Besides issues related to economics and policy, and despite being competitive with petroleum diesel fuel, biodiesel has continued to face several technical issues, including unfavorable cold flow and oxidative stability, as well as questionable NOx emissions. With the introduction and increasing market penetration of new emissions control technologies such as selective catalytic reduction (SCR), the issue of NOx may fade with time, but the issues of cold flow and oxidative stability, both of which are addressed by specifications in biodiesel standards, will remain. This article briefly discusses addressing these problems without compromising other fuel properties in biodiesel standards, such as cetane number (CN) and kinematic viscosity.

As fatty acid mono-alkyl esters, the major components of biodiesel are constructed of two moieties, the fatty acid and the alcohol, both of which can be considered for “tailoring” to improve biodiesel properties.

Five approaches exist for modifying biodiesel fuel composition for improvement. These are a) additives; b) using an alcohol other than methanol to produce biodiesel; c) physical procedures such as fractionation; d) using “alternative” feedstocks with an inherently different fatty acid profile; e) genetic modification of the fatty acid profile. 

Additives are probably the most common approach. However, several additives are likely necessary since specific additives usually address a single problem. Issues here are additive compatibility, effects on other properties besides the one to be addressed, storage and more. For example, antioxidants are consumed with time, thus they are actually oxidation delayers. Oxidation of biodiesel will begin once the antioxidants have been consumed.

Physical procedures include fractionation, for example, to remove higher-melting saturated fatty esters to improve cold flow. However, this lowers both oxidative stability by enriching the unsaturated fatty esters in biodiesel, and the CN, as saturated fatty esters possess higher CNs than unsaturated fatty esters. 

Changing the alcohol moiety has no or little effect on properties such as oxidative stability or CN. Any effect is likely marginally beneficial. Kinematic viscosity, however, slightly increases. Esters other than methyl have better cold flow properties as shown by the melting points (MP) of ethyl stearate (33 degrees Celsius) and propyl stearate (28.1 C) vs. methyl stearate (37.7 C). Isopropyl esters have even lower melting points, reflected in lower cloud points of the corresponding vegetable oil esters. The major disadvantages of using esters other than methyl as biodiesel are the higher cost of the alcohol, and changes to the transesterification reaction. 

The other approaches, “alternative” feedstocks and genetic modification of the fatty acid profile, are partially related. For these approaches, it is useful to first define fatty acids that are most likely to impart favorable properties to biodiesel. Acids such as decanoic (C10:0; capric acid) and hexadecenoic (C16:1; palmitoleic) are prime candidates for enrichment as their esters compromise between the various fuel properties. Methyl decanoate has a MP of around minus 13 C, a CN of 51 to 52, oxidative stability exceeding 24 hours in the Rancimat test, and a kinematic viscosity of 1.72 mm2/sec. Methyl octanoate, with an even lower MP of minus 37.4 C, however, has a CN of approximately 42, below that prescribed in biodiesel standards. Accordingly, methyl esters prepared from cuphea oil enriched in C10:0 (about 65 percent) exhibited a CN in the range of 55 to 56, kinematic viscosity around 2.4 mm2/sec and a cloud point around  minus 9 to minus 10 C, probably the lowest reported for a vegetable oil-derived biodiesel fuel. The oxidative stability, however, only slightly exceeded the 3-hour minimum prescribed in ASTM D6751, showing that an antioxidant additive would likely still be necessary. Cuphea oil methyl esters also exhibited improved combustion properties. Methyl palmitoleate has a MP of minus 34 C, lower than that of methyl oleate (minus 20 C), and a CN in the lower to mid-50s. Breeding feedstocks with a modified fatty acid profile highly enriched in such acids is therefore likely a promising approach to improving biodiesel fuel properties. An interesting aspect that has been receiving increasing attention in this respect is the use of nonlipid feedstocks (carbohydrates) to ultimately produce biodiesel. Minor components affecting biodiesel fuel properties, however, will need to be taken into account in any case. 

In summary, modifying biodiesel composition to improve fuel properties is a promising approach that is likely to receive increasing attention by researchers in the future. It may ultimately prove to be the most viable approach to improving biodiesel fuel properties, thereby ensuring its future commercial marketability and success.

Author: Gerhard Knothe
Research Chemist, USDA-ARS-NCAUR
(309) 681-6112
gerhard.knothe@ars.usda.gov