Physical properties and chemical composition of biofuels

CHAPTER

Physical properties and chemical composition of biofuels

11

Mohd Hafizil Mat Yasin1,2, Mohd Affandi Ali1,2, Rizalman Mamat1,2, Ahmad Fitri Yusop1,2 and Mohd Hafiz Ali1,2 1

Department of Mechanical Engineering, Politeknik Kota Kinabalu, Sabah, Malaysia 2Faculty of Mechanical Engineering, University of Malaysia Pahang, Pahang, Malaysia

11.1 INTRODUCTION Biofuels are among the most promising fuels of renewable energy with various forms of possible applications. Most applications of biofuels are mainly based on the use of products in internal combustion engines and power generation industrial sectors. These biofuels are organic products which can be derived from oil-producing crops, to produce vegetable oils either pure or transesterified into biodiesel, or from sugary and starchy feedstock crops to produce alcohol forms of ethanol. This chapter focuses in detail on some aspects of biofuel characteristics regarding their physical and chemical characteristics as well as the chemical composition of biofuels. A few critical positive elements are described regarding these biofuels, including being self-sustainable in contrast to fossil fuels, the entrepreneurial system of cultivation, and higher yields in agricultural production outcomes, the absence of sulfur, biodegradability, lower carbon monoxide emissions, as well as particulate matters (PMs) and unburnt products. However, there are a few limitations connected to these biofuel utilizations, including higher production costs, which is not competitive when compared to fossil fuels, and secondly, the chemical and physical properties of biofuels, which have less equivalence to engine specifications and standards. Therefore, it is necessary to consider the high variability of chemical and physical characteristics of different biofuels and their blends. These biofuels are made up of 98% triglycerides, and the remaining 2% consists of phospholipids with different types of hydrocarbons. A molecule of esterified glycerol bonds with molecules of fatty acids with a range from one to three to form glycerides, dependent on the different number of carbon atoms and the configuration of variable chemicals. The differences in chemical structure contribute to the variations of the different oil characteristics, which strongly

Second and Third Generation of Feedstocks. DOI: https://doi.org/10.1016/B978-0-12-815162-4.00011-2 Copyright © 2019 Angelo Basile and Francesco Dalena. Published by Elsevier Inc. All rights reserved.

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Table 11.1 Types of Biofuels (Zaharin et al., 2017; Sakthivel et al., 2018) Types of Feedstock Generation

Types of Biofuels

First generation

Vegetable oil

Second generation

Nonedible vegetable oil

Third generation

Animal fat oil

Waste/used cooking oil

Algae

Examples

Explanation

Edible vegetable oil: rapeseed, soybean, coconut, palm, sunflower, peanut, olive and sesame seed, mustard oil, pistachio oil, cashew nut oil, walnut oil, radish oil, tigernut oil, cottonseed oil, rice bran oil, hazelnut oil, and castor oil Nonedible vegetable oil: Jatropha curcas, Karanja or Pongamia, jojoba, cottonseed, sea mango, Calophyllum inophyllum, mahua indica, neem, rubber seed, nicotiana tabacum, aleutites fordii, crambe abyssinica, sapindus mukorossi, cerbera odollam, thevettia peruviana, nagchampa, silk cotton tree, tall oil milk bush, petroleum nut, and babassu tree Animal tallow oil, chicken fat oil, poultry fat oil, fish oil

Edible vegetable oil: competes with food materials and sources for use in diesel engines

Oil and grease from households, restaurants, and food processing factories Biomass: pyrolysis oil Dunaliella salina algae, Chlorella vulgaris algae, Botryococcus braunii

Nonedible vegetable oil: second-generation feedstocks are substituted for edible sources

A layer of fats derived from animals. Made of triacylglycerols, diacylglycerols, and monoacylglycerols Recycled waste products and prevents wastage disposal problems

influence the physical properties of the oil. Table 11.1 summarizes the types of biofuels from the first generation, second generation, and third generation of biofuel feedstocks when used as a fuel for diesel engines. Internal combustion engine fuels consist of several hundred different hydrocarbons of various groups (CnHnOn). There are differences in the molecular size and structure of these fuel mixtures due to the varying properties of the fuels. Also,

11.2 Main Chemical and Physical Properties of Biofuels

their compositions are diverging, with different kinds of separation and treatment methods in producing the existing fuels including gasoline, diesel, and other biofuels. However, there are specific limit values among the properties of engine fuels which are compulsory to the existing standards including American Society for Testing and Materials (ASTM), European Standard (EN), Deutsches Institut fu¨r Normung (DIN), and others, which guarantee consistent quality and composition, to ensure the engine operation reliability. Fuel physical properties and chemical composition are among the most critical parameters to verify the quality of biofuel. Discreet observation and measurement is required to obtain the viscosity, density, acid value, water content, and other properties, to define the variability of characteristics of different biofuels. These data are beneficial for use in any numerical biofuel comparisons. The properties of biofuels are also significant for experimental and simulation work on engine operation, to analyze the outcome from the engine cylinder. The physical properties are very critical in the atomization process, whether in an internal combustion engine or a gas turbine engine. This is because viscosity affects the quality of atomization of fuel injection into the combustion chamber, droplet size distribution, and the uniformity of the mixture, while the surface tension affects the disintegration of a liquid jet into droplets. Table 11.2 lists the effects of different leading chemical and physical properties of biofuels on engine operation.

11.2 MAIN CHEMICAL AND PHYSICAL PROPERTIES OF BIOFUELS In general, biofuels that originated from different feedstocks have different properties as compared to other biofuels made from other organic sources as well as diesel fuel. Most biofuels, including biodiesel, have specific characterizations compared to diesel fuel. Therefore, a comprehensive dataset of biofuels with physical fuel properties is required to analyze the characteristics of biofuels when operated with conventional diesel engines, as listed in Tables 11.2 and 11.3. The physical fuel properties are very critical parameters in the atomization process in compression ignition (CI) engines.

