Catalytic Hydroprocessing of Brassica carinata Oil to Produce Bio-Jet Fuel: Investigation of Alternative Feedstocks, and Synthesis, Characterization, and Evaluation of Supported Mesoporous Catalysts
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Date
2024-08-20
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Addis Ababa University
Abstract
The aviation industry, which is one of the biggest drivers of economic growth, offers a considerable
contribution to the global economy. However, over the past few decades, the industry has played a
significant role in driving up a significant portion of greenhouse gas emissions into the atmosphere,
contributing significantly to global CO2 emissions. The transition from fossil-derived jet fuels to
sustainable aviation fuels is one of the most viable strategies to decarbonize the industry and
mitigate CO2 emissions generated by fossil fuel combustion. Consequently, as environmental
concerns pertaining to the effects of CO2 emissions from fuels derived from carbon resources that
originate from fossil fuels are growing, the endeavor to explore renewable carbon sources has been
spurred. As a fossil fuel substitute, bio-jet fuel, one of the most significant forms of renewable and
green energy that can be potentially derived from renewable and alternative feedstocks, has the
potential to improve energy security, reduce carbon footprint, and promote agricultural economy
and social development.
In this doctoral study, comprehensive experimental investigations were conducted on a variety of
Brassica carinata oilseed crops (indigenous to Ethiopia) derived coproducts, including non-food
vegetable oils, oilseed meals, and bio-oils to evaluate their potential industrial applications,
particularly in the aviation industry. Following their successful synthesis, supported transition
metal carbide catalysts, distinguished for their superior properties, were evaluated for their ability
to produce bio-jet fuel. Under appropriate reaction conditions, a lab-scale designed fixed-bed
reactor system was used to transform the oil feedstock, Yellow Dodolla oil (one of the most
remarkable inedible Brassica carinata vegetable oils), into the bio-jet fuel.
The non-food Brassica carinata vegetable oils were extracted using a solvent extraction method
involving the Response Surface Methodology with Box-Behnken Design in an isothermal batch
reactor. After extraction, physicochemical characterization, fatty acid profiling, ultimate analysis,
metal and phosphorus concentration analysis, Fourier-transform infrared spectroscopy
characterization, and calorific value studies were used to investigate the qualities of the oils. As a
result, it was found that oil yields varied between 35.93 and 45.25%. Erucic acid was the most
prevalent fatty acid in all oils, representing 42–50% in Derash and Yellow Dodolla oils,
respectively, making Yellow Dodolla oil a super-high erucic acid oil. The characterization results
also showed that the Brassica carinata oils have better physicochemical qualities, exceptional fatty acid profiles, and extremely low concentrations of metals, phosphorous, and heteroatoms (nitrogen,
and sulfur). As a result, the oils—most notably Yellow Dodolla oil—are of the highest caliber and
offer a viable alternative feedstock for upgrading them into bio-jet fuel using a hydroprocessing
route. However, the bio-jet fuel plant's output streams, such as solvent defatted oilseed meals, may
be utilized as an effective resource utilization approach, to produce a wide range of co-products.
Thus, a comprehensive study was conducted on these meals using bomb calorimetric digestion,
proximate analysis, ultimate analysis, inductively coupled plasma optical emission spectroscopic
analysis, and determination of energy densities. Characterization results verified that the meals'
highly distinguishing characteristics enable them better options than other oilseed meals.
However, through valorization, the solvent-defatted oilseed meals can be employed as a useful
resource utilization strategy which could subsequently support the development of a carinata-based
circular bio-economy. Four different hexane-defatted meals were characterized and valorized for
their substantial potential valorization options. Proximate analysis, ultimate analysis, inductively
coupled plasma optical emission spectroscopic analysis, bomb calorimetric digestion, and energy
densities, were used to characterize the meals. As a result, analysis results demonstrated that the
meals had distinct features, making them ideal alternative feedstocks for valorization into a variety
of industrial applications.
The solvent-defatted oilseed meals were further used to produce bio-oils using a non-catalytic slow
pyrolysis approach at various temperatures (350−500 ℃). The pyrolysis experiments showed that
the highest temperature (550 ℃) yielded the maximum bio-oil product (55.01%), while the lowest
temperature (350 ℃) yielded the maximum bio-char (34.93%) and gas (45.84%) yields. The
properties of the bio-oils were studied using physicochemical characterization, ultimate analysis,
atomic ratios analysis, heating value analysis, inductively coupled plasma-optical emission
spectrometry analysis, gas chromatograph-mass spectroscopy, and Fourier-transform infrared
spectroscopy. Analysis results showed that the bio-oils had moisture (35.38−48.64%), pH (8.50),
kinematic viscosity (14.10−16.05 cSt), ash content (0.17−0.208%), carbon (55.4−62.3%),
hydrogen (9.02−9.29%), nitrogen (6.08−6.20%), sulfur (0.61−0.69%), oxygen (21.47−28.56%),
and higher heating value (26.98−30.45MJ/kg). Furthermore, it was found that the major classes of
compounds identified include saturated hydrocarbons (13.56−14.52%), saturated fatty acids
(2.33−3.67%), monounsaturated hydrocarbons (30.28−34.62%), monounsaturated fatty acids (6.54−11.23%), polyunsaturated fatty acids (1.41−2.82%), and Others (such as nitrogenated
compounds) (38.44−39.62%).
