Biodiesel fuels are methyl or ethyl esters derived from a broad variety of renewable sources such as vegetable oil, animal fat and cooking oil. Esters are oxygenated organic compounds that can be used in compression ignition engines because some of their key properties are comparable to those of diesel fuel. “Rape Methyl Ester” diesel (“RME”), derived from rapeseed oil, is the most common biodiesel fuel available in Europe. “Soy Methyl Ester” diesel (“SME” or “SOME”), derived from soybean oil, is the most common biodiesel in the United States.  Collectively, these fuels are sometimes referred to as “Fatty Acid Methyl Esters” (“FAME”).

Biodiesel fuels are produced by a process called transesterification, in which various oils (triglycerides) are converted into methyl esters through a chemical reaction with methanol in the presence of a catalyst, such as sodium or potassium hydroxide. The by-products of this chemical reaction are glycerols and water, both of which are undesirable and need to be removed from the fuel along with traces of the methanol, unreacted triglycerides and catalyst. Biodiesel fuels naturally contain oxygen, which must be stabilized to avoid storage problems.


Biodiesel is a light to dark yellow liquid. It is practically immiscible with water, has a high boiling point and low vapor pressure. Typical methyl ester biodiesel has a flash point of ~ 150°C, making it rather non-flammable. Biodiesel has a density of ~ 0.8 g/cm3, less than that of water. Biodiesel uncontaminated with starting material can be regarded as non-toxic. Biodiesel has a viscosity similar to petrodiesel, the industry term for diesel produced from petroleum. It can be used as an additive in formulations of diesel to increase the lubricity of pure Ultra-Low Sulfur Diesel (ULSD) fuel, although care must be taken to ensure that the biodiesel used does not increase the sulfur content of the mixture above 15 ppm.

Biodiesel offers many advantages:   • It is renewable   • It is energy efficient   • It displaces petroleum derived diesel fuel   • It can be used in most diesel equipment with no or only minor modifications   • It can reduce global warming gas emissions   • It can reduce tailpipe emissions, including air toxics   • It is nontoxic, biodegradable, and suitable for sensitive environments   • It is made from either agricultural or recycled resources.

Transesterification chemistry

A reaction scheme is as follows:


Animal and plant fats and oils are typically made of triglycerides which are esters of free fatty acids with the trihydric alcohol, glycerol. In the transesterification, the alcohol is deprotonated with a base to make it a stronger nucleophile. Commonly, ethanol or methanol is used. As can be seen, the reaction has no other inputs than the triglyceride and the alcohol.

Normally, this reaction will proceed either exceedingly slowly or not at all. Heat, as well as an acid or  base are used to help the reaction proceed more quickly. It is important to note that the acid or base are not consumed by the transesterification reaction, thus they are not reactants but catalysts.Almost all biodiesel is produced using the base-catalyzed technique as it is the most economical process requiring only low temperatures and pressures and producing over 98% conversion yield (provided the starting oil is low in moisture and free fatty acids). For this reason only this process will be described below.

Batch proccess

  • Preparation: care must be taken to monitor the amount of water and free fatty acids in the incoming biolipid (oil or fat). If the free fatty acid level or water level is too high it may cause problems with soap formation (saponification) and the separation of the glycerin by-product downstream.

  • Catalyst is dissolved in the alcohol using a standard agitator or mixer.

  • The alcohol/catalyst mix is then charged into a closed reaction vessel and the biolipid (vegetable or animal oil or fat) is added. The system from here on is totally closed to the atmosphere to prevent the loss of alcohol.

The reaction mix is kept just above the boiling point of the alcohol (around 70 °C, 158°F) to speed up the reaction though some systems recommend the reaction take place anywhere from room temperature to 55 °C (131°F) for safety reasons. Recommended reaction time varies from 1 to 8 hours; under normal conditions the reaction rate will double with every 10 °C increase in reaction temperature. Excess alcohol is normally used to ensure total conversion of the fat or oil to its esters.

