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April 2014 Issue

Transforming Seawater into Designer Fuel
Heather Willauer

U.S. Naval Research Laboratory (NRL) scientists are developing and demonstrating novel technologies for the recovery of carbon dioxide (CO2) and the production of hydrogen (H2) from seawater. These feedstocks are combined in an NRL innovative GTL (gas to liquids) process to produce value-added hydrocarbons. These hydrocarbons could one day be used to augment industrial chemical processes and produce designer fuel (LNG, CNG, F-76 and JP-5) stocks for the Navy.

The potential longer term payoff for the Navy is the ability to produce fuel at or near the point of use when needed, reducing the logistics tail on fuel delivery, enhancing combat capabilities, and providing greater energy security by fixing the cost of fuel and its availability while having a minimal impact on the environment. From an environmental perspective, such a combination of integrated NRL-developed technologies could be considered CO2 neutral. The carbon dioxide, produced from combustion of the synthetic fuel, is returned to the atmosphere where it re-equilibrates with the ocean to complete the natural carbon cycle.

CO2 in the air and in seawater is an abundant resource, but the concentration in the ocean (100 milligrams per liter) is about 140 times greater than that in air and one-third the concentration of CO2 from a stack gas (296 milligrams per liter). Two to 3 percent of the CO2 in seawater is dissolved CO2 gas in the form of carbonic acid, 1 percent is carbonate, and the remaining 96 to 97 percent is bound in bicarbonate.

In close collaboration with Office of Naval Research P38 Naval Reserve Program, NRL has developed for CO2 recovery and H2 production from seawater. This is the first time technology of this nature has been demonstrated with the potential for transitioning from the laboratory to full-scale commercial implementation. Using a novel and proprietary NRL electrolytic cation exchange module (E-CEM), both dissolved and bound CO2 are removed from seawater at 92 percent efficiency by re-equilibrating carbonate and bicarbonate to CO2 gas at a seawater pH below 6. In addition to CO2, the module produces hydrogen gas at the cathode. The energy required to obtain H2 gas from seawater needed for the GTL synthesis process far exceeds the energy needed to re-equilibrate carbonate and bicarbonate in seawater to CO2 gas; therefore, CO2 is a free byproduct. The process efficiencies, the capability to produce large quantities of H2, and the ability to process seawater without additional chemicals or pollutants, has made E-CEM far superior to membrane and ion exchange technologies previously developed and tested for recovery of CO2 from seawater or air.

NRL has made progress in scaling-up and integrating the carbon capture technology into an independent platform. At the heart of this platform, located at NRL's Center for Corrosion Science & Engineering facility in Key West, Florida, is the three-chambered E-CEM module. The module uses electricity to exchange hydrogen ions produced at the anode with sodium ions in the seawater stream. As a result, the bicarbonate and carbonate in the seawater is re-equilibrated to CO2 gas. At the cathode, water is reduced to H2 gas.

The platform has been tested using seawater from the Gulf of Mexico to simulate conditions that will be encountered in an open-ocean process for capturing large quantities of CO2 and producing H2 gas from seawater. Currently one E-CEM module can process up to 7,200 gallons of seawater a day, thus requiring about three modules to produce enough feedstock to make 1 gallon of fuel a day at an energy consumption of 25.7 kilowatt-hours per cubic meter of H2. The data from these studies has been of paramount importance in the next steps to develop a commercial prototype E-CEM module that will increase the volume of seawater processed per module and decrease the energy consumption needed to produce both CO2 and hydrogen (below 6 kilowatt-hours per cubic meter of H2). The time line for development of the prototype depends highly on funding.

Using CO2 and H2 to produce value-added hydrocarbons for chemicals or designer fuels is a challenge NRL scientists continue to study at the basic science level. NRL has made advances in the development of a GTL process to convert CO2 and H2 from seawater to a fuel-like fraction of C9 to C16 molecules. In the first patented step, an iron-based catalyst has been developed that can achieve CO2 conversion levels up to 60 percent and decrease unwanted methane production in favor of longer-chain unsaturated hydrocarbons (olefins) to serve as building blocks for industrial chemicals and designer fuels.

In the second step, these olefins can be converted to compounds of higher molecular structure using controlled polymerization. The resulting liquid contains hydrocarbon molecules in the carbon range C9 to C16, suitable as a possible renewable fuel replacement for petroleum-based jet fuel. NRL operates a lab-scale, fixed-bed catalytic reactor system, and the outputs of this prototype unit have confirmed the presence of the required C9 to C16 molecules in the liquid. This lab-scale system is the first step towards transitioning the NRL technology into commercial modular reactor units that can easily be scaled-up by increasing the length and number of reactors.

Despite an unfavorable energy balance, this process offers a unique approach to converting electricity into portable, storable, versatile, high-energy-density hydrocarbons. The predicted cost of jet fuel using these technologies is $3 to $6 per gallon. With sufficient funding and partnerships, this approach could be commercially viable in the next seven to 10 years. The minimum modular carbon capture and fuel synthesis unit can be scaled-up with individual E-CEM modules and reactor tubes to meet fuel demands.

Heather Willauer has been a research chemist at the U.S. Naval Research Laboratory since 2002. She is currently leading research in the development of technologies for capturing carbon dioxide and hydrogen from seawater and their subsequent synthesis to hydrocarbons. She has published more than 53 papers in referred journals, has made major contributions to seven books, and holds five patents.


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