11.2.1 KINEMATIC VISCOSITY Viscosity is one of the main fuel properties used to estimate the fuel quantity that is injected into the combustion chamber for advanced combustion. It is defined as the resistance to the flow of liquids or the measurement of internal friction between molecules. In engine operation, liquid fuel is sprayed into the highpressurized air and atomized spontaneously into smaller droplets near the fuel injector exit. The presence of free fatty acid (FFA) in the chemical bonding of the biodiesel contributes to the higher viscosity that can result in injector coking, ring

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Table 11.2 Description of the Leading Chemical and Physical Properties of Biofuels as a Source of Energy (Toscano and Maldini, 2007) Property

Description

Effects

Density

The weight of the unit volume of fuel (g/m3)

Viscosity

The resistance between the adjacent layers of a fluid, which hinders the sliding of one over the other

Calorific value

The fuel releases its energy in units of Joule/kg during the combustion process to deliver work. All the energy produced by the fuel is known as the higher calorific value (HCV), as the lower calorific value does not consider the latent heat of vaporization of water Determines the concentration of free fatty acid (FFAs)

• Influences the performance of the oil in the injectors • The minimum value of density is desirable to obtain the maximum engine power through the fuel flow control in the injection pump • The atomization quality of fuel injection within the combustion chamber • Distribution of fuel droplet size • The uniformity of the mixture • Influences specific consumption of the fuel and combustion quality in the engine

Acid value

Cetane number

Melting point

Flash point

Iodine value

Saponification value (SV)

The ignition quality indicator of the fuel. This defines whether the fuel has a longer or shorter ignition delay during the combustion period Determines the temperature at which the transformation from solid to liquid state can be observed in standard conditions The fuel vapor with the lowest possible temperature that can be directly flammable if in contact with fire Indicates the level of unsaturation of the oil which is related to the presence of double bonds between carbon atoms Assesses the fatty acids content, either free or bound, to the glycerol of the oil

FFAs form salts with the metal at high temperatures and can possibly damage the engine or tanks Cold start combustion is achieved and produces engine noise

Causes problems with flow in the fuel supply at higher melting point values Requires higher flash point value to ensure the safe storage, transport, and distribution of the product The oxygen tends to react with the double bonds compared to single bonds, with the formation of gum as a result Higher SV relates to the lower molecular weight of the acids (Continued)

11.2 Main Chemical and Physical Properties of Biofuels

Table 11.2 Description of the Leading Chemical and Physical Properties of Biofuels as a Source of Energy (Toscano and Maldini, 2007) Continued Property

Description

Effects

Oxidation stability

Determines the amount of gum and macromolecules which are formed from a sample at certain pressure conditions with the presence of oxygen Determines the phosphatide content

An increase in viscosity is found due to the generated compounds/ residues

Phosphorus content

Carbon residue

Indicates some natural compounds with higher molecular weight as temperatures increase

Sediment content

Measure the sediment content

Water content

Measure the water content

Na 1 K content

Measure the Na and K content

Ca 1 Mg content

Measure the Ca and Mg content

Formation of gum is found in the tanks, the feed pipes, and the filters attributed to the phosphatides An increase in carbon residues due to some compounds decomposing at high temperatures The developed sediment shortens the filter life or plugs the fuel filter, which affects the quality of fuel The presence of water in the fuel can shorten the filter life or plug the fuel filter and allows fuel corrosion and microbial growth Existing Na and K residuals may form deposits in the fuel injection system components, suffer high ash levels in the engine, and reduce the efficiency of emission control after treatment systems These Ca and Mg residuals may form deposits and clog the fuel injection system components, thus forming high ash levels in the engine and influencing the emissions produced by the engine

sticking, and gumming in diesel engines. Also, higher values of viscosity lead to poorer fuel atomization of the fuel spray, which causes incomplete combustion and carbon deposition inside the injectors. It also causes less accurate operation of the fuel injectors, especially during cold weather, whereas decreasing temperature causes viscosity to increase and affects the fluidity of the fuel. Conversely, fuels with lower viscosities cannot provide sufficient lubrication for the required standards in fuel injection pumps, which is mainly attributed to leakage and wear. According to the European standard EN 14214, kinematic viscosity at 40 C is between 3.5 5.0 cSt (European Standards, 2013), while ASTM D6751, the limit is between 1.9 6.0 cSt (ASTM International, 2015) utilizing a kinematic

295

Table 11.3 Properties of Various Biofuels (Zaharin et al., 2017; Sakthivel et al., 2018) Tyes of Biofuel First generation (edible oils)

Density at 40 C

Viscosity at 40 C

Flash Point

Cloud Point

Pour Point

Biofuels

(kg/m3)

(mm2/s)

( C)

( C)

( C)

Crude soybeans (Glycine max) Rapeseed (Brassica napus L.) Sunflower (Helianthus annuus) Coconut (Cocos nucifera) Canola (Brassica napus) Peanut (Arachis hypogaea) Palm (Arecaceae) Corn (Zea mays)

913.8

28.87

254

882.5

4.48

885

4.53

173

1

26

908.9

27.64

264.5

17

19

884.4

4.526

177.6

880

4.258

855.5 876.37

4.56

906.95

Crude rice bran oil (Oryza sativa L.) Palm kernel (Elaeis guineensis) Cashews (Anacardium occidentale)

Cetane Number

Calorific Value (MJ/kg)

References

37.9

39.6

54.7

39.92

Karmakar et al. (2010) Pinzi et al. (2013)

60

37 37.806

Abuhabaya et al. (2013) Atabani et al. (2013)

28

54.3

38.6

Öztürk (2015)

5

53.21

38.708

Tosun et al. (2014)

167.3 169

39.8 39.93

52.225

300.5

39.548

Abedin et al. (2014) Gülüm and Bilgin (2015) Wakil et al. (2014)

899.8

41.932

254.5

39.867

Atabani et al. (2013)

906.4

13.78

66

38.4

Kasiraman et al. (2016)

23

12 50

Second generation (nonedible oils)

Jatropha (Jatropha curcas L.) Karanji Cottonseed (Gossypium hirsutum) Terebinth Mahua (Madhuca longifolia) Neem (Azadirachta indica) Jojoba (Simmondsia chinensis) Moringa (Moringa oleifera) Tobacco seed (Nicotiana tabacum) Rubber seed tree (Hevea brasiliensis) Pork lard

