Recent Advances in Biodiesel from Plants

Abstract

Due to population explosion, and increased industrialization with urban and rural development, the need for increased energy utilization has become more intense. Petrodiesel that has been the main energy source for heavy-duty automobiles or machines has contributed immensely to environmental pollution leading to climate change, an increase in illnesses, and reduced lifespan. To combat this ugly situation arising from the utilization of Petrodiesel, biodiesel is produced from plant oil or animal fats to substitute for Petrodiesel in internal combustion engines, either as neat biodiesel or as a blend with Petrodiesel. Different scientists and researchers have produced biodiesel from edible and non-edible plant oils. Their reports show that biodiesel properties depend on the nature of the parent plant oil and the production procedures taken. These properties that are due mainly to its production procedures determining their performance in internal combustion engines. In this chapter, recent findings on biodiesel properties with their effects on performance in internal combustion engines are reviewed. Researchers’ reports show that the most suitable blend of biodiesel with Petrodiesel is B20. This blend consisting of 20% biodiesel with 80% Petrodiesel has equivalent performance as Petrodiesel with fewer pollutants and only 1–7% nitrogen oxide emission.

1. Introduction

Biofuels are fuels that are produced from biomass in a short period. They are different from fossil fuels which are produced very slowly from biomass by natural processes [1]. Examples of fossil fuels are coal, crude oil or petroleum, and natural gas [2]. Examples of biofuel are fuel briquettes, bioethanol, wood, biodiesel, biobutanol, and biogas [3]. Refining of crude oil or petroleum produces fossil or conventional diesel which is the main fuel for compression ignition engine. The utilization of Petrodiesel in compression ignition engines has led to the release of greenhouse gases and particulate matter [3, 4, 5, 6, 7, 8, 9, 10]. The release of greenhouse gases into the atmosphere is the major cause of global warming, leading to climate change and so many environmental degradations. One biofuel which has captured the interest of so many scientists and researchers is biodiesel. Biodiesel is the alkyl ester of vegetable oil or animal fats produced by transesterification under suitable reaction conditions [8, 9, 11, 12, 13].

Biodiesel sometimes called the new age fuel can proffer solutions to our global and modern energy needs either 100% or partly with Petrodiesel as a blend [9, 14, 15, 16, 17]. Currently, biodiesel makes an insignificant contribution to the total global fuel utilization and combustion processes [18, 19]. Petrodiesel accounts for over 80% of fuel used in almost all compression ignition engines. This situation is presently unfavorable because the combustion of Petrodiesel in compression ignition engines contributes immensely to the release of greenhouse gases and severe pollution leading to global warming and climate change [6, 20, 21]. The effects of global warming and climate change are higher temperatures, rising seas, drought, floods, stronger storms, food spoilage, more heat-related illnesses such as typhoid fever, malaria fever, meningitis, etc., and economic losses [6, 22, 23].

Biodiesel burns more smoothly in compression ignition engines and produces cleaner emissions [14, 15, 16, 24, 25, 26]. Although biodiesel cannot entirely replace petroleum-based diesel fuel, it decreases a country’s dependence on petroleum. Researcher’s reports show that a blend of 20% biodiesel with 80% Petrodiesel in a compression ignition engine produces far less carbon IV oxide fumes with little or no greenhouse gases as compared to 100% usage of Petrodiesel which releases large emissions of carbon IV oxide fumes and other greenhouse gases [9, 21, 24, 27, 28, 29, 30, 31, 32]. Total dependence on biodiesel or its blends with Petrodiesel in compression ignition engines is the sure way out of global warming and severe climate conditions experienced today [1, 6, 33, 34, 35].

Several researchers [17, 19, 21, 27, 28, 29, 30, 31, 32, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59] have reported that the nature of plant oil together with the production processes taken affects the properties of biodiesel and its performance in compression ignition engines. Even the corrosive effects of biodiesel on aluminum, one of the metals used for engine parts were reported [60, 61, 62]. Biodiesel causes more severe pitting corrosion on aluminum than Petrodiesel [63, 64, 65, 66, 67]. Over sixty researchers some of whom include [17, 19, 23, 32, 36, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 61, 62, 64, 65, 66, 67, 68, 69, 70] studied alkali catalyst transesterification of different plant oil concerning their fuel properties such as density, viscosity, cetane number, cold flow properties, flash point, and calorific value. Their studies show the best quality alkyl ester under different transesterification circumstances [1, 14, 17, 19, 24, 35, 36, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 71, 72, 73]. A review on biodiesel fuel specifications by [61] found that over 80% of scientists and researchers failed to extensively report on the fuel properties of biodiesel and their influence on performance in compression ignition engines.

