Although “impressive progress” has been made in developing renewable replacement fuels for widely used transportation fuels such as gasoline and diesel, the researchers noted, high-density, petroleum-based, tactical fuels such as JP-10 and RJ-5 are difficult to replace given their high densities of 0.94 and 1.08 g/mL.

Bulk agricultural waste products, such as cellulose and lignin, are often targeted as feedstocks for the production of renewable fuels. However, even branched chain saturated hydrocarbon fuels, which can be derived from cellulosic butanol (Biojet), only have a density of 0.78 g/mL, while saturated linear fuels produced from syngas have a density of 0.75 g/mL. Although these alternative fuels compare favorably to high flashpoint jet fuels, such as JP-5, their low densities are reflected in their relatively poor volumetric heating values, with renewable Biojet capable of producing only 34.3 MJ/L compared to JP-10 at 39.6 MJ/L and RJ-5 at 44.9 MJ/L.

—Harvey et al.

To meet the requirements of a tactical fuel replacement, then, abundant, specialized, and renewable feedstocks will be necessary, the Navy researchers note. One promising technological route for addressing this challenge would be to bioengineer plants and microbes to selectively produce specific organic molecules in a continuous process, they suggest.

In effect, this approach would result in the upgrading of low-value feedstocks, such as CO₂, glucose, and cellulose, to custom organic molecules and high-value fuels that lie along an established biosynthetic pathway. Potential targets for this bioengineering approach are α- and β-pinene. These molecules are versatile natural products that are produced by a wide variety of trees and other plant life...Because of their compact structures and reactive olefin functionalities, these molecules have significant potential as feedstocks for high-density renewable fuels.

—Harvey et al.

The volumetric heat of combustion of the pinenes can further be improved by dimerization (the process of combining two molecules with the same chemical composition into a polymer). To selectively produce dimers mixtures using environmentally friendly catalysts, the authors tried the acidic heterogeneous catalysts Montmorillonite K10 (MMT-K10), Amberlyst-15, and Nafion.

Of the three, Nafion performed the best, and was capable of producing dimers in up to 90% yield primarily through isomerization followed by dimerization under moderate conditions (100 °C and atmospheric pressure). The dimer mixtures were upgraded through hydrogenation over PtO₂ and fractional distillation.

The density of the hydrogenated dimer mixture prepared with Nafion was 0.938 g/cm³, similar to JP-10 at 0.94 g/cm³, while the net heat of combustion of the dimer mixture was 141,745 BTU/gallon, virtually identical to JP-10 (142 000 BTU/gallon). However, the pour point of the renewable high density fuel was measured to be -30 °C, substantially higher than JP-10, which has a freezing point of -79 °C.

Clearly, fuels made solely from pinene dimers by an acid-catalyzed process are not yet suitable for high altitude applications given current engine configurations. Of course specialty fuels based on norbornadiene dimers, which have high melting points, have been considered as rocket fuel components in combination with a low freezing component. In a similar manner, one could envision pinene dimer/JP-10 blends as a partially renewable fuel or pinene dimer/monomer mixtures as a fully renewable replacement for petroleum-based, high-density fuels.

—Harvey et al.

Resources

• Benjamin G. Harvey, Michael E. Wright and Roxanne L. Quintana (2009) High-Density Renewable Fuels Based on the Selective Dimerization of Pinenes. Energy Fuels, Article ASAP doi: 10.1021/ef900799c

• Thermochemical and Thermophysical Properties of JP-10 (NISTIR 6640, June 2006)