A review of the effect of the composition of biodiesel on NOx emission, oxidative stability and cold flow properties

Renewable and Sustainable Energy Reviews 54 (2016) 1401–1411

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A review of the effect of the composition of biodiesel on NOx emission, oxidative stability and cold flow properties R.D. Lanjekar, D. Deshmukh n Spray and Combustion Laboratory, Discipline of Mechanical Engineering, Indian Institute of Technology Indore, India

art ic l e i nf o

a b s t r a c t

Article history: Received 12 May 2015 Received in revised form 15 October 2015 Accepted 16 October 2015

This paper will review and attempt to discover the ideal fatty acid composition of biodiesel which exhibits lower NOx emissions, better oxidative stability and cold flow properties. The physicochemical properties of biodiesel strongly depend on their fatty acid composition. A high percentage of unsaturated fatty acid in biodiesel is correlated with higher NOx emissions, poor oxidative stability and better cold flow properties. The presence of saturated fatty acids (SFA), in particular the long chain type, exhibits good oxidative stability and produces lower NOx emissions. SFA do however demonstrate poor cold flow properties. The polyunsaturated fatty acids (PUFA) exhibit better cold flow properties but produces higher NOx emissions and poorer oxidative stability. The ideal requirements of biodiesel properties impose contradictory conditions on the fatty acid composition of biodiesel. For example, coconut and palm kernel oils which have a high percentage of lauric fatty acid are reported to circumvent all three drawbacks of biodiesel. The monounsaturated fatty acids (MUFA), specifically oleic acids, a major component in almost all biodiesel, display the positive characteristics of both SFA and PUFA. Biodiesel properties can therefore be improved by using various remedial methods including genetic engineering, reformulated biodiesel and additives. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Fatty acid composition Cold flow property Oxidative stability NOx emissions Biodiesel formulation

Contents 1. 2. 3. 4.

5. 6. 7. 8.

n

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodiesel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The influence of biodiesel composition on properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NOx emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Advanced injection timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Flame temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Combustion phasing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxidative stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cold flow properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Inter-correlation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Remedial approaches. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1. Modification of composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.1. Physical method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.2. Chemical method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.3. Genetic engineering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2. Reformulated biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3. Additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Corresponding author. E-mail address: [email protected] (D. Deshmukh).

http://dx.doi.org/10.1016/j.rser.2015.10.034 1364-0321/& 2015 Elsevier Ltd. All rights reserved.

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9. Conclusions and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1409 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1409

1. Introduction As a result of its high thermal efficiency, a Compression Ignition (CI) engine is a popular choice for industrial and domestic power generation applications. However, emissions from CI engine such as Particulate Matter (PM), Unburnt Hydrocarbon (UHC), Carbon Monoxide (CO) and Oxides of Nitrogen (NOx) are hazardous for human health. As demand for power increases and fossil fuels become more limited, it is important to search a renewable fuel for CI engines. This gives direction for the discovery of an alternative fuel for CI engines, one which is renewable in nature, has a similar engine performance to that of diesel fuel and has less engine emissions. Biodiesel is a renewable fuel which has similar properties to that of diesel, comparable engine performance and less UHC and PM emissions. However, biodiesel is found to produce higher NOx emissions and exhibits poor cold flow properties and oxidative stability. The level of higher NOx emissions when using biodiesel is attributed to the difference in the physicochemical properties of biodiesel and diesel. The compositions of diesel and biodiesel are markedly different. Biodiesel is a mixture of a narrow range of fat and oil esters with relatively similar chemical composition, whereas diesel is a complex mixture of a wide range of hydrocarbon compounds [1].

and oxidative stability. This work will review the literature based around the correlation of chemical composition with engine emissions and performance along with stability and cold flow properties of biodiesel together as a hurdle for their widespread use.

2. Biodiesel Biodiesel is produced using oil from various sources, including edible and non-edible vegetable oils, waste oils, fats. These oils are triglycerides heavy molecules consisting of three long-chain fatty acids attached to glycerol. They have very high kinematic viscosity in the range of 27.2–53.6 mm2/s, which is 10–20 times higher than that of diesel [19–23]. The high viscosity of these oils leads to various problems in the operation of CI engines, including poor atomisation, injector coking, engine deposits and piston ring sticking. The viscosity of these triglycerides can be closely reduced to that found in diesel by converting them into mono-alcohol ester, also known as biodiesel. The transesterification process is the most widely used process to produce biodiesel from oils, and the transesterification reaction (Eq. (1)) replaces a heavy molecule of glycerol with a comparatively lighter molecular weight alcohol molecule, such as methanol or ethanol [1,23-26].

ð1Þ

There have been many studies concerning the use of biodiesel as a fuel in CI engines. It is known that biodiesel produces slightly lower engine power and higher fuel consumption with reduced engine emissions such as UHC, CO and PM. However, NOx emissions are found to be higher in biodiesel compared to that of diesel fuel [2–13]. Experimental studies conducted using various biodiesels have identified an increase in NOx emissions, poor cold flow properties and oxidative stability as the drawbacks of biodiesel fuel [3–8,10–18]. Biodiesel, from various origins, displays similar trends in engine performance, emissions and other properties. This is due to the similarities in their chemical compositions and therefore their physicochemical properties. The chemical composition of biodiesel plays a key role in determining performance and emission characteristics. This gives direction in order to find the ideal biodiesel composition which will provide minimum engine emissions with optimum engine performance and fuel properties. Previous studies were conducted in order to identify the correlation of the compositional features of biodiesel with an observed increase in NOx, poor cold flow properties and oxidative stability. However, these studies are segregated, placing increased emphasis on NOx emissions as opposed to the cold flow properties