  • The glycerin phase is much more dense than biodiesel phase and the two can be gravity separated with glycerin simply drawn off the bottom of the settling vessel. In some cases, a centrifuge is used to separate the two materials faster.

  • Once the glycerin and biodiesel phases have been separated, the excess alcohol in each phase is removed with a flash evaporation process or by distillation. In other systems, the alcohol is removed and the mixture neutralized before the glycerin and esters have been separated. In either case, the alcohol is recovered using distillation equipment and is re-used. Care must be taken to ensure no water accumulates in the recovered alcohol stream.

  • The glycerin by-product contains unused catalyst and soaps that are neutralized with an acid and sent to storage as crude glycerin (water and alcohol are removed later, chiefly using evaporation, to produce 80-88% pure glycerin).

  • Once separated from the glycerin, the biodiesel is sometimes purified by washing gently with warm water to remove residual catalyst or soaps, dried, and sent to storage.

In - Line Reactor  (Ultra- and High Shear Reactors)

Ultra- and High Shear in-line reactors allow to produce bio-diesel continuously, therefore, reduces drastically production time and increases production volume. Ultra – Shear, up to three sets of rotor and stator which converts mechanical energy to high tip speed, high shear stress, high shear-frequencies. Droplet size range expected in the low micron until sub-micron range after one pass.

The reaction takes place in the high - energetic shear zone of the Ultra- and High Shear mixer by reducing the droplet size of the immiscible liquids such as Oil or Fats and Methanol. Therefore, the smaller the droplet size the larger the surface area the faster the catalyst can react.

Ultra- and High Shear mixers are used for the pre-treatment of crude vegetable oil or animal fats such as:

   - the dispersion of citric/phosphoric acid and crude oil within the de-gumming process to remove Phosphatides (Gums)

   - the dispersion of caustic and de-gummed oil within the neutralization process to remove FFA (Free Fatty Acid)

Furthermore, for the transesterification of pre-treated vegetable oil or animal fats into Methyl Ester. Finally, for the water wash process of Methyl Ester.

Biodiesel Specifications

Europe’s Committee for Standardization (“CEN”) is in the final stages of setting a technical standard for biofuels to be referred to as EN 14214. ASTM International approved a specification for biodiesel referenced as D 6751. In addition, German authorities have issued a provisional specification for FAME under DIN 51606. The European specifications include more stringent limits for sulfur and water, as well as a test for oxidation stability, which is absent from the current ASTM specification. Depending on the biomass feedstock and the process used to produce the fuel,B100 fuels should meet the requirements of either ASTM D 6751 or an approved European specification, such as DIN 51606 or EN 14214. The standards ensure that the following important factors in the fuel production process are satisfied:

  • Complete reaction
  • Removal of glycerin
  • Removal of  catalyst
  • Removal of  alcohol
  • Absence of free fatty acids
  • Low sulfur content.

Basic industrial tests to determine whether the products conform to the standards typically include gas chromatography, a test that verifies only the more important of the variables above. More complete tests are more expensive. Fuel meeting the quality standards is very non-toxic, with a toxicity rating (LD50) of greater than 50 mL/kg.

Technical Specifications:


Biodiesel  EN 14214


Biodiesel (DIN51606)


Density @ 15°C  (g/cm³)



Viscosity @ 40°C  (mm²/s)



Flashpoint  (°C)       



Sulphur  (% mass)



Sulphated Ash  (% mass)



Water (mg/kg)



Carbon Residue  (% weight)



Total Contamination  (mg/kg)



Copper Corrosion  3h/50°C

Class 1

Class 1

Cetane Number



Methanol  (% mass)



Ester Content  (% mass)



Monoglycides  (% mass)



Diglyceride (% mass)



Tridlyceride  (% mass)



Free Glycerol (% mass)



Total Glycerol  (% mass)



