Open access peer-reviewed chapter

A Comparative Evaluation of Biodiesel and Used Cooking Oil as Feedstock for HDRD Application: A Review

Written By

Josiah Pelemo, Kayode Timothy Akindeji, Freddie L. Inambao, Omojola Awogbemi and Emmanuel Idoko Onuh

Submitted: 29 January 2022 Reviewed: 08 March 2022 Published: 18 May 2022

DOI: 10.5772/intechopen.104393

From the Edited Volume

Diesel Engines and Biodiesel Engines Technologies

Edited by Freddie L. Inambao

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Abstract

The search for clean energy for transportation fuel across the globe has grown in intensity. The use of biodiesel as a fuel for compression ignition (CI) engines has shown some deficiencies, e.g., poor storage, and poor pour point. The carbon chain of biodiesel is one of the factors to be considered; the longer carbon chain length leads to decreased ignition delay, which leads to the formation of OH during the premixed combustion phase. The major challenges that render biodiesel inefficient are discussed, like higher viscosity, lower energy content, higher nitrogen oxide (NOX) emissions, lower engine speed and power, injector coking, engine compatibility, high cost, and higher engine wear. The novelty of this work is that it shows that biodiesel conversion to green diesel is possible using a biowaste heterogeneous catalyst to obtain quality and high yield of HDRD with lower cost. This renewable energy (HDRD) possesses properties that are directly compatible with CI engines and transportation engines. This research reviewed biodiesel and UCO as feedstocks for the production of HDRD, including the cost–benefit of these feedstocks. Hydrogenation of biodiesel has the potential to overcome the drawbacks of conventional chemically catalyzed processes.

Keywords

  • hydrogenation
  • biodiesel
  • HDRD
  • hydrogen
  • UCO

1. Introduction

The global population growth and the resulting development of commercial and industrial activities, especially in the transportation sector, have stimulated scholars in various institutions to search for sustainable renewable energy. With the continual depletion of conventional primary energy, the need for renewable alternative energy sources becomes more and more important for energy utilization in social and industrial activities. Biodiesel has as a renewable energy has received much attention and research over the year. Biodiesel in neat form or mixed with conventional diesel can be used in compression ignition (CI) engines and stationary engines [1, 2]. The production of biodiesel can be from vegetable oils or animal fats by means of a transesterification reaction, which uses alcohols in the presence of a catalyst [3]. A catalyst is used to chemically convert triglyceride molecules into alkyl esters, generally known as biodiesel fuels [4, 5]. Methanol and ethanol are the most commonly used alcohols for transesterification due to their low cost and high activity [6]. A free fatty acid (FFA) content of higher than 0.5% in vegetable oil renders it a low-grade feedstock due to saponification under alkali catalyzed reaction. The production of conventional biodiesel comprises two stages: the first stage is acidic catalytic esterification and the second stage is the transesterification method using an alkaline or base catalyst. The production process is a time-consuming and tedious procedure because of certain mandatory stages, i.e., time taken for the water to settle at the base and the transesterification process in the presence of an alkaline catalyst in the second stage. The volume of wastewater generated during esterification is high. The drying process of the esterified mixture and further application of transesterification is also a highly taxing process. The catalytic conversion process of low-grade vegetable oil with a high percentage of FFA or high moisture content into biodiesel requires a high-temperature reaction as reported in the literature [7, 8]. The high content of oxygen in biodiesel results in deficiencies such as low oxidative stability, high viscosity, low cloud point, and high pour point in the cold region [9]. Biodiesel also shows lower stability during storage and it attacks metals like copper, zinc, tin, and lead, which can lead to corrosion of some parts of the engines. Another shortcoming of biodiesel is the low energy content and nitrogen oxide (NOX) [10] which reduces thermal and break power efficiency [11]. The deficiencies mentioned above limit the application of biodiesel in CI engines.