Among the plant oil biodiesel studied, Jatropha curcas biodiesel and algae biodiesel stands out in terms of quality and good fuel properties for use in compression ignition engines [36, 47, 48, 56, 71, 74, 75]. Some studies [5, 25, 56, 76, 77, 78] even report that Jatropha seed oil or its blend with conventional diesel fuel can be used as diesel in compression ignition engines without engine modification.

Jatropha is a non-edible plant, hence its usage in fuel production does not compete with food. Moreover, J. curcas has a yield per hectare of more than four times that of soybean and ten times that of corn [79]. Recent studies on the evaluation of 75 non-edible oils as biodiesel feedstocks, identified J. curcas as one of the most promising plants apart from algae [71, 79, 80]. J. curcas plant is very prolific, it grows on any type of soil, gets matured within a very short period, and produces large quantities of seed almost throughout the year. The percentage oil content in J. curcas seed is between 28 and 60% [79, 80, 81]. This chapter reports the viability and sustainability of J. curcas biodiesel and algae biodiesel over several plant biodiesel. The biodiesel fuel properties of some common plant oils in comparison to those of J. curcas biodiesel and algae biodiesel are reviewed.

2. Biodiesel feedstocks from plants

There are several ways to obtain biodiesel feedstocks from plants. They include extraction of the plant oil by solvent extraction or by hydraulic press, supercritical fluid extraction, ultrasound-assisted extraction, and microwave-assisted extraction [61, 74, 81]. These methods are suitable for different plant materials, depending on the nature of the plant material and the desired yield [1, 71, 75, 81].

Several oils extracted from plants including algae oil have been used as biodiesel feedstocks [24, 71, 79]. To assess the quality of oil for biodiesel production, the fatty acid composition and free fatty acid content of the oil must be determined. Oils with high free fatty acids of over 3% must be esterified to reduce their free fatty acids before transesterification to biodiesel [74, 76, 77].

The fatty acid composition of the plant oil is an indication of its biodiesel quality and performance in compression ignition engines especially its cold flow properties [8, 71, 74, 75, 76]. Plant oils whose structural fatty acid content has a sufficient number of double carbon bonds show promising biodiesel qualities such as good cetane number, good oxidative stability, lower viscosity with good engine atomization, and higher calorific value [1, 71, 72, 74]. Table 1 shows some common biodiesel feedstocks with their fatty acid content.

Oil or fat	C14:0 Myristic acid	C16:0 Palmitic acid	C16:1 Palmitoleic acid	C18:0 Stearic acid	C18:1 Cis Oleic acid	C18:1 trans oleic acid	C18:2 Linoleic acid	C18:3 Linoleic acid	C20:0 Arachidic acid	C22:0 Erucic acid
Soybean		6–10		2–5	20–30		50–60	5–11
Corn	1–2	8–12		2–5	19–49		34–62	Trace
Peanut		8–9		2–3	50–65		20–30
Olive		9–10		2–3	73–84		10–12	Trace
Safflower		4.8		1.4	74.1		19.7
Rapeseed		3.0		0.8	13.1		14.1	9.7	7.4	50.7
Lard	1–2	28–30		12–18	40–50		7–13	0.1
Tallow	3–6	24–32		20–25	37–43		2–3
Yellow grease	1.27	17.44		12.38	54.67		7.96	0.69	0.25	0.52
Linseed oil		4–7		2–4	25–40		35–40	25–60
Jatropha	0.26	14.398	0.704	6.65	43.39		32.98	0.351	0.218	0.506
Algae	1.0	16.6	1.9	1.9	26.9	3.7	22.8	16.1	1.5
Cotton seed	0–2	20–25		1–2	23–35		40–50	Trace
Sunflower		6.6	0.1	3.08	17.31	73.31

Table 1.

