Biodiesel

Definitions. According to the American biodiesel standard ASTM D6751, biodiesel “is a fuel comprised of mono‑alkyl esters of long chain fatty acids derived from vegetable oils or animal fats, designated B100”. This kind of definition is generally accepted worldwide for biodiesel. Biodiesel is miscible with petrodiesel in all ratios. The nomenclature “BX” is commonly used in connection with blends of biodiesel and petrodiesel, with the number, X, indicating the amount of biodiesel. Thus, B2 is a blend of 2% biodiesel with 98% petrodiesel. Neither the vegetable oil or animal fat feedstock nor blends with petrodiesel should be called biodiesel.

The major components of vegetable oils and animal fats are triacylglycerols (triglycerides). Chemically, triacylglycerols are esters of fatty acids with glycerol (1,2,3-propanetriol; glycerol is often also called glycerine). The triacylglycerols of vegetable oils and animal fats typically contain several different fatty acids. Different fatty acids can be attached to one glycerol backbone. The different fatty acids that are contained in the triacylglycerols comprise the fatty acid profile (or fatty acid composition) of the vegetable oil or animal fat. The fatty acid profile is probably the most important parameter influencing the properties of a vegetable oil or animal fat because different fatty acids can have very different physical and chemical properties. Fatty acid profiles of some common vegetable oils are given in Table 1 (tables can be found at the end of this text).

Standards for biodiesel have been or are being established in many countries around the world. Probably the most commonly referred to standards are the American biodiesel standard ASTM D6751 (ASTM = American Society for Testing and Materials) mentioned briefly above and the European biodiesel standard EN14214. Other standards exist also, for example, in Europe the standard EN 14213 specifies biodiesel when used as heating oil and in the United States recently a standard for blends of B6 to B20 has been approved by ASTM.

Biodiesel production. Biodiesel is produced from vegetable oils or other triacylglycerol-containing materials by means of a transesterification reaction. In the transesterification reaction, the vegetable oil or animal fat is reacted in the presence of a catalyst (usually a base such as NaOH or KOH; however, alkoxides such as CH ₃ONa are preferable due to reduced tendency to form water; approximately 1 wt.-% catalyst relative to the oil or fat) with an alcohol (usually methanol) to give the corresponding alkyl esters (when using methanol, the methyl esters) of the fatty acid mixture that is found in the parent vegetable oil or animal fat. The transesterification reaction is depicted below in Figure 1. The transesterification reaction formally requires a molar ratio of alcohol to oil of 3:1 as shown in Figure 1. However, in practice a molar ratio of 6:1 needs to be applied in order for the reaction to proceed properly to high yield. The reaction system should be as free of water and free fatty acids as possible. The transesterification usually requires about 1 h at normal pressure with the reaction temperature at 60-65EC (for methanol). The transesterification reaction starts as a two-phase system of immiscible reactants (oil or fat and alcohol) and ends as a two-phase system (ester product and glycerol; with higher alcohols there may be a tendency to form emulsions). Approximate weights of the reactants in the transesterification process (for methyl esters) are also given in Figure 1.

The transesterification reaction occurs stepwise, proceeding from the triacylglycerols (triglycerides) as found in the oil or fat via di- and monoacylglycerols. Glycerol is a co-product of biodiesel production. Thus all these materials, the various acylglycerols (glycerides) and glycerol itself, are present in minor amounts in biodiesel. Their content in biodiesel is limited by biodiesel standards. Residual catalyst is also a minor component (contaminant) of biodiesel. How the conversion to biodiesel is carried out is also affected by the nature of the feedstock. Feedstocks with high free fatty acid content, for example used cooking oils or specific vegetable oils, require acid pretreatment before carrying out the conventional base-catalyzed transesterification. The major reason that vegetable oils and animal fats are transesterified to alkyl esters (biodiesel) is that the kinematic viscosity of the biodiesel is much closer to that of petrodiesel (usually in the range of 2-3 mm ²/s). The high viscosity of untransesterified oils and fats leads to operational problems in the diesel engine such as deposits on various engine parts. While there are engines and burners that can use untransesterified oils, the vast majority of engines require the lower viscosity fuel. Typical kinematic viscosity values of vegetable oils and biodiesel (in form of methyl esters) are also shown in Figure 1.

Figure 1. The transesterification reaction. R is a mixture of various fatty acid chains. Methanol (R′ = CH ₃) is the most commonly used alcohol for biodiesel production.

