|Heather D. Willauer
NRL scientists are developing and demonstrating novel technologies for the recovery of carbon dioxide (CO2) and production of hydrogen (H2) from seawater. These feedstocks are combined in an NRL innovative GTL (gas to liquids) process to produce value added hydrocarbons. It is envisioned that these hydrocarbons will 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 it is needed, thereby reducing the logistics tail on fuel delivery, enhancing combat capabilities, and providing greater energy security by fixing fuel cost and its availability. 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.
Carbon Capture: Using novel and proprietary NRL electrolytic cation exchange modules (E-CEM), up to 92% of CO2 both dissolved and bound can be removed from seawater. The concentration of carbon dioxide in seawater is 140 times greater than that in air, yet harvesting large quantities of CO2 fast and efficiently from seawater had not been demonstrated prior to the recently announced patented NRL process. In addition to recovering CO2, the E-CEM HC (hydrogen and carbon dioxide) module simultaneously produces H2 gas at the cathode. The energy required to obtain these feedstocks from the ocean is primarily for the production of hydrogen and the carbon dioxide is a free byproduct. The process of both recovering CO2 and concurrently producing H2 gas eliminates the need for additional large and expensive electrolysis units.
The E-CEM C (carbon dioxide) module offers the opportunity to recover only CO2 from both seawater and alkaline water sources, therefore significantly reducing the overall energy requirements for the process. This second novel and proprietary technological approach is envisioned to produce only CO2 for use in a growing number of processes seeking to increase product efficiencies. These processes include enhanced biological carbon fixation and new strategies involving CO2 in Low Temperature Solidification Processes.
NRL has made significant 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 (NRLKW), is the E-CEM module. Extraction of CO2 utilizing these to two different E-CEM approaches offers a pure and concentrated source of CO2 from the environment, while reducing the effects of anthropogenic CO2 on the ocean. This unique source of CO2 is extremely advantageous when compared to CO2 recovered from flue and stack gasses. These sources require energy intensive hardware and further cost to purify the gasses to be used in any biological carbon fixation applications where pH, concentration, and purity affect the living organisms and designer fuel GTL process where purity affects the catalyst. The process efficiency, the capability to produce large quantities of H2 using the E-CEM HC module if needed, and the ability to process the seawater without the need for additional chemicals or pollutants, has made these technologies far superior to membrane and ion exchange technologies previously developed and tested for recovery of CO2 from seawater and air.
CO2 + H2 to High Value Hydrocarbons: NRL has made significant advances in the development of a GTL 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 percent. These value-added hydrocarbons serve as building blocks for the production of industrial chemicals and designer fuels.
In the second step these olefins are converted to compounds of higher molecular using controlled polymerization. The resulting liquid contains hydrocarbon molecules in the carbon range, C9- C16, suitable for use as a possible renewable replacement for petroleum based jet and diesel 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- C16 molecules in the liquid. This finished liquid hydrocarbon from carbon dioxide and hydrogen was recently used to power an internal combustion (IC) engine of a model aircraft at Blossom Point, Maryland. 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.
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 in the range of $3-$6 per gallon.