The viscosity of mono-alcohol esters is a third of that of source oil. These esters are also termed as Fatty Acid Methyl Ester (FAME) or Fatty Acid Ethyl Ester (FAEE), based on the alcohol moiety (methanol or ethanol) attached to the fatty acid chain [24,20,25]. The fatty acid composition of biodiesel however, is same as that of the source oil. Table 1 compares the fatty acid composition of key oils for biodiesel production. It shows that most of these oils contain five fatty acids, namely palmitic (hexadecanoic; C16:0), stearic (octadecanoic; C18:0), oleic (9(Z)-octadecanoic; C18:1), linoleic (9(Z), 12 (Z)-octadecanoic; C18:2) and linolenic (9(Z), 12(Z), 15(Z)- octadecanoic; C18:3) acids [1,26–28]. These fatty acids can be classified by the number of double bonds or the degree of unsaturation and chain length. The saturated fatty acid has zero double bond (palmitic and stearic fatty acids), mono-unsaturated fatty acid has one double bond (oleic fatty acid) and polyunsaturated fatty acids have more than one double bond in its fatty acid chain (linoleic and linolenic fatty acid) [1,26]. The olive and peanut oils are of MUFA type containing 65–85% of oleic (C18:1) fatty acids, whereas safflower, sunflower, corn and soybean are of PUFA type oils having 43–79% of linoleic (C18:2) fatty acids. These oils fall in long chain unsaturated fatty acid

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Table 1 Fatty acid composition of oils [1,26]. Common name

Caproic Caprylic Capric Lauric Myristic Palmitic Stearic Arachidic Behenic Palmitoleic Oleic Gadoleic Erucic Linoleic Linolenic

Carbon no.:double bond no.

6:0 8:0 10:0 12:0 14:0 16:0 18:0 20:0 22:0 16:1 18:1 20:1 22:1 18:2 18:3

Percent composition of oil type PN

RS

OL

CO

CR

P

PK

SA

SU

SB

TL

— — — — T–1 6–9 3–6 2–4 1–3 T–1.7 53–71 — — 13–27 T

— — — — — 1–3 0.4–3.5 0.5–2.4 0.6–2.1 0.2–3 12–24 4–12 40–55 12–16 7–10

— — — — 0.1–1.2 7–16 1–3 0.1–0.3 — T 65–85 — — 4–15 T–1

0–0.8 5–9 6–10 44–52 13–19 8–11 1–3 0–0.4 — 0–1 5–8 — — T–2.5 —

— — — — T–1.7 8–12 2–5 T T 0.2–1.6 19–49 — — 34–62 T

— T T T 0.5–6 32–45 2–7 T — 0.8–1.8 38–52 — — 5–11 T

T–1.5 3–5 3–7 40–52 14–18 7–9 1–3 T–1 — T–1 11–19 — — 0.5–2 —

— — — — T 3–6 1–4 T–0.2 — — 13–21 — — 73–79 T

— — — — — 3–6 1–3 0.6–4 T–0.8 T 14–43 — — 44–75 T

— — T T T 7–11 2–6 0.3–3 T T 15–33 — — 43–56 5–11

— — — T–0.2 2–8 24–37 14–29 T–1.2 — 1.9–2.7 40–50 — — 1–5 —

PN - peanut; RS - rapeseed oil; OL - olive; CO - coconut; CR - Corn; P - palm. PK - palm kernel; SA - safflower oil; SU - sunflower; SB - soybean; TL - tallow; T - trace.

Table 2 Fatty acid composition of oil from algae [29]. Algae species

SFA C:0 (%)

MUFA C:1 (%)

PUFA C:2 (%)

Amphora sp.

14:0 (9.7); 16:0 (19.6); 18:0 (1.7)

16:2 (6.8); 18:2 (3.4)

Chlorella sp.

14:0 (2.0); 15:0 (0.9); 16:0 (19.6); 17:0 (0.3); 18:0 (3.3) 14:0 (1.6); 16:0 (32.1); 18:0 (2.4)

14:1 (0.3) ; 16 :1 (28); 18:1 (0.5) 14:1 (0.1) ; 16 :1 (0.8); 18:1 (5.7) ; 20:1 (0.1) 16 :1 (24.6); 18:1 (2.6)

Fragilaria sp.

Tetraselmis sp. 14:0 (1.01); 16:0 (27.33); 18:0 (0.80)

16:1 (2.76); 18:1 (28.10); 20:1 (1.70); 22:1 (1.48)

category. The palm oil with 32–45% of palmitic acid (C16:0) and tallow oil with 14–29% of stearic acid (C18:0) are in the category of long chain saturated oils. The coconut and palm kernel oils are found to be exception and contain 60–90% of C:12–C:16 medium chain saturated fatty acids [26,27]. Most vegetable oils have long chain fatty acids. Another method of producing biodiesel is through using oil derived from algae. Algae derived biodiesel has been studied extensively in recent years due to its high yield. The fatty acid composition of algae derived biodiesel is provided in Table 2. The algal oil consists of PUFA which have two to six number of double bonds, with the chain length varying from C:14−C:22. The chain length of MUFA in algal oil varies from C14:1−C22:1 and the chain length of SFA varies from C10:0−C24:0. The algae-derived oils have a much wider range of chain lengths and number of double bonds in each category of fatty acids compared to that found in traditional biodiesel [29].

3. The influence of biodiesel composition on properties The various compositional features of biodiesel include: type of fatty acid, alcohol moiety, chain length and number, position and isomers of double bond influencing properties of biodiesel [30,31]. The correlations between these compositional features and the properties of biodiesel are proposed in the literature. The correlation between unsaturation (the average number of double bonds) of biodiesel and their properties such as density, cetane number (CN), iodine value (IV), kinematic viscosity, cloud point (CP), pour point (PP) and heating value have been proposed by Hoekman et al. [32] and Giakoumis [33]. They found a negative

PUFA C:3 (%)

16:3 (1.1); 18:3 (2.7) 16:2 (3.6); 18:2 (11.8); 16:3 (12.0); 18:3 20:2 (0.2) (22.3) 16:2 (3.85); 18:2 (1.4) 16:3 (5.2); 18:3 (1.1) 16:2 (2.11); 18:2 (7.02) 16:3 (2.78); 18:3 (18.52)

PUFA C : Z 4 (%) 16:4 (0.9); 18:4 (2.4); 20:4 (4.9); 20:5 (13.9); 22:6 (0.3) 20:5 (1.3) 16:4 (1.1); 18:4 (1.9); 20:4 (8.7); 20:5 (6.8); 22:6(1.0) 18: 4 (11.12); 20:4 (2.58); 20:5 (10.27)

