Feasibility of Fe-based nitrogen carrier for chemical looping ammonia synthesis: thermodynamics.

Chemical looping ammonia synthesis (CLAS) is a novel and energy-efficient technology to generate ammonia. The synthesis system includes dual chemical processes, namely nitridation and ammoniation. The nitrogen carrier absorbs nitrogen in the nitridation reactor, and ammonia is synthesized in the ammoniation reactor. The continuous ammonia production process is achieved depending on the capability of the nitrogen carrier. In this paper, we propose a new chemical reaction path to achieve the CLAS and the feasibility of the Fe-based nitrogen carrier (Fe₄N/Fe₂N) used for the reaction was investigated through the thermodynamic method. According to the analysis of Gibbs free energy change during the synthesis of ammonia (NH₃), the nitridation cannot proceed spontaneously at high temperatures. The ammoniation can proceed spontaneously at a temperature not exceeding 500 °C, generating NH₃ and Fe, thereby realizing the chemical looping cycle. Thermodynamic equilibrium calculation, based on Gibbs free energy minimization principle, was used to simulate the influence of several important parameters, viz. temperature, pressure, and molar ratio, on the yields of nitrogen carrier and ammonia in nitridation and ammoniation reactions, respectively. The result shows that the most suitable conditions for the nitridation reaction are at a temperature of 200–500 °C, a reaction pressure of 5–20 bar, and a molar ratio of Fe and N₂ of 8:1. For the amination reaction, the optimized reaction temperature is 200–400 °C, the reaction pressures should fall below 5 bar, and the molar ratio of the nitrogen carrier and H₂ is 2:3. [ABSTRACT FROM AUTHOR]