Thermochemical and Techno-Economic Evaluation of Ammonia Synthesis Via Lithium-Molten Salt Processes

Abstract

Ammonia is an attractive carbon-free energy carrier as it is easily liquified, transported and stored at a lower energy penalty than other non-carbon containing fuels. Anhydrous ammonia production occurs today in large Haber-Bosch (H-B) plants fueled from natural gas. Industrial (H-B) plant techno-economics are poorly suited for remote small-scale ammonia production from renewable energy. Highly efficient ammonia synthesis in small-scale H-B systems does not exist, thus stimulating research towards a low temperature and pressure processes. The thermodynamics of low temperature ammonia synthesis are quite favorable however, the high activation energy for nitrogen reduction requires efficient catalysts and high temperatures. Electrochemistry offers a means to activate the process at lower temperatures however this has proven to be very challenging [1]. Direct electrochemical protonation of nitrogen at low temperatures has produced rates on the order of 10-9 to 10-10 moles/cm2-sec, many orders of magnitude too low for industrial production [2]. Nitrogen reduction on lithium metal occurs spontaneously; hence molten lithium salt processes may prove to be attractive for ammonia generation [3,4,5]. We have evaluated the thermodynamics and techno-economics of intermediate temperature NH3 synthesis processes, based on the spontaneous electro-reduction of nitrogen in lithium ion systems where the potential for lithium is close to metal deposition and the subsequent formation of lithium nitride. The advantage of the molten salt lithium processes is the reaction is autocatalytic however the direct protonation of Li3N with hydrogen yields lithium amide. Protonation with water does yield ammonia however the reaction is very exothermic. The electrolysis of the LiOH product is also a major obstacle as the reaction between LiOH and Li produces Li2O[6]. To overcome this difficulty, we have also evaluated alternative iodide and bromide cycles were lithium electrowinning is known to be viable. The techno-economics of the processes were also considered in a preliminary study.