Table 3 Influence of double bond position, configuration and chain length on kinematic viscosity of fatty acids [37]. Chain length and double bond position

14:1; Δ9 16:1; Δ9 18:1; Δ6 18:1; Δ9 18:1; Δ11 18:2; Δ9,12 20:1; Δ11 22:1; Δ13

Double bond configuration cis

trans

2.73 3.67 4.64 4.51 4.29 3.65 5.77 7.33

– – 5.51 5.86 5.41 5.33 – –

correlation of unsaturation with CN, viscosity, CP and PP, whereas there is a positive correlation was reported with density, heating value and IV [33]. The density of biodiesel is reported to be directly proportional to the average number of double bonds present [32,34–36]. For example, high oleic sunflower oil (HO) and rapeseed oil (RO) have lower PUFA content than soybean oil, thus, the density of HO and RO is lower than that of soybean oil [36]. Biodiesel exhibits a narrow range of boiling point (325–350 °C) compared to No.2 diesel (180–320 °C) [1]. Biodiesel, however, has a higher average boiling point ( 331 °C) than that of diesel fuel ( 263 °C) [1]. The energy density and lower heating value of biodiesel is less than that of diesel which is attributed to the presence of oxygen in biodiesel (around 10% by weight) [24]. Knothe and Steidley [37] evaluated the influence of the fatty acid chain length, configuration and position of double bonds on the viscosity of biodiesel. Table 3 compares the kinematic viscosity

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of MUFA in relation to a different chain length, including cis and trans isomers of methyl octadecenoate along with double bonds at three different positions of the chain [37]. It is observed that increasing the fatty acid chain length increases the viscosity of pure fatty acids. The configuration of double bond such as cis and trans also influences the viscosity of biodiesel. The trans and cis are geometric isomers of fatty acids, which differ by the orientation of the double bond. The presence of a cis double bond induces a 30° bend in the fatty acid chain that obstructs the proper packing of fatty acid chains. The observed reduction in the intermolecular forces is due to the poor packing in fatty acid with cis double bonds, resulting in decreased viscosity. The viscosity of fatty acid chain having trans double bond configuration is found to be similar to that of saturated fatty acids [27]. The trans configuration of the double bond has a higher kinematic viscosity, irrespective of the fatty acid chain length and position of the double bond [38,37]. It can be said that moving the position of the double bond towards the centre of the chain has no effect on the viscosity of the methyl octadecenoate. Therefore, no added advantage in viscosity will be gained by the double bond isomerisation of MUFA [37]. Teeter and Cowan [38] studied the influence of polar groups (carboxyl and hydroxyl) on the viscosity of fatty acid derivatives and hydrocarbons. They observed that the polar groups tend to associate more with each other as a result of hydrogen bonding than their non-polar counterparts. Therefore, non-polar hydrocarbons have the lowest viscosities for a given chain length than those of polar hydrocarbons [38]. The high viscosity of fatty acid derivatives can therefore be attributed to their polar groups and the highest viscosity of glyceride is due to its molecular weight [38]. Allen et al. [39] tested methyl and ethyl esters at 40 °C to observe the influence of the molecular weight and the degree of unsaturation on viscosity. They reported that the kinematic viscosity is proportional to the molecular weight of the saturated ester and has inverse relation with number of double bonds [38,33,39]. The viscosity was found to decrease by 21% with addition of one double bond in C18:0. The ethyl esters have 5.4% higher viscosity than that of the methyl esters which was attributed to the difference in their alcohol moiety [39]. The above discussion shows that the physical properties of biodiesel are influenced by the compositional features of fatty acids. The difference in properties of biodiesel and diesel, results in a change in combustion and emission characteristics. The lower energy density (calorific value) of biodiesel leads to reduced engine power and increased fuel consumption. Biodiesel spray atomisation, air-fuel mixing, combustion and emissions are affected by its higher density, viscosity, bulk modulus, presence of oxygen, and surface tension. The major concerns with biodiesel use are the increased NOx emissions, poor oxidative stability and cold flow properties. These properties are therefore important to consider while using biodiesel as fuel in engine. Hence, these properties are discussed in more detail in next sections.

4. NOx emission The NOx emission in a typical diesel engine exhaust is composed of oxides of nitrogen with major component being NO and lesser amount of NO2 [31,40,41]. The mechanisms of NO formation are: Extended Zeldovich (Thermal NO), Prompt NO (Fenimore) and Fuel NO mechanism [40,41]. For non-premixed combustion of CI engine, Zeldovich and Fenimore mechanisms are found to be responsible for NO formation [41]. High temperature ( 4 1800 K) and mixture with equivalence ratio ( o 1) present at the periphery of diffusion flame are favourable conditions for thermal NOx formation [42]. Additionally, residence time of mixture at these

