James G. Speight - Encyclopedia of Renewable Energy

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 and ASTM D5773 methods], and oxidation stability (normally evaluated using Differential Scanning Calorimetry and Oxidation Stability Index). In addition, according to the EMA (Engine Manufacturers Association), a blend of crude oil diesel fuel meeting ASTM D975 and 100% (neat) biodiesel fuel meeting either ASTM 6751 or EN 14214, where the biodiesel content of the blended fuel is no more than 20% biodiesel by volume (B20), shall meet the requirements identified in at the point of delivery of the fuel to the end user.

In terms of the effects of biodiesel on fuel filters, use of biodiesel blends which do not meet up to the designated specifications, have shown to drastically reduce filter life. Blends greater then B20 may have enough of a solvent to break down the varnish deposits on the walls of existing fuel storage tanks or fuel systems. The breakdown of these varnish deposits will contaminate the fuel with particulate, which can cause fuel filters to plug rapidly.

Another disadvantage of biodiesel is that it tends to reduce fuel economy. Energy efficiency is the percentage of the thermal energy of the fuel that is delivered as engine output, and biodiesel has shown no significant effect on the energy efficiency of any test engine. Volumetric efficiency, a measure that is more familiar to most vehicle users, is usually expressed as miles traveled per gallon of fuel (or kilometers per liter of fuel). The energy content per gallon of biodiesel is approximately 11% lower than that of crude oil diesel. Vehicles running on B20 are therefore expected to achieve 2.2% (20 % x 11 %) fewer miles per gallon of fuel.

Areas of concern and interest are for the biofuels industry to have in place a good quality control protocol for the measurement of bioalcohols, to avoid metal corrosion from water and acid corrosion (due to weak and strong acids and inorganic chlorides in solution). Also of importance are the limits set on phosphorous content (less than 5.0 mg/L in ethanol) to prevent engine catalyst deterioration, and copper content (less than 0.1 mg/kg), along with a sulfur content less than 10 mg/kg.

Up to a 10% blend level, the performance of bioethanol-blended gasoline is similar to ordinary gasoline. At higher levels, however, some engines may begin to exhibit problems, for example, stumbling under slight acceleration. The fuel also has more aggressive properties at higher concentrations of bioethanol which increases the possibility of deterioration of some components. Gasoline must be volatile enough to move from the carburetor or injectors into the cylinders and to vaporize prior to combustion. However, gasoline cannot have such a volatility that allows it to vaporize and boil in the injectors, carburetor, fuel lines, or fuel pump, which could prevent it from being metered correctly. Also, if the gasoline is too volatile, more evaporates into the air adding to environmental problems. There are a number of volatility specifications to ensure suppliers get this balancing act right. Adding bioethanol to gasoline as low-level blends increases the volatility of the blended fuel.

The Engine Fuel Specifications Regulations specify volatility measures for bioethanol-blended petrol. The limits for blends are similar to those for gasoline so as to ensure no changes in vehicles are required. Bioethanol introduces more oxygen into the fuel. In vehicles with simple fuel metering systems such as carburetors, this causes the mixture to become a little leaner. Leaning is good for fuel economy and is generally good for lowering some types of exhaust emissions. However, it may cause some engines to stumble if they are already tuned reasonably lean. If a vehicle stumbles on bioethanol-blended gasoline, re-tuning should solve the problem. A vehicle tuned correctly for use on ordinary gasoline would normally not exhibit problems when using bioethanol blends.