Iodine Number



Phosphor  (mg/kg)



Alcaline Metals  Na, K (mg/kg)



Acid value  (mgKOH/g)



Oxidative stability  (hrs, 110°C)

>6 hours



* country specific


Biodiesel Blends

Biodiesel is produced in a pure form (100% biodiesel fuel referred to as “B100” or “neat biodiesel”) and may be blended with petroleum-based diesel fuel. Such biodiesel blends are designated as BXX, where XX represents the percentage of pure biodiesel contained in the blend (e.g., “B5,” “B20”). Much of the world uses a system known as the "B" factor to state the amount of biodiesel in any fuel mix, in contrast to the "BA" or "E" system used for ehanol mixes. If blends exceeding B5 are desired, vehicle owners and operators should consult their engine manufacturer regarding the implications of using such fuel.

Biodiesel blends up to a maximum of B5 should not cause engine or fuel system problems, provided the B100 used in the blend meets the requirements of ASTMD 6751, DIN 51606, or EN 14214. Engine manufacturers should be consulted if higher percentage blends are desired. Biodiesel blends may require additives to improve storage stability and allow use in a wide range of temperatures.

Engine Operation, Performance and Durability

The energy content of neat biodiesel fuel is about eleven percent (11%) lower than that of petroleum-based diesel fuel (on a per gallon basis), which results in a power loss in engine operation. The viscosity range of biodiesel fuel, however, is higher than that of petroleum-based diesel fuel (1.9 – 6.0 centistokes versus 1.3 – 5.8 centistokes), which tends to reduce barrel/plunger leakage and thereby slightly improve injector efficiency. The net effect of using B100, then, is a loss of approximately five to seven percent (5-7%) in maximum power output. The actual percentage power loss will vary depending on the percentage of biodiesel blended in the fuel. Neat biodiesel and higher percentage biodiesel blends can cause a variety of engine performance problems, including filter plugging, injector coking, piston ring sticking and breaking, elastomer seal swelling and hardening/cracking, and severe engine lubricant degradation. At low ambient temperatures, biodiesel is thicker than conventional diesel fuel, which would limit its use in certain geographic areas. In addition, elastomer compatibility with biodiesel remains unclear; therefore, when biodiesel fuels are used, the condition of seals, hoses, gaskets, and wire coatings should be monitored regularly. There is limited information on the effect of neat biodiesel and biodiesel blends on engine durability during various environmental conditions.

Emission Characteristics

Use of neat biodiesel and biodiesel blends in place of petroleum-based dieselfuel may reduce visible smoke and particulate emissions, which are of special concern in older diesel engines in non-attainment areas. In addition, B100 and biodiesel blends can achieve some reduction in reactive hydrocarbons (“HC”) and carbon monoxide (“CO”) emissions when used in an unmodified diesel engine. Those reductions are attributed to the presence of oxygen in the fuel. Oxygen and other biodiesel characteristics, however, also increase oxides of nitrogen (“NOx”) in an unmodified engine. As a result, B100 and biodiesel blends produce higher NOx emissions than petroleum-based diesel fuel.

Figure: Average emission impacts of biodiesel fuels in CI engines

B100 Cleaning Effect

Methyl esters have been used as low VOC (volatile organic compound) cleaners and solvents for decades. Methyl esters make an excellent parts cleaner, and several companies are offering methyl esters as a low VOC, non-toxic replacement for the volatile solvents used in parts washers. B100 has a tendency to dissolve the accumulated sediments in diesel storage and engine fuel tanks. These dissolved sediments can plug fuel filters and in some cases cause the fuel filters to burst, sending all the sediment through the fuel injection system. If this happens, it can cause injector deposits and even fuel injector failure. If will be use or store B100 for the first time, clean the tanks and anywhere in the fuel system where sediments or deposits may occur before filling with B100.