Biodiesel Feedstocks with their fatty acid composition [1, 71, 72, 79, 81, 82].

3. Biodiesel processing

According to Rudolf Diesel [83], pure vegetable oil could be used as a biofuel but for its high viscosity which causes lots of problems to internal combustion engines. Some researchers have proven through research that by blending a suitable amount of pure vegetable oil with a suitable amount of Petrodiesel, one could overcome the challenges of using pure vegetable oil in internal combustion engines [15, 16, 24, 26]. This was proven to be true for pure J. curcas seed oil. The fuel produced by the blending of a suitable amount of J. curcas seed oil with Petrodiesel possesses the ideal quality of fuel that burns smoothly in an internal combustion engine with little or no emission of dangerous fumes [56, 74]. For vegetable oil to possess properties similar to that of Petrodiesel and to burn smoothly in internal combustion engines, it has to be modified by transesterification [4, 74, 75, 84].

Transesterification is a chemical reaction between the high molecular weight triglyceride in vegetable oil or animal fat with low molecular weight or short-chain alcohol such as methanol or ethanol to produce methyl or ethyl esters and a glycerol molecule. Transesterification is an equilibrium reaction that takes place in three consecutive steps. One of the reasons for determining the fatty acid contents of oil is to enable one to calculate the oil’s average molecular weight. Knowing the average molecular weight of an oil is an important step in knowing the required amount of alcohol needed for transesterification reaction for a given oil to alcohol molar ratio. For example, the average molecular weight of Jatropha oil is 900 moles, while that of algae oil is 274.4 moles [11, 12]. Certain variables are known to affect the transesterification reaction of vegetable oil or animal fats. These variables include temperature, catalyst type, catalyst amount, the molar ratio of oil to alcohol, reaction time, stirring speed, type of oil, free fatty acid of oil or fats, and so on. Many researchers have reported how each of these variables affects the transesterification reaction of vegetable oil or animal fats to biodiesel. Their reports also include the effect of these variables on the quality of biodiesel produced and how these variables can be modified to give higher yield and best quality biodiesel [21, 40, 53, 77, 85]. Of these variables oil to alcohol molar ratio, temperature, catalyst type, and catalyst concentration or catalyst amount play vital roles [12, 77, 82]. To cause the reaction to proceed at a reasonable speed, a catalyst must be involved. There are three main types of catalyst, namely liquid catalyst examples are a solution of sodium hydroxide or potassium hydroxide, solid catalyst examples are calcium oxide, zinc oxide, zeolite, etc. and enzyme catalyst example is lipase. Researchers have observed that liquid catalysts cause transesterification reactions to proceed fasters and products are formed in a shorter time than solid and enzyme catalysts [4, 12, 82].

Figure 1 shows the biodiesel chemical reaction while Eq. (1)–(23) shows the chemical reaction mechanism.

Figure 1.

Transesterification of triglycerides with methanol to form fatty acid methyl esters [77].

The biodiesel or transesterification reaction is of the form [3, 50].

$$A+B\stackrel{\mathit{\text{Catalyst}}}{\rightleftharpoons }C+D$$E1

Where $A$ = Triglyceride (TG), $B$ = alcohol ($\mathit{ROH}$).

C = Glyceride and D = methyl esters ($\mathit{ME}$).

Therefore Eq. (1) becomes

$$\mathit{TG}+\mathit{ROH}\stackrel{\mathit{\text{Catalyst}}}{\rightleftharpoons }\mathit{\text{Glyceride}}+\mathit{ME}$$E2

Where the glyceride could be diglyceride, monoglyceride, or glycerol.

Eq. (2) [3, 12] is an equilibrium reaction that occurs in three consecutive steps forming intermediates. Stoichiometrically, 3 moles of alcohol were required to react with 1 mole of TG to form 3 moles of esters and 1 mole of Glycerol [12, 77, 82]. Therefore, we have [3],

$$\mathit{TG}+\mathit{ROH}\underset{K_2}{\stackrel{K_1}{\rightleftharpoons }}\mathit{DG}+\mathit{ME}$$E3

$$\mathit{DG}+\mathit{ROH}\underset{K_4}{\stackrel{K_3}{\rightleftharpoons }}\mathit{MG}+\mathit{ME}$$E4

$$\mathit{MG}+\mathit{ROH}\underset{K_6}{\stackrel{K_5}{\rightleftharpoons }}\mathit{GL}+\mathit{ME}$$E5

The overall reaction is, therefore [12, 82].

$$\mathit{TG}+3\mathit{ROH}\rightleftharpoons \mathit{GL}+3\mathit{ME}$$E6

From Eq. (3).

