Thermodynamic analysis of novel vanadium redox materials for solar thermochemical ammonia synthesis from N2 and CH4

Abstract Owing to the wide applications of ammonia in hydrogen field and high energy consumption of the Haber-Bosch process, developing economic and environmentally benign ammonia synthesis process has attracted great interests. This work focuses on the moderately high two-step solar water-splitting of VN to produce ammonia, thus avoiding the reliance of fossil-fuel based heating source and pure hydrogen. Based on the equilibrium composition analysis, we find that V2O3, CH4 and N2 with mole ratio of 1:3:1.5 at TH = 1050 °C is enough for complete methane reforming and sufficient nitrogen fixation of V2O3. As for the water-splitting of VN, the production of NH3 is only possible at TL ≤ 400 °C, and inputting excessive water vapors is found to exert little effect on ammonia production at H2O:AlN>3:2. At the temperature range of full conversion between V2O3 and VN, the cycle efficiency, ηcycle, and solar-to-fuel efficiency, ηsolar-to-fuel, under different operating temperatures are compared, in which the highest ηcycle and ηsolar-to-fuel are 31.9% and 35.3% respectively. Moreover, efficiencies could be increased up to more than 37% with consideration of heat recuperation, demonstrating the great solar energy storage and fuel production potential of the proposed system.