History. The diesel engine was developed in the 1890's by Rudolf Diesel (1858-1913). His goal was to develop an engine that was more efficient than the steam engine. The first recorded use of a vegetable oil, peanut oil, occurred in 1900 at the Paris World Expo in a diesel engine produced by the French Otto Company. The reason was that the French Government at the time was interested in providing a local source of fuel for its African colonies. Diesel himself described experiments related to the use of vegetable oil fuels and appeared supportive of the concept. The rather modern theme of energy independence can be observed throughout the technical literature from the 1920's through the early 1950's dealing with vegetable oil fuels. The apparently first description of what is today termed biodiesel is contained in the Belgian patent 422,877 granted August 31, 1937 to C.G. Chavanne. In the summer of 1938 a passenger bus operating on this fuel (ethyl esters of palm oil) served the commercial line between Brussels and Leuven (Louvain), according to an extensive report published in Bulletin Agricole du Congo Belge in 1942. In the 1940’s and 1950's the age of inexpensive petroleum commenced. With the oil embargo of 1973 and rising petroleum prices, the interest in alternatives to petroleum-based fuels was renewed, including vegetable oils and their derivatives. Vegetable oil esters (derived from sunflower oil) were then described again in 1980. Recent years have seen significant growth in research, the biodiesel industry, development of standards and the search for additional feedstocks.

Sources of biodiesel. Biodiesel can be produced from a great variety of feedstocks. These feedstocks include most common vegetable oils (soybean, cottonseed, palm, peanut, rapeseed / canola, sunflower, safflower, coconut, etc.) and animal fats (tallow) as well as waste oils (used frying oils, etc.). Which feedstock is used depends largely on geography. Thus soybean oil is the major biodiesel feedstock in the United States, rapeseed oil is the major source in Europe, and palm oil is of significance for countries with tropical climate. It may be noted that rapeseed oil as used in Europe and canola oil as used in North America are very similar regarding their fatty acid profiles. Depending on the origin and quality of the feedstock, changes to the production process may be necessary. Since not enough vegetable oils are available to supply the whole diesel market with biodiesel, the search for other feedstocks has become very significant. Besides the “classic” feedstocks mentioned above, other sources or potential sources of biodiesel that are being investigated and / or used include animal fats, used cooking or frying oils, greases, algae, as well as “non-classic” feedstocks such as castor oil and less common vegetable oils such as jatropha. Interest in some “non-classic” inedible feedstocks such as jatropha exists because they avoid the food vs. fuel issue. Not all biodiesel fuels derived from these feedstocks are suitable for application in the neat form in all countries because of issues such as poor cold flow properties. This is another reason why blends of biodiesel and petrodiesel are of interest.

Methanol is commonly used as alcohol for producing biodiesel because it is the least expensive alcohol in most countries, although other alcohols, for example ethanol or iso-propanol, may afford a biodiesel fuel with better fuel properties. Often the resulting product is also called FAME (fatty acid methyl esters) instead of biodiesel. Although other alcohols can by definition give biodiesel, many now existing standards are designed in such a fashion that only methyl esters can be used as biodiesel when observing the standards.

Comparison to petrodiesel. Biodiesel has several distinct advantages compared to petrodiesel besides being fully competitive with petrodiesel in most technical aspects:

• Derived from a renewable domestic resource, thus reducing dependence on and preserving petroleum as well as enhancing the agricultural economy.

• Biodegradability.

• Reduces most exhaust emissions (with the exception of nitrogen oxides, NO _x ).

• Higher flash point leading to safer handling and storage.

• Excellent lubricity. This fact is steadily gaining significance with the advent of low- sulfur petrodiesel fuels, which have significantly reduced lubricity. Adding biodiesel at low levels (1-2%) restores the lubricity.

Some problems associated with biodiesel are its inherent higher price, which in many countries is offset by legislative and regulatory incentives or subsidies in form of reduced excise taxes, slightly increased NO _x exhaust emissions, stability when exposed to air (oxidative stability), and cold flow properties which are especially relevant in North America. The higher price can also be (partially) offset by the use of less expensive feedstocks which has sparked the interest in materials such as waste oils (for example, used frying oils).

Fuel properties. Esters of fatty acids are the major components of biodiesel and therefore have significant influence on fuel properties. Some common fatty acid methyl esters are listed in Table 1 together with some fundamental fuel properties. Important fuel properties determined by the fatty acid profile of a biodiesel fuel are the cetane number, oxidative stability, cold flow and kinematic viscosity. It must be noted, however, that for some properties, especially oxidative stability and cold flow, relatively small amounts of minor components (contaminants) can have a significant effect of fuel properties.

