl
fatty esters, which are termed Fatty Acid Methyl Esters (FAME) somewhat generically, or more
inclusively, Fatty Acid Alkyl Esters (FAAE). At one time, they were under consideration as a
jet fuel-blending component, but their poorer cold-weather properties and specific energy are
inadequate for jet fuel. The new standards for bio-derived jet are only expected to include HEFA
fuel. In this work, we will refer to vegetable oils and greases as “green crude”, to the methyl ester
blends created from green crude as “FAAE”, to the blends of FAAE and conventional diesel which
conform to the standards of EN 14214 and ASTM 6751 as “biodiesel”, and biofuel consisting
of mixtures of pure hydrocarbons derived from renewable sources as “HEFA”. Only the latter is
under consideration as a blending component for jet fuel.
Some requirements on biodiesel properties in its neat state are identified in Table 11.2. Note
that these requirements are not targeted toward HEFA, but they are provided here as a reference.
Although the European EN14214 specifications place a range limit on biodiesel density for
FAME and both standards identify a permissible range on viscosity, most of the requirements have
to do with fuel composition. The presence of excess mono-, di- and triglycerides, methanol, and
glycerol would indicate an incomplete reaction in the refining process or flaws in the separation
process or both. Relative to the specifications for jet fuel in Table 11.1, one important difference
is that the standards for FAME/FAAE in Table 11.2 specify a range on kinematic viscosity at
40 ◦ C, while jet fuel specifications are referenced to a much lower temperature,

20 ◦ C. This will
be discussed in more detail in the next section. Relative to petroleum fuel, biodiesel derived from
FAAE have the following:

−

• Advantages : cleaner burn; high lubricity; reduced carbon monoxide (CO), sulfuric gases

(SO x ) and particulate emissions; better thermal stability; carbon-negative (provided the
manufacturing process is sustainable); biodegradable, and
• Disadvantages : higher freeze point; lower energy density; higher NO x emissions; poorer
oxidative and storage stability; high cost and low availability of feedstock and refining plants.