Flying a radio-controlled replica of the historic WWII P-51 Mustang red-tail aircraft—of the legendary Tuskegee Airmen—NRL researchers (l to r) Dr. Jeffrey Baldwin, Dr. Dennis Hardy, Dr. Heather Willauer, and Dr. David Drab (crouched), successfully demonstrate a novel liquid hydrocarbon fuel to power the aircraft's unmodified two-stroke internal combustion engine. (Photo: US Naval Research Laboratory).
Jim Lane, Biofuels Digest
Hydrocarbon fuel from seawater and nothing else? Is it possible? Not only possible, but demonstrated as a process, and tested in a model plane. But what are the red lights flashing?
In Washington, researchers at the US Naval Research Laboratory have completed a miniature-scale flight demonstration using a liquid hydrocarbon fuel, produced entirely from seawater.
In the demonstration, the research team demonstrated sustained flight of a radio-controlled (RC) P-51 replica, powered by an off-the-shelf (OTS) and unmodified two-stroke internal combustion engine.
The fuel was produced using a proprietary NRL "electrolytic cation exchange module" via which both dissolved and bound CO2 are removed from seawater at 92 per cent efficiency by re-equilibrating carbonate and bicarbonate to CO2 and simultaneously producing H2. The gases are then converted to liquid hydrocarbons by a metal catalyst in a reactor system.
The production of CO2 and hydrogen
NRL has made significant advances in the development of a gas-to-liquids (GTL) synthesis process to convert CO2 and H2 from seawater to a fuel-like fraction of C9-C16 molecules. In the first patented step, an iron-based catalyst has been developed that can achieve CO2 conversion levels up to 60 per cent and decrease unwanted methane production in favour of longer-chain unsaturated hydrocarbons (olefins). These value-added hydrocarbons from this process serve as building blocks for the production of industrial chemicals and designer fuels.
In the second step these olefins can be converted to compounds of a higher molecular using controlled polymerisation. The resulting liquid contains hydrocarbon molecules in the carbon range, C9-C16, suitable for use a possible renewable replacement for petroleum based jet fuel.
Seawater-based CO2 as a feedstock
CO2 in the air and in seawater is an abundant carbon resource, but the concentration in the ocean (100 milligrams per litre [mg/L]) is about 140 times greater than that in air, and 1/3 the concentration of CO2 from a stack gas (296 mg/L). Two to three per cent of the CO2 in seawater is dissolved CO2 gas in the form of carbonic acid, one per cent is carbonate, and the remaining 96 to 97 per cent is bound in bicarbonate.
But $3-$6 per gallon fuels? When, where?
The Navy says that "the predicted cost of jet fuel using these technologies is in the range of $3-$6 per gallon, and with sufficient funding and partnerships, this approach could be commercially viable within the next seven to ten years. Pursuing remote land-based options would be the first step towards a future sea-based solution."
Scale-up? The modular carbon capture and fuel synthesis unit "is envisioned to be scaled-up by the addition individual E-CEM modules and reactor tubes to meet fuel demands."
Right now, it's a lab-scale, with the Naval Research Lab operating a fixed-bed catalytic reactor system. The outputs of the prototype unit "have confirmed the presence of the required C9-C16 molecules in the liquid. This lab-scale system is the first step towards transitioning the NRL technology into commercial modular reactor units that may be scaled-up by increasing the length and number of reactors."
The hydrogen key
According to the Navy, the key is in the hydrogen. Though membrane and ion exchange technologies have been developed for the recovery of CO2 from seawater or air, this system has higher potential, the Navy believes, because of the process efficiencies — and, especially, the capability to simultaneously produce large quantities of H2, and process the seawater without the need for additional chemicals or pollutants.
"In close collaboration with the Office of Naval Research P38 Naval Reserve program, NRL has developed a 'game-changing' technology for extracting, simultaneously, CO2 and H2 from seawater," said Dr Heather Willauer, NRL research chemist. "This is the first time technology of this nature has been demonstrated with the potential for transition, from the laboratory, to full-scale commercial implementation."
The red lights
Before we break out the bubbly, let's keep an eye on all that seawater.
The good news is that it is essentially a free and permanently abundant feedstock, and we could really, really use a technology that utilises the sea's dissolved CO2.
The bad news is that — like algae — you have to get the water out of the CO2 or get the CO2 out of the water, and any time you move water, it takes energy. Now, the amount of carbon you need in a gallon of diesel fuel is around 3 kilos. If you have 80 mg of CO2 per litre of seawater you have to process 39,000 gallons of water to make a gallon of fuel, operating at 80 per cent processing efficiency (in real-world, scaled-up conditions, vs. lab).
Now — "mileage" on a naval warship will greatly depend on running speed, number of engines employed and so on, but you can start with 1,000 gallons per hour for a destroyer and be in range. So, consider a technology that needs to process 39 million gallons of seawater per hour to cover the energy budget (leaving aside the need to produce KWh to move the water, say 20 feet up from sea level). It's a weight of something like 130,000 tons per hour.
To put this into design terms, the biggest ballast tanks in the US Navy run in the 3 million gallon range for the largest diesel-operated ships.
The bottom line
We're not sure how the math is going to work out for on-board fuel generation, for this technology — without a consideration for ship redesign, and possibly slowing the ships down or making them sluggish in response, with all that seawater sloshing around. So, we'll wait and see on that.
For land-based application, it will all come down to how far the water has to be transported — for the same reason that biomass is (ideally) a little dried out before transporting, because water is expensive to move. $6 per gallon — in today's dollars — well, that will be challenging today, but should oil jump past $200 a barrel, you never know. And the Navy may well wish to have a reserve fuel-production capacity, even with a higher cost.
But the lower end of the range — around $3 per gallon. That's attractive right now. So, it's a technology to watch closely as it moves from the lab and towards scale-up.
