Green ammonia from air, water, and renewable electricity: Energy costs using natural gas reforming, solid oxide electrolysis, liquid water electrolysis, chemical looping, or a Haber–Bosch loop

Abstract

The purpose of this work is to quantitatively compare the energy cost of design alternatives for a process to produce ammonia (NH3) from air, water, and renewable electricity. It is assumed that a Haber–Bosch (H–B) synthesis loop is available to produce 1000 metric tons (tonnes) of renewable NH3 per day. The overall energy costs per tonne of NH3 will then be estimated at U.S.$195, 197, 158, and 179 per tonne of NH3 when H2 is supplied by (i) natural gas reforming (reference), (ii) liquid phase electrolysis, (iii) solid oxide electrolysis (SOE) of water only, and (iv) simultaneous SOE of water and air. A renewable electricity price of U.S.$0.02 per kWhelectric, and U.S.$6 per 106 BTU for natural gas is assumed. SOE provides some energy cost advantage but incurs the inherent risk of an emerging process. The last consideration is replacement of the H–B loop with atmospheric pressure chemical looping for ammonia synthesis (CLAS) combined with SOE for water electrolysis, and separately oxygen removal from air to provide N2, with energy costs of U.S.$153 per tonne of NH3. Overall, the most significant findings are (i) the energy costs are not substantially different for the alternatives investigated here and (ii) the direct SOE of a mixture of steam and air, followed by a H.–B. synthesis loop, or SOE to provide H2 and N2 separately, followed by CLAS may be attractive for small scale production, modular systems, remote locations, or stranded electricity resources with the primary motivation being process simplification rather than significantly lower energy cost.