Impact of Renewable Fuels Standard/MTBE Provisions of S. 1766
Appendix B: Technical Characteristics of Biodiesel
Biodiesel is a solution of chemicals known as esters. Esters are characterized by a carbon atom double bonded to an oxygen atom and single bonded to another oxygen atom. Other organic groups occupy the remaining bonds on the carbon and oxygen. If soybean oil is the raw material for the biodiesel, the organic groups attached to the esters are palmitic, stearic, oleic, linoleic, and linolenic acids.25 Biodiesel can be produced by several processes. The oil or fat may be converted to fatty acids which are in turn converted to biodiesel. The oil or fat can also be converted to biodiesel directly, using an acid or base to catalyze, or accelerate, a reaction known as transesterification. Base catalyzation is preferred, because the reaction is quick and thorough. It also requires lower temperature and pressure than other processes, which translates to lower capital costs for the biodiesel plant. The most common method is to react animal fat or vegetable oil with methanol in the presence of sodium hydroxide (a base, known as lye or caustic soda). This reaction is a base-catalyzed transesterification. The products of the reaction with methanol are methyl esters and glycerine.26If ethanol is substituted for methanol, ethyl esters and glycerine are produced. Methanol is preferred because it is less expensive than ethanol.27
All esters contain at least two oxygen atoms per molecule. Thus, biodiesel is an oxygenated fuel. It may be blended with petroleum diesel or used in pure form. The blend most often studied is 20 percent biodiesel and 80 percent petroleum diesel, known as B20. Benefits to B20 and B100 include lower hydrocarbon, carbon monoxide, and particulate emissions.28 Nitrogen oxide emissions may increase or decrease, depending on the biodiesel properties.29 Oxides of nitrogen and hydrocarbons are ozone precursors. Carbon monoxide is also an ozone precursor, though to a lesser extent than unburned hydrocarbons or nitrogen oxides. Additional research is needed to determine whether the use of biodiesel will increase or decrease ground-level ozone on balance. The biodiesel emission studies were carried out on existing heavy-duty vehicles. The effects of biodiesel on the emissions of Tier II heavy diesels slated for introduction in model year 2007 has not been determined.
Biodiesel exceeds the performance of petroleum diesel on several important measures. Petroleum diesel typically has a cetane index in the low 40’s. The cetane index is a correlate of the cetane number, which is a measure of the ease with which a fuel is ignited. A higher cetane number is more desirable. Graboski and McCormick summarize several experimental studies of biodiesel characteristics. They report a range of 40 to 52 cetane index for petroleum diesel. The reported cetane number for biodiesel ranges from 45.8 to 56.9 for soybean oil methyl esters, with an average of 50.9. The authors imply that careful production control will result in product with cetane number on the high end of the range.30 By contrast, petroleum diesel tends toward the low end of the range as high-cetane streams are instead directly or indirectly blended into gasoline.
Biodiesel is also better at lubrication of diesel fuel system components than petroleum diesel. Fuel injectors and some types of fuel pumps rely on diesel fuel for lubrication. Lubricity is a measure of the lubricating properties of fuel. One study cited by the National Biodiesel Board found that half of its samples of U.S. petroleum diesel sold did not meet the recommended minimum standard for lubricity.31 Biodiesel has better lubricity than current low-sulfur (500 parts per million sulfur by weight) diesel. The lubricity problem is expected to get worse when ultra-low sulfur (15 parts per million sulfur by weight) diesel is introduced, starting in 2006. A 1 or 2 percent blend of biodiesel, by volume, in low-sulfur diesel improved lubricity substantially.32
B100 is less toxic and less flammable than petroleum diesel, making it safer to handle. There are a couple of minor additional benefits to using B100. The exhaust smells better than conventional diesel.33 Persons who service engines run on biodiesel may experience less skin irritation than with conventional diesel.34
One drawback with biodiesel is that it has higher cloud and pour points. Cloud point is the temperature at which a sample of the fuel starts to appear cloudy. The reason for this is wax crystals begin to form. In a vehicle's fuel system, the wax crystals can clog fuel lines and filters. Pour point is always lower than cloud point; it is the temperature slightly below which the fuel will not flow. If the ambient temperature is below the pour point, diesel engines will not run without special precautions. That the cloud and pour points for biodiesel are higher means that vehicles running on biodiesel blends may experience drivability problems at higher temperatures than vehicles running on petroleum diesel.35 This is a potential concern during the winter in much of the United States. More work is needed to develop biodiesel blends with cold-flow properties comparable to petroleum diesel. Transitioning a vehicle to B20 or B100 requires some care as well. Over time, petroleum diesel forms deposits in vehicle fuel systems. Biodiesel can loosen these deposits, allowing them to migrate and clog fuel lines and filters.36 Other disadvantages include high production cost per gallon and approximately 11 percent lower energy content per gallon relative to petroleum diesel.37