Synthetic fuel production utilizing CO₂ recycling as an alternative to sequestration

Pressing environmental and energy security concerns have brought much needed attention to economic production of hydrogen as the secondary energy carrier for non-electrical markets as well as lo meet increasing demand for crude upgrading and desulfurization. While steam reforming of natural gas is currently the dominant method of hydrogen production, it is depleting a valuable fossil fuel while emitting green house gases. Thus, in the long run, efficient, environmentally friendly, and economic means of hydrogen production using nuclear and renewable energy needs to be developed. High temperature electrolysis using solid oxide electrolyte cells is an exceptionally efficient means of generation of high purity hydrogen. It is also capable of directly converting steam and CO₂ into synthesis gas. This coelectrolysis process enables conversion of CO₂ recovered from concentrated industrial sources into a feedstock for synthetic fuels production. Use of CO₂ provides a means of storing hydrogen in a concentrated and convenient form - a hydrocarbon. This is also an effective storage medium for intermittent renewable energy sources such as wind and solar. Ceramatec, Inc and the Idaho National Laboratory are applying solid oxide fuel cell technology to the production of synthesis gas by high temperature coelectrolysis of steam and carbon dioxide. This technology utilizes solid oxide fuel cell stacks to electrochemically remove oxygen from steam and CO₂. leaving a clean stream of hydrogen and carbon monoxide suitable for fuel synthesis. By varying the relative concentrations of carbon dioxide and steam, the hydrogen to carbon monoxide product ratio can be controlled as required by the desired type of synthetic fuel being produced from the syngas. The resultant synthesis gas has been used as the feedstock to produce synthetic methane and Fischer Tropsch liquid fuels. Alkaline and proton exchange membrane electrolysis cells typically operate below I00° while solid oxide electrolysis cells operates in the 750-850° range. This higher temperature improves the efficiency significantly. Practically, it translates into a lower cell operating voltage, 1.1 -1.4V for high temperature electrolysis vs. ̃1.8V for water electrolysis. Hydrogen production is stoichiometric with current in all types of electrolyzers. Carbon dioxide is a valuable resource as a feedstock for energy storage as a hydrocarbon. There is a vast amount of renewable energy that cannot be effectively utilized as grid electricity, but which could be used to convert CO₂ to hydrocarbons, offering a solution to the even more difficult (than supplying grid power) challenge of production of sustainable, domestic transportation fuels. Conventional electrolysis and chemical processes could be deployed to accomplish this conversion. However, high temperature co-electrolysis of CO₂ and steam to produce synthesis gas jsing solid oxide fuel cell lechnology has the potential of improving the efficiency by nearly 50%. A domeslic synfuei industry will enable a much larger reliance on intermittentrenewable energy than can be accommodated by conventional electric demand profiles while increasing our energy security.