Industrial Oil Crops. http://dx.doi.org/10.1016/B978-1-893997-98-1.00002-6 15
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Biodiesel and Its Properties
Gerhard Knothe
U.S. Department of Agriculture, Agricultural Research Service, National Center for Agricultural Utilization Research, Peoria, IL, United States
INTRODUCTION
Biodiesel (Knothe et al., 2010; Mittelbach and Remschmidt, 2004) is a biogenic alternative to conventional diesel fuel (DF) obtained from vegetable oils, ani- mal fats, or other materials consisting largely of triacylglycerols (triglycerides).
Reacting an oil or fat with an alcohol, usually methanol, in the presence of a catalyst, commonly sodium methoxide, affords the corresponding mono- alkyl esters. These mono-alkyl esters are defined as biodiesel. Glycerol is obtained as a co-product. Fig. 2.1 depicts the principle of the transesterifica- tion reaction.
While the suitability of any material as fuel, including biodiesel, can be influenced by contaminants arising from production or other sources, the nature of the major fuel components ultimately determines the fuel proper- ties. Some of the properties included in standards can be traced to the struc- ture of the fatty esters comprising biodiesel. Since biodiesel consists of fatty acid esters, not only the structure of the fatty acids but also that of the ester moiety derived from the alcohol can influence the fuel properties of biodiesel.
Furthermore, the aforementioned mono-alkyl esters that comprise biodiesel are a mixture corresponding in its fatty acid profile to that of the parent oil or fat from which it is produced with each ester component contributing to the properties of the fuel.
The properties of a biodiesel fuel that are determined by the structure of its component fatty esters include ignition quality, cold flow, oxidative stability, viscosity, and lubricity. The present work discusses the influence of the structure of fatty esters on these properties. Not all of these properties have been included in biodiesel standards, although all of them are essen- tial to the proper functioning of the fuel. This article begins, however, with brief summaries on the historical background, production, and analysis of biodiesel.
HISTORICAL BACKGROUND
The use of vegetable oils or their derivatives in diesel engines is nearly as old as the diesel engine itself. While the engine was developed in the 1890s by Rudolf Diesel (1858–1913), the apparently first recorded run of a diesel engine using a vegetable oil as fuel, peanut oil, occurred at the World Exposition in Paris in 1900 at the request of the French government. The French government at that time was interested in developing alternative fuels for its European colonies that could be obtained or produced locally. Diesel himself made remarks supporting the concept of using vegetable oils as fuel in later writings as well as carry- ing out some experiments himself and describing the first test in 1900 (Diesel, 1912). Through the 1940s, numerous references can be found that describe the use of vegetable oils as diesel fuel. Probably under current aspects of using biodiesel, the most remarkable document in this connection is a Belgian patent granted in 1937 to C.G. Chavanne of the University of Brussels in which the synthesis of ethyl esters of vegetable oils for fuel use is discussed (Chavanne, 1937). A related publication details the production and use of ethyl esters in the Belgian Congo and in Belgium, including the probably first use of biodiesel in commercial buses (van den Abeele, 1942). Interest in alternative fuels waned after approximately 1945 and this lack of interest continued into the 1970s. In the United States, a few reports from The Ohio State University and Georgia Institute of Technology, after approximately the end of World War II, through the 1970s, apparently little interest existed in vegetable oil–based fuels, or alter- native fuels in general.
The energy crises of the 1970s then spawned renewed interest in alternative fuels, among them vegetable oil–based fuels. In this connection, the alkyl esters of vegetable oils were “rediscovered” with the probably first reports dealing with
FIGURE 2.1 The transesterification reaction. R1, R2, and R3 is a mixture of various fatty acid chains. The alcohol used for producing biodiesel is usually methanol (RN]CH3).
the production of methyl esters of sunflower oil (Bruwer et al., 1980a,b). A vari- ety of legislative and regulatory incentives in many countries around the world assisted in bringing biodiesel to market beginning in the mid to late 1990s and accelerating a few years later. Most biodiesel, however, is used in blends with petrodiesel, these blends often being termed “BXX” in which “XX” denotes the percentage level of biodiesel blended with the petrodiesel. The successful introduction and commercialization of biodiesel in many countries worldwide has been accompanied by the development of standards to ensure high product quality and instill user confidence. The first biodiesel standard was developed in Austria in 1991 as ÖNORM C1190. Major biodiesel standards that have served as guidelines for other standards around the world are ASTM D6751 (ASTM, American Society for Testing and Materials) and the European standard EN 14214 (CEN). Table 2.1 is a list of standards that have been mostly developed for application to biodiesel. Table 2.2 contains fuel property specification from the standards ASTM D6751 and EN 14214.
BIODIESEL PRODUCTION
The most common process for the production of biodiesel is the transesteri- fication of the vegetable oil (or other oil or fat) with an alcohol, most commonly methanol, in the presence of a catalyst. Methanol is the most commonly used alcohol as it is the least expensive alcohol. As mentioned, glycerol is obtained as a co-product. This transesterification reaction with methanol is straightfor- ward as it can be carried out at 60–65°C, slightly below the boiling point of methanol, with a molar ratio of alcohol:oil = 6:1 (Freedman et al., 1984). In case of the use of an alcohol other than methanol, the reaction temperature is raised accordingly. The reaction is usually complete after 1 h, with most of the conversion occurring during the first 10 min. Base-catalyzed transesterification is usually considerably faster than acid-catalyzed transesterification (Freedman et al., 1984). Catalyst sodium methoxide (or other alkoxide corresponding to the alcohol used) is preferable compared to hydroxide because the water-forming reaction
XOH + ROH → ROX + H2O (X = Na, K; R = alkyl)
is not possible when using methoxide. The transesterification reaction yield- ing biodiesel needs to be as free of water as possible to prevent the formation of salts of fatty acids (soap). Likewise, the free fatty acids should be kept to a minimum. For feedstocks high in free fatty acids, notably often used cooking oils or animal fats but also some vegetable oils, a two-step conversion is suitable in which the free fatty acids are esterified first by acid-catalyzed esterification and then the triacylycerols transesterified to the alkyl esters (Canakci and Van Gerpen, 1999).