$\mathit{TG}+\mathit{ROH}\stackrel{K_1}{\to }\mathit{DG}+\mathit{ME}$ (7)

and

$$\mathit{DG}+\mathit{ME}\stackrel{K_2}{\to }\mathit{TG}+\mathit{ROH}$$E7

From Eq. (4).

$\mathit{DG}+\mathit{ROH}\stackrel{K_3}{\to }\mathit{MG}+\mathit{ME}$ (9)

and

$$\mathit{MG}+\mathit{ME}\stackrel{K_4}{\to }\mathit{DG}+\mathit{ROH}$$E8

from Eq. (5),

$\mathit{MG}+\mathit{ROH}\stackrel{K_5}{\to }\mathit{GL}+\mathit{ME}$ (11)

and

$$\mathit{GL}+\mathit{ME}\stackrel{K_6}{\to }\mathit{MG}+\mathit{ROH}$$E9

The reactions above are initiated and terminated as follows:

Formation of alkoxide from methanol and sodium hydroxide [12].

$$\underset{\mathit{\text{alcohol}}}{\mathit{ROH}}+\underset{\mathit{\text{catalyst}}}{\underset{\mathit{\text{from}}}{{\mathit{OH}}^{-}}}\rightleftharpoons \underset{\mathit{\text{alkoxide}}}{{\mathit{RO}}^{-}}+\underset{\mathit{\text{water}}}{H_{2}O}$$E10

The alkoxide ion attacks the glyceride molecules to form methyl esters and glycerol as shown in Eqs. (14)–(19).

Transesterification [12, 82]:

$$\mathit{TG}+{\mathit{RO}}^{-}\rightleftharpoons {\mathit{DG}}^{-}+\mathit{ME}$$E11

$${\mathit{DG}}^{-}+\mathit{ROH}\rightleftharpoons \mathit{DG}+{\mathit{RO}}^{-}$$E12

$$\mathit{DG}+{\mathit{RO}}^{-}\rightleftharpoons {\mathit{MG}}^{-}+\mathit{ME}$$E13

$${\mathit{MG}}^{-}+\mathit{ROH}\to \mathit{MG}+{\mathit{RO}}^{-}$$E14

$$\mathit{MG}+{\mathit{RO}}^{-}\to {\mathit{GL}}^{-}+\mathit{ME}$$E15

$${\mathit{GL}}^{-}+\mathit{ROH}\to \mathit{GL}+{\mathit{RO}}^{-}$$E16

If saponification, an almost inevitable side reaction associated with transesterification when using a homogeneous catalyst such as sodium hydroxide is allowed to take place, the following reaction equations Eqs. (20)–(23) would be observed [12, 76].

$$\mathit{ME}+{\mathit{OH}}^{-}\to \mathit{ROH}+\mathit{\text{Soap}}$$E17

$$\mathit{TG}+{\mathit{OH}}^{-}\to \mathit{DG}+\mathit{\text{Soap}}$$E18

$$\mathit{DG}+{\mathit{OH}}^{-}\to \mathit{MG}+\mathit{\text{Soap}}$$E19

$$\mathit{MG}+{\mathit{OH}}^{-}\to \mathit{GL}+\mathit{\text{Soap}}$$E20

Saponification reactions of Eqs. (20)–(23) become prominent during transesterification when using vegetable oil with high free fatty acids and high moisture content with a homogenous catalyst like sodium hydroxide [76]. Therefore, transesterification of high-free fatty acid vegetable oil with sodium hydroxide catalyst would result in a lot of soap formation with little or no biodiesel [4, 76, 77]. Hence it is recommended that if sodium hydroxide catalyst must be used in the transesterification of oil with high free fatty acid the oil must be esterified to a free fatty acid of below 3% and the oil must be dried or free from moisture [12, 76, 77].