The focus of this study is the processing of biodiesel as a feedstock to obtain a green diesel product which can offer sufficient properties without any adverse effects on the CI engine and environment. The following vegetable oils have been discovered and have been used as a feedstock for the production of biofuel for decades, viz., rapeseed, palm, cottonseed, sunflower, peanut oil, soybean oil [12, 13] and animal fats like butter, fish oil, and tallow. These renewable energy sources from biomass sources are the major feedstock sources of biofuel production. However, vegetable oil cannot be used directly to fuel CI engines because it is not compatible due to its high viscosity. The triglycerides and fatty acids present in vegetable oil are the promising components of vegetable oil feedstocks for the production of sustainable biofuel. These feedstocks produce diesel and gasoline type of hydrocarbons via hydroprocessing that can be used in CI engines [14]. HDRD is produced via hydroprocessing of triglycerides contained in edible oil such as used cooking oils and vegetable oils (e.g. rapeseed, soybean, cottonseed, palm, corn, sunflower, coconut, peanut, camelina, carinata, and jatropha oils), fats, and micro-algal oils [15]. These vegetable oils cannot be applied directly in the modern CI engine due to their high viscosity but can be used as a fuel source after some modifications in the fuel properties [16]. Feedstock sourced from edible oil for the production of biofuel has become a problem because of the threat to food security. The need for large land space for farming, the cost, and the resulting threat of deforestation is a major challenge for edible oil, therefore, UCO as a feedstock for HDRD application has recently been adopted. Sunflower oil constitutes about 40–50% of vegetable oil produced in Europe, Russian Ukraine, Turkey, and Argentina. It was reported in literature that sunflower and rapeseed oil are the major sources of feedstock for renewable energy in Europe [17]. The percentage production of the main vegetable oils across the globe are sunflower (10%), rapeseed (55%), cottonseed (10%), and soybean (55%) [18]. Palm oil has been discovered as a potential feedstock for biofuel production in Malaysia [19, 20]. This novel research study focuses on the potential of biodiesel fuel as a better feedstock for production of green diesel.

The main feedstock for the production of biodiesel is vegetable oil [21]. Biodiesel is an alternative fuel that has similar properties to conventional or ‘fossil’ diesel. Conventional homogeneously catalyzed processes for fatty acid methyl esters (FAME) biodiesel production can be used to convert waste vegetable oils but is limited to oils with a relatively low FFA content. Some of the shortcomings of biodiesel that makes it necessary to convert it first to green diesel are: variation in the quality of biodiesel, food shortages, clogging in engine, not suitable for use in low temperatures, water shortages, slight increase in nitrogen oxide emissions. Biodiesel can be hydrotreated to obtain a quality green diesel fuel.

Da Rocha Filho et al. reported that more than 600,000 tons of used cooking oil are generated in South Africa per/annum [22, 23]. HDRD could be produced annually from this waste; given a yield rate of 80%, this will provide 205 million liters. However, this would cater for 50% supply targeted for renewable fuel in the biofuel policy of the government of the South Africa government. The current price of UCO is R3/liter and diesel is R14/liter. A value-added industry generating R2.04 billion can be created producing premium diesel (HDRD) along with the creation of thousands of jobs. Other potential secondary sources of feedstock are cellulose from pulp and paper industries plus a diverse range of agricultural waste with a far greater capacity than UCO [24]. UCO is an oil generated from vegetable oils after frying. UCO is readily available and abundant from food industries, restaurants, households, and fast food outlets using vegetable oils for cooking and frying. The demand for vegetable oil is on the increase in the continent. The yearly consumption of edible vegetable oils in China is approaching 22 million tons, and the country produces more than 4.5 million tons of used oil and grease per year [25]. Vegetable oil used for cooking undergoes various form of chemical and physical changes. Some unwanted compounds like FFAs and some polymerized triglycerides are formed during frying which causes a rise in the molecular mass and condenses the volatility of the oil. Used cooking oils are renewable and do not contain any aromatics, metal, or sulfur contaminants. Reuse of UCO can exacerbate environmental problems, health challenges including hypertension, diabetes, vascular inflammation, and other health effects [26]. Vegetable oil is an oil used for cooking a various type of food items which include, chicken, beef, yam/potatoes. Currently, the major factor that hinder the commercialization of renewable fuel the high cost of feedstock compared to fossil fuel. It has been reported in literature that about 70–85% cost of production of HDRD arises from the raw materials. However, the use of UCO as a feedstock for production of HDRD will enhance the commercialization of green diesel due to the availability at a low price. The HDRD production process involves the conversion of fatty acids in triglycerides into normal and/or iso-paraffin which can be obtained by hydrodeoxygenation, decarbonylation, decarboxylation, isomerization and hydrocracking or a combination of two or more thereof (Tables 1 and 2).