869.2

4.75

180

889 871

4.3

181 199

1.7

880 850

4.12 3.98

75 208

5

884

5.21

920

5.2

186

877.5

4.91

888

210 to 215

53.5

40

52 41.2 59.5

39.13 35.66

55 56.61

40.1

870.98

Prbakaran and Viswanathan (2016) I˙lkılıç et al. (2015) Yang et al. (2014)

57.83

Yang et al. (2014)

55

Yang et al. (2014)

206

62.12

Yang et al. (2014)

4.23

165.4

51.6

Yang et al. (2014)

3.12

128

16

Yang et al. (2014)

39.53

Beef tallow Poultry fat Chicken fat Waste anchovy fish oil

Abedin et al. (2014)

39.5

889.7 893.4

4.7 4.44 5.3 4.435

169 156.2 169 182.4

Waste cooking oil

877

4.9

129

Waste frying cottonseed oil

885.1

4.547

174.8

39.933 9

6

56 52.3

4 49 3

37.1 37.3 37.951 37.25

Chakraborty et al. (2014) Mangus et al. (2015) Joshi et al. (2010) Alptekin et al. (2015) Altun and Lapuerta (2014) Attia and Hassaneen (2016) Altun and Lapuerta (2014) (Continued)

Table 11.3 Properties of Various Biofuels (Zaharin et al., 2017; Sakthivel et al., 2018) Continued Tyes of Biofuel Third generation (microalgal oils)

Density at 40 C

Viscosity at 40 C

Flash Point

Cloud Point

Pour Point

Biofuels

(kg/m3)

(mm2/s)

( C)

( C)

( C)

Lyngbya kuetzingii Isochrysis sphacrica Microalgae (Spirulina sp.) methyl ester Microalgae (Chlorella variabilis) Kirchneriella lunaris Microalgae (Chlorella protothecoides) Microalgae (Chlorella vulgaris) Melanothamnus afaqhusainii Euglena saingunea

882 874 831.2

4.18 5.07 5.76

867 at 15 C 772 882 880

4.38

2.44

870

3.67

21

868 at 15 C 877 at 15 C 863.7 at 15 C 880 at 15 C 875

4.545

172

4.354

160

12.4

189

23

5.22

186.53

19.12

68.80

4.90

13.49

874 877

5.06 4.72

878 879 875

Auxenochorella protothecoides Spirulina platensis Schizochytrium mangrovei Selenastrum capricornutum Staursatrum sp. Scenedesmus obliqnus Navicula sp. Phaeodactylum tricornutum Batra-Chospermum sirodotia

Cetane Number

Calorific Value (MJ/kg)

References

145

52 61.4 46.78

41.4 38.9 41.2

4.875

157

58.6

38.78

Song et al. (2013) Song et al. (2013) Hariram and Mohan Kumar (2013) Devendra et al. (2015)

75 4.43

115

44 39

Chen et al. (2012)

40.8

Song et al. (2013)

21.79 17.02

54.1 22 13

Majeed and Noureen (2016) Kings (2016)

65 52.6

29

70

45.63

Mostafa and ElGendy (2013) Hong et al. (2013)

59.6

39.4

Song et al. (2013)

16.84 9.65

61.3 57.7

38.9 39.9

Song et al. (2013) Song et al. (2013)

4.62 4.47

7.58 4.47

56.7 55.1

40.2 40.6

Song et al. (2013) Song et al. (2013)

4.79

13.27

59.5

39.4

Song et al. (2013)

11.2 Main Chemical and Physical Properties of Biofuels

FIGURE 11.1 Digital constant temperature kinematic viscosity bath (Yasin et al., 2013).

viscosity bath tester as shown in Fig. 11.1. The compound structures of the fuel influence the kinematic viscosity of fatty compounds at 40 C. This limit is imposed within the standard EN International Standard Organization (ISO) 3104:1996 (British Standard, 2009), which determines the kinematic viscosity and calculates the dynamic viscosity. Among parameters that influence the fuel viscosity are the number, chain length, position, and nature of double bonds, in addition to the character of oxygenated moieties (Martı´nez et al., 2014). The hydrocarbons in biodiesel exhibit greater viscosity than fossil fuel due to the long chain of hydrocarbon and fatty esters, but less than that of straight vegetable oil (SVO) or fat. Therefore, a few methods have been proposed, including transesterification which converts the SVO into methyl esters, microemulsification (cosolvent bonding) and pyrolysis to reduce the viscosity of the biodiesel and vegetable oils which are currently associated with higher production costs and extensive production time. However, more straightforward approaches are feasible to reduce the density and viscosity of biodiesel, for example, by blending with mineral diesel and alcohols at different proportions, as depicted in Fig. 11.2. Also, the presence of related fatty compounds with

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CHAPTER 11 Physical properties and chemical composition of biofuels

FIGURE 11.2 Alcohol blending with mineral diesel and palm biodiesel.

dibenzothiophene may reduce the viscosity of the fossil fuel. Another point that may affect the viscosity of fatty acids is variations in biofuel feedstock sources that result in changes in esters.

11.2.2 DENSITY Another essential fuel property of biofuel is density, which directly affects the characteristics of engine performance and emissions as density relates to cetane number (CN) and heating value. It is the weight of the unit volume of fluid. The requirement for the biodiesel density is in the range of 860 900 g/cm3 for the European standard EN 14214 and ASTM D6751, as listed in Table 11.4. The density is measured at 15 C according to EN ISO 12185 (British Standard, 1996), ASTM D1298, and EN 3675 test methods. An example of a measuring device for density is shown in Fig. 11.3. The minimum value of density is desirable due to the need for obtaining a maximum engine power through the fuel flow control in the injection pump. It is also needed to prevent the formation of smoke when it operates with maximum power. It is precisely shown that the rise of the biodiesel content in the fuel blend will increase the density of the fuel. Fossil fuel and biodiesel have very similar densities, but it should be considered that the density of biodiesel is affected by the sources of raw material (feedstock) in their production, as listed in Table 11.3. Also, biodiesel density depends on the composition of methyl ester and their purities. Thus, fuel density increases as chain length (the number of carbon atoms) decreases, and the number of double bonds (unsaturation degree) increases. Moreover, the presence of low-density substances may reduce the fuel density.

