Publication Details (including relevant citation information):
Ronald Michalsky, Peter H. Pfromm: Thermodynamic rationale for designing metal reactants for reactive synthesis of ammonia from steam, nitrogen and biomass at atmospheric pressure (submitted, AIChE Journal).
Catalytic ammonia synthesis at approximately 30 MPa and 800 K is technically demanding and consumes about 5% of the global annual natural gas production causing significant CO2 emissions. A conceptual solar thermochemical reaction cycle to produce NH3 at near atmospheric pressure without natural gas is explored here and compared to solar thermochemical steam/air reforming to provide H2 utilized in the Haber-Bosch process for NH3 synthesis. Mapping of Gibbs free energy planes quantifies the trade-off between the yield of N2-fixation via metal nitridation and NH3 liberation via steam hydrolysis vs. the temperatures required for reactant recovery from undesirably stable metal oxides. Equilibrium composition simulations suggest that reactants combining an ionic nitride-forming element with a transition metal (e.g., MgO*Cr2O3, MgO*Fe2O3, or MgO*MoO3) may enable the concept near 0.1 MPa (approximately 64 mol% yield of Mg3N2 through nitridation of MgO*Fe2O3 at 1300 K, and 72 mol% of the nitrogen in Mg3N2 as NH3 during hydrolysis at 500 K). To stabilize the metal nitride at elevated temperatures cerium is considered as alternative reactant constituent.