Fuel propertiesUsed cooking oilBiodieselCommercial
Kinetics viscosity (mm2/s, @313 k)36.45.31.9–4.1
Flash points (k)485469340–358
Cetane Number495440–46
Density (kg/l, @ 288 k)0.9240.8970.75–0.840
Pour points(k)284262254–260
Sulfur contents (%)0.090.060.35–0.55
Water contents (%)0.420.040.02–0.05
Ash contents (%)0.060.0040.008–0.010
Free fatty acid (mgkoH/g oil)1.320.10
High heating value (MJ/kg)41.4042.6545.62–46.48

Table 1.

Comparison of properties of UCO, biodiesel, and commercial diesel fuel [27].

S/NName of industryFeedstock/TechnologyTargeted product/tonProperties of productReferences
1UOP/EcofiningTriglyceride, hydro-processing of UCOGreen dieselSimilar to the properties of fossil fuels, Good cool flow[28, 29, 30, 31]
2Tyson Foods Inc. Gesmar LA, USAHydrotreating of non-edible and animal fatsGreen diesel, 75 Mil.P/annumGood storage stability, High cetane numbers, properties similar to petroleum fuel[32]
3Haldor TopsoeNew hydrotreating raw tallGreen dieselSimilar to fossil fuel properties[33]
4ConcocoPhillipsHydrogenation of Vegetable oilGreen diesel 365,000 barrel/annumSulful free fuel content, emission of less NOx[33]
5Valero Energy Corporation, St. CharlesHydrogenation of UCO/Animal fatsRenewable dieselSimilar to fossil fuel properties[34]

Table 2.

Commercialization of green diesel by selected industries.

The high acid value of UCO is due to the high content of FFAs [23]. In recent years, several petroleum companies have directed their resources into the production of renewable green fuels from hydro-processing of vegetable oils feedstock, with considerable commercial success.

The Neste Oil Co. developed a technology to convert vegetable oil and animal fat into high-quality hydrocarbons. The plant start operation by Neste Oil in Singapore in 2012 using NExBTL technology targeted production of over 800,000 tons renewable diesel per annum from feedstocks [35]. The experimental analysis of the samples by Neste Oil shows a high cetane value of between 84 and 99, a low cloud point value (as low as minus 30°C), and can withstand storage for extended periods. These properties enhance its performance in both car and truck engines [36].

The commercialization of bio hydroformed diesel (BHD) by the joint effort of Toyota Motor Corporation (TMC), Hino Motors, the Tokyo Metropolitan Government, and Nippon Oil Corporation (NOC) have commenced operation in recent time, a second-generation renewable diesel fuel produced by hydrogenating a vegetable oil feedstock. Nippon Oil and Toyota have worked jointly on the development of BHD technology since 2005. The use of refinery-based for hydro processing of vegetable oil for the production of a synthetic, second-generation biofuel depends on several issues, including the properties and effects of first-generation FAME (storage, oxidation, possible effect on fuel handling systems). In its studies, Nippon Oil explored reaction temperatures ranging from 240–360°C, with reaction pressures of 6 MPa and 10 MPa, and used a common hydrodesulfurization catalyst. The resulting fuel is claimed to be aromatics- and sulfur-free, with a cetane number of 101 [33].