Cetane number and combustion. The cetane number is a dimensionless descriptor related to the tendency of a fuel to ignite in the combustion chamber of a diesel engine. The shorter the ignition delay time, i.e., the time between injection of the fuel into the combustion chamber and onset of ignition, the greater the tendency of the fuel to ignite leading to a higher the cetane number and vice versa. Alkanes in the C ₁₀ to C ₁₆ range are nearly “ideal” components of petrodiesel fuels. Indeed, hexadecane (trivial name cetane; hence the cetane scale) has a short ignition delay time and has an assigned cetane number of 100. A highly branched alkane, 2,2,4.4,6,8,8-heptamethylnonane, has a long ignition delay time and has an assigned cetane number of 15. Saturated long-chain esters such as methyl palmitate or methyl stearate also have high cetane numbers. Thus the cetane scale can also be used to clarify why fatty esters can be used as diesel fuel, namely the structural similarity of fatty esters with alkanes imparted by the long hydrocarbon chain. It is important to note that the cetane number of fatty esters decreases with increasing unsaturation and shorter chain lengths. This is reflected in the cetane numbers given for some esters in Table 1. Values for some biodiesel fuels are given in Table 3. Minimum cetane numbers are specified in biodiesel standards, namely 47 in ASTM D6751 and 51 in EN 14214. Exhaust emissions are the products of combustion of a fuel in an engine. While the ideal products of combustion of a hydrocarbon are water and CO ₂, small amounts of other species are formed because combustion, like other reactions, is not completely ideal. These other species often pose health hazards and therefore are limited by regulations. The species that are limited are oxides of nitrogen (NO _x), particulate matter (PM), hydrocarbons (HC), and carbon monoxide (CO). Numerous studies have shown that biodiesel reduces PM, HC and CO when compared to petrodiesel, however, NO _x exhaust emissions are slightly increased. NO _x exhaust emissions are significant because they are precursors of ozone in urban smog. The effect on exhaust emissions when blending biodiesel with petrodiesel is approximately linear to the blend level. Figure 2 depicts regulated exhaust emissions of biodiesel vs. petrodiesel and Figure 3 shows these emissions for B20 blends vs. petrodiesel.

Figure 2. Comparison of regulated exhaust emissions of biodiesel vs. petrodiesel (Source: USEPA report 420-P-02-001).

Figure 3. Comparison of regulated exhaust emission of B20 vs. petrodiesel (Source: USEPA report 420-P-02-001).

Kinematic viscosity. As mentioned above, kinematic viscosity is a major reason why vegetable oils are transesterified to biodiesel. The high viscosity of vegetable oils causes poorer atomization of the fuel upon injection into the combustion chamber of the engine, leading to increased formation of engine deposits. Kinematic viscosity increases with chain length and increasing saturation of a fatty acid chain. Some data for neat esters are given in Table 1 and values for biodiesel fuels are presented in Table 3. Kinematic viscosity is also specified in biodiesel standards, 1.9-6.0 mm ²/sec in ASTM D6751 and 3.5-5.0 mm ²/sec in EN 14214. The tighter specification in EN 14214 can lead to biodiesel derived from certain feedstocks not being useable as fuel in the neat form.

Cold flow. One of the major technical problems facing biodiesel is its relatively poor cold properties. The reason is the presence of saturated fatty esters with relatively high melting points (see Table 2), leading to relatively high cloud and pour points for biodiesel (see Table 3). The cold filter plugging point is another test method for biodiesel cold flow used in the European standard EN 14214. Cold flow is a property that can be influenced by minor components such as mono- and diacylglycerols present in the biodiesel, which have high melting points and limited solubility in biodiesel. Other materials known to cause cold flow problems are sterol glucosides, which also have high melting points. Additives, winterization or esters other than methyl, for example ethyl or iso-propyl, are methods for improving cold flow. Cold flow is contained in biodiesel standards as a “soft” specification due to the fact that requirements differ. Therefore, in ASTM D6751 a “report to customer” is prescribed and in EN 14214 the requirements differ depending on the geographical location and time of year.

