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Navy Looks to Seawater to Fuel the Fleet

Refueling U.S. Navy vessels, at sea and underway, is a costly endeavor in terms of logistics, time, fiscal constraints, and threats to national security and sailors at sea.

Scientists at the U.S. Naval Research Laboratory are developing a process to extract carbon dioxide (CO2) and produce hydrogen gas (H2) from seawater, subsequently catalytically converting the CO2 and H2 into jet fuel by a gas-to-liquids process.

NRL has successfully developed and demonstrated technologies for the recovery of CO2 and the production of H2 from seawater using an electrochemical acidification cell, and the conversion of CO2 and H2 to hydrocarbons (organic compounds consisting of hydrogen and carbon) that can be used to produce jet fuel.

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 can be envisioned.

NRL has made significant advances developing carbon capture technologies in the laboratory. Using the novel cells both dissolved and bound CO2 were recovered from seawater by re-equilibrating carbonate and bicarbonate to CO2 gas at a seawater pH below 6. In addition to CO2, the cells produced H2 at the cathode as a by-product.

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.

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