conditions determine the amount of NOx formed. Therefore, any parameter or property which influence combustion temperature, equivalence ratio or residence time of air–fuel mixture can be responsible for NOx formation. Increase in NOx emission with biodiesel as fuel is attributed to advance in injection timing, combustion phasing and flame temperature in most of the studies. The detailed analysis of these parameters is discussed in next section. 4.1. Advanced injection timing The timing at which fuel is injected inside the cylinder has significant effect on combustion efficiency and engine NOx emission [40,43–46]. Old diesel engines, which used pump-line-nozzle (PLN) injection system, observed advance in injection timing with biodiesel as compared to diesel fuel due to higher bulk modulus of biodiesel. The injection advance upto 1° CA has been reported for soyester biodiesel owing to its higher speed of sound and bulk modulus than that of diesel [47,46]. Many researchers have reported increase in NOx emission due to advanced fuel injection with biodiesel [34,46,13,48,49]. However, the increase in NOx emission is much higher than that can be expected from advance in injection timing [47]. The CRDI system for biodiesel give negligible advance in injection timing, however produce higher NOx emission than that of diesel [34]. The bulk modulus of biodiesel is proportional to density, which has positive correlation with unsaturated fatty acid content in biodiesel [35,48,33]. Thus, biodiesel with higher unsaturated fatty acid content will result in advanced injection. In summary, high bulk modulus and speed of sound for biodiesel lead to advance in injection timing in PLN injection system. However, increase in NOx cannot be completely attributed to advanced injection timing in biodiesel [45]. 4.2. Flame temperature The NOx formation in diesel is largely attributed to high combustion temperature. Ban-Weiss et al. [50] studied combustion of propane and propene, methyl butanoate and methyl trans-2butenoate numerically. The adiabatic flame temperature of propene and methyl trans-2-butenoate was higher than that of their saturated counterparts. Consequently, following the hypothesis of high flame temperature, both propene and methyl trans-2butenoate showed higher NOx emission. Graboski et al. [35] conducted detailed experimental investigation to analyse influence of fatty acid chain length and unsaturation on NOx emissions. It was found that, emissions of NOx increased with increase in proportion of unsaturated fatty acids in biodiesel derived from soybean, canola, and soap-stock oils [35,51,52]. The unsaturation of fatty acids is also indicated by IV. The NOx emissions are linearly related to IV of the biodiesel [32,33]. Regression analysis study predicted that a biodiesel with IV of 38 (corresponding to 1.5 double bonds/molecule) would be a NOx neutral fuel relative to diesel [35]. McCormick and Alleman [53] also reported that fuel having IV of 40 would be NOx equivalent to diesel. Wyatt et al. [54] investigated NOx emission of soymethyl ester and methyl esters of animal fat origin in 20% blend with petroleum diesel. They observed that animal fat origin methyl esters with lower unsaturation produced lower NOx emissions. In summary, studies indicate that higher unsaturation of biodiesel results in higher NOx emissions [32,33,35,51,52, 54,50,55]. Long chain saturated esters (C16:0 and C18:0) produce lower NOx emissions compared to short chain saturated esters. However, saturated methyl laurate (C12:0) is an exception, which produces NOx equal or below that of diesel fuel [35]. Methyl esters of

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coconut oil and palm kernel oils, which has methyl laurate more than 50% by weight, may act as NOx neutral fuel. The experimental investigation on pure coconut oil gave 12% lower NOx than that of diesel fuel [56]. Kalam et al. [57] also observed reduction in NOx emissions for 50% blend of coconut oil with diesel. They attributed observed decrease to lower combustion temperature based on temperature of exhaust gas. The exhaust gas temperature for biodiesel may be low due to its lower heating value [58]. Experimental investigation with pure and 50% CME blend with diesel resulted in consistent reduction of NOx emission [59]. It was attributed to lower peak pressure value of premixed combustion due to their shorter ignition delay periods, which was further related to its saturated nature. Kinoshita et al. [60] compared NOx emissions from CME, PME and RME biodiesel and found lowest NOx emission for CME (8% less than that of diesel). This was due to its highest saturated fatty acid contents (90% in CME compared to 50% in PME, and 6% in RME) and lower combustion temperature of CME [60–62]. Similar results and attributions are reported for palm kernel methyl esters and coconut oil biodiesel [61,63]. It has been suggested that exhaust gas emissions of FAME can be improved by mixing relatively short chain saturated FAME and the ignitability can be adjusted by varying the composition ratio of saturated and unsaturated FAME [58]. However, investigations by Satyanarayana and Muraleedharan [64] observed 26% and 7% higher NOx emission for coconut and palm methyl esters respectively, compared to diesel at 100% load condition. Similarly, How et al. [65] reported 20.8% higher NOx for 50% coconut biodiesel blend at mid load condition. Nevertheless, studies did not report the fatty acid composition of coconut and palm methyl esters used. 4.3. Combustion phasing The combustion process in CI engine comprises of ignition delay, period of uncontrolled combustion (premixed) and diffusion controlled combustion. The lower ignition delay results in lesser premixed and higher diffusion controlled combustion period [43,44,66]. Significant amount of NOx is formed during premixed combustion duration, hence in modern CI engine amount of fuel burned in premixed phase is reduced [50]. The ignition delay is influenced by physical and chemical properties of the fuel such as viscosity, density, surface tension, liquid thermal conductivity, heat capacity and thermal diffusivity [13,43,67]. The lower compressibility, high density and high CN of the biodiesel are responsible for shorter ignition

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delay and earlier injection timing that results in advanced combustion [43,44,46,68,69]. The researchers have investigated different mechanisms of NOx formation through combustion phasing [31,43,46]. Sahoo and Das [69] observed that pure biodiesel and their blends with diesel have shorter ignition delay than that of diesel. Ignition delay reduced with increase in engine load for pure biodiesel of jatropha, karanja and polanga and their blends with diesel. The maximum heat release rate of biodiesel was lower and occurred earlier than that of diesel due to shorter ignition delay. Cheng et al. [70] observed 10% increase in NOx emission of pure biodiesel for same premixed burn period and start of combustion as that of diesel. These observations show that factors other than start of combustion and premixed-burn fraction are responsible for increase in NOx emission of biodiesel. Mueller et al. [71] reported that combustion process of biodiesel was faster than that of diesel irrespective of ignition delay. It leads to higher in-cylinder temperature and longer residence time at high temperature, which may result in higher NOx than that of petroleum diesel. The shorter ignition delay (4°) and early start of combustion (5°) could allow the fuel mixture and initial combustion products to have a longer residence time at elevated temperature, thereby increasing formation of thermal NOx [31,32,40,43,46,69]. The NOx emission of SME remained higher than that of PME, even when the rate of heat release of SME was matched to that of PME by adding the cetane number improver. These studies indicate that cetane number is not the only factor which influences the NOx emission in biodiesel [72]. Studies in the literature have observed that lower ignition delay of biodiesel alone have minimal effect on NOx emission. Long chain saturated components of biodiesel with higher CN give lower NOx or act as NOx neutral. Biodiesel with higher percentage of saturated components produce lower NOx compared to that of high unsaturated components. It is observed that biodiesel gives advanced injection timing due the their higher bulk modulus and speed of sound. NOx emission is directly proportional to percentage of unsaturated fatty acid present in the biodiesel. The unsaturated fatty acids in biodiesel are responsible for higher adiabatic flame temperature and leads to higher NOx emissions. It is observed that long chain saturated fatty acid produce lower NOx with methyl laurate (C12:0) being an exception. Biodiesel from coconut oil and palm kernel oil contains high percentage of lauric fatty acid and are reported to produce lower NOx than that of diesel fuel. Shorter

