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.
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