Navy Secretary Ray Mabus says the development of such fuels is needed for reasons of energy security.
Earlier this month the US Navy said it recently tested two alternative "drop in" fuels aboard its Self Defense Test Ship (SDTS), and although initial indications are that the trial was a success, others say the exercise only further highlights the high cost of using alternative bunkers.
"While under the power of the alternatively-sourced fuels the ship successfully completed multiple engine starts and speed changes. There were no mechanical, operational or qualitative differences when operating on either fuel. The data collected from the trial will be fully analyzed and coupled with borescope results into a final report," the Navy said.
The exercise was part of the US Navy's polarising plan to get half its fuel from non-petroleum sources by 2020, something Navy Secretary Ray Mabus says is needed for reasons of energy security.
Critics argue the cost - both in financial and energy terms - of producing enough fuel to achieve this makes it untenable, and as early as 2012 a report by the RAND Corporation (RAND) concluded that renewable marine fuels will remain "far more expensive than petroleum-based fuels" unless there is a "major technical breakthrough."
So what exactly are these latest alternative bunkers, and do they represent such a breakthrough?
The US Navy identified the two fuels used in the trial as being Synthetic Iso-Paraffin (SIP) and Catalytic Hydrothermolysis Conversion Diesel (CHCD).
Conventional MGO, by comparison, is currently under $500 pmt
The cost of the fuel was not discussed, but contracts seen by Ship & Bunker indicate it paid $50.00/gal for pure CHCD, and $25.73/gal for pure SIP, or about $15,000 and $7,700 per metric tonne (pmt) respectively.
Conventional MGO, by comparison, is currently under $500 pmt ($1.60/gal) in the primary bunker ports, according to Ship & Bunker data.
T. A. Kiefer, retired U.S. Navy Captain, energy consultant, and long-time critic of the Navy's "Green Fleet" programme, spoke to Ship & Bunker about the latest trial and explained that both SIP and CHCD are biofuels.
"CHCD is a synthetic marine diesel fuel similar to distillate No. 2 fuel oil. It was developed by Applied Research Associates (ARA) as a variation of their commercial ReadiDiesel product," he said.
ARA makes the fuel by converting plant oils into a "biocrude"
"The CHCD-76 fuel, to give it its full name, is 100 percent drop-in which means it can be and was used in pure form and does not need to be blended with petroleum fuel."
ARA makes the fuel by converting plant oils into a "biocrude" using a process it calls "Catalytic Hydrothermolysis," which it then refines into a finished product.
"There's nothing really new in this process. While the choice of process catalyst is usually what is patented, the pathway and the end result are well-understood, and there have been many other companies doing a similar thermal conversion biocrude processes including Cool Planet, Virent, Sasol, Syntroleum, and Shell," said Kiefer.
SIP is known to the Navy as Direct Sugar to Hydrocarbon (DSH) fuel and commercially is also known under the proprietary name of farnesane.
Farnesane was developed by California-based Amyris as part of a joint venture with French energy giant Total, and in June 2014 a version of the product was launched as a renewable jet fuel for commercial aviation.
Production involves converting plant sugars into a variety of hydrocarbon molecules that are then hydroprocessed into paraffins.
T. A. Kiefer, retired U.S. Navy Captain & Energy Consultant
It is long-established science and technology that we can synthesize 100 percent drop-in hydrocarbon fuels from coal, natural gas, plant sugars, plant starches, animal and plant lipids, etc
"DSH-76 is the full designation for the alternative fuel being tested, and it is quite different from the complex blend of hydrocarbon molecules in conventional fuel. The paraffins do not meet the fuel specs for cold-flow, volatility, elastometric seal compatibility, etc., so they generally have to be blended with at least 50 percent conventional fuel, depending upon their specific composition and characteristics," said Kiefer.
In the latest trial, the Navy indicated it used a blend of 80 percent conventional F-76 Naval Distillate and 20 percent SIP.
But this is not the first time the Navy has tested SIP; trials of a 50/50 blend of DSH-76 and F-76 were successfully concluded in 2013.
"They previously purchased 40,000 gallons of DSH-76 from Amyris for $25.43/gal, so the cost for this latest trial actually went up slightly, it is just the ratios that changed for either overall cost or compatibility reasons," said Kiefer.
"It is long-established science and technology that we can synthesize 100 percent drop-in hydrocarbon fuels from coal, natural gas, plant sugars, plant starches, animal and plant lipids, etc. It is also established science that hydrocarbon synthesis is extremely expensive. It is only worth doing for boutique quantities of very valuable substances such as fragrances or oils for perfumes and cosmetics. It is not suitable for making bulk quantities of substances intended to be burned."
Others have been much blunter in their view on biofuel costs: "There's more important shit to spend money on in the military, period," Vice News this month quoted US Representative Duncan Hunter as saying.
For its part, the Navy maintains that the programme is useful.
"The goal is to qualify as many alternative fuel sources as possible," said Diane Mattingly, in-service engineer with Naval Surface Warfare Command, Philadelphia Division, who was commenting on the latest trial.
"A range of operational fuel sources will give our mission planners and commanders increased operational flexibility."
Eric McAfee, Chairman and CEO, Aemetis
We plan to utilize the low cost hydrogen in the production of 100% drop-in renewable jet fuel, F-76 marine diesel and renewable diesel
The Navy is also pushing ahead with the programme as part of the biennial Rim of the Pacific (RIMPAC), a twenty-seven nation military exercise taking place in and around the Hawaiian Islands and Southern California, that began on June 30 and runs until August 4.
California-based fuels and biochemicals company Aemetis, Inc (Aemetis) last week said it would be a guest at the event "to observe biofuels refuelling and other operations that represent significant milestones for the US Navy Great Green Fleet."
Nine countries will be operating ships and equipment on alternative fuel blends, it added.
Unlike Hunter and Kiefer, Aemetis Chairman and CEO Eric McAfee is far more optimistic about the future cost implications for the fuels.
"The pending acquisition of Edeniq and the recently licensed Lanzatech technology will enable Aemetis to produce low carbon, low cost cellulosic ethanol and hydrogen derived from agricultural waste, forest waste and even hazardous waste," said McAfee.
"We plan to utilize the low cost hydrogen in the production of 100% drop-in renewable jet fuel, F-76 marine diesel and renewable diesel using the Chevron/ARA process that Aemetis licensed in 2011 as the first global licensee of the technology."
Correction: A previous version of this story named the CHCD manufacturer, ARA, as "Advanced Research Associates", and has been corrected to Applied Research Associates".