(659f) Solar-Thermal Ammonia Production: A Renewable, Carbon-Neutral Route to Ammonia

Ammonia (NH₃) is an energy-dense chemical and a vital component of fertilizer. In addition, it is a carbon-neutral liquid fuel and a potential candidate for thermochemical energy storage for high-temperature concentrating solar power (CSP). NH₃ is currently synthesized via the Haber-Bosch process, which requires pressures of 15- 25 MPa and temperatures of 400-500 °C. Nitrogen (N₂) and hydrogen (H₂) are essential feedstocks for producing NH₃: H₂ is generally derived by steam reforming of methane, and N₂ is sourced from air, after O₂ removal, via combustion of methane. Burning hydrocarbons produces the heat and mechanical energy required to drive the NH₃ reaction. All three processes (sourcing the energy, producing H₂ and N₂) produce CO₂ emissions. The development of a renewable pathway to NH₃ synthesis that utilizes concentrated solar irradiation for the process heat instead of hydrocarbon combustion and operates under relatively low pressure, will result in both a decrease (or elimination) in greenhouse gas emissions and avoid the cost, complexity, and safety issues inherent in high-pressure processes.

The aim of the Solar-Thermal Ammonia Production (STAP) project is to develop a solar thermochemical looping technology to produce and shuttle N₂ from air for the subsequent production of NH₃ in an advanced two-stage process. The endothermic thermal reduction of redox-active metal oxide particles is driven by concentrated solar irradiation; subsequent exposure to air re-oxidizes the particles, removing O₂ and producing relatively pure N₂ gas. The N₂ serves as a feedstock for a renewable NH₃ production process in an advanced two-step, low pressure, looping process. In the first step, reduced metal (or nitrogen deficient metal nitride) particles react with N₂ to form a nitride. In the second step, H₂ reacts with the resulting nitride to produce NH₃. The net result is NH₃ produced from sunlight, air, and (green) H₂, while the metal oxide and nitride particles are recycled.

The STAP project consists of four thrusts: (1) Systematic synthesis, characterization, and thermodynamic analysis of oxides for N₂ recovery from air; (2) materials investigation and development (synthesis, characterization, and thermodynamic analysis) of nitrides for NH₃ production and re-nitridation; (3) process and reactor design, modeling, fabrication, and small-scale demonstration, and (4) full system and techno-economic analyses. The system concept will be summarized in this presentation with a brief summary of the results to date.