11.2.3 CALORIFIC VALUE Calorific value or heating value (also called energy content) is an essential factor in selecting the fuel for the engine. The fuel calorific value is defined as the

Table 11.4 ASTM D6751 and EN 14214 Standards for Biodiesel Fuels and ASTM D 975 for Petroleum Diesel Fuel (Sakthivel et al., 2018) Diesel ASTM D975 Property Specification

Units

Flash point



C

Cloud point



C

Our point



C

Cetane number Density at 15 C

kg/m3

Kinematic viscosity at 40 C Iodine number

mm2/s

Acid number Cold filter lugging point Oxidation stability Carbon residue

Test Method ASTM D975 ASTM D975 ASTM D975 ASTM D4737 ASTM D1298 ASTM D445

Limits 60 to 80 215 to 25 235 to 215 46 820 to 860

2.0 to 4.5

Biodiesel ASTM D6751 Test Method ASTM D93 ASTM D2500 ASTM D97 ASTM D613 ASTM D1298 ASTM D445

Limits 30 minimum

% m/m

EN 590

28

ASTM D2274 ASTM D4530

25 mg/L maximum 0.2 maximum

ASTM D664 ASTM D6371

ASTM D4530

Test Method

Limits

EN ISO 3679

23 to 2 12 215 to 216 47 minimum 880

1.9 to 6.0

g I2/100 g mm KOH/g  C

EN 14214

0.5 maximum Maximum 15

0.050 maximum

EN ISO 5165 EN ISO 3675/ 12185 EN ISO 3104 EN 14111 EN 14104 EN 14214 EN 14112 EN ISO 10370

3.5 to 5.0

0.5 v maximum

3h minimum 0.3 maximum (Continued)

Table 11.4 ASTM D6751 and EN 14214 Standards for Biodiesel Fuels and ASTM D 975 for Petroleum Diesel Fuel (Sakthivel et al., 2018) Continued Diesel ASTM D975 Property Specification Copper corrosion Distillation temperature Lubricity (HFRR) Sulfated ash content Ash content

Units



C

m

Test Method ASTM D130 ASTM D86 I 450

Limits Class 1 maximum 370 maximum 0.460 mm (max.) (all diesel containing less than 500 ppm-sulfur)

%mass %mass

ASTM D482 ASTM D2709

Biodiesel ASTM D6751 Test Method

Limits

ASTM D130 ASTM D1160 ASTM D6079 ASTM D874

No.3 maximum 360

ASTM D2709

0.005 vol% maximum

520 maximum 0.002 maximum

EN 14214 Test Method

Limits

EN ISO 2160

Class 1

EN ISO 3987

0.02 maximum

EN ISO 12937 EN 14105 EN 14105 EN 14105 EN 14105/ 14016 EN 14105

500 mg/ kg 0.8 maximum 0.2 maximum 0.2 maximum 0.02 maximum

100 maximum

Water and sediment Monoglycerides

0.05 maximum

%mass

Diglycerides

%mass

Triglycerides

%mass

Free glycerine

%mass

ASTM D6584

0.02 maximum

Total glycerine

%mass

ASTM D6548

0.24

0.25

Phosphorus Sulfur (S grade) Sulfur (S grade) Sulfur (S grade) Sulfur (S grade) Carbon

%mass

10

ASTM D5453

15

ppm

50

ppm

500

ppm wt%

Hydrogen

wt%

Oxygen

wt%

BOCLE scuff

g

Conductivity at ambient temperature Total contamination Boiling point

pS/m

SV

mg KOH/ g

ASTM D5453 ASTM D5453 ASTM D975 ASTM D975

ASTM D975 ASTM D2624

mg/kg 

C

HFRR, High-Frequency Reciprocating Rig.

ASTM D4951

0.001 maximum

ASTM D5453

150 maximum

ASTM D5453 ASTM PS121 ASTM PS121 ASTM PS121 ASTM PS121

500 maximum 77

EN 14107

0.001 maximum

EN 12662

24

10 maximum

50 maximum 500 maximum 87 13

2000 to 5000

12 11 .7000

50 m minimum at ambient temperature (all diesel held by a terminal or refinery for sale or distribution) ASTM D5452 ASTM D7398 ASTM D555895

24 100 to 615 370 maximum

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CHAPTER 11 Physical properties and chemical composition of biofuels

FIGURE 11.3 Portable density/gravity meter DA-130 (Yasin et al., 2013).

amount of heat being released through combustion when a unit of fuel is burnt. The calorific value of fuel contributes significant effects on fuel consumption, thermal efficiency, power output, and combustion characteristics. The heating value is expressed in the unit kJ/kg, which is the heat of combustion or the calorific value. This heating value is determined when a specific known amount of fuel is burned in the bomb calorimeter with specific controlled conditions. This calorific value follows the standard method of testing in DIN 51900-1. The calorific value property of any fuel or biofuels is mainly related to several factors including oxygen, hydrogen, carbon, and water content of the fuel, as well as fatty acid. Most alcohols have three times lower calorific value (LCV) compared to gasoline. Several studies have concluded that the causes of LCVs for alcohols are influenced by the higher oxygen content and carbon concentration (Zhang et al., 2016; Zahos-Siagos et al., 2018; He et al., 2018). A few problems are related to the LCV including miscibility and stability as well as low CN, high autoignition temperature, and poor lubricating characteristics since the shorter chains of alcohols have a LCV compared to diesel fuel. Biofuels contain methyl esters with different saturation levels, although aromatics are not present. For that reason, an increase in the chain length of fuels causes an increase in heating value but also a rise in the number of double bonds. Therefore, the increase in the calorific value results from the rise in the number of the carbon numbers and hydrogen (lower unsaturation), in addition to the increases in the ratio of these elements relative to oxygen.

