Techno-Economical Analysis of Solar Thermochemical Ammonia Production At near Atmospheric Pressure

Techno-economical analysis of solar thermochemical ammonia production at near atmospheric pressure

Ronald Michalsky ¹, Peter H. Pfromm ¹, Bryon Parman ², Vincent Amanor-Boadu ²

¹ Department of Chemical Engineering, ² Department of Agricultural Economics,

Kansas State University, Manhattan, Kansas, USA

 

Ammonia is the basis for modern agriculture and ensures the food supply for a growing world population. It is used widely as base chemical for the chemical industry. Over 100 million metric tons NH₃ are produced world wide per year. NH₃ has been proposed recently as sustainable transportation fuel and may enable efficient H₂ storage.

At industrial scale the Haber-Bosch process synthesizes NH₃ catalytically at high pressure and elevated temperature with natural gas and nitrogen as main inputs. Globally this process consumes up to 5% of all natural gas produced and 2% of the total world energy production, with significant fossil-based CO₂ emissions. Aiming at reactive NH₃ synthesis without natural gas from air and steam at ambient pressure, this work studies a solar thermochemical cycle employing a molybdenum-based reactant.

Based on Gibbs free energy computations, a process sequence of Mo₂N formation from its elements, nitride hydrolysis forming NH₃ and MoO₂ at ambient pressure, and metal oxide reduction using solar heat and H₂ as chemical reducing agent below 1200 °C, is proposed. Experimental data on a Mo-based transition element reactant utilized for the fixation of N₂ and formation of NH₃ from the solid nitride hydrolyzed with steam will point out process limiting steps such as diffusion in the solid state and ionicity of an interstitial nitride reactant. To assess sustainability and economic competitiveness compared to the state of the art Haber-Bosch process, the mass and energy balance for Mo-based solar thermochemical NH₃ synthesis including on-site production of H₂ used as reducing agent will be presented along with an economic analysis determining the price of NH₃ required for breaking even with plant investment and operational costs.