From This Week in Petroleum, September 16, 2009
There's a good chance you've been hearing more about biodiesel lately. This article, the first of several short discussions about biodiesel that will appear in the next few months, lays out some of the basic terminology and qualities associated with this fuel. It also begins to illustrate important factors that vary among biodiesel fuels and fuel blends with petroleum diesel - variations that require fuel suppliers to make adjustments in order to ensure that these fuels perform well.
Biodiesel is a renewable fuel for diesel engines derived from natural oils like soybean oil, and which meets the specifications of the ASTM International (formerly American Society for Testing and Materials) in ASTM D6751. But the term is sometimes also used to describe a blend of biodiesel with petroleum diesel. Blends are designated with the letter "B" followed by a number that represents the volumetric percentage of biodiesel in the blend. For example, B100 refers to 100 percent biodiesel, while B20 refers to a blended product of 20 percent biodiesel and 80 percent petroleum diesel. B20 is the minimum blend required for alternative fuel fleet compliance with the Energy Policy Act of 1992. However, many vehicle and engine manufacturers currently require the use of B5 blends or less to maintain vehicle warranties. Also, Federal regulations do not require biodiesel labeling for blends of B5 or less. B5 is therefore the most common biodiesel blend sold in retail markets, although some higher blends are also sold, particularly in States with biodiesel tax incentives.
B100 can be created from a variety of oils and fats coming from plants or animals and recycled greases. A fat or oil molecule contains a fatty acid component, which is what is used to create biodiesel. Examples of B100 feedstock, in order of increasing saturated fatty acid content, include: canola, safflower, sunflower, corn, olive, soybean, peanut, cottonseed, lard, beef tallow, palm, and coconut. Most biodiesel in the United States is soy-derived.
The differing levels of fat saturation in feedstocks affect such B100 fuel properties as cold-flow properties, cetane number, oxidative stability, and NOx emissions, although some uncertainty has arisen recently about NOx variation with fat saturation. The remainder of this article focuses on these and other properties of B100 and how blending B100 with petroleum diesel changes the qualities of the blended fuel, requiring fuel suppliers to make appropriate fuel adjustments to ensure the fuel will meet emission and drivability requirements.
Cold-Flow Properties. One important cold flow property that affects diesel use is the cloud point, which measures the temperature at which small crystals form, giving the liquid a cloudy appearance. These crystals can plug filters or settle to the bottom of a storage tank. A second cold-flow property, the pour-point, is the lowest temperature at which the liquid stops flowing. At this temperature, the fuel gels to a point that it becomes too thick to pump. The cloud point for B100 can range from 26°F for B100 derived from canola, to 32°F for soy, and all the way up to 66°F for animal fats. Clearly, B100 cannot be used in many parts of the United States in the colder months.
Cetane Number. The cetane number quantifies the ability of diesel fuel to auto-ignite, one of the most important characteristics of this fuel. The higher the cetane number, the shorter the lag time between the injection of the fuel into the combustion chamber and ignition of the fuel. The minimum cetane number required of petroleum diesel in the United States is 40. California has additional requirements that result in typical diesel cetane values over 50. The ASTM D6751 specification requires B100 to have a minimum cetane number of 47. B100 created from soy, canola, or sunflower has a cetane number of 50 to 52, while B100 created from highly saturated feedstock, such as tallow or lard can have a cetane number of 60 or higher.
Stability. Stability refers to the tendency of fuels to resist undergoing chemical change. Three types of stability are associated with biodiesel: thermal stability addresses changes that may occur when fuel is circulated through elevated fuel system temperatures; oxidative stability refers to the tendency of fuels to react with oxygen at ambient temperatures; and storage stability refers to the changes that may occur when fuel is stored for prolonged periods of time. Storage stability is directly influenced by oxidative stability. For B100, oxidative stability is of primary concern. Poor stability causes the biodiesel molecules to react with oxygen and produce sediments, gums, and unwanted acids. Both the degree of saturation (higher is better) and the presence of natural or synthetic antioxidants affect oxidative stability. A minimum stability requirement is a part of the ASTM D6751 specifications. Synthetic antioxidants are the most common approach for stabilization of B100 and blends. Biodiesel blends are much more stable than the parent B100, so the best way to stabilize biodiesel is to blend it with petroleum diesel.