11.2.4 CETANE NUMBER Each engine fuel possesses different CN, which refers to the ignition quality, a time of ignition lag of fuel to the injection in the combustion chamber. The

11.2 Main Chemical and Physical Properties of Biofuels

60 Octane 80 Octane

Slow burning

100 Octane 60 Octane Slow burning

40 Octane 20 Octane

Octane rating (gasoline)

Cetane rating (diesel)

FIGURE 11.4 Octane and cetane number ratings.

percentages of heptamethylnonane (HMN) and n-cetane in a baseline fuel blend that achieves the equivalent compression ratio of the test fuel in a cetane engine are required to determine the CN for the fuel. The CN defines the longer or shorter ignition delay of the fuel within the combustion duration. The CN is the primary indication for the quality of the diesel fuel, where high values of CN improve stability in the combustion with an increase in engine efficiency. An increase in CN value is mainly attributed to the increasing carbon chain length. Fig. 11.4 shows the ratings for the octane number and CN, which reflect the slow or fast burning of the fuel. Typical diesel engines accept CN values between 40 and 55, whereas the presence of a higher ignition delay is observed when the CN is below 38. As an example, alcohols have lower CN values (8 for ethanol and 3 for methanol) compared to mineral diesel and biodiesel fuels, which contribute to higher engine noise with longer ignition delay in the combustion period. Therefore, the CN for alcohols blending with mineral diesel mainly depends on the quality of base diesel fuel ignition, the percentage value of the alcohols in the fuel blend, and the addition of cetane improver additives. However, higher CN tends to shorten ignition delay which limits the injection of the fuel quantity during the premix combustion and complete combustion cycle. Moreover, fuels with higher CN produce a cold start, with low noise due to slow motion with lower gas emissions. These consequences may lower the cylinder pressure rise that possibly results in lower cylinder temperature. Since a higher CN of the test fuel may modify the ignition timing that increases the combustion temperature and pressure simultaneously. Most of the emission effects due to CN are largely dependent on the engine type. Older engines are significantly affected by the CN, while existing engines with more advanced combustion technology are less affected regarding emissions.

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CHAPTER 11 Physical properties and chemical composition of biofuels

11.2.5 LATENT HEAT OF VAPORIZATION The latent heat of vaporization (LHOV) for the fuel is the amount of required energy which changes the state of fuel from liquid to gas without a change in temperature. This parameter is expressed in unit kJ/kg, which is the value of heat energy to turn one kilogram of fuel from the liquid state to a gaseous vapor. A higher LHOV in the fuel could create a more considerable cooling effect and difficulties in evaporation, which reduces the cylinder temperature, thus decreasing the rate of ignition delay in the combustion chamber. Also, higher heat release and prolonged combustion duration are observed in the combustion (Wang et al., 2016). In addition, there are more improvements in NOx and soot emissions in diesel engines fueled with lower LHOV fuels due to lower combustion temperature (Wang et al., 2016; Lin et al., 2016). However, a higher LHOV causes a higher intake charge density to fill up the air volume in the combustion chamber as well as suffering cold start problems at lower loads (Vallinayagam et al., 2015). Both butanol and diesel fuels have a similar LHOV as compared to other alcohols including ethanol and methanol (Yilmaz et al., 2014). There is a strong correlation between the LHOV on fuel temperature which relates to the engine performance and emission (Gautam and Agarwal, 2015).

11.2.6 OXYGEN CONTENT A higher oxygen content in alcohols could lead to a higher in-cylinder temperature which achieves more advanced and clean fuel combustion. Most alcohols, including butanol, have higher oxygen content compared to diesel and gasoline. The oxygen content of methanol is higher than ethanol or butanol, these being 49.9, 34.8, and 34, respectively. Thus, methanol provides less dilution compared to other alcohols when blending at the same level of oxygen. Some researchers have confirmed that the relative oxygen content is mainly associated with the engine knock phenomenon (Taghizadeh-alisaraei and Rezaei-asl, 2016; Zhou et al., 2015). However, alcohols offer the highest combustion efficiency, which enhances the completeness of combustion compared to other fuels due to the higher relative oxygen content. In contrast, the oxygen content in alcohols does not contribute to fuel consumption since it has no additional energy.

11.2.7 OXIDATIVE STABILITY Oxidative stability is another essential fuel property related significantly to the engine performance of biofuel, which affects the viscosity, formation of gums, sediment, and other deposits. The literature has described further these degradation processes related to oxidative stability (Yaakob et al., 2014; Giwa et al., 2014). Besides fatty acid methyl ester (FAME) compositional properties, oxidative stability is also determined by the biodiesel storage period and storage conditions. Therefore, most biofuel samples contain additives to improve stability,

11.2 Main Chemical and Physical Properties of Biofuels

which does not affect the gross composition. In general, lack of saturation influences this oxidative stability, in which higher saturation leads to poorer stability, although the autoxidation of unsaturated fatty compounds proceeds at different rates depending upon the number and position of the double bonds. Allylic position or extraction of a hydrogen atom from a carbon adjacent to a double bond is mainly attributed to these oxidative degradation processes. Thus, the removal of the hydrogen atom and the rapid reaction of molecular oxygen together form the allylic hydroperoxide. Hence, similar reactions involving isomerization and radical chain propagation generate a number of oxidation products including alcohols, aldehydes, and carboxylic acids. FAME molecules are developed from carbon which is bonded together with two double bonds (a bis-allylic group) which is mainly attributed to the oxidative instability. Therefore, the European biodiesel standard, EN 14214, provides a separate specification for linolenic acid methyl ester, which contains two bis-allylic groups.

11.2.8 WATER CONTENT Water content is the quantity of water contained in materials, such as fuels, made from different feedstock sources. The content of water in petroleum products plays a vital role in predicting the superiority and performance of the product. If water is present, some phenomenon has occurred, which may include oxide formation, premature corrosion and wear, diminished lubrication, filter plugging, decreased effectiveness of additives, and bacterial growth that can happen in the interface between fuel and free water. The water content of a biodiesel is significantly high as compared to mineral diesel and SVO. The water content is found to be decreased when the percentage of biodiesel blend is reduced. The European standard, EN 14214, sets a limit for a water content of 0.05 wt.%, which is equivalent to 500 mg/kg, which does not exceed the water solubility in biodiesel (1500 mg/kg). Another standard, EN ISO 12937, is also used for measuring the water content in petroleum products (British Standard, 2001).