The daily consumption of vegetable oil has witnessed a tremendous increase globally due to the increasing population and modernization. The global total primary energy consumption(GTPEC) has recorded over 150,000,000 GW h and this is expected to increase by 57% in the year 2050 [37]. This significant growth of energy consumption will eventually result in more environmental problems [38]. Currently, over 80% of the total energy used across the globe is sourced from fossil fuels, leading to their high contribution to environmental and health challenges [39]. UCO, which is considered waste, is collected before disposal. The annual collection of UCOs is evidence of the high consumption rate of UCO in some countries. The Energy Information Administration (EIA) of the United States reported an estimate of 100 million gallons of UCO produced per day in the USA [40]. About 135,000 and 140,000 tons of UCO are generated per annum in Canada [41, 42]. In South Africa, 0.6 million tons of UCO are collected annually from bakeries, takeaway outlets, and restaurants [43, 44]. The UK and the European Union countries produced 0.7 million tons to1.0 million tons and 0.2 million tons of UCO per annum, respectively [45]. The generation of a large amount of used cooking oil is a panacea to food security and fuel sustainability if properly harnessed for hydrogenation purposes. UCO is readily available, sustainable and cost–effective. Reports show that 17% of UCO offer a yield of 11.92 million tons while about 9% of the feedstock for the production of 26.62 million tons of biofuel is obtained globally in 2015 [46]. UCO was investigated, and the outcome of the chemical properties show that oleic acid has the highest value of 43.67%, followed by palmitic acid with 38.35%, and linoleic acid, 11.39% [47]. These properties of used cooking oils make it viable as a l feedstock for conversion into hydrocarbon. Table 3 shows the properties of UCO samples of sunflower oil, palm oil and sunfoil.

PropertiesUCO Samples
Sunflower oilPalm oilSunfoil
Density (Kg/m3)920.4913.4923.2
pH value5.346.196.61
Viscosity (mm2/s)31.38138.40735.236
Acid value2.291.131.44
Congealing temperature °C−5.1514.7−3.4
Molecular weight (g/mol)51.94586.05395.28
Iodine value (cg/g)111.154.254.2

Table 3.

Properties of feedstock compared to other vegetable oil [46].

The feedstock is one of the key resources in determining the production costs of biofuel. The adoption of biodiesel as feedstock will reduce the total cost of production and this will support the profitability and commercialization of an HDRD product. Biodiesel can be hydrotreated to combat the challenges of storage stability, cetane number to obtain superior HDRD as known as renewable fuel at a lesser cost and labor. UCO oil makes up approximately 80% of the total production expenses [29]. Biodiesel oil is readily available, does not affect food security, is cheap, requires not much effort to source, and offers a good yield when used. Green diesel is oxygen-free; hence oxidation stability is high, and has a high cetane number (CN) similar to fossil fuel. Hydrogenation derived renewable diesel(HDRD) possess high pour point better than biodiesel, It reduces NOX emissions, and has a high heating value which is a significant property of diesel fuel because it gives the energy content of the fuel and aid the performance of CI engines. Furthermore, green diesel produced by the hydro-processing of triglycerides has propane as a by-product which is a gaseous fuel of good market value. This property makes HDRD production more feasible in economic terms when compared to the production of FAME [48]. The composition of biodiesel products can be improved by hydro-processing techniques to obtain HDRD.

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2. Parameters and metrics of study