Storage and Handling

Biodiesel fuels have shown poor oxidation stability, which can result in long-term storage problems. When biodiesel fuels are used at low ambient temperatures, filters may plug, and the fuel in the tank may thicken to the point where it will not flow sufficiently for proper engine operation. Therefore, it may be prudent to store biodiesel fuel in a heated building or storage tank, as well as heat the fuel systems’ fuel lines, filters, and tanks. Additives also may be needed to improve storage conditions and allow for the use of biodiesel fuel in a wider range of ambient temperatures. To demonstrate their stability under normal storage and use conditions, biodiesel fuels, tested using ASTM D 6468, should have a minimum of 80% reflectance after aging for 180 minutes at a temperature of 150°C. The test is intended to predict the resistance of fuel to degradation at normal engine operating temperatures and provide an indication of overall fuel stability.

Biodiesel fuel is an excellent medium for microbial growth. In as much as water accelerates microbial growth and is naturally more prevalent in biodiesel fuels than in petroleum-based diesel fuels, care must be taken to remove water from fuel tanks. The effectiveness of using conventional anti-microbial additives in biodiesel is unknown. The presence of microbes may cause operational problems, fuel system corrosion, premature filter plugging, and sediment build-up in fuel systems.

Health, Safety and Environmental Issues

Pure biodiesel fuels have been tested and found to be nontoxic in animal studies. Emissions from engines using biodiesel fuel have undergone health effects testing in accordance with EPA Tier II requirements for fuel and fuel additive registration. Tier II test results indicate no biologically significant short term effects on the animals studied other than minor effects on lung tissue at high exposure levels. Biodiesel fuels are biodegradable, which may promote their use in applicationswhere biodegradability is desired (e.g., marine or farm applications). Biodiesel is as safe in handling and storage as petroleum-based diesel fuel.

Biodiesel contains no hazardous materials and is generally regarded as safe to use. Like any fuel, certain fire safety precautions must be taken. A number of studies have found that biodiesel biodegrades much more rapidly than conventional diesel. Users in environmentally sensitive areas such as wetlands, marine environments, and national parks have taken advantage of this property.


Engine manufacturers are legally required to provide an emissions warranty on their products and, typically, also provide commercial warranties. Individual engine manufacturers determine what implications, if any, the use of biodiesel fuel has on the manufacturers’ commercial warranties. It is unclear what implications the use of biodiesel fuel has on emissions warranty, in-use liability, anti-tampering provisions, and the like. As noted above, however, more information is needed on the impacts of long-term use of biodiesel on engine operations. Economics The cost of biodiesel fuels varies depending on the base stock, geographic area, variability in crop production from season to season, and other factors. Although the cost may be reduced if relatively inexpensive feedstock, such as waste oils or rendered animal fat, is used instead of rape, soybean, corn or other plant oil, the average cost of biodiesel fuel nevertheless exceeds that of petroleum-based diesel fuel. That said, users considering conversion to an alternative fuel should recognize that the relative cost of converting an existing fleet to biodiesel blends is much lower than the cost of converting to any other alternative fuel because no major engine, vehicle, or dispensing system changes are required.


