FAME biodiesel cannot be blended with diesel fuel in high concentrations without substantial risks for fuel quality, engine operation, exhaust emissions and infrastructure. FAME is not so called "drop-in" fuel. FAME may cause problems also at low concentrations, for example, deterioration of oil quality, clogging of particulate filters and dissolving materials. Injection performance and cold-start properties of FAME may be poor. Storage stability of FAME is challenging. Flash point of FAME is sufficiently high, but if there is any alcohol as impurity the flash point lowers rapidly (Graboski et al. 1998, WWFC 2006). Therefore maximum limits for blending of FAME are defined in many regions. In Europe, maximum 7% FAME is allowed in diesel fuel and 5% in U.S. In order improve local air quality. Neat FAME (100%) can be used as such in fleet operations like city buses, taxis or mine vehicles.
For low level blends, the standards for petroleum diesel fuels such as EN 590 and ASTM D 975 apply, provided that the biodiesel fuel does meet the biodiesel standards for B100. There is great interest in a standard for higher blending ratios of FAME to help with biofuel targets in some countries. Standards organizations in various countries have been struggling for some time now with the process of establishing a 20% FAME standard. That specification moved closer to reality in early 2007 when new limits in ASTM D6751 were approved for oxidation stability and other parameters for the 2007-2010 diesel engines. Essentially, the B100 standard has been redesigned so that it is “protective” of the B20 and lower blends.
Blending FAME with paraffins
When blending diesel fuel, paraffins (HVO/XTL) and FAME, the following instructions should be followed: diesel fuel and paraffins are mixed first, and then FAME is added into blend. Paraffins and diesel fuel are chemically close to each other. A risk for precipitation of impurities increases when FAME is blended with low aromatic fuel. Less than 7% FAME may be mixed with HVO, otherwise the risk of precipitation increases. (Mikkonen et al. 2011).
Two forms of stability are important for biodiesel fuels; one is stability during long-term storage and the other relates to stability while the fuel is subjected to high temperatures and/or high pressure, as is the case with an engine’s fuel injection system.
FAME in diesel fuel presents risk on engine durability. Biodiesel esters have tendency to form sludge and engine deposits, and to plug diesel particulate filters (DPF). Today, diesel cars and vehicles are increasingly equipped with DPF to meet new, stringent emission legislation. In biodiesel esters, for example, K, Na, P, glycerides, and glycerine may be present originating from feedstock and catalysts used in production of the biodiesel. These elements may cause filter plugging and engine deposits.
Biodiesel esters are high-boiling compounds, which may lead into fuel dilution of the engine oil especially in engines using post-injection for particulate filter regeneration. The high boiling range of FAME results in fuel condensation on the cylinder walls when fuel is injected late in the working cycle. High-boiling components of FAME do not burn completely, which may also lead to engine deposits and increased exhaust emissions, especially at low temperatures.
As polar compounds FAME esters dissolve materials more efficiently than diesel fuel and this may lead to material compatibility problems. As mentioned before FAME may also contain peroxides and acids, which can damage or degrade plastics, elastomers and metals (WWFC 2009).
If there is any methanol or ethanol present in biodiesel esters, flash point lowers to an extent that fuel cannot be handled in the same way as diesel fuel. In addition, low flash point may lead to decreased lubricity, corroded injectors and degraded materials in the vehicle and fuel distribution system (WWFC 2009).
FAME biodiesel is in principle compatible with the current vehicles, logistic systems and practices, however, stability issues are challenging. During long-term storage FAME biodiesel have a “use before” date. FAME that meets the specification should provide six months of storage capability before unacceptable degradation occurs, but this depends on storage conditions (WWFC 2009). Heat and sunlight can accelerate the oxidation process. FAME biodiesel should not be stored in systems that contain copper, brass, bronze, lead, and tin. Those metals will accelerate the aging process and will contribute to higher sediments. Also avoiding exposing the biodiesel fuel to oxygen during the storage can help to extend shelf life.
FAME may contain risky impurities, and its tendency to pick up and solve water is also higher than that of traditional diesel fuels. This raises consideration of measures to prevent microbiological growth.
Methyl esters have been used for many years as cleaners and solvents. It should not be surprising, then, that biodiesel will have a tendency to dissolve sediments and residues that might accumulate over time in storage tanks. The released sediments then can lead to filter plugging. Therefore, when storing FAME biodiesel it is advisable to clean the storage tank if has been used previously for other fuels. With biodiesel blends of less than 35% FAME biodiesel, the problem is not as great. As polar compound, biodiesel dissolve also materials of cars and vehicles, such as seals of fuel system or paint coatings, more efficiently than diesel fuel. Many engine and vehicle manufacturers have taken this into account, but for many cars and vehicles problems may occur. (McGill et al. 2008).
Flash point of FAME is well above +55 °C, which is the limit set for diesel fuel, meaning that FAME can be stored and transported like standard diesel fuel. However, even a small amount of methanol remnant lowers the flash point rapidly. (WWFC 2009).