Biodiesel fuel has been investigated by researchers with varying outcomes. Table 4 shows some problems and possible solutions related to biodiesel fuel. Some important properties of biodiesel are cloud and pour point, storage stability, viscosity, acid value, cetane number. These are the properties of biofuel that must meet the set ASTM standard of biofuel. These properties are deficient when considering using biodiesel in CI engines, particularly in a cold, temperate, regions. The cloud points of ethyl ester produced from use cooking oil, linseed oil, canola, sunflower, and rapeseed oil are −1°C, −2°C, −1°C, −1°C, and −2°C, respectively. Lang et al. (2001) [49] reported that the cloud point of ethyl esters of linseed oil, canola, sunflower, and rapeseed oil were −2°C, −1°C, −1°C, and −2°C, respectively,whereas the corresponding methyl esters had cloud points of 0°C, 1°C, 1°C, and 0°C. Currently, internal combustion engine are controlled by compression ignition(CI) engine and the spark ignition (SI) engine. The operations of Spark engine (SI) is done by premixed charge near stoichiometric air-fuel ratio (Ø ~ 1), the flame is spread with the aid of a spark plug, the throttling effect of the charges into the cylinder leads to low thermal efficiency at partial load. Moreover, with the high temperature at peak load, cause generation of massive NOX, this have advert effect on the environment, is form a major challenge of the SI engine applications. The compression ratio of CI engine is between 12 and 24, in addition CI engine, is characterized by turbulent flame, diffused flame, auto-ignition via elevated pressure and temperature around the top dead center. The CI engine has a higher thermal efficiency compared to a SI engine. The major part of its operation is ignition delay (this is the time difference between the start of injection and the self-ignition). The ignition delay mechanism is controlled by physical and s chemical kinetic processes. The physical process proceeds sequentially through droplet formation, collision, break-ups, evaporation, and vapor diffusion. The chemical kinetic process involves species and radical formation proceeding through low-temperature reaction (LTR), negative temperature coefficient (NTC), and high-temperature reaction (HTR). The challenge is that the time scales for the physical processes are often larger than those of the chemical processes, hence ignition often commences before the physical processes are completed. This inevitably leads to a complex system of charge, flame, and thermal stratification that produces both high NOX in some regions and high soot precursors and unburnt hydrocarbon (UHC) in others. Research focus, therefore, has been to hydroprocess biodiesel fuel into HDRD to mitigate NOX, UHC, and soot formation (as is the case with the SI engine). From the foregoing, it is clear that there are many challenges facing the efforts to utilize biodiesel as a fuel in transportation.

ProblemCausesPossible solutions
Cold weather startingHigh viscosity, low cetane, and low flash point of vegetable oilsPreheat fuel before fuel injection. Chemically alter fuel to an ester
Plugging and gumming of filters lines and injectorsNatural gums (phosphatides) in vegetable oil. Other ashPartially refine the oil to remove gums. Filter to 4-microns
Excessive engine wearThe high viscosity of vegetable oil, incomplete combustion of fuel. Poor combustion at part load with vegetable lubricating oil due to blow-by of vegetable oilHeat fuel before injection. Switch engine to diesel fuel when operating at part load. Chemically alter the vegetable oil to an ester. Increase motor oil changes. Motor oil additives to inhibit oxidation
Engine knockingVery low cetane number of some oils. Improper injection timingAdjust injection timing. Use higher compression engines. Preheat fuel before injection. Chemically alter fuel to an ester
Coking of injectors on piston and head of engineThe high viscosity of vegetable oil, incomplete combustion of fuel. Poor combustion at part load with vegetable oilsHeat fuel before injection. Switch engine to diesel fuel when operating at part load. Chemically alter the vegetable oil to an ester
Failure of engine lubricating oil due to polymerizationCollection of polyunsaturated vegetable oil blow-by in the crank case to the point where polymerization occursHeat fuel before injection. Switch engine to diesel fuel when operating at part load. Chemically alter the vegetable oil to an ester. Increase motor oil changes. Motor oil additives to inhibit oxidation

Table 4.

Problems, causes, and possible solutions regarding biodiesel products.

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3. Potential feedstock for biodiesel production

The rapid urbanization, abundant land resources, and vegetable oil in Africa offer adequate opportunity for biodiesel production on a large scale. FAME product obtained from this resource provide insight into the new option of sourcing for renewable energy. Traditionally, biodiesel had been produced from vegetable oil; about 93% of biodiesel is produced from edible oil, and the feedstock sourced from agricultural sector [50]. However, the edible vegetable oil is unsustainable due to the food security threat [51]. Biodiesel production uses around 4.4 million hectares of arable land in the European Union [52]. The consequence of deforestation is greenhouse gas (GHG) which great effect on man, plant, animal. The alternative option to these is the use of non-edible oils, hence the focus on biodiesel and used cooking oil as feedstock. Other vegetable oils can be propagated easily in drought-prone areas and are highly adaptable in tropical areas [53]. Biodiesel fuel converted to HDRD is set to play an important role in the future around the world. Africa’s population is projected to reach 1.5 billion by 2030, with 53.5% of this population living in developed countries [54]. This trend is set to accelerate and a greater supply of green diesel will be necessary in order to meet the growing demand for HDRD. The only economically beneficial and sustainable ways to achieve this goal is to embark on aggressive production of HDRD using biodiesel as a feedstock.

