Navy researchers produce high-density, high-cetane bio-hydrocarbon fuels from sesquiterpenes; jet and diesel
Researchers at the Naval Air Warfare Center, Weapons Division, China Lake have produced three new high-density, high-cetane biofuels from sesquiterpene feedstocks. In an open-access paper published in the RSC journal Sustainable Energy & Fuels, they describe the preparation of the three fuels from sesquiterpene components of cedarwood oil.
The three biofuels described in the work could outperform conventional fuels. The researchers, Kale Harrison and Benjamin Harvey, note that with recent advances in metabolic engineering, the generation of multicyclic sesquiterpenes from biomass sugars could allow for the production of these new fuels on a commercial scale.
Sesquiterpenes are naturally occurring trimers of isoprene that can be isolated from trees and other biomass sources. The China Lake team had previously successfully used sesquiterpenes in the production of renewable high-density fuels, reported in a paper in 2015. (Earlier post.)
Based on the earlier work on these high-density sesquiterpene-based fuels, the China Lake team started considering how to further enhance the density and volumetric net heat of combustion of diesel fuels, Harvey explained.
The 2015 paper used a blended approach of combining multicyclic sesquiterpanes with a synthetic paraffinic kerosene (5-methylundecane). The sesquiterpanes increased the net heat of combustion (NHOC) of the blends, while the alkane fraction imparted a short ignition delay to allow for use of these fuels in diesel engines. It would be more elegant, they then theorized, to combine both functionalities in the same molecule.
They started with the premise that they could synthesize molecules with multicyclic cores and then functionalize those cores with short alkyl chains; the cores would increase the density while the alkyl chains would increase the cetane number of the fuels.
In a 2016 paper in the ACS journal Energy & Fuels (Harvey et al., 2016), they described the production of four alkyl diamondoids from adamantanes and their resulting fuel properties, including density, net heat of combustion, low-temperature viscosity, and derived cetane number (DCN).
Adamantanes or diamondoids are present in trace amounts in petroleum and have also been found in large quantities in exhausted natural gas wells. However, adamantane has a melting point of 269 °C, making it unsuitable as a significant component of turbine or diesel fuels.
The fuel molecules they prepared from adamantanes had densities up to 17% higher than conventional jet fuel and volumetric net heats of combustion comparable to or exceeding that of the synthetic missile fuel JP-10.
Further, the alkyl diamondoid fuels had DCNs (derived cetane numbers) in the range of 42−49—allowing for efficient combustion in diesel engines. The alkyl diamondoids described in that paper report were the first examples of multicyclic hydrocarbons that combine extraordinary densities (>0.9 g/mL) with DCNs comparable to or exceeding that of conventional diesel fuel.
With these results, then then looked for a way to generate adamantanes from bio-derived feedstocks.
Cedarwood oil is an intriguing starting material for advanced biofuel development. It is primarily comprised of the tricyclic sesquiterpenes α-cedrene, β-cedrene, and thujopsene along with a significant quantity of cedrol. Cedarwood oil derived from redcedar has previously been investigated as a fuel product, and epi-cedrol, a potential precursor to cedrenes, has been generated from glucose with metabolically engineered yeast. The latter work suggests that significant quantities of cedrenes and thujopsene can be obtained from biomass sugars via fermentation.
…Alkyl adamantanes are an intriguing class of molecules that have been explored as high density turbine fuels to enhance the range of aircraft. In a recent paper our group showed that attachment of short alkyl chains to the adamantane core resulted in fuels with high densities, NHOCs [net heat of combustion], and cetane numbers.
… To explore the potential of cedarwood oil and sustainable alkyl-diamondoid mixtures as high density fuels, this paper describes the preparation and characterization of hydrogenated cedarwood oil, a single component fuel containing only cedrane, and a complex mixture of adamantanes prepared by the AlCl3 catalyzed isomerization of cedrane.—Harrison and Harvey (2017)
The China Lake researchers hydrogenated cedarwood oil to generate a fuel blend (HCWO) with a cetane number of 31 and a volumetric net heat of combustion (NHOC) more than 12% higher than conventional jet fuel.
They then prepared a single component high density fuel containing only hydrogenated α-cedrene (cedrane) by dehydration and hydrogenation of α-cedrol. They found that this fuel had an even higher NHOC and lower viscosity than HCWO.
Cedrane was then isomerized to 1-ethyl-3,5,7-trimethyladamantane (ETMA) and a mixture of other alkyl adamantanes. The adamantane mixture had a cetane number of 46 and a viscosity suitable for use in a conventional diesel engine.
These fuels exhibit NHOCs substantially higher than conventional jet or diesel fuel, while the viscosity of the fuels can be reduced by a factor of three through acid catalyzed isomerization. Despite the multicyclic structures of the hydrocarbons comprising the fuel blends, the DCN for HCWO is one of the highest yet observed for a high density sesquiterpane mixture, while the DCN of ETMA is high enough to allow for efficient combustion in a diesel engine. The results presented in this paper show that it is possible to create sustainable fuels that have both exceptional densities and high cetane numbers.—Harrison and Harvey (2017)
Kale W. Harrison and Benjamin G. Harvey (2017) “Renewable high density fuels containing tricyclic sesquiterpanes and alkyl diamondoids” Sustainable Energy Fuels doi: 10.1039/C6SE00108D
Benjamin G. Harvey, Kale W. Harrison, Matthew C. Davis, Andrew P. Chafin, Joshua Baca, and Walter W. Merriman (2016) “Molecular Design and Characterization of High-Cetane Alkyl Diamondoid Fuels” Energy & Fuels 30 (12), 10171-10178 doi: 10.1021/acs.energyfuels.6b01865