11.2.9 AUTOIGNITION TEMPERATURE The autoignition temperature for regular diesel can range from 300 C to as low as 230 C, whereas butanol has an autoignition temperature of 300 C. The autoignition temperature is applicable for vapors and gases. It can be defined as the temperature at which the vapor ignites simultaneously in a confined space. Different kinds of alcohol, including butanol and ethanol, offer similar combustion and ignition characteristics to existing known conventional fuels including diesel and gasoline. For example, methanol has a higher autoignition temperature compared to gasoline. Most alcohols including butanol have a higher autoignition temperature, which is safer for storage and transportation.

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11.2.10 ACID NUMBER The acid number is defined as the amount of potassium hydroxide (KOH) in milligrams, which is necessary to neutralize FFA contained in 1 g of oil. This acid value number is the quality indicator for the biofuels to monitor oil degradation during the storage period as imposed in the specification of the EN 14104:2003 (British Standard, 2003). This parameter is a measurement of the FFA concentration, which is mainly attributed to the type of biofuel raw feedstock sources and particular modifications. The acid number value can also be increased due to hydrolytic cleavage of methyl ester bonds during an extensive period. It is an essential indicator of vegetable oil quality as well as monitoring oil degradation during storage. The maximum value of an acid number according to ASTM D6751 is 0.5 mg KOH/g. It shows that the palm methyl ester biodiesel has relatively high potential to be degraded during an extended storage period as compared to mineral diesel. The presence of acidity is strongly related to the use of mineral acids as catalysts during the transesterification process and the existence of FFA from the acid formation from soap. A high fuel acidity number value is mainly associated with the corrosion and establishment of deposits in the engine, specifically in fuel injectors.

11.2.11 SAPONIFICATION VALUE The saponification value (SV) is the amount of KOH in milligrams which is required to saponify an oil sample of 1 g. This parameter relates to the average molecular weight, as the molecular weight increases with a decrease in the SV of oil. However, acids from triglycerides are similar to those that form the FAMEs or biodiesel. Therefore, there is no significant change in molecular weight average, and the employed feedstock sources for biodiesel have strongly influenced the SV for each produced fuel. According to the European biodiesel standard, EN 14214, as shown in Table 11.4, there is no limit for this parameter for biodiesel producers to conform to.

11.2.12 COLD FLOW PROPERTIES Biofuels under cold climatic conditions suffer condensation and gel formation, which result in crystallization of fuel. This is mainly attributed to the strong intermolecular interaction below the melting point. Thus, poor cold flow properties may cause clogging of fuel tubings and the fuel injection system, causing inferior engine operation. As the temperature decreases, the presence of the solidified wax is observed and thickens the fuel, which resists fuel flow at low temperatures. The cold flow properties are defined as the cloud point (CP), pour point (PP), and cold filter plugging point (CFPP).

11.2 Main Chemical and Physical Properties of Biofuels

11.2.12.1 Cloud point CP is the temperature at which the fuel turns into a crystallized solid at a specified temperature. The existence of solidified wax or gelling at a specific temperature explains the CP of the fuel. Therefore, the presence of the solidified wax influences the liquidity of the fuel as well as clogging the fuel filters and fuel injectors in engines. It has been discovered that prediction of the CP value can be achieved using the composition of fatty acid, by which CP increases as saturated fatty acids increase. This finding concludes that higher saturated oils from raw feedstock sources will have an inferior CP, which affects the stability of the engine operation.

11.2.12.2 Pour point The PP is the lowest temperature at which a fuel turns into a semisolid state or gel and affects the fuel’s ability to flow, and is the measurement of the fuel gelling point. The PP gives a very rough indication of when the fuel is ready to be pumped at the lowest temperature. This severe condition prevents the fuel from being pumped and injected into the combustion chamber. The PP is always lower than the CP for the fuel. Therefore, all the PPs of all methyl esters are within the standard ASTM D6751.

11.2.12.3 Cold filter plugging point The CFPP is the specific temperature at which a fuel resists flow due to its crystallization or gelling formation within a particular period. Hence, the fuel suffers a plug in the fuel filter which prevents the fuel from flowing smoothly to the fuel injection system. Therefore, this parameter has a significant influence on the engine performance and emissions. According to the European standard EN 14214, there is a limit for a moderate climate and cold climate, which gives the opportunity for each country to select one of two options whether mild or cold for seasonal classes (summer and winter) and this specification is selected based on their meteorological data. Lower purity with higher content of tri-, di- and monoglycerides as well as higher content esters are mainly attributed to a high value of CFPP at a temperature of 5 C. This is the temperature limit that allows liquid fuel to flow through a standard fuel filter in a period when the temperature decreases related to the cooling under certain conditions. CFPP considers the lowest possible temperature that gives difficulty for fuel to pass freely in a fuel injection system. Therefore, it is very crucial for cold climatic countries which suffer from clogging of engines due to the higher CFPP value. For that reason, it is suggested that for cold climatic regions, fuel composition requires more unsaturation in the oil to gain superior CFPP results.