Table 4 Summary of literature on NOx emission from biodiesel. Advance in injection timing

References

Influence of advance in injection timing on NOx emission Injection timing advance for soyester correlated to higher bulk modulus and speed of sound NOx emissions attributed to advance in injection Bulk modulus and density of biodiesel correlated to degree of unsaturation

[40,43–46] [47,46] [34,46,13,47–49] [35,48,33]

High flame temperature

References

High flame temperature correlated to degree of unsaturation of biodiesel NOx emission increases with unsaturation or IV of biodiesel Fuel with IV 38–40 act as a NOx neutral fuel (coconut and palm kernel oil) Saturated esters (C16:0, C18:0, C12:0) produce NOx emissions lower than or equal to diesel

[50] [32,33,35,51,52,54,50,55] [35,34,56] [35,60–63,57]

SOC and premixed combustion duration

References

Combustion phasing (shorter ID and earlier SOC) of biodiesel resulted in higher NOx emission Difference in ignition delay leads to change in premixed and diffusion combustion period Ignition delay correlated to physicochemical properties

[31,32,40,43,46,69] [43,44,66] [13,43,67]

Contradictions to combustion phasing theory

References

Higher NOx emission observed for pure biodiesel for same premixed burned and SOC as that of diesel SME found to emit higher NOx than that of PME even when the rate of heat release of SME is matched with PME

[70] [72]

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ignition delay and higher CN of biodiesel cannot be directly correlated to NOx emission. Various theories of NOx formation discussed are summarised in Table 4.

The above discussion indicates that the oxidative stability of the biodiesel, having higher levels of saturated fatty acid and lower levels of free fatty acids, is a desirable composition. Unsaturated fatty acids therefore are not desirable for good oxidative stability.

5. Oxidative stability 6. Cold flow properties When exposed to atmosphere, biodiesel absorbs and reacts with oxygen to produce insoluble gums, aldehyde, alcohols, shorter chain carboxylic acids and sediments [17]. On precipitation, these insoluble products result in plugging, fouling and corrosion of injection system and fuel storage tank [73,16,74]. Biodiesel ideally should exhibit high oxidation stability for use as fuel in CI engine. The stability of biodiesel includes both oxidation and thermal stability [16]. The factors which influence the oxidation rate of oil are temperature, oxygen pressure, prior oxidation, metal ions, lipoxygenases, hematin compounds, loss of natural antioxidants, the absence of metal deactivators, storage time and exposure to ultraviolet or visible light [1,16,74–78,17]. Knothe and Dunn [76] observed that unsaturated fatty acids have low oxidative stability. Metal containers consisting of copper, iron and nickel decrease oxidative stability, although their effect was less compared to the presence of unsaturated fatty acids. Copper had the strongest catalytic effect on oxidative stability [76]. Thermal instability signifies the increased rate of oxidation at higher temperatures. The plant derived oils are composed of polyunsaturated chains which contain allylic site (i.e. CH2 moiety adjacent to a single double bond) and bis-allylic site (i.e. CH2 moiety located directly between two double bonds) [76,79–82]. These PUFA are prone to oxidation as the double bonds and adjacent allylic sites are the reactive sites for the oxidation reaction [27,75,76,15,79,81]. Zuleta et al. [83] reported on the bond energies of hydrogen at various positions in the carbon chains of fatty acids. The bond energy of hydrogen at bis-allylic sites (272 kJ/mol) was found to be lowest, followed by the allylic sites with (322 kJ/mol). The lower bond energy of bis-allylic sites and allylic sites makes them highly reactive. The rate of oxidation reaction was found to be proportional to the degree of unsaturation and the position of the double bond (i.e. bis-allylic site and allylic site) [75,76,84,85,81,82]. McCormick et al. [86] reported that oxidation stability does not correlate to the total number of double bounds, but instead with the total number of bis-allylic sites. It can be argued therefore that linolenic fatty acid (C18:3) is more susceptible to oxidation than linoleic (C18:2) [75,76,84,85]. The specific relative rate of oxidation found is 1 for oleate, 41 for linoleate, and 98 for linolenate [27,76,79,81,86]. Knothe [87] suggested two indices namely allylic position equivalent (APE) and bis-allylic position equivalent (BAPE) as a way to differentiate the reactivity observed by the presence of allylic and bis-allylic positions in fatty acid chains. The limitation of the unsaturation of fatty acids is found to be of greater significance for the oxidative stability of biodiesel due to the fact that heating highly unsaturated fatty acids results in the polymerisation of glyceride, which could lead to the formation of deposits [85,88]. Vicente et al. [89] have experimentally shown that the iodine, peroxide and acid values influence the oxidative stability of biodiesel. They observed that an increase in viscosity with the oxidation of oil is due to the polar nature of hydro-peroxides, and its products are formed due to its polymeric decomposition [38,90]. Hydro-peroxides are the primary products of oxidation of biodiesel and tend to attack elastomers, which may lead to deterioration in the fuel lines of the injection system [85,1,15]. It may also be produced as a result of feed-stock impurities such as unconverted mono and diglycerides and glycerine that can lead to engine deposit formation [1].

