There are standards for three different varieties of biodiesel, which are made of different oils: (i) rapeseed methyl ester, DIN E51606, (ii) vegetable methyl ester, purely vegetable products, DIN E51606, and (iii) fat methyl ester, vegetable and animal products, according to DIN V51606. The standards ensure that the following important factors in the fuel production process are satisfied; there needs to be (i) complete reaction, (ii) removal of glycerin, (iii) removal of catalyst, (iv) removal of alcohol, (v) absence of free fatty acids, and (vi) 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. Fuel meeting the quality standards is very non-toxic, with a toxicity rating (LD50) of greater than 50 mL/kg.
Biodiesel has promising lubricating properties and cetane ratings compared to low sulfur diesel fuels. Fuels with higher lubricity may increase the usable life of high-pressure fuel injection equipment that relies on the fuel for its lubrication. Depending on the engine, this might include high pressure injection pumps, pump injectors (also called unit injectors), and fuel injectors.
Generally, the heat content (calorific value) of biodiesel is lower than that of the regular crude oil-derived diesel, and variations in biodiesel energy density are more dependent on the feedstock used than the production process. Biodiesel can provide better lubricity and more complete combustion, thus increasing the engine energy output and partially compensating for the higher energy density of petrodiesel. The color of biodiesel ranges from golden to dark brown, depending on the production method. It is slightly miscible with water, has a high boiling point, and, consequently, a low vapor pressure. The flash point of biodiesel exceeds 130°C (266°F), which is significantly higher than that of crude oil-derived diesel which may be as low as 52°C (126°F). Biodiesel has a density of on the order of 0.88 g/cm³, higher than petrodiesel (which is approximately 0.85 g/cm³). Typically, biodiesel does not contain sulfur, and it is often used as an additive to ULSD fuel to aid with lubrication, as the sulfur compounds in petrodiesel provide much of the lubricity.
ASTM International (ASTM), formally known as the American Society for testing materials, is an international organization which develops and publishes information on the technical standards of various products, materials, systems, and services. It is one of the largest and most highly regarded standards development organizations in the world. The available literature on the performance of biofuels when compared with traditional fossil fuels normally uses ASTM and ISO (International Standards Organization) specifications and parameters. The specifications provide details on requirements for fuel characteristics as well as the relevant standard test methods to use for each.
Because of the focus on alternate fuel standards (particularly the standards for biodiesel) over the past two decades, the development of biodiesel standards started in the 1990s in order to support the increasing use of alkyl esters-based biodiesel and its blends as automotive fuels. The first ASTM standard (ASTM D6751) was adopted in 2002 while in Europe; EN 14214 biodiesel standard (based on former DIN 51606) was finalized in October 2003. The US and EU standards have international significance; they are usually the starting point for biodiesel specifications developed in other countries. However, in the United States, the standard (ASTM D6751) establishes specifications for a biodiesel blend stock for middle distillate fuels and, while the specification was written for B100, it is not intended for neat biodiesel used as automotive fuel. Rather, it is for the biodiesel component that is to be blended to produce biodiesel/diesel fuel blends. Since 2012, the ASTM D6751 standard has defined two grades of biodiesel (i) grade 2-B, which is identical to biodiesel defined by earlier versions of the standard, and (ii) grade 1-B, which has more strict controls on monoglycerides and cold soak filterability. In addition, the ASTM Standard Specification for Diesel Oil (ASTM D975) was modified in 2008 to allow up to 5% biodiesel to be blended into the fuel while a standard for biodiesel blends (ASTM D7467) is a specification for biodiesel blends from B6 to B20.
Most countries 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 bioalcohol. Pure biodiesel is referred to as B100, while fuel containing 20% biodiesel is labeled “B20.” The common international standard for biodiesel is EN 14214, while ASTM 6751 is most referenced in the U.S. In Germany, the requirements for biodiesel are fixed in the DIN EN 14214 standard.
One of the most important fuel properties of biodiesel and conventional diesel fuel derived from crude oil is viscosity, which is also an important property of lubricants. Ranges of acceptable kinematic viscosity are specified in various biodiesel and crude oil standards. Reducing viscosity is one of the main reasons why vegetable oils or fats are transesterified to biodiesel because the high viscosity of neat vegetable oils or fats ultimately leads to operational problems such as engine deposits.
The viscosity of biodiesel is slightly greater than that of petrodiesel but approximately an order of magnitude less than that of the parent starting material (vegetable oil or fat). Biodiesel and its blends with petrodiesel display temperature-dependent viscosity similar to that of neat petrodiesel. Influencing factors are chain length, position, number, and nature of double bonds as well as the nature of the oxygenated moieties.
Generally, biodiesel has a higher cloud point (temperature at which a fuel becomes hazy or cloudy and starts to gel) than petrodiesel. This makes its use impractical in cooler climates and limits its potential market. Other important chemical and physical properties described in ASTM standards for biodiesel are acid number (TAN – total acid number, indicates the presence of free fatty acids and carboxylic acids present), corrosion (describes the potential for copper corrosion, measured using ASTM method D130), low temperature performance [describes pour points (PP), and cloud points (CP) using ASTM D5949
