Thermodynamics of metal reactants for ammonia synthesis from steam, nitrogen and biomass at atmospheric pressure

Catalytic ammonia synthesis at approximately 30 MPa and 800 K consumes about 5% of the global annual natural gas production causing significant CO 2 emissions. A conceptual solar thermochemical reaction cycle to produce NH 3 at near atmospheric pressure without natural gas is explored here and compared to solar thermochemical steam/air reforming to provide H 2 used in the Haber‐Bosch process for NH 3 synthesis. Mapping of Gibbs free energy planes quantifies the tradeoff between the yield of N 2 reduction via metal nitridation, and NH 3 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 (e.g., Mg or Ce) with a transition metal (e.g., MgCr 2 O 4 , MgFe 2 O 4 , or MgMoO 4 ) may enable the concept near 0.1 MPa (at maximum 64 mol % yield of Mg 3 N 2 through nitridation of MgFe 2 O 4 at 1,300 K, and 72 mol % of the nitrogen in Mg 3 N 2 as NH 3 during hydrolysis at 500 K).