The biodiesel produced by transesterification from Jatropha curcas seed oil with methanol using sodium hydroxide as a catalyst has properties close to those of conventional diesel fuel [3, 8]. It was suitable to be used as neat biodiesel in internal combustion engines. Its blend with conventional or Petrodiesel also could result in low carbon emissions during combustion. Biodiesel produced from vegetable oils or animal fats with a catalyst concentration of 1%, the molar ratio of oil to alcohol of 6:1 or 8:1, and at a temperature of 60°C or 65°C gave a good quality fuel [3, 7, 8, 9].

Biodiesel produced by this method has properties close to that of Petrodiesel. Its viscosity becomes lower than that of pure vegetable oil to enable it to burn smoothly in an internal combustion engine. It has a high cetane number and higher flash point than Petrodiesel which makes it safer to transport than Petrodiesel [8, 71, 74, 75].

Table 2 compares the properties of biodiesel produced by various researchers with those of conventional diesel fuel and ASTM biodiesel standards.

Parameters	Jatropha Biodiesel @ 65°C, 1% NaOH, Molar ratio of methanol to oil 8:1 [76]	Jatropha Biodiesel @ 65°C, 1% CaO, Molar ratio of methanol to oil 8:1 [76]	Palm Kernel Oil Biodiesel [8]	B20 Biodiesel blend of 20% biodiesel with 80% fossil diesel [8]	Conventional Diesel ASTM D975 [9]	Biodiesel Fuel Properties (ASTM D6751) [9, 61]
Flash point, °C	191	192	171	66	60–80	100–170
Cloud point, °C	7	9	9.9	6	−15 to 5	−3 to 12
Pour point, °C	11	12	−1.0	1	−35 to −15	−15 to 10
Kinematic Viscosity @ 40°C, mm⅖	4.91	4.86	4.87	8.79	4.0–6.0	1.3–4.1
Calorific value(MJ/Kg)	42.56	42.22			45.5	39-43
Cetane number	49.17	48.94			48–65	40–55
Specific gravity	0.891	0.893	0.898	0.832	0.88	0.85
Acid number (mgKOH/g)	0.45	0.47

Table 2.

Biodiesel properties of some vegetable oils compared with standards and conventional diesel properties.

4. Biodiesel fuel properties

The fuel properties of biodiesel and its performance in a compression ignition engine can be predicted mainly from the type of fatty acids contained in it. Plant oils or fats which contain a substantial number of double carbon bonds in their structure are a sure indication of producing quality biodiesel [36, 71, 72, 79, 86]. Biodiesel properties such as cetane number, calorific value, viscosity, oxidative stability, flash point, pour point, and cloud point are connected with the type and nature of the carbon structure of the parent oil. The free fatty acid of the parent oil is also a factor capable of affecting the biodiesel fuel property and production cost [71, 76, 87]. Oils or fats with high free fatty acid usually must go through a two-step transesterification process with a homogenous catalyst [8, 76]. The additional process of esterification of high free fatty acid oils to reduce the free fatty acids before trans esterifying it to biodiesel affects its properties and increases the production cost [11]. Some Jatropha oil and algae oil have low free fatty acid making them suitable for a one-step transesterification to biodiesel [36, 71, 74, 75]. Table 3 shows the reviewed biodiesel fuel properties of some common feedstocks.

Properties/Units	Jatropha Biodiesel [56, 61, 74, 76]	Sunflower Biodiesel [7, 61]	Algae Biodiesel [61, 71, 87]	Pongamia Pinnata Biodiesel [34, 61, 72]	Almond Biodiesel [9, 61]
Flash point °C	148–170	110–183	150	90–172	172
Pour point °C	−12 to 2.0		−2.67
Cloud point °C	4–10.2		1
Kinematic Viscosity @ 40°C (mm^2/s)	4.23–5.65	4.10–4.90	3.16	4.16–10.64	4.2
Density (g/cm^3)	0.870–0.880	0.860–0.885	0.880	0.878–0.920	0.876–0.891
Cetane number	4–57.4		47	53–55.7
Calorific value (MJ/kg)	38.5–45.5	38.47–45.3	40.66	35.560
Oil content (in %)	28–60	40		30–70
Acid number (mgKOH/g)				0.497

Table 3.