The cold flow properties of biodiesel are of greater importance in regions with low ambient temperatures [91]. Poorer cold flow properties of fuel leads to start up problem due to the formation of solid crystals in the fuel tank, resulting in the plugging of filters and fuel lines. The cold flow properties are influenced by the CP and PP of biodiesel. The cloud point is the temperature at which the first crystal is formed in the fuel [23,73,16]. The CP of biodiesel is observed as being in the range of 262–289 K, whereas the CP of No.2 diesel is in the range of 256–265 K [92]. Pour point (ASTM D-97) is the measure of the fuel gelling point, the temperature at which the fuel can not be pumped [23,73,16]. The PP of biodiesel is observed in the range of 258–286 K, whereas the PP of the No.2 diesel is in the range of 237–243 K [92]. The onset crystallisation temperature (Tco) is the highest temperature at which the substance remains in a liquid state [93,94]. Crystallisation is highly dependent on both molecular packing and the interactions between molecules. The chain branching interferes with crystal packing. Thus branched fatty acids have lower Tco and better cold properties compared to straight chain alkyl esters [93]. Rodrigues et al. [94] observed a decrease in Tco with the presence of double bonds in the fatty acid chain. The linseed oil containing high linolenic acid has low Tco due to the presence of three double bonds. The babusa coconut oil has high lauric acid (medium chain fatty acid) content which provides low Tco due to the weaker molecular interaction [94]. The saturated esters have higher melting point and poor cold flow properties than that of their unsaturated counterparts [73,95]. Stearic acid is solid at 39 °C, while unsaturated methyl oleate melts at  19 °C and methyl linoleate melts at  35 °C. The tallow methyl esters have poor cold flow properties with a CP of 14 °C and PP of 10 °C because they have a higher degree of saturation than soybean and rapeseed esters with an average CP of 0 °C and  5 °C, and PP of  4 °C and  10 °C, respectively [73,54,93,96]. In addition to saturated esters, the monoacylglycerols (MAG) of saturated FAs (melting point 4 70 1C) and sterol glucosides (SG) (melting point 4 240 1C) that may be present in small quantities, are responsible for poor cold flow properties [96]. However, Echim et al. [97] did not observe any improvement in the cold flow properties on removal of minor components such as free SG and MAG. It has been well-established that the presence of higher amounts of saturated components results in an increase of the CP and PP of biodiesel [54,16,93,95,97,98]. The summary of the literature on cold flow properties and oxidative stability of biodiesel is presented in Table 5.

7. Inter-correlation The degree of unsaturation of fatty acid in biodiesel is found to correlate with NOx emissions, oxidative stability and the cold flow properties of biodiesel. The biodiesel, with a high percentage of saturated fatty acid esters, produces lesser NOx emissions and ensures a longer stability duration during storage. However, saturated biodiesels have poor cold flow properties. The higher unsaturation favours the cold flow properties of biodiesel, which is unfavourable for its NOx emissions and stability. Therefore, the

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Table 5 Summary of oxidative stability and cold flow properties of biodiesel. Biodiesel oxidative stability

References

Biodiesel oxidative stability influenced by: temperature, oxygen pressure, prior oxidation, metal ions, lipoxygenases, hematin compounds, [1,16,74–78,17] loss of natural anti-oxidant, absence of metal deactivators, storage time and exposure to ultraviolet or visible light The rate of oxidation reaction is proportional to the degree of unsaturation and the position of double bond (allylic and bis-allylic sites) [15,27,75,79,76,84,85,81,82] The specific relative rate of oxidation is 1 for oleate, 41 for linoleate and 98 for linolenate [27,76,79,81,86] Biodiesel cold flow properties

References

Cold flow properties of fuel (CP,PP,Tco) Unsaturated esters exhibit lower Tco, CP and PP

[23,16,73,93,94] [73,54,93–96]

Table 6 Requirements of fatty acid composition for better cold flow properties, oxidative stability and lower NOx emissions of biodiesel. Cold flow property CFPP (°C)

Fatty acids

Biodiesel

0 to  10

Monounsaturated fatty acids (C16:1, C18:1) [94,33,104,30,36,105,97,91,106,107] and Polyunsaturated fatty acids (C18:2, C18:3) Saturated fatty acids (C12:0, C14:0, C16:0, C18:0) [94,104,30,36,105,97,106,107,33]

soybean, rapeseed, sunflower, high oleic sunflower, corn, coconut, olive, karanja and jatropha [104,36,16,105,97,91,106,107,33] peanut, beef tallow, palm, lard, castor and cottonseed [104,36,16,105,97,106,107,93,33]

Oxidative stability Induction time in hours

Fatty acids

Biodiesel

Z3

Saturated fatty acids (C12:0, C14:0, C16:0, C18:0) [83,76,78,33]

o3

Monounsaturated fatty acids (C16:1, C18:1) and polyunsaturated fatty acids (C18:2, C18:3) [83,76,33]

coconut, palm, castor, karanja , rapeseed, corn, soybean, sunflower and olive [17,83,33] cottonseed, linseed and beef tallow [17,83,33]

40

NOx emissions Compared to diesel Lower Higher

Fatty acids

Biodiesel

Saturated fatty acids (C12:0, C14:0, C16:0, C18:0) [35] Monounsaturated fatty acids (C16:1, C18:1) and polyunsaturated fatty acids (C18:2, C18:3) [35,54]

coconut and palm kernel [60,56–59] soybean, rapeseed, jatropha and karanja [108–110,54,34]

optimum composition of saturated and unsaturated fatty acids is required in order to satisfy lower NOx emissions and good oxidative stability and cold flow properties. The MUFA may be the mid-way approach, in that it is neither saturated nor having higher degree of unsaturation [99–101]. The enrichment of MUFA specifically, C18:1, has been used in the production of biodiesel for obtaining desired properties and emissions with biodiesel. The melting point of C18:1 is around 20°C, which satisfies the cold flow property requirements of fuel in most regions which have cold climate [102,28]. NOx emissions are found to be lower with C18:1 blends of diesel fuel. The lower oxidation stability is a problem for MUFA (C18:1) utilisation, which is around 2.5 h less than the requirements of ASTM D6751 and EN 14214 standards [14,102,28,103]. Saturated fatty acids are required alongside MUFA to increase the oxidative stability of biodiesel. The saturated counterpart of C18:1, stearic fatty acid (C18:0), may be suitable for increasing the stability. It should be noted however that the addition of any fatty acid should not adversely affect the melting point and cetane number of the resulting mixture. The ideal composition of oil for biodiesel that may satisfy both performance and emission requirements, may comprise of MUFA in the range of 60–80% and SFAs 10–20%. The percentage of PUFA in biodiesel can be fixed by limiting the iodine value to 120, as per European Standard EN 14214 [80]. Rigorous testing is needed to get the desirable blend of fatty acids which may give good cold flow properties, stability and fewer NOx emissions. The fatty acid composition requirements to provide acceptable cold flow properties, oxidative stability and NOx emissions of biodiesel are shown in Table 6.