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4. Production and feedstock value chain

The technology for petroleum-based fuel production has been in existence for many years. The same catalyst, reactor type, and distillation facilities used for the production of fossil-based fuel are also applicable for hydro-processing of vegetable oil-derived feedstocks to obtain high-quality hydrocarbon. Thus, massive savings are achievable since the same production plant facilities can be used for the purpose. Conversion of used vegetable oil via hydro processing using stand-alone units can be achieved by optimizing and controlling the facilities to obtain high yield green diesel. The design and construction of this facility as an attachment to existing plants and hydrogen in the refinery can be streamlined as recycled gas. The only shortcoming of a stand-alone unit as a production facility is the high cost of construction. In recent times the feedstock used for the production of HDRD has been plant-derived oils such as rapeseed, soybeans, and palm [55, 56, 57, 58] which are edible oils, with the non-edible oils like Jatropha, algal oils and waste cooking oil products being the most popular feedstock ion recent times [57, 59]. Many researchers have confirmed UCO as being the most viable, cost-effective, and available feedstock. Researchers are still investigating the most suitable technology for the production of green diesel that is cost-effective and CI engine compatible. No research has been done yet on the viability of biodiesel oil as a potential feedstock for HDRD production. When this is explored it will boost the supply of renewable fuel in energy sector. Green diesel obtained from biomass can be processed through four technologies: (i) Pyrolysis and upgrading of bio-oil, (ii) Hydro-processing, (iii) Catalytic upgrading of sugars, starches, and alcohols. (iv) Biomass to liquid (BTL) thermochemical processes. FT green diesel is produced by the Fischer-Tropsch [60, 61]. Hydrocracking and hydrotreating are the two major step in hydroprocessing technique [62]. The following yield was obtained via hydro processed used vegetable oil; biofuel (85%), non-condensable gases (10%), and water(5%) [63]. Green diesel is a biofuel product that comprises of branched saturated hydrocarbons and straight chain which contain carbon atoms of C15–C18. The properties of green diesel is similar to fossil fuel making it compatible with CI engines [64, 65]. Therefore, green diesel has superior fuel properties like high cetane number, oxidation stability, and cold flow and cloud point compared to FAME and petroleum based diesel [66]. Othman et al. investigated the value of cetane number, the outcome shows that green diesel has value of cetane number between 80 to 90 which is higher than the petrol based diesel standard [67]. The density and the net heating value of green diesel are in the range 0.77 g/ml to 0.83 g/ml and 42 MJ/kg and 44 MJ/kg respectfully, which also meets the biodiesel and petrol diesel standard [68].

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5. Sourcing of UCO for production of FAME

The global population and consumption of vegetable oils have a direct relationship. The increase in population has triggered the consumption of vegetable oil that gives rise to massive volumes of UCO. The largest percentage of these vegetable oils is used in households, restaurants, and fast-food outlets for cooking and frying. Table 5 shows the estimated amount of UCO collected by some countries. Canada is reported to generate between 0.120 and 0.135 million tons of UCO per year [4269], while the United States of America produced 0.6 million tons of yellow grease in 2011. The United Kingdom and the European Union countries generate ∼0.7 million tons to 1.0 million tons and 0.2 million tons of UCO per year, respectively [45, 70]. In South Africa 0.6 million tons of UCO are collected per year, while more than an estimated 0.2 million tons of UCO is produced but not collected from households, bakeries, takeaway outlets, and restaurants per year [44, 70], which contributes to soil and water contamination, sewage blockages, and damage to aquatic life [71]. China, Malaysia, and Japan generated 0.6 million tons, 0.5 million tons, and 0.6 million tons of UCO, respectively, annually. It is reported that more than 60% UCO generated globally is indiscriminately disposed of [24].

CountryUCO collection (million ton/annum)
Canada0.14
China0.15
Malaysia0.5
South Africa0.6
United Kingdom0.2

Table 5.

Annual collection of UCOs in some countries.