Reviewed biodiesel fuel properties for some common feedstocks.

5. Biodiesel tests on engines

Many scientists and researchers [1, 8, 9, 61, 71, 75, 76, 88, 89] have assessed the performance of biodiesel produced on compression ignition engines. Their reports show that once the biodiesel fuel properties fall within the standard ASTM D6751 and EN14214 diesel fuel properties, the biodiesel is sure to have good performance in any compression-ignition engines.

The work of over 60 biodiesel researchers on biodiesel fuel quality and performance on compression ignition engines was reviewed [1, 14, 15, 16, 20, 24, 71, 72]. Their reports confirmed that all produced biodiesel with fuel properties conforming to ASTM D6751 or EN14214 have significant superiority to conventional diesel fuel due to total hydrocarbon volume and other vital fuel properties [7, 71, 77, 88]. These biodiesels have better performance in compression ignition engines with very low carbon monoxide and carbon dioxide emission, low particle substance, and ultra-low Sulfur content [1, 71, 72]. The cold flow properties of biodiesel were also found to be very encouraging, with that of algae and Jatropha biodiesel being the best amongst all others [71, 76].

The kinetic viscosity of fuel determines its flow, spray, and atomization properties. High density creates high viscosity leading to inefficient combustion, poor engine performance, and excess carbon monoxide emission [71, 72, 74]. The density of algae and Jatropha biodiesel from several research show conformities to standards [71, 74, 76].

The performance of a blend of biodiesel with conventional diesel fuel was also examined. The findings show that an increase in the proportion of biodiesel in the blend decreases carbon monoxide and carbon dioxide emissions, but also increases nitrogen oxide emissions which can be reduced with ionic liquid additives [62]. The emission of oxides of nitrogen is increased with an increase of biodiesel in the blend, especially algae biodiesel [15, 26]. This is not surprising because algae are known to assimilate nitrogen from their marine environment [2]. Biodiesel is known to increase the corrosion of vessels because of its high oxygen content, especially pitting corrosion [63].

Soladiesel BDR company is among other companies reported to have used different biodiesels with standard diesel engines without modification. The company reported that biodiesel emits fewer pollutants than conventional diesel and has equivalent performance to Petrodiesel with lesser engine wear [71, 74, 75, 87].

Several automobile manufacturers including Caterpillar, DEUTZ, MAN, and Volvo have built trucks, buses, and engines that can run on pure and blended biodiesel [90, 91]. A report shows that using biodiesel in heavy-duty vehicles reduces greenhouse gases emission by up to 90% [91]. Biodiesel raises the cetane number of blended fuel and improves fuel lubricity [67, 92] which is usually low in conventional low sulfur diesel [93]. A higher cetane number makes the engine easier to start and reduces ignition delay, together with improved lubricity, premature engine wear is prevented. Hence, the blending of biodiesel with Petrodiesel in internal combustion engines is highly recommended.

6. Economics of biodiesel production

A major hurdle facing commercial biodiesel production is the cost of fats and oils. About 60–80 percent of the cost of producing biodiesel arises from oil seed procurement, transport, seed storage, and oil extraction [11]. The cost depends mostly on the type and nature of feedstock used.

The use of lower-cost feedstock such as waste cooking oil, tallow, and non-edible oils would reduce the production cost of biodiesel and biodiesel blend fuel. A blended fuel of 20 percent biodiesel and 80 percent Petrodiesel would reduce cost. For bioethanol production, the use of sugarcane bagasse, cassava peels instead of tubers, corn cobs, or other non-edible cellulosic biomass helps to reduce production costs [84].

The cost of biodiesel can also be reduced by the use of an appropriate catalyst. Biodiesel produced with liquid catalysts usually contain impurities, excess alcohol, and parts of the catalysts used in the process [4]. The excess alcohol can be removed by using a flash evaporation process or by distillation. The biodiesel fuel after being separated from glycerin is further purified by washing the fuel with hot or warm water to remove unreacted alcohol, alkaline, oil, or glycerol/glycerides. The cost of this additional refining process adds to the already high manufacturing cost of biodiesel due to the high cost of feedstock [4, 75]. Akhihiero [11] calculated the approximate cost of production of a liter of Jatropha biodiesel to be $9.01 (₦3740.50). This cost which was obtained in 2019, could be reduced by adopting a better production process such as the continuous process instead of the batch process [12]. The cost can also be minimized by using the fuel as a blend with Petrodiesel. Research shows that a fuel produced with 20% biodiesel with 80% Petrodiesel performs excellently well in internal combustion engines by emitting low carbon deposits with smooth burning and complete combustion [15, 87].