8. Remedial approaches The problem associated with optimum biodiesel properties and emissions has been addressed through various remedial approaches. Some of the remedial approaches mentioned in the literature feature inherent modification of the fatty acid composition via physical, chemical or genetic modification of raw oil. Biodiesel properties can be improved through the use of different alcohols other than methyl alcohol, as well as additives such as cetane enhancers, cold-flow improvers and antioxidants. 8.1. Modification of composition Modifying fatty acid composition has been identified as a promising way for improving biodiesel fuel properties [27,111]. The fatty acid profile can be modified by physical, chemical and genetic modification methods. 8.1.1. Physical method “Winterisation” concerns the physical method of separating solid and liquid fractions of refined and bleached oil stored in tanks exposed to cold weather during the winter months [75,91,95,106]. The part of the oil which, remained liquid without clouding during winter is removed from the top of the tank. The clear liquid is proportional to the unsaturated fatty acid in biodiesel [75,91,95,106]. “Fractionation” is another physical method in which oil is separated into two or more portions [27,75,106]. Fractionation technology, specifically solvent fractionation, has been used to

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produce specialised edible oils in the food industry, having a minimum of 350 h stability without adding any kind of external antioxidants [75]. These methods produce biodiesel which has better cold flow properties, but with poor cetane numbers (ignition quality) and oxidative stability as the portion of saturated fatty acid is removed [27,91,106,1,95]. Another disadvantage as pointed out by Dunn et al. [91] is that even if just a small fraction of saturated esters remained after these processes, there was an increase in the cloud point of biodiesel. A presence of 5.6% (by weight) of saturated esters in winterised soy methyl esters increases its cloud point by 7 °C and the pour point by 32 °C [91]. Furthermore, the loss of original material, which is around 75% occurred in these processes makes them uneconomical [106]. Park et al. [112] investigated the method of blending two biodiesel to improve both oxidative stability and cold flow properties. They demonstrated this method using palm, rapeseed and soybean biodiesel with oxidative stability of 11 h, 6.94 h and 3.87 h and cold flow plugging properties of 10 °C,  20 °C and  3 °C respectively. An improvement in the oxidative stability of soybean biodiesel was achieved by blending it with palm and rapeseed biodiesel featuring higher oxidative stability. On the other hand, the poor cold flow properties of palm biodiesel is improved by blending it with rapeseed and soybean biodiesel having better cold flow properties. Thus, the blending of biodiesel can be a simple and effective method to improve the cold flow properties and oxidative stability of biodiesel. 8.1.2. Chemical method “Hydrogenation” is a chemical process in which hydrogen is added to the double bond of unsaturated fatty acids and the movement of the double bond. This reaction is carried out by thoroughly mixing heated oil and hydrogen gas in the presence of catalyst. Saturated fatty acids such as stearic and palmitic acids do not undergo any change in this reaction [111]. In this selective reaction, the linoleic fatty acid is fully converted to oleic or other 18:1 isomers before any hydrogen reacts with oleic acid. Furthermore, this oleic acid and its isomers will absorb hydrogen and are converted into saturated stearic fatty acids. However, this type of perfect degree of selectivity can never be attained in practice. The perfection of reaction depends upon the type of catalyst, dosage, operating temperature and pressure [111]. The hydrogenation process with biodiesel results in increases in oxidative stability, although it worsens its cold flow properties [1]. 8.1.3. Genetic engineering Genetic engineering has been applied to plant oils in order to modify fatty acid composition to achieve desirable oil properties [111,91]. Duffield et al. [73] suggested that inherent change in fatty acid profiles via plant breeding and molecular genetics is the way to achieve the desirable properties of biodiesel [27,106]. The combination of certain genes may lead to higher saturation, which improves ignition quality, oxidative stability and the NOx emissions of biodiesel, whereas other genes may give superior cold flow properties [73]. The methyl oleate (MUFA) features a balance between saturation and unsaturation levels and is suggested as a favourable compound for improving biodiesel properties [99–101,106]. The development of an oil featuring a higher percentage of oleic acid and lower levels of saturated fatty acids such as palmitic and stearic acid and PUFA such as linolenic acid may result in higher oxidative stability and better cold flow properties [73]. Szybist et al. [113] increased the content of methyl oleate (C18:1) from 23 to 76% while decreasing methyl linoleate (C18:2) from 52.1 to 6.7% to increase the cetane number from 48.2 to 50.4 [81,113]. Liu et al. [114] had developed improved cottonseed oil with enhanced levels

