Converting vegetable oils using co2 as a reactant into aviation

European Bioenergy Research Institute (EBRI), Aston University, UK
Introduction
The inexhaustible, clean, and absence of greenhouse gas emissions are
properties that renewable energy sources have over fossil fuels. Fossils
are most applied worldwide but are not sustainable since they emit CO2
gas in the atmosphere leading to accumulation of greenhouse gases
hence climate change. Renewable friendly sources of energy such as
biofuels that are friendly to the environment are needed to be produced
and applied to improve the health of environment and make it habitable
to both human, plants and animals. One way to be rid of the negative
impacts these fossil energy sources have is by electrification. Batteries
charged by renewable energy sources such as wind, hydro, solar.
Charging by non-renewable energy sources is also plausible option as the
potential harmful emissions will be away from the dense population areas.
However, planes require a lot of power, batteries are heavy and planes
need to fly. Another line of reasoning is the economic impact, the positive
correlation between the fuel quantity and the operating costs of the
aircraft. Complete electric propulsion is still being proposed,
nonetheless, a hybrid system is an approach used today. The energy from
the hydrocarbon fuel and power for the aircraft systems from the
batteries (Atkinsglobal.com, 2019).
Biofuels are known as the liquid fuel generated from the biomass of
dissimilar forest materials, agricultural materials and some biodegradable
wastes (Yan, 2017). It is extracted from the vegetable oil algae, bio
butanol. Vegetable oil is an upcoming major source of biofuel generation
in the whole world since they have a close energetic content to
hydrocarbon fuels for transport, but the direct use is not acceptable
mainly due to the relatively low viscosity required by the engines (Neuling
and Kaltschmitt, 2017). Transesterification is a chemical process that
converts oil into esters in the presence of a catalyst. This conversion to
biodiesel helps to reduce the viscosity of the vegetable oil and allows use
without any modification to conventional engines. Almost 400 oil crops
are potential sources for diesel engines, but aircraft engines require more
specific characteristics as defined by ASTM D1655 in terms of energy
density, viscosity, lower heating value, boiling point.
Algae are also an upcoming source of biofuel generation as they are
known to be safe, non-competitive and grow very fast, but they have a
relatively low solid content and require more pre-treatment steps before
oil extraction (SAIFUDDIN et al., 2016).
Therefore, exploring methods for direct production of bio kerosene is
necessary. One of these is fisher –tropsch processes which involve high-
temperature gasification of biomass to produce a syngas which is later
converted catalytically to hydrocarbons. The HEFA process is another, it
is more famous and already being commercialized. However, with the
discovery of solutions and solving problems being the sole aim of science
and engineering, there goal of this research will be to find a way to make
this process more efficient (Biofuels International, 2019).
Methodology
This proposed research will focus mainly on the hydrothermal conversion
of vegetable oils using into aviation fuel . The yield and quality of product
depend on the process conditions and feedstock. The application of
heated water works as a form of catalyst, minimising the formation of
gaseous products from degraded oil because of the lower temperature
used relative to conventional methods. The procedure is carried out in
four stages : i) Hydrolysis ii) Decarboxylation iii) Cracking iv)
Hydrogenation.
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. Biofuel 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 biofuel in the presence
of alkali-based catalysts (Demirbas 2008).
Next, decarboxylation. 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 °C ) 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).
Following this, the large hydrocarbon molecule is broken down into
smaller molecules. In hydrothermal cracking, high temperatures as high
as 750 °C 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.
Finally, 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 〖 Al 〗 _2 O_3.
Conclusion
This processes of generating biofuel is greener, simpler and cost-effective
hence more sustainable. More innovations should be done on how to use
the vegetable oil, forest and agricultural products and wastes to generate
more clean and friendly energy to the environment. Algae are most
preferred since they are not edible and therefore will reduce the
competition of energy generation and food industry. Generation of biofuel
maintain clean, healthy and friendly environment, reduces the levels of
diseases caused by burning fossil fuels, and also ensure sustainability.
Biofuel reduced the emission of greenhouse gases into atmosphere hence
reduces global warming and climate change, therefore it will try to solve
the environmental problems and at the same time meet the need of
people in terms of energy production.
ReferencesAtkinsglobal.com. (2019). [online] Available at: https://www.atkinsglobal.com/~/media/Files/A/Atkins-Corporate/Electrification%20White%20Paper%20-%20digital.pdf [Accessed 23 Jul. 2019].
Attaphong, C., Do, L. & Sabatini, D., 2012. Vegetable oil-based microemulsions using carboxylate-based extended surfactants and their potential as an alternative renewable biofuel. Fuel, Volume 94, pp. 606-613.
https://www.sciencedirect.com/science/article/pii/S0016236111006661.
Chiaramonti, D. et al., 2017. Oilseed pressing and vegetable oil properties and upgrading in decentralised small scale plants for biofuel production. International Journal of Oil, Gas and Coal Technology, Volume 14, p. 91.
https://www.researchgate.net/publication/312013174_
Keera, S., Sabagh, s. & Taman, A., 2011. Transesterification of vegetable oil to biodiesel fuel using an alkaline catalyst. Fuel, Volume 90, pp. 42-47. https://www.sciencedirect.com/science/article/pii/S0016236110004059
Lapuerta, M., Villajos, M., Agudelo, J. & Boehman, A., 2011. Key properties and blending strategies of hydrotreated vegetable oil as biofuel for diesel engines. Fuel Processing Technology, Volume 92, pp. 2406-2411.
https://www.sciencedirect.com/science/article/pii/S0378382011003171
Neuling, U. and Kaltschmitt, M. (2017). Biokerosene from Vegetable Oils – Technologies and Processes. Biokerosene, pp.475-496.
Rathore, V. & Badoni, R., 2016. Processing of vegetable oil for biofuel production through conventional and non-conventional routes. Energy for Sustainable Development, Volume 31, pp. 24-49.
https://www.researchgate.net/publication/291423239