Oxidative stability. Oxidative stability is another major technical problem facing biodiesel. Especially during prolonged storage, biodiesel fuel quality can deteriorate when the fuel is in contact with oxygen in the air. Unsaturated fatty acid chains, especially those of linoleic and linolenic acids are responsible for this autoxidation process. Therefore the fatty acid profile of the stored biodiesel is an important factor influencing storage stability. Certain extraneous materials such as metals can catalyze the process as well as factors such as elevated temperature and presence of light can affect it. Appropriate storage conditions such as minimizing exposure to air, low temperature and exclusion of light can prolong the shelf life of biodiesel. Antioxidant additives such as butylated hydroxytoluene (BHT) or tert.-butylhydroquinone (TBHQ) also extend the time biodiesel can be stored without degradation. However, with time the antioxidants are consumed and eventually the oxidation of biodiesel will commence. Thus, antioxidants are oxidation delayers, not oxidation preventers.

Lubricity. Historically, diesel engines had relied on the inherent lubricity of petrodiesel fuel to lubricate parts such as injectors and fuel pumps. Modern ultra-low sulfur diesel fuels (ULSD), however, exhibit poor lubricity. Neat biodiesel, on the other hand, possesses excellent lubricity. This feature has led to blends such as B2 becoming “popular” since at such blend level biodiesel restores the lubricity to ULSD. It may be noted that some minor components (contaminants) of biodiesel such as monoacylglycerols and free fatty acids possess excellent lubricity and are to a large part responsible for any lubricity of low level blends such as B1.

Commercial aspects. One of the major issues facing biodiesel is the fact that there is not enough vegetable oil or other triacylglycerol-containing feedstock to replace the whole petrodiesel market with biodiesel. This has led to the search for potential feedstocks, as mentioned above. Algae are claimed to have the greatest yield potential per unit of land for biodiesel production but the issue is fraught with problems of its own and any significant supply of algal oils is still in the future. Nevertheless, the issue of dwindling petroleum reserves coupled with rising energy prices has spurred the development and use of biodiesel, although the prices of biodiesel appear to move in tandem with petroleum prices. The issue of biodiesel use is also being affected by the issues of food vs. fuel and carbon footprint. These relate to, for example, the use of edible oils for non-edible purposes such as fuel or deforestation of rain forest for the sake of planting agricultural commodities such as soy or palm. However, as several studies have shown, biodiesel derived from “classic” vegetable oil feedstocks has a positive energy balance of approximately 3:1.

Biodiesel production has risen significantly in many countries around the world, including the United States and Europe. However, other countries such as Brazil and Malaysia have developing biodiesel industries also. In the United States, biodiesel production has increased from about 500,000 gallons (approximately 1,667 metric tons) in 1999 to around 450,000,000 gallons (approximately 1,500,000 metric tons) in 2007 (source: National Biodiesel Board). In Europe, total biodiesel production in 2007 was approximately 5,713,000 metric tons, with Germany producing 2,890,000 tons, France 872,000 tons, Italy 363,000 tons, Austria 267,000 tons, Portugal 175,000, Spain 168,000 tons, Belgium 166,000 tons, UK 150,000 tons, Greece 100,000 tons with sixteen other countries producing less than 100,000 (source: European Biodiesel Board). The total production capacity for biodiesel in Europe in 2008 is projected to be approximately 16,000,000 tons. The production and use of biodiesel are strongly affected by issues such as commodity prices, i.e. prices of petroleum and biodiesel feedstocks, taxation as well as subsidies and other incentives. Legislation and regulations affecting these issues are undergoing almost constant change.

Table 1. Fatty acid profiles of some common vegetable oils or animal fats. ^a)

a) Source: The Biodiesel Handbook and references therein.

b) Definition of acronyms: 12:0 = Methyl laurate (methyl dodecanoate); 16:0 = Methyl palmitate (methyl hexadecanoate); 18:0 = Methyl stearate (methyl octadecanoate); 18:1= Methyl oleate (methyl 9( Z)-octadecenoate); 18:2 = Methyl linoleate (methyl 9,12( Z,Z)-octadecenoate); 18:3 = Methyl linolenate (methyl 9,12,15( Z,Z,Z)-octadecenoic).

Table 2. Properties of some common fatty acid methyl esters. ^a)

a) Source: G. Knothe, Energy & Fuels 22 (2), 1358-1364 (2008). Oxidative stability by Rancimat test (standard EN 14112).

Table 3. Fuel-related properties of some vegetable oil esters. ^a)

a) Source: The Biodiesel Handbook and references therein.