of oleic acid (increased to 78% from 13% at the expense of linoleic acid). Buhr et al. [115] developed transgenic soybean event, 335– 13, for a reduced level of palmitic acid content ( o5%) and higher oleic acid content ( 4 85%). This oil demonstrated improved oxidative stability and enhanced cold flow properties without compromising agronomic performance, including overall yield, total protein/oil and amino acid profile [116]. The oil featuring a higher percentage of stearic acid have been found to give higher cetane number at the expense of cold flow properties, which can be used during the warmer season and in hot climates [73,106]. There are limiting factors to this technology, such as identity preservation (IP) and economics [75]. The IP needs segregated fields, storage, handling, transportation and seed extraction. This leads to an increase in the cost of the final product. Genetic engineering is economical if the modified product becomes a commodity product, e.g., canola oil with low erucic fatty acid replaced rapeseed oil with high erucic fatty acid and became a commodity product [75]. The fatty acid profile of algae can be optimised to enhance oil production and display favourable properties [27]. 8.2. Reformulated biodiesel Changing the type of alcohol (ethyl, methyl, isopropyl and butyl) moieties or the percentage of favourable fatty acids in fatty ester composition can produce the required properties of the biodiesel [23,101]. Demirbas [23] reported that the cloud point and pour point of esters of ethyl alcohol are lower than the esters of methyl alcohol. An investigation of palm oil biodiesel featuring a different alcohol moiety (methyl, ethyl, butyl and isobutyl) has shown that an increase in the carbon number of alcohol decreases the pour point [117]. Palm oil isobutyl ester has the lowest PP of 0 °C compared to palm oil butyl ester (PP of 5 °C). However, the pour point increases when the carbon number of alcohol moiety exceeds 4 [117,118]. The ethyl esters also have the advantage of producing a lower volume of smoke, lower exhaust temperatures and pour points; but feature more injector coking than the methyl esters [23]. Ethanol can be preferable for the transesterification of oil compared to methanol as it is renewable and biologically less objectionable for the environment. However, higher price makes this approach economically less attractive and it requires a modification in the transesterification reaction [23,101]. The Tco can be reduced significantly (7–14 °C) by replacing methyl esters with branched chain such as isopropyl and 2-butyl in biodiesel. A reduction in the palmitate content of isopropyl esters of soybean oil to 3.8% resulted in the reduction of its Tco by 5–6 °C [119]. Lang et al. [22] reported a lower yield of 2-propyl ester (around 87%) compared to linseed butyl ester (96%). A lower yield was attributed to the lower reactivity of the branched-chain alcohol (2-propyl) compared to butyl alcohol. The lower volatility and yield is observed with the esters of branched alcohol. Moreover, the branched chain alcohols, like propyl and butyl alcohols, are presently obtained from fossil sources, which would increase the cost of the production of biodiesel. 8.3. Additives The additives used for improving the properties and emission profile of biodiesel are flow-improvers, antioxidants and cetane improvers. The flow-improver additives do not change the cloud point. They instead inhibit the growth and combination of existing wax crystals, which improves the pour point and Cold Filter Plugging Point (CFPP) [73,91,97]. The pour point of soymethyl esters is found to reduce to  40 °C with 1000 ppm of commercial additive [73,91]. Bhale et al. [120] observed decrease in pour point

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Table 7 Summary of remedial approaches. Modification

References

Physical methods (winterisation and fractionation): produce biodiesel with poor oxidative stability and uneconomical Chemical method (hydrogenation): difficult to achieve in practice and produce biodiesel with poor cold flow property Genetic engineering: focus on increasing the content of oleic acid, limitation identity preservation and economics

[91,106] [1,111] [73,91,111,27,106,113–116,75]

Reformulated biodiesel

References

Alcohol moieties (ethyl, methyl, isopropyl and butyl) of biodiesel significantly affect cold flow properties Changing alcohol moieties have limitations due to change in transesterification process, cost, lower yield and non-renewable source

[117–119] [22,23,101]

Use of additives

References

Flow improver additives only affect PP of fuel, it does not change CP Antioxidants are useful for enhancing oxidative stability Use of CN enhancing additives is limited to older engines Compatibility of various additives required for different issues is the limitation of this approach

[73,91,97] [27,122,123] [124]

of mahua methyl ester (7 °C to  5 °C) by doping it with Lubrizol up to 2%. Wang et al. [121] used (0.2 wt%) of polyglycerol ester to reduce CFPP of waste cooking oil biodiesel (from  10 °C to 16 °C). A variety of commercial antioxidants such as BHT (butylated hydroxytoluene), TBHQ (tert.-butylhydroquinone), PG (propyl gallate), PA (pyrogallol) and BHA (butylated hydroxyanisole) have been studied and found to be effective in enhancing stability of biodiesel [27,122,123]. Karavalakis and Stournas [122] reported that the efficiency of antioxidant with respect to improvement in oxidative stability of biodiesel follows the order (PG 4PA 4 TBHQ 4 BHT 4BHA). The PG and PA were among the most effective antioxidants for pure and blended biodiesel. The higher effectiveness of TBHQ, PG and PA has been attributed to their molecular structure having two OH groups attached to the aromatic ring compared to one OH group attached to both BHT and BHA aromatic ring. The required concentration of additives for biodiesel varies with their degree of unsaturation [122]. The CN of biodiesel can be enhanced by selecting a source featuring long chain saturated fatty acids. It is reported that neat biodiesel with CN Z 68 produces less or equal NOx emissions compared to diesel [35]. The use of cetane enhancing additives results in the reduction of NOx for B20 blends specifically for older engines, which are more cetane sensitive than that of modern engines [124]. To summarise, additives have the potential to improve various biodiesel properties of interest. However, each additive targets only one property, which results in the requirement of many additives to improve the overall biodiesel properties. This may lead to problems such as additive compatibility and the effect of additives on other fuel properties [101]. The summary of remedial approaches discussed in the paper is presented in Table 7.

9. Conclusions and recommendations The biodiesel is found to have higher NOx emissions, poor oxidative stability and cold flow properties. These drawbacks are related to the physicochemical properties and composition of biodiesel. Hence, there is a need for an ideal biodiesel composition which will satisfy all the requirements of biodiesel as a fuel for CI engines. The following points have been observed based on the literature review: 1. Advanced injection timing, high flame temperature and combustion phasing are thought to be the reasons behind high NOx emissions from biodiesel. 2. High bulk modulus and the speed of sound for biodiesel are responsible for the advance in injection timing and these properties are proportional to the unsaturation of fatty acids.

3. Unsaturated fatty acids have higher adiabatic flame temperature compared to their saturated counterparts. 4. Biodiesel with an iodine number of less than 40 is expected to produce lesser or equal NOx emission than that of diesel. 5. A high percentage of unsaturated fatty acids in biodiesel is correlated with higher NOx emissions, poor oxidative stability and better cold flow property. 6. Coconut and palm kernel oils featuring high content of lauric acid are found to produce lower NOx emissions, better oxidative stability and acceptable cold flow properties. 7. Long chain saturated fatty acids produce lower NOx emissions and have high oxidative stability. 8. NOx emissions, oxidative stability and cold flow properties impose contradictory requirements on the fatty acid composition of biodiesel. 9. MUFA is accepted as a better solution to meet the ideal requirements of biodiesel composition. 10. Various remedial methods including genetic engineering, reformulated biodiesel and additives may be experimented with in order to enhance biodiesel properties. 11. Algae-derived biodiesel is a promising technology in order to produce the ideal fatty acid composition for biodiesel due to the presence of a wide range of fatty acids. 12. As per local availability, a blend of various biodiesels can be used to alter biodiesel properties for optimum performance and emission production.

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