Despite the high production cost of biodiesel by transesterification, its discovery and production are a welcome development. The environmental damage caused by the complete utilization of Petrodiesel or fossil fuels together with their effects on human health far outweighs the high cost of biofuels. A compromise is reached by blending which reduces cost drastically while still overcoming the negative effect of fossil utilization [9, 14, 16].

7. Advantages and disadvantages of biofuels

Benefits of biofuel usage include fewer carbon emissions, when burned they release as much carbon as they absorbed during the growth of plants. This is not the case with fossil fuels which release excess carbon and oxides of carbon.

Other benefits include energy efficiency, reducing petroleum oil dependency, a healthy environment, positive economic impact, reducing greenhouse gases and reducing global warming, sustainability, high-quality engine performance, and lowering particulate matters [1, 20, 71, 72].

Disadvantages of biofuel include the need for a lot of labor, the high cost of production, and various health problems that may be encountered during processing. Cases of heart disease, respiratory symptoms such as asthma, chronic bronchitis, or even premature death may result from biofuel processing [94]. Sodium methoxide the popular liquid catalyst for biodiesel production is very toxic to humans. To avoid the menace of sodium methoxide, solid catalysts such as calcium oxide, zinc oxide, doped or modified eggshells, or animal bones are used in biodiesel production [8, 39, 40, 41, 42, 43].

8. Challenges posed by biodiesel

Biodiesel utilization is not without challenges. These challenges include;

1. Nitrogen oxides emission.

2. Corrosion of aluminum

3. High production cost

4. Production health hazards

8.1 Nitrogen oxides emission

One major challenge of biodiesel utilization in compression ignition engines is the high emission of Nitrogen oxides. Nitrogen oxides are pollutants that are responsible for the corrosion of metals and acid rain [62]. Neat biodiesel is known to emit higher oxides of Nitrogen than Petrol diesel [21, 27, 28, 29, 30, 31, 32]. The emission of Nitrogen oxide by biodiesel is relatively 10–30% higher than that of Petrodiesel in compression ignition engines [27, 28, 29, 68, 69]. A blend of 20% biodiesel to Petrodiesel-only emits 1–7% Nitrogen oxide [23, 62, 68, 69]. As the number of biodiesel increases in the blend, Nitrogen oxide emission increases. Nitrogen oxide emission depends on the type of biodiesel feedstock [27, 62, 68, 69, 70]. The highest Nitrogen oxide emission was reported with the most highly unsaturated biodiesel such as Jatropha, neem, castor, Karanja, algae, etc. [62, 68, 69, 70].

Biodiesel produced from feedstocks with a high saturation of fatty acids such as palm oil or animal fats like tallow emits fewer Nitrogen oxides in Compression ignition engines. An increase in Nitrogen oxide emission also depends on engine technology [64, 65, 66, 67, 85, 95]. Studies reveal that Nitrogen oxide emission increases with newer engines [73, 85, 96]. However, a blend of 20% biodiesel with Petrodiesel shows little or no emission of Nitrogen oxides [17, 35, 62, 64, 65, 66, 67, 69, 70, 73, 85, 95, 96].

8.2 Corrosion of aluminum

Biodiesel causes corrosion in aluminum, especially pitting corrosion, more than Petrodiesel. This is because its oxygen content is higher than that of Petrodiesel [63]. The corrosion of aluminum by biodiesel can be controlled with the use of plant extracts like the ethanol extract of Rosemary leaves [66] and Vitex negundo leaf extract [67] and ionic liquids as corrosion inhibitors [85, 96]. Corrosion inhibitors also help to reduce nitrogen oxide emission [73, 85, 95, 96].