Abbreviations and Acronyms
AFV AFV alternative fuel vehicle   NOx nitrogen oxide
ASTM American Society for Testing and Materials   NPAH NPAH nitrated polyaromatic hydrocarbons
B100 100% biodiesel   OSHA Occupational Safety and Health Administration
B20 20% biodiesel, 80% petroleum diesel   PAH polyaromatic hydrocarbons
BTU British Thermal Unit   ppm parts per million
CFPP cold filter plug point   RME Rape Methyl Ester
CI compression ignition   SME; SOME Soy Methyl Ester
FA; FFA fatty acid; free fatty acids   ULSD ultra low sulfur diesel
FAME fatty acid methyl esters   VOC volatile organic compound
HC hydrocarbon      
MSDS material safety data sheet      
additive: material added in small amounts to finished fuel products to improve certain properties or characteristics
antioxidant: substance that inhibits reactions promoted by oxygen
aromatic compound: a hydrocarbon based on a six-carbon benzenoid ring
biodiesel: methyl esters of fatty acids meeting the requirements of specification EN14214 and/or ASTM specification D6751
biodegradable: capable of being broken down by the action of microorganisms
boiling range: the spread of temperature over which a fuel, or other mixture of compounds, distills
cetane index: an approximation of cetane number based on an empirical relationship with density and volatility parameters such as the mid-boiling point. This approximation is not valid for biodiesel or biodiesel blends
cetane number: a measure of the ignition quality of diesel fuel based on ignition delay in an engine. The higher the cetane number, the shorter the ignition delay and the better the ignition quality
chelating compound: a fuel additive that deactivates the catalytic oxidizing action of dissolved metals, notably copper, on fuels during storage
cloud point: the temperature at which a sample of a fuel just shows a cloud or haze of wax (or in the case of biodiesel, methyl ester) crystals when it is cooled under standard test conditions
detergent: a fuel detergent is an oil-soluble surfactant additive that maintains the cleanliness of engine parts by solubilizing deposits or materials likely to deposit in the engine fuel system
dispersant: a surfactant additive designed to hold particulate matter dispersed in a liquid
elastomer: synthetic rubber-type material frequently used in vehicle fuel systems (but not necessarily natural or synthetic rubber, may also apply to other polymers)
energy content: the heat produced on combustion of a specified volume or mass of fuel, also known as heating value or heat of combustion
EN 14214 specification: the standard for biodiesel (B100) by the Europen Standard organisation (CEN). It is broadly based on DIN 51606
fatty acid methyl esters (FAME): Mono alkyl ester of long-chain fatty acids from naturally occurring vegetable oil, animal fats, and recycled greases
fatty acid: any of the saturated or unsaturated monocarboxylic acids that occur naturally in the form of triglycerides (or mono or diglycerides) or as free fatty acids in fats and fatty oils
Free fatty acids: any saturated or unsaturated monocarboxylic acids that occur naturally fats, oils or greases but are not attached to glycerol backbones. These can lead to high acid fuels and require special processes technology to convert into biodiesel
flash point: the lowest temperature at which vapors from a fuel will ignite on application of a small flame under standard test conditions
Hydrocarbon (HC): a compound composed of hydrogen and carbon. Hydrocarbons can refer to fuel components and to unburned or poorly combusted components in vehicle exhaust
kerosene: a refined petroleum distillate of which different grades are used as lamp oil, as heating oil, blended into diesel fuel, and as fuel for aviation turbine engines
lubricity: the ability of a fuel to lubricate
microbial contamination: containing deposits or suspended matter formed by microbial degradation of the fuel
multifunctional additive: an additive or blend of additives with more than one function
OEM: riginal engine manufacturer
oxidation: the chemical combination of oxygen to a molecule
oxidative stability: the ability of a fuel to resist oxidation during storage or use
oxygenate: a fuel component that contains oxygen; i.e., biodiesel or ethanol
particulate matter (PM): the solid or semi-solid compounds of unburned fuel that are emitted from engines
polycyclic aromatic hydrocarbons (PAH): aromatic compounds with more than one benzenoid ring (PAH). Also, NPAH for nitro-polyaromatic compounds
polyunsaturated fatty acids: fatty acids with more than one double bond
pour point: the lowest temperature at which a fuel will just flow when tested under standard conditions as defined in ASTM D97
saturation: or saturated compound. A paraffinic hydrocarbon or fatty acid, i.e. with only single bonds and no double or triple bonds
solvency: he quality or state of being a solvent
specific gravity: the ratio of the density of a substance to the density of water
splash blending: the fuels to be blended are delivered separately into a tank
storage stability: the ability of a fuel to resist deterioration on storage due to oxidation
torque: a force that produces rotation
viscosity: a measure of the resistance to flow of a liquid