11.2.13 IODINE VALUE The iodine value (IV) is an unsaturation degree of fuel, which represents the percentage of the mass of iodine absorbed by the mass of the sample. This parameter

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Table 11.5 The Iodine Value for Different Oils Used in Biodiesel Production (Yaakob et al., 2014) Fat/Oil

Iodine Value (mg KOH/g Oil)

Grape seed oil Alm oil Olive oil Coconut oil Alm kernel oil Cocoa butter Jojoba oil Cottonseed oil Corn oil Wheat germ oil Sunflower oil Linseed oil Soybean oil Peanut oil Rice bran oil Lard Rapeseed oil Crude fish oil Tung oil Beef fat (tallow) Canola oil

124 143 44 51 80 88 7 10 16 19 35 40 280 100 117 109 133 115 134 125 144 136 178 120 136 84 105 99.1 57.6 11.4 108.5 163.1 46.9 188 193

follows the standard testing method and the limit is regulated in EN 14111:2003. Hence, an increase in IV is mainly attributed to an increasing number of double bonds. For that reason, the distribution of fatty acid in the raw feedstock sources influences the IV value in fuels. However, the iodine value is dependent on the degree of conversion since the transesterification degree does not affect the number of double bonds. A higher IV determines the instability in the chemical substances since the double bonds are entirely reactive zones of the fuel molecules. Table 11.5 lists the IV for different oils used to produce biodiesel.

11.2.14 FLASH POINT AND VAPOR PRESSURE The flash point and vapor pressure are directly associated with the flammability or combustibility of a fuel but have a lower impact on engine combustion. The lower the flash point and the higher the vapor pressure for the fuel, the higher the tendency for the fuel to ignite. For that reason, these parameters are taken into account for the safety steps to be implemented during storage, processing,

11.2 Main Chemical and Physical Properties of Biofuels

FIGURE 11.5 Pensky Martens closed tester (Yasin et al., 2013).

transportation, and use of fuel. The content of light hydrocarbons in the fuel signifies the exact values for flash point and vapor pressure, which relates to the initial distillation point of the fuel. The flash point defines the tendency of the liquid fuel to form a mixture of air and ignite with an ignition source at the lowest temperature under the controlled conditions of a laboratory. An example of the flash point tester is shown in Fig. 11.5. In addition, the combustion point defines the lowest temperature at which that liquid fuel vaporizes to develop a flammable mixture in air. For that reason, the combustion point is usually higher than a flash point, and both points are strongly related to the safety levels for fuel handling and storage. High values of both points lessen the potential fire hazard and provide superior storage conditions with low risk. Also, the addition of a small quantity of residual alcohol in the biodiesel significantly reduces the flash point, which signifies the direct correlation between those parameters. Therefore, there is a limit for an alcohol concentration which requires less than 0.1 wt.% to allow the minimum (120oC) % flash point for biodiesel according to the standard EN 14214.

11.2.15 BOILING POINT The boiling point is a significant parameter for the fuel since it is directly linked to the fuel volatility. Any fuel that has a low boiling point will easily evaporate in

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a short period, which means that if any fuel spill occurs, there will be no indication of the spill. For example, since gasoline has a low boiling point, when a gasoline spill occurs, it will quickly evaporate and leave no liquid trace in a short time. However, as for diesel which has a high boiling point, the spill will remain for extended periods. Interestingly, alcohols have low boiling points which could lead to an increase in energy release. Volatility is the disposition of fuel to evaporate at a relatively low temperature. This parameter is mainly related to vaporization since if vaporization does not occur under a temperature range available to the engine and its fuel delivery components, the carburetor and injection system could not deliver the exact air fuel mixture which causes the fuel not to burn entirely. This adverse condition could not only result in fuel waste but also significantly increase the emission levels of carbon monoxide (CO) and hydrocarbons (HC).

11.2.16 PHASE SEPARATION TEMPERATURE Phase separation may happen when alcohol is blended with diesel or gasoline at the specific temperature operating in an engine. In this phase, the fuel blends (alcohol and diesel or gasoline) will be separated into two different liquid layers (lower density liquid will be at the bottom of the higher density liquid) due to several factors including water content temperature, type of alcohol in the blend, and baseline fuel composition. In some cases, alcohol gasoline blends consist of three components, which are alcohol, gasoline, and water due to the hygroscopic characteristic of the alcohol. Some research has agreed that the main technical problem which occurs during alcohol gasoline blending is a phase separation form with the presence of water content which causes a misfiring problem in the fuel injector. Water solubility in diesel is very low and enhances combustion.

11.2.17 BULK MODULUS The value of bulk modulus at atmospheric pressure and 40 C is calculated using both values of the speed of sound, a (m/s), and fuel density, ρ (kg/m3). This parameter relates to the decrease in compressibility with increasing length of chain and increased unsaturation degree of the methyl ester (Je˛˙zak et al., 2016). The following relation equation for bulk modulus is defined as: Bulk modulus; B 5 ρ

@P @ρ

(11.1)

where pressure, P (Pa) and fuel density, ρ (kg/m3). An improvement in power recovery using biodiesel results from the higher density and viscosity with greater bulk modulus.

11.3 Chemical Composition of Biofuels

11.2.18 SPEED OF SOUND Characterization of diesel engines, including fuel energy, utilization, and environmental indicators, relies on the method of fuel preparation and ignition of the mixture, which refers to the injection or delivery system of fuel and air mixture into the combustion chamber. Therefore, fuel injection characteristics are strongly related to both fuel properties and the type of injection system (direct or indirect injection). Therefore, to ensure the consistency of fuel injection operation, it requires a certain limit of fuel properties including speed of sound, density, viscosity, and bulk modulus. The relationship between bulk modulus, fuel density, and speed of sound is defined as: sffiffiffi B The speedof sound; a 5 ρ

(11.2)

where bulk modulus, B and fuel density, ρ (kg/m3). For that reason, these fuel properties strongly depend on variations in pressure and temperature of the fuel at a certain atmospheric pressure. The speed of sound is an essential fuel thermophysical property that strongly contributes to the different characterizations of the fuel injection and NOx emissions in diesel engines involving pressure-activated injectors. However, some of the literature describes the speed of sound for biofuels in detail. The speed of sound is estimated from the composition of FAMEs in the biofuel.

