University of Minnesota (UMN)

Most liquid fuels used in transportation today are derived from petroleum and burned in internal combustion engines. These fuels are attractive because of their high energy density and current economics, but they remain partially reliant on imported petroleum and are highly carbon intensive. Domestically produced carbon-neutral liquid fuels (CNLFs), such as ammonia (NH3), can address both of these challenges. Chemical manufacturers commonly use the Haber-Bosch (HB) process to produce NH3 for use in agriculture or the chemical industry. The HB process involves first separating nitrogen (N2) from air, then breaking the very stable nitrogen-nitrogen bond, and finally combining the nitrogen atoms with hydrogen to form NH3. The HB process requires huge capital investments for reactors to operate at high pressure and temperature, large amounts of base-load power to keep the process running continuously (HB uses 1-2% of global energy), and distribution infrastructure to ship the resulting ammonia around the world. Technology enabling the small- and medium-scale synthesis of ammonia can move the production of the fuels closer to the consumer, and - if renewable sources are used - the fuels can be produced in a carbon neutral manner. However, significant technical challenges remain in either adapting the HB process for smaller scale use or developing alternative electrochemical processes for fuel development. New methods would also have to employ variable rates of production to match the intermittent generation of renewable sources. Improvements in these areas could dramatically reduce the energy and carbon intensity of liquid fuel production. By taking better advantage of intermittent renewable resources in low-population areas and transporting that energy as a liquid fuel to urban centers, we can more fully utilize domestically available resources.

The University of Minnesota (UMN) will develop a small-scale ammonia synthesis system using water and air, powered by wind energy. Instead of developing a new catalyst, this team is looking to increase process efficiency by absorbing ammonia at modest pressures as soon as it is formed. The reactor partially converts a feed of nitrogen and hydrogen into ammonia, after which the gases leaving the reactor go into a separator, where the ammonia is removed and the unreacted hydrogen and nitrogen are recycled. The ammonia is removed completely by selective absorption, which allows the synthesis to operate at lower pressure. This reduced pressure makes the system suitable for small-scale applications and more compatible with intermittent energy sources. The success of preliminary experiments suggests that this new approach may be fruitful in reducing capital and operating costs of ammonia production.