Most of the recent research on full-performance renewable fuels for jet and diesel propulsion has focused on synthetic paraffinic kerosenes (SPKs). These fuels have a number of characteristics that make them attractive for use in both diesel and turbine engines. SPKs burn cleanly and generate little in the way of coke or polyaromatics due to the absence of aromatic compounds in the fuel.

The lack of aromatics and naphthenes also has a negative impact, evidenced by the decreased density of the fuel. Further, aromatic compounds are essential to engine integrity in a number of fielded systems due to their ability to swell o-rings and seals. Naphthenes typically account for ~45% of diesel fuel, and aromatics comprise ~25%; similarly, jet fuel contains ~35% naphthenes and 20% aromatics. The combination of naphthenes, aromatics and paraffins in conventional diesel fuel results in densities from 0.825-0.850 g/mL, while petroleum-derived jet fuels have densities of about 0.81 g/mL.

In contrast, renewable fuels containing purely paraffinic or isoparaffinic hydrocarbons in the C₁₀-C₁₄ range have densities from about 0.73-0.76 g/mL, while renewable diesel range hydrocarbons with their average higher molecular weights have densities of up to ~0.78 g/mL. A potential solution to the low density of these fuels is to blend high density naphthenes with the paraffins. Terpenoids are particularly compelling sources of renewable naphthenes that can be isolated from pine resin or generated via biosynthetic approaches. Toward this end a number of researchers have been investigating bio-derived naphthenes based on monoterpenes, sesquiterpenes, and diterpenes. Some of these molecules have densities as high as 0.94 g/mL, with their viscosities dependent on their structures and molecular weight.

—Harvey et al.

NAWCWD researchers have been pursuing high-density biofuels for a number of years (e.g., earlier post, earlier post). In 2014, a NAWCWD team demonstrated that renewable high density fuels with net heats of combustion ranging from ~133,000 to 141,000 Btu gal^-1—up to 13% higher than commercial jet fuel (~125,000 Btu)—can be generated by combining heterogeneous catalysis with multicyclic sesquiterpenes produced by engineered organisms. (Earlier post.)

This latest study explored the use of sesquiterpanes as blendstocks to enable 100% bio-derived high-density diesel and jet fuel. The sesquiterpanes impart high density and volumetric net heat of combustion to the blends; the modestly branched paraffin component decreases the viscosity and increases the cetane number.

In the paper, they reported achieving a surrogate diesel fuel comprising 65% sesquiterpanes and 35% 5-methylundecane with a cetane number of 45.7, a density of 0.853 g/mL, and a volumetric net heat of combustion (NHOC) of 134.0 kBtu/gal. By increasing the amount of paraffin to 60% by volume, they prepared a jet fuel surrogate with a cetane number of 57.0, a density of 0.806 g/mL, a -20 °C kinematic viscosity of 8.3 mm²/s, and a NHOC of 124.6 kBtu/gal.

The ability to synthesize renewable fuels that outperform petroleum-derived fuels is an emerging trend in biofuel development. Virtually all full-performance biofuels are generated via direct biosynthesis or some combination of biosynthesis and chemical catalysis. This provides the opportunity to improve the performance of renewable fuels through custom design of fuel composition. In addition to their utility in high-performance fuel blends, the individual components studied in this work can be used to upgrade the properties of other biofuels as well as conventional jet and diesel fuels. Both components of the fuel blends studied in this work can be derived from biomass sugars by combining fermentation processes with chemical catalysis. This suggests that fuels of this type could potentially be generated on a scale that would make them practical for widespread military or commercial use.

—Harvey et al.

Resources

• Benjamin Grant Harvey, Walter W Merriman, and Thomas A Koontz (2015) “High Density Renewable Diesel Fuels Prepared from Multicyclic Sesquiterpenes and a 1-Hexene Derived Synthetic Paraffinic Kerosene ” Energy Fuels doi: 10.1021/ef5027746

• US Patent Application Nº 20150031927: Renewable High Density Turbine And Diesel Fuels From Sesquiterpenes (29 Jan 2015)