The transesterification reaction mixture to give methyl esters consists of two phases at the outset of the reaction, methanol and vegetable oil, and
TABLE 2.1 Major Standards (ASTM and EN) Related to Biodiesel
D975 Diesel Fuel EN 590 Diesel Fuel
D6584 Total monoglycerides, total diglycerides, total triglycerides, and free and total glycerin in B-100 by Gas Chromatography
EN 141078 FAME in diesel fuel by IR
D6751 Biodiesel blend stock (B100) for middle distillate fuels
EN 14103 Ester and
linolenic acid ester content D7321 Particulate
contamination of B100 and biodiesel blends by laboratory filtration
EN 14104 Acid value
D7371 Biodiesel (FAME) content in diesel fuel by mid infrared spectroscopy
EN 14105 Free and total glycerol, acylglycerols by GC
D7398 Boiling range distribution of FAME in the range from 100 to 615°C by GC
EN 14106 Free glycerol in the range 0.005–
0.07% (m/m)
D7462 Oxidation stability of B100 and blends of biodiesel with diesel fuel
EN 14107 Phosphorus by ICP
D7467 B6–B20 blends EN 14108 Sodium content
by AAS D7501 Determination of fuel
filter blocking potential of B100 by cold soak filtration test (CSFT)
EN 14109 Potassium content by AAS
D7591 Free and total glycerin in biodiesel blends by anion exchange chromatography
EN 14110 Methanol content by GC
D7797 FAME content in jet fuel using flow analysis by FTIR
EN 14111 Iodine value
D7806 FAME content of a blend of biodiesel and petroleum-based diesel Fuel oil using mid-IR
EN 14112 Oxidation
stability
two phases at the end, methyl esters and glycerol, although in case of higher esters there exists an increased tendency toward the formation of emulsions.
The methyl ester and glycerol can generally be easily separated after the reaction. The methyl ester phase can be washed with water (temperature of the wash water may be slightly elevated) to remove remaining glycerol and catalyst. Minor components remaining in the biodiesel after comple- tion of all steps include starting material (triacylglycerols), intermediates (di- and monoacylglycerols), glycerol, catalyst, and extraneous materials.
These minor components can significantly influence biodiesel fuel proper- ties, probably the most prominent examples being cold flow and oxidative stability.
The development of other catalysts or catalytic systems as well as varia- tions of the transesterification reaction has received considerable attention by the research community in recent years. Countless other catalysts, the enu- meration of which transcends this article, are to address issues with the con- ventional transesterification reaction such as increasing tolerance for water, reducing problems associated with the use of other alcohols or easy removal of the catalyst. Most notably, heterogeneous catalysts or catalyst systems as well as enzymatic catalysis have been researched extensively. Some review articles that deal with conventional transesterification or alternative catalytic systems are Ma and Hanna (1999), Lotero et al. (2005), Di Serio et al. (2008), Fjerbaek et al. (2009), Andrade et al. (2011), Abbaszaadeh et al. (2012), He and Van Gerpen (2012b), Kouzu and Hidaka (2012), Motasemi and Ani (2012), Veljkovic et al. (2012), Davison et al. (2013), Hama and Kondo (2013), Kralisch et al. (2013), Narwal and Gupta (2013), and Ramachandran et al. (2013).
EN 14213 FAME as heating fuels
EN 14331 FAME analysis by LC/GC
EN 14538 Ca and Mg by
ICP-OES
EN 15751 Modified
oxidation stability EN 15779 Determination of polyunsaturated (≥4 double bonds) in FAME by GC TABLE 2.1 Major Standards (ASTM and EN) Related to Biodiesel—cont’d
Other approaches to modifying the transesterification reaction include the use of a solvent to achieve a one-phase system or in situ transesterification in which the oil is not removed from the original feedstock, rather the feedstock containing the oil directly subjected to the transesterification reaction. Such procedures entail changes to the reaction such as a higher stoichiometric ratio
TABLE 2.2 Fuel Property Specifications in Biodiesel Standards
Property Unit ASTM D6751 EN 14214
Cetane number – 47 min 51 min
Kinematic viscosity at 40°C
mm2/s 1.9–6.0 3.5–5.0
Oxidation stability at 110°C
h 3 min 8 min
Cloud point °C Report –
Cold soak filterability
s 200 or 360 –
Density kg/m3 – 860–900
Flash point °C 93 min 101 min
Free glycerol % Mass 0.02 0.02
Total glycerol % Mass 0.24 0.25
Linolenic acid content
– 12% (m/m)
FAME ≥4 double bonds
– 1% (m/m)
Sodium μg/g (ppm) 5 max, combined 5 mg/kg combined
Potassium
Calcium μg/g (ppm) 5 max, combined 5 mg/kg combined
Magnesium
Sulfur % Mass (ppm) 0.0015 or 0.05a 10 mg/kg
Phosphorus % Mass 0.001 max 4 mg/kg
Acid value mg KOH/g 0.5 max 0.5 max
Water content % Volume 0.05 max 500 mg/kg
Alcohol content % Mass 0.2 or flash point 130°C max
0.2% (m/m)
aDepends on grade of petrodiesel fuel to be blended with.
