MSU-Ford team evaluates 12 biofuel compounds for effects on cold flow properties of diesel and jet blends
|Cloud point temperatures of a high aromatic diesel (HAD) in mixtures with various biofuel compounds. Lown et al. Click to enlarge.|
Researchers from Michigan State University and Ford Motor Company's Research and Advanced Engineering Group recently tested 12 potential biofuel compounds containing oxygen in different functional groups in mixtures with three diesel fuels and one jet fuel to determine the effects of the functional groups on low-temperature fuel properties.
Groups evaluated included diesters, esters, ketones and ethers; alkanes were used for comparison. Fuels included a standard #2 US diesel (USD); a European standard diesel (ESD); and a high aromatic diesel (HAD), as well as JP-8 donated by the US Air Force.
Although some of the evaluated compounds are unlikely to be commercial biofuels, they were compared to understand the effect of functional groups on cold flow behavior, the team noted in a paper published in the journal Fuel.
While FAME [fatty acid methyl ester, biodiesel] from soybean oil is the most commonly used form of biodiesel in the United States, the use of this fuel is limited in cold climates due to its high cloud point temperature, even when mixed with petroleum diesel. FAME biodiesel is more problematic as a bio-jet fuel, due to the low temperatures encountered at high flight altitudes. Without additional processing, such as distillation or the use of urea to precipitate the saturated esters, FSME is not able to achieve the required low cloud point temperatures.
…Second-generation biofuels are often defined as biofuels made from non-food feedstocks, including ethanol from corn stover and other lignocellulosic sources, Fischer-Tropsch oils from wood, and butanol or mixed alcohols form renewable sources. In addition, these fuels can be reacted together or upgraded to produce fuels with more desirable qualities. Chemical reactions create compounds with a wide variety of functional groups and oxygen content, and can usually be tailored to produce a specific molecule type. … Evaluations of the functional group's effects on cloud points and other cold flow properties are needed to streamline the development of these fuels, and to identify which types of molecules would be desirable as fuels.
In the study, the team used ASTM D7683 and D2500 to determine cloud point temperatures, and ASTM D6371 for cold filter plugging point temperatures. Most of the evaluated compounds have the potential to be produced through bio-derived pathways. Most are also not produced directly by fermentation, but as products of the upgrading of fermentation products.
Generally, the team found that alkanes, ethers, esters and ketones with a low melting point temperature decreased the cloud point temperature of a fuel. Adding diesters resulted in low temperature liquid-liquid immiscibility; they used multiple methods to confirm the presence of two discrete liquid phases with diesters, including cold filter plugging point temperatures.
They also found that the general behaviors are independent of chain length and branching—as long as the melting point temperature of the compound is not significantly higher than the cloud point temperature of the fuel.
When diesel fuels were mixed with compounds of low molecular weight, the oxygen moieties present had more effect on miscibility than the degree of branching of the carbon chains.
Functional groups in order from the most effective to least effective at decreasing cloud point were alkanes, ethers, esters, ketones and diesters.
High molecular weight compounds exhibited more consistent effects, despite up to 60 °C differences in melting temperatures, over the composition ranges until the eutectic temperature was encountered.
Anne L. Lown, Lars Peereboom, Sherry A. Mueller, James E. Anderson, Dennis J. Miller, Carl T. Lira (2014) “Cold flow properties for blends of biofuels with diesel and jet fuels,” Fuel, Volume 117, Part A, Pages 544-551 doi: 10.1016/j.fuel.2013.09.067