8.3 High production cost

The high production cost of biodiesel has been the major reason why biodiesel utilization in compression ignition engines is unpopular in many countries, especially Nigeria [13, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109]. The feedstock cost is the major contributor to this high cost [11]. Hence, it’s been recommended to use waste feedstocks and oils of non-edible plants [11, 53, 54, 97]. Three ways to reduce biodiesel production costs are stated thus:

1. The use of low-cost feedstocks with high oil content such as low-cost J. curcas and algae for biodiesel production [97, 110, 111]

2. The use of heterogeneous catalysts and plant oils with low free fatty acids [8, 107, 108, 109]. The use of heterogeneous catalysts prevents the additional cost of product washing which is associated with homogeneous catalysts. Also, oils with free fatty acids lower than 3% are suitable with homogeneous catalysts in one-step transesterification. Oils with high free fatty acid must undergo the additional cost of esterification to prevent soap formation [39, 40, 41, 42, 43, 44, 45, 76], before transesterification to biodiesel [8, 108].

3. Blending of biodiesel with Petrodiesel in a ratio of 20% biodiesel to 80% Petrodiesel. For example, from the work of [11], a 100% Jatropha biodiesel has a production cost of $9.01/L. By producing a 20% blend with Petrodiesel, which has a global average cost of $1.40/L [112] the estimated cost of the B20 blend is about $2.92/L.

8.4 Production health hazards

The use of acid or alkaline heterogeneous catalysts such as calcium oxide, zinc oxide, zeolite, doped aluminum oxide, doped eggshell, etc. [39, 40, 41, 42, 43, 44, 45, 46] in transesterification eliminates the health hazard in biodiesel production [8, 35, 58, 59]. The only drawback with heterogeneous catalysis is a longer reaction time [8]. The reaction could take additional 30 minutes or over an hour to complete with heterogeneous catalysts [8, 59, 76].

In biodiesel production, the most popular initial catalyst is sodium hydroxide [4, 76]. Sodium hydroxide in methanol produces sodium methoxide which is very toxic to humans [8]. Hence, the usage of face masks is recommended during the production of biodiesel, especially with homogeneous catalysts.

9. Conclusion

Research has revealed and is still revealing what is embedded in the numerous biomass all around us. Biofuel produced from biomass has a lot to offer in combating global warming, reducing the effects of climate change, and reducing disease and environmental degradation. Sole utilization of neat biodiesel or a blend of biodiesel with Petrodiesel is recommended in all compression ignition engines to maintain a cleaner and safer environment. However, the high cost of biodiesel is a challenge that can be controlled by adopting a better production process and by using a blend of biodiesel with Petrodiesel. Biodiesel quality and performance characteristics have been traced to the chemical structure of its parent oil and production procedures adopted. Alkali catalyst transesterification of low-free fatty acid oils is one of the most promising methods to produce cheaper and better-quality biodiesel for compression ignition engines. Low free fatty acid J. curcas oil and Algae oils have shown promising features in biodiesel production for compression ignition engines. The performance of Jatropha biodiesel and algae biodiesel in compression ignition engines has been outstanding. They both have good cold flow properties because of the presence of palmitoleic acid and high oleic and linoleic acids in them. However, the increase in emission of Nitrogen oxides by biodiesel or its blend can be controlled or prevented by using plant extracts or ionic liquids as corrosion inhibitors. The optimal biodiesel blend for a more tolerable Nitrogen oxide emission for compression ignition engines is B20. Also, the use of low-cost feedstock with high oil content such as low-cost J. curcas and algae for biodiesel production is recommended. However, the use of corrosion inhibitors such as Rosemary leaves extract on Jatropha and algae biodiesel is a must due to the presence of high unsaturated fatty acids which lead to high nitrogen oxides emission.

Usage of acid or alkaline heterogeneous catalysts in transesterification of plant oils is the sure way to prevent health hazards associated with homogenous catalysis in biodiesel production. Also, proper use of safety masks during processing is another option to avoid the risk of inhaling poisonous Sodium methoxide.

The production cost of a liter of Jatropha biodiesel was calculated to be $9.01/L (₦3740.50/L) but this cost can be potentially reduced to $2.92/L by producing a B20 blend. Because the cetane number of Jatropha and algae biodiesel is higher, it becomes easier to blend either of them with Petrodiesel at higher concentrations.