**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

## *A Comparative Evaluation of Biodiesel and Used Cooking Oil as Feedstock for HDRD… DOI: http://dx.doi.org/10.5772/intechopen.104393*

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


#### **Table 1.**

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


#### **Table 2.**

*Commercialization of green diesel by selected industries.*

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**).

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

### *A Comparative Evaluation of Biodiesel and Used Cooking Oil as Feedstock for HDRD… DOI: http://dx.doi.org/10.5772/intechopen.104393*

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.

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.


#### **Table 3.**

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

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 hydroprocessing 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.