UCO is produced when vegetable oils sourced from palm, soybean, sunflower, cottonseed, olive, palm kernel, and rapeseed or animal fats like butter, fish oil, and tallow, are used for cooking or frying food [72]. With feedstock accounting for between 80% and 85% of the production cost of biodiesel, the use of UCO can result in a substantial reduction in production costs, thereby significantly reducing the cost of biodiesel fuel. This makes biodiesel more viable as a feedstock for green diesel production as a substitute fuel for internal combustion engines, particularly unmodified CI engines. The focus of this research work has been on the conversion of biodiesel product into HDRD via the hydrogenation technique.

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6. Evaluation of properties of UCO and biodiesel

The pH value of UCO varies between 5.13 and 6.61, indicating a weak acid for a biodiesel feedstock. The pH value reported in the literature were not uniform but depended on the type and degree of usage. Food such as sausage triggered higher pH values than fish etc. This is a result of fats from fish being more acidic than those of beef [39]. The value of acidity is reduced in used palm oil after repeated frying due to the effects of thermal degradation and contamination from the food items. UCO which is normally subjected to heating witnesses a reduction in pH as a result of usage. Generally, due to repeated and high cooking temperatures, the acid value of the oil tends to reduce. However, the transformation and the mechanism for the generation of cyclic and noncyclic hydrocarbon in vegetable oil during high-temperature repeated cooking can be difficult to predict as a result of the many reactions that produce many unstable intermediate hydrocarbons. In contrast, the results obtained as reported in the literature show that biodiesel is a feasible feedstock for the production of HDRD which is also known as green diesel. From the previous report research analysis, EN 14214 and other standard test methods were used for the analysis of biodiesel. Density at 15°C and kinematic viscosity at 40°C of the fuel were measured under EN ISO 3675:1998 and EN ISO 3104:1994,with 926 kg/cm3 and 37.3 1mm2/s values being reported respectively [73]. The acid value and iodine value analysis were done by titration under EN 14104:2003 and EN 14111:2003, with 0.63 mgKOH/g and 109.98 mgI2/100 g being reported respectively [74]. The flashpoint was measured using the Pensky-Martens method ISO 2719:2002 [75], and the sulfur content was quantified under EN ISO 13032:2012 [76]. The aforementioned properties prove the viability of biodiesel as a potential feedstock for HDRD production. The application of HDRD derived from biodiesel fuel offers high yield, excellent storage stability, high cetane number, and flashpoint.

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7. Conclusions

The continuous search for sustainable renewable energy has stimulated researchers to develop solutions to the global environmental crises and demand for renewable biofuel. HDRD has received significant attention across the global economy due to its higher stability, higher heating value, and emission of harmless pollutants. The use of biodiesel as a feedstock is more viable as it reduces the production cost and has the potential to settle the concerns related to the edible oil market. This critical review has identified biodiesel fuel as a potential feedstock for hydrogenation into HDRD. The main conclusions are:

  • Commercialization of HDRD production requires government attention without delay. Implementation of government policies and regulations that directly address the challenge of demand, production, and trade of this HDRD must be spearheaded by ministries and governmental bodies.

  • The outcome of this research shows that biodiesel fuel can be harnessed as a potential feedstock for the commercial production of HDRD.

  • HDRD possesses superior fuel qualities such as CN, low pour point, and excellent oxidation stability which offers a great benefit for the CI engine and the environment.

  • The goal of producing cost-efficient HDRD is still far from being reached. The economic feasibility of the production of HDRD using biodiesel is sustainable.

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Acknowledgments

The authors are grateful to the leadership of Green Energy Solutions, Discipline of Mechanical Engineering, Howard College, University of KwaZulu-Natal, Durban for their contributions towards the success of this work.

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Conflict of interest

No conflict of interest.

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Notes/thanks/other declarations

The Authors hereby acknowledge the funding support from TETFUND.

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Written By

Josiah Pelemo, Kayode Timothy Akindeji, Freddie L. Inambao, Omojola Awogbemi and Emmanuel Idoko Onuh

Submitted: 29 January 2022 Reviewed: 08 March 2022 Published: 18 May 2022