However, since a catalytic hydroconversion technique will be applied for transforming the oil
feedstock, high-efficiency, high-stability, and low-cost deoxygenation and hydrocracking catalytic
materials must be developed. Aluminophosphate (AlPO4-18), a zeolite-like molecular sieve, was
used as a catalyst support for the synthesis of carbon-coated β-Mo2C, Ni3C, and WC nanoparticle
catalytic materials. The nanoparticles were synthesized using an incipient wetness impregnation
followed by a temperature-programmed reduction-carburization approach, which involved
cracking a hydrocarbon gas, propane, in a hydrogen environment. The synthesis parameters were
a 1:7 propane/hydrogen reductive-carburizing gas stream, 15 wt. % metal loading, an 800 ℃
carburization temperature ramped-up at a heating rate of 10 ℃ min-1, 2 hours holding time, and a
1-hour holding time in a hydrogen environment. The nanoparticles were characterized by
thermogravimetry mass spectroscopy coupled with temperature-programmed oxidation (TPO TGMS),
nitrogen physisorption at 77 K, X-ray diffraction (XRD), and transmission electron
microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS) methods.
Consequently, TPO TG-MS, nitrogen physisorption, TEM, and XRD characterization results
proved that atomic carbon was successfully incorporated into the lattice interstitials, resulting in
the development of thermally stable AlPO4-18 supported well-dispersed, crystalline and
mesoporous β-Mo2C, Ni3C, and WC nanoparticles. Moreover, XRD analysis showed how the
structural evolution occurred during the reduction-carburization, revealing average crystallite sizes
of the metal-containing particles to be 8.2–9.22, 6.64–8.50, and 6.03–7.56 nm for β-Mo2C/AlPO4-
18, Ni3C/AlPO4-18, and WC/AlPO4-18, respectively, with these values did not significantly
deviate from the results of high-resolution TEM analysis. After the reduction-carburization
process, the nanoparticles' surface areas dropped and were categorized in decreasing order as
WC/AlPO4-18 > Ni3C/AlPO4-18 > β-Mo2C/AlPO4-18, with values of 193.79, 169.05, and 66.57
m2 g-1, respectively.
Following the successful synthesis of the nanoparticles, a single pot oil-to-jet catalytic
hydroprocessing upgrading approach was employed to evaluate the nanoparticles' catalytic
activities. The catalytic activities were carried out on Yellow Dodolla oil to transform it into biojet
fuel. The deoxygenation and hydrocracking reactions were carried out at different reaction temperatures (300 and 500 ℃) and an elevated hydrogen pressure (21 bar) in a laboratory-scale
designed three-phase continuous fixed-bed reactor system. The remaining variables, including the
volumetric flow rates of the oil feedstock (0.30 mL min-1), hydrogen gas (350 mL min-1), the
hydrogen gas-to-oil ratio (1,667 mL H2 gas per mL oil), the catalyst-to-oil ratio (0.14 g catalyst per
g oil), the liquid hourly space velocity (LHSV) (2.41 h-1), the weight hourly space velocity (WHSV)
(2.78 h-1), and the residence time (2.5 h), were all kept constant throughout the experiments.
Consequently, the hydrodeoxygenation, hydrodecarbonylation, decarboxylation, and
hydrocracking/polymerization reactions resulted in a conversion of 71.57–79.76 wt. %, with the
highest conversion was achieved by Ni3C/AlPO4–18 catalyst at the maximum temperature. The
results of these catalytic reactions showed that the rate of deoxygenation varied from 8.08 to 11.67
wt.% at 300 ℃, with nickel catalyst achieving the highest rate and molybdenum having the lowest.
Yet, the rate of deoxygenation rose sharply to 96.67, 62.44, and 57.31 wt. %, respectively, via
molybdenum, nickel, and tungsten catalysts as the temperature rose to 500 ℃. Moreover, it was
determined that bio-jet fuel (C8-C16) exhibited remarkably higher yields (23.34–27.31 wt.%) and
selectivity (37–45 wt.%) at the maximum temperature when compared to biogasoline (2.63–8.72
wt.%) and biodiesel (1.18–4.58 wt.%). The WC/AlPO4-18 catalyst produced the highest yields and
selectivity of the jet fuel. Furthermore, characterization findings of products revealed that, in
comparison to conventional jet fuels, they had nearly identical physicochemical properties,
chemical compositions, hydrogen-to-carbon atomic ratio (H/C) (1.90–1.92), oxygen-to-carbon
atomic ratio (O/C) (0.002–0.030), and gravimetric energy density, net, (41.35–42.89 MJ kg-1)
compared to conventional jet fuels.
In conclusion, the comprehensive investigation carried out on the various Brassica carinata
coproducts—vegetable oils, oilseed meals, and bio-oils—showed that they are highly significant
and viable alternative feedstocks for various industrial uses, particularly in the aviation industry.
Because of the fierce competition for resources on a global scale, which is rendering aviation fuels
unsustainable in the long run, the use of a hydroconversion approach involving low-cost, highefficiency,
and high-stability transition metal carbide nanocatalyst alternatives on Brassica
carinata coproducts can be considered one of the most promising decarbonization strategies.
Description
Keywords
Brassica carinata Oil, Oilseed Meal, Catalyst Synthesis, Catalyst Characterization, Transition Metal Carbide Catalyst, Nanoparticle, Fixed-Bed Reactor, Sustainable Aviation Fuel.