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Easing refuelling logistics: producing jet fuel from seawater

26 September 2012

Researchers are developing a process to extract carbon dioxide and produce hydrogen gas from seawater, subsequently catalytically converting the CO2 and H2 into jet fuel.

NRL's Electrochemical Acidification Carbon Capture Skid (photo courtesy of the US Naval Research Laboratory)

The US Naval Research Laboratory (NRL) has successfully developed and demonstrated technologies for the recovery of CO2 and the production of H2 from seawater using an electrochemical acidification cell (pictured), and the conversion of CO2 and H2 to hydrocarbons that can be used to produce jet fuel.

"The reduction and hydrogenation of CO2 to form hydrocarbons is accomplished using a catalyst that is similar to those used for Fischer-Tropsch reduction and hydrogenation of carbon monoxide," says research chemist, Dr Heather Willauer. "By modifying the surface composition of iron catalysts in fixed-bed reactors, NRL has successfully improved CO2 conversion efficiencies up to 60 percent."

A renewable resource
CO2 is an abundant carbon (C) resource in the air and in seawater, with the concentration in the ocean about 140 times greater than that in air. Two to three percent of the CO2 in seawater is dissolved CO2 gas in the form of carbonic acid, one percent is carbonate, and the remaining 96 to 97 percent is bound in bicarbonate.

If processes are developed to take advantage of the higher weight per volume concentration of CO2 in seawater, coupled with more efficient catalysts for the heterogeneous catalysis of CO2 and H2, a viable sea-based synthetic fuel process may be possible. "With such a process, the Navy could avoid the uncertainties inherent in procuring fuel from foreign sources and/or maintaining long supply lines," Willauer said.

The major component of the carbon capture skid is a three-chambered electrochemical acidification cell. This cell uses small quantities of electricity to exchange hydrogen ions produced at the anode with sodium ions in the seawater stream. As a result, the seawater is acidified.At the cathode, water is reduced to H2 gas and sodium hydroxide (NaOH) is formed.

This basic solution may be re-combined with the acidified seawater to return the seawater to its original pH with no additional chemicals. Current and continuing research using this carbon capture skid demonstrates the continuous efficient production of H2 and the recovery of up to 92 percent of CO2 from seawater.

How it Works: CO2 + H2 = jet fuel
NRL has developed a two-step process. In the first step, an iron-based catalyst has been developed that can achieve CO2 conversion levels up to 60 percent and decrease unwanted methane production from 97 percent to 25 percent in favour of longer-chain unsaturated hydrocarbons (olefins).

In the second step these olefins can be oligomerized (a chemical process that converts monomers, molecules of low molecular weight, to a compound of higher molecular weight by a finite degree of polymerization) into a liquid containing hydrocarbon molecules in the carbon C9-C16 range, suitable for conversion to jet fuel by a nickel-supported catalyst reaction.

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