NOx Emissions. The presence of oxygen in a fuel ensures more complete combustion, which reduces various emissions, including hydrocarbons and toxic compounds. According to the U.S. EPA, B100 emits, on average, 10 percent more NOx than petroleum diesel. As the level of fat saturation decreases in the feedstock, NOx emissions from pure biodiesel tend to increase, so coconut oil is better in terms of NOx emissions than safflower or sunflower oils. Changes in NOx emissions from biodiesel blends are small and so have been difficult to ascertain. Some studies show slight increases in NOx emissions while others show slight decreases. Additional research is ongoing to determine the impact of fuel feedstock, engine design, and emission control systems on NOx emissions from biodiesel.
Most biodiesel currently in use involves blends of B20 or lower. Blending biodiesel with petroleum diesel combines the qualities of both fuels. Petroleum diesel helps to temper some of biodiesel's problematic qualities and vice versa. For example:
- Blending increases lubricity (reduction in friction) over that of petroleum diesel fuel alone. Diesel engine parts, especially fuel pumps and injectors, require sufficient fuel lubricity or slipperiness to avoid premature wear. Federal regulations lowering the sulfur content of diesel to 15 parts per million have had the unintended side effect of reducing petroleum diesel lubricity. Blending petroleum diesel with low concentrations of biodiesel can impart adequate lubricity to help protect the engines.
- Blending reduces the cleaning effect of B100. Biodiesel feedstocks are excellent cleaners and solvents. B100 made from a variety of different feedstocks will dissolve accumulated sediments in diesel storage tanks. While this may sound like a good thing, the dissolved sediments mix with the fuel, then plug filters and fuel injectors in diesel engines. The use of low-biodiesel blends reduces any cleaning or solvent issues.
- Blending increases fuel compatibility with various engine materials. B100 degrades common materials found in diesel engine systems, such as hoses, seals, gaskets, plastics, and glues.
- Blending minimizes loss in engine performance and fuel economy of B100. The energy content of B100 (15,940 Btu/lb for soy feedstock) is lower than typical No. 2 petroleum diesel (18,300 Btu/lb). The lower energy content reduces power, torque, and fuel economy.
- Blending B100 with petroleum diesel moderates the low temperature operability problems of B100. Proper care must be taken with biodiesel blends in preparing a wintertime fuel. As is done with petroleum diesel, No. 1 diesel (kerosene) is frequently used in B20 and other blends to decrease the cloud point temperatures. Cold flow additives that reduce the size of the crystals or inhibit crystal formation are of limited effectiveness for B100, but will work to varying degrees with B20. Research to create more effective low temperature additives for biodiesel is ongoing.
Second Generation Biomass-Based Diesel Fuels
For improved economics and fuel properties, different processes are being developed to create second generation biomass-based diesel fuels. One advanced production process involves treating the source fats and oils with hydrogen in the presence of a catalyst. The resulting renewable diesel has less oxygen and higher energy content than biodiesels created through the current process. Another second generation production process involves converting organic material from animals and vegetables into hydrogen and carbon monoxide and applying the Fischer-Tropsch process, a catalyzed chemical reaction that converts synthetic gas into liquid hydrocarbons. The resulting biomass-based diesel has no oxygen and an energy content that is higher than conventional biodiesel, though still lower than petroleum diesel. Since neither resulting fuel has the structure found in conventional biodiesel, these second generation fuels are referred to as biomass diesel according to Federal Trade Commission labeling guidelines for biodiesel and biomass-based diesel pumps.