11.3 CHEMICAL COMPOSITION OF BIOFUELS Different variations in chemical composition are found in fuels for principal elements consisting of carbon (C), hydrogen (H), and oxygen (O), the ratio of C/H and the chemical formula of biofuels when they originate from different feedstocks. These variations of chemical characteristics have influenced the fuel physical properties listed in Table 11.6. These principal compositions of biofuels are strongly related to the origin of the feedstocks, such as edible and nonedible crops, waste or biomass. Biodiesels tend to have a higher oxygen content which is between 10% and 30% with the addition of being sulfur-free when compared with conventional diesel. The elemental analysis of the biofuel chemical composition was determined using GC-MS according to ASTM D6584. It can be observed that methyl laurate followed by methyl myristate, methyl palmitate, methyl palmitoleate, methyl stearate, methyl oleate, and methyl linoleate have been obtained from biofuels, depending on the type of oil. Also, Table 11.7 lists examples of biofuels with their own different chemical compositions. Therefore, the most prominent chemical composition fatty acids in palm oil were methyl palmitate (16:0) 42.80%, methyl oleate (18:0) 40.50%, and methyl linoleate (18:2) 10.10% which is equal to 93.4% methyl esters in biofuel. According to Atabani et al.

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Table 11.6 Influence of Chemical Characteristics on Physical Properties (Toscano and Maldini, 2007) Chemical Characteristics Physical Properties

Unsaturation

Chain Length

Branching

Viscosity Higher calorific value Cetane number (CN)a Flash point Oxidation stability Filtrability limit Cloud point Specific heat

2 2 2 1 2

1 1 1 1 /

1 / / /

2 2

2 1

/ /

Notes: (1) positive relationship; ( ) negative relationship; (/) no relationship. a The CN is also a function of other chemical parameters such as: the presence of aromatic compounds (the greater the number of aromatic compounds, the lower the CN will be) and the position of the double bonds (when the double bond is closer to the end, there will be a greater CN).

Table 11.7 Typical Chemical Compositions of Biofuels Type of Biofuels Methyl (Cn:b)

Molecular Weight

12:0 (Lauric) 14:0 (Myristic) 16:0 (Palmtic) 18:0 (Stearic) 18:1 (Oleic) 22:1 (Erucic) 18:2 (Linoleic) 18:3 (Linolenic) 20.0 (Arachidic) % methyl MW ester MW oil

200.32

Canola

Jatropha curcas

Sunflower

Rapeseed

Palm 0.10

228.38

1.40

1.50

1.00

256.43

6.00

11.30

6.08

3.49

42.80

254.41

2.50

17.00

3.26

0.85

4.50

284.48 282.47

66.90

12.80

16.93

64.40

40.50

47.30

73.73

22.30

10.10

8.23

0.20

101 283 894

99 268 835

280.45 278.44

14.10

312.54

4.70 90 251 713

95 259 773

100 279 874

11.3 Chemical Composition of Biofuels

FIGURE 11.6 Common fatty acid methyl ester biodiesel molecules (Yaakob et al., 2014).

(2013), the fatty acid composition is in a range, such as myristic (2 8), palmitic (24 37), stearic (14 29), oleic (40 50), and linoleic (1 5), which were changed to the number of methyl esters through the esterification and transesterification processes. Examples of common FAME biodiesel molecules are presented in Fig. 11.6. The percentage or amount of methyl esters produced was influenced by the effect of catalyst type, molar ratio of alcohol to oil, time, and temperature. Fundamentally, a proper process of esterification and transesterification appears to solve many problems associated with biofuels as a diesel fuel in modern engines. Biofuels have their advantages, such as being produced from vegetable oils that areavailable around the world, renewable, and “greener” for the environment. However, there are disadvantages to biofuel as diesel fuel which need to be overcome, including higher viscosity, lower volatility, and the reactivity of unsaturated hydrocarbon chains (Shahid and Jamal, 2008). By being properly blended, biofuel and diesel fuel with different proportions is one of the options to overcome these problems. The consensus has been made by researchers worldwide according to biofuel combustion to the environmental pollution. As a result, it was reported that a decrease is found in the emissions of HC, CO, PM emissions, and sulfur dioxide toward the use of biofuel in engines (Zhang et al., 2016; Lapuerta et al., 2008; Mosarof et al., 2015). Therefore, an awareness of the serious threats to the environment is forcing scientists to adopt biofuel to reduce pollutant emissions. This is the right time to use biofuel for CI engines, whether in pure or blended form, with a reduced amount of CO, CO2, total hydrocarbon, toxic compounds, and polycyclic aromatic hydrocarbons released to the environment.

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11.4 CONCLUSIONS AND FUTURE TRENDS Biofuels with satisfactory and constant qualities can only be achieved if the fuel properties meet biofuel quality standards. Thus, it is necessary for biofuel producers to monitor fuel quality during the biofuel production processes, from harvesting the feedstock to commercialization, including at distribution stations or pump stations. The nature and composition of raw biofuel feedstocks strongly contribute to variation in the physicochemical properties for biofuels within the production process. Since the raw biofuel feedstocks originate from different regions, therefore, each region or country has different quality requirements or standards for commercializing biofuels for the consumer. Some fuel properties including density, viscosity, CN, calorific value, IV, and oxidation stability have significant differences for a variety of biofuel feedstocks. Also, the variation in weather conditions in different cold and hot regions also contributes to these differences, which affects the fuel property regulations when the biofuels operate at low temperature. Therefore, it is not feasible to unify the different requirements for biofuel according to these significant differences in fuel properties. At the present time, there is a resilient challenge for the engine manufacturers in producing engines which compatible with different qualities of biofuels as well as facing the complexity in biofuel trades in different regions.

ACKNOWLEDGMENTS Universiti Malaysia Pahang is greatly acknowledged for technical and financial support under UMP short grant (RDU1603126).

LIST OF ABBREVIATIONS ASTM B CFPP CI CO CN CP DIN EN FAME FFA HC HCV ISO

American Society for Testing and Materials bulk modulus cold filter plugging point compression ignition carbon monoxide cetane number cloud point Deutsches Institut fu¨r Normung (German Institute for Standardization) European Standard fatty acid methyl ester free fatty acid unburned hydrocarbons higher calorific value International Standard Organization

References

IV KOH LHOV NOx P PM PP SV SVO ρ

iodine value potassium hydroxide latent heat of vaporization nitrogen oxides pressure (Pa) particulate matter pour point saponification value straight vegetable oil fuel density (kg/m3)

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