Analysis of Aviation Fuel Production from Vegetable Oils: A Report

This report provides a comprehensive analysis of converting vegetable oils into aviation fuel. It details the hydrothermal conversion process, including hydrolysis, decarboxylation, cracking, and hydrogenation stages, highlighting the role of catalysts and reaction conditions. The report also explores biochemical routes like microbial conversion and gasification, comparing their yields and energy densities. A recommendation section evaluates the feasibility of each method, emphasizing the advantages of biochemical platforms due to their relatively pure products and potential for commercialization. The report references various sources to support the feasibility of the conversion processes and identifies areas for improvement, such as enhancing hydrocarbon yields and selectivity towards bio-jet fuel.

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CONVERSION OF VEGETABLE OILS INTO AVIATION FUEL
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SECTION 1
Stage 1: hydrothermal hydrolysis of vegetable oils
Hydrolysis reactions can be performed using heat (thermally) as liquid-liquid reactions or as gas-
liquid reactions utilizing superheated steam. Another method involves the use of lipolytic enzymes at
room temperature. Biodiesel can be produced via two routes beginning with vegetable oils or fats. In
the first process, the reaction occurs in two stages. The vegetable oil/fat is first converted into fatty
acids through hydrolysis and then the fatty acid is converted to the fuel via esterification. The second
method which is the conventional method involves transesterification of vegetable oils into biodiesel
in the presence of alkali-based catalysts (Demirbas 2008).
Stage 2: Hydrothermal decarboxylation of fatty acids
Decarboxylation is a process in which a carboxyl group is eliminated from a molecule. The thermal
decarboxylation of fatty acids at moderate temperatures less than 400 degrees Celsius produces a
low yield of hydrocarbons which then means it is necessary to use a catalyst. Hydrothermal
decarboxylation of fatty acids at moderate temperatures (less than 400 ℃ ) gives low yields if no
catalyst is used (Heimann, Karthikeyan, & Muthu 2016). This indicates that a catalyst is necessary to
drive the reaction and to boost the yield. Commonly used catalysts are metals such as platinum Pt,
nickel Ni and palladium Pd (Demirbas 2008).
Stage 3: Hydrothermal cracking of long-chain alkanes (in the presence of CO2)
Cracking is a chemical process in which a large hydrocarbon molecule is broken down into smaller
molecules. For instance, the hydrocarbon C15 H32 is broken down into ethane, propene, and octane
as shown below:

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C15 H32 → 2C2 H 4 +C3 H6 +C8 H18
In hydrothermal cracking, high temperatures as high as 750 ℃ are used (Boyadjian & Lefferts 2018).
The pressure used is also very high (in the range of 70 atm) (Boyadjian & Lefferts 2018). It is a
combined process of both thermal and catalytic cracking. The major steps in hydrothermal cracking
include:
i) Thermal reactions break down the carbon-carbon (C-C) bonds of the alkane into free reactive
radicals.
ii) This is followed by the dissociation of the free radicals.
iii) The free radicals are then converted into hydrocarbon molecules through catalytic
hydrogenation.
The two reactions (thermal and catalytic) occur in parallel and the thermal reaction dominates at
higher reaction temperature range.
Stage 4: Hydrogenation of alkenes to make biojet fuel (biokerosene)
Hydrogenation involves the addition of hydrogen atoms to carbon-carbon double bonds in the case
of alkenes. In order to meet the Jet A1 specification, the double bonds in the alkenes must be
saturated (Demirbas 2008). The overall effect of the addition of hydrogen is the elimination of the
double bond in the alkene. Despite the fact that the reaction is exothermic, the activation energy is
quite high and the reaction cannot take place under normal conditions. Therefore, a catalyst is
normally used to lower the activation energy and to speed up the reaction. A commonly used
catalyst is aluminium (iii) oxide Al2 O3.

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SECTION 2
i) Biochemical routes
These involve the conversion of biomass, for example, starch or sugars into long-chain hydrocarbons
and alcohols such as butanol. These are less oxygenated and more energy-dense. One method
proposed by Amyris involves the use of microorganisms to convert sugars into terpenes and then
diesel-like fuels and kerosene (Gupta & Demirbas 2010). This is a method that directly converts
sugars into hydrocarbon fuels. Another method proposed by Virent involves the use of catalysts to
produce alkanes and other hydrocarbons.
ii) Gasification
In this route, small feedstock particles are heated at high temperatures producing synthesis gas. This
gas is mostly comprised of hydrogen and carbon (ii) oxide and is usually referred to as syngas
(Agency 2017). The FT process is then used to convert syngas into numerous other gases and
chemicals or fuels in the presence of a catalyst. Gasification results in a mixture of hydrocarbons
which are then used to extract various fuels and chemicals (Chuck 2016).
SECTION 3: Recommendation
All of the above routes result in different yields of the desired products. In addition, the routes
produce fuels of different energy densities. For instance, gasification leads to the formation of tar in
considerable amounts. Furthermore, biomass has high oxygen content and this has an impact on the
ratio of hydrogen and carbon (ii) oxide in the synthesis gas. Therefore the product must be cleaned
which calls for more expenses. A method called plasma gasification can be used to produce very
clean syngas but it is expensive too (Agency 2017). The yields from most of the other routes depend
on reaction conditions such as temperature, pressure and the presence of catalysts.

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Compared to thermochemical processes, biochemical based jet fuel production systems produce
relatively pure products which is an advantage over the latter, furthermore, the potential scale of
commercialization is higher for biochemical platforms hence these should be considered over
thermochemical processes.

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References
Agency, I. R. (2017). Biofuels for Aviation: Technology Brief.
Boyadjian, C., & Lefferts, L. (2018). Catalytic Oxidative Cracking of Light Alkanes to
Alkenes. European Journal of Inorganic Chemistry, 2018(19), 1956-1968.
doi:10.1002/ejic.201701280
Chuck, C. (2016). Biofuels for Aviation: Feedstocks, Technology and Implementation.
Cambridge, MA: Academic Press.
Demirbas, A. (2008). Biofuels: Securing the Planet’s Future Energy Needs. Berlin,
Germany: Springer Science & Business Media.
Gupta, R. B., & Demirbas, A. (2010). Gasoline, Diesel, and Ethanol Biofuels from Grasses
and Plants. Cambridge, England: Cambridge University Press.
Heimann, K., Karthikeyan, O. P., & Muthu, S. S. (2016). Biodegradation and Bioconversion
of Hydrocarbons. Basingstoke, England: Springer.