Small-scale ammonia production: a superstructure approach

Ammonia is an important chemical for the energy transition as it can serve as a hydrogen carrier. Small-scale ammonia can be produced from decentralized or waste resources thereby integrating and expanding its use by delivering the product directly to the site of consumption. Nonetheless, the cost of small-scale ammonia is usually higher than largescale ammonia due to the lack of economy of scale even though new technologies for H₂ and NH₃ synthesis are available. This work presents a superstructure (SE) as a decisionmaking tool to select the best set of technologies delivering the lowest ammonia cost. The SE is formulated as a mixed-integer linear program (MILP) comparing three different routes: electrolysis, gasification, and thermal decomposition of methane (TDM). The optimal route for a base case of 1000 kgNH₃/h is electrolysis with a cost of 1067 $/t of NH₃. The capital expenditure (CAPEX) of electrolysis and ammonia synthesis, as well as the electricity cost, are the main contributors to this cost. The electrolysis route is the most promising for small-scale ammonia since it envisions the highest potential for cost reduction from lower electrolyzer and electricity prices. Gasification can also become competitive if the price of biomass is low but the cost of carbon capture and storage (CCS) needs to be considered. The TDM route is not suited for small scale due to large CAPEX contribution. The presented tool advances the spread and use of small-scale ammonia.