This review examines the cost of green ammonia production and intercontinental transport to assess the complete supply chain for energy in a global, decarbonised ammonia economy. It further identifies the major constraints on supply and demand.

Ammonia production currently contributes 1.3% of global carbon dioxide emissions. In the context of global emission targets and growing demand, decarbonisation of this process is environmentally imperative. We present a method to calculate a first estimate for the optimum size of an ammonia production plant (at the process level), the required renewable energy (RE) supply, and the levelised cost of ammonia (LCOA) for islanded operation with a hydrogen buffer. A model was developed to quantitatively identify the key variables that impact the LCOA (relative to a ±10 GBP/tonne change in LCOA): levelised cost of electricity (±0.89 GBP/MWh), electrolyser CAPEX (±65 GBP/kW), minimum Haber-Bosch (HB) load (±12% of rated power), maximum rate of HB load ramping, and RE supply mix. Using 2025/2030 estimates results in a LCOA of 588 GBP/tonne for Lerwick, Scotland. The application of the model will facilitate and improve the production of carbon-free ammonia in the future.

Decarbonising ammonia production is an environmental imperative given that it independently accounts for 1.8% of global carbon dioxide emissions and supports the feeding of over 48% of the global population. The recent decline of production costs and its potential as an energy vector warrant investigation of whether green ammonia production is commercially competitive. Considering 534 locations in 70 countries and designing and operating the islanded production process to minimise the levelised cost of ammonia (LCOA) at each, we show the range of achievable LCOA, the cost of process flexibility, the components of LCOA, and therein the scope of LCOA reduction achievable at present and in 2030. These results are benchmarked against ammonia spot prices, cost per GJ of refined fuels and the LCOE of alternative energy storage methods. Currently a LCOA of $473 t⁻¹ is achievable, at the best locations the required process flexibility increases the achievable LCOA by 56%; the electrolyser CAPEX and operation are the most significant costs. By 2030, $310 t⁻¹ is predicted to be achievable with multiple locations below $350 t⁻¹. At $25.4 GJ⁻¹ currently and $16.6 GJ⁻¹ by 2030 prior combustion, this compares favourably against other refined fuels such as kerosene ($8.7–18.3 GJ⁻¹) that do not have the benefit of being carbon-free.

Due to the global need to decarbonize, it is increasingly clear that we are heading toward a new era of massive renewable energy generation and utilization to substitute fossil fuels. However, pragmatic means for renewable energy storage, transportation, and utilization at a very large scale are not yet fully developed and require careful scrutiny. In this Focus Review, we will consider the thermodynamics, kinetic barriers, material challenges, current science and technology readiness, and geographical locations for green hydrogen versus green ammonia as energy vectors for suitable sustainability of our future economy.

Green ammonia is a promising derivative of hydrogen with the capability to decarbonise the fertiliser and maritime industries, and to supplement clean energy supply in nations where renewable energy potential cannot satisfy demand. However, green ammonia production requires significant hydrogen and electricity storage infrastructure in order to convert variable renewable energy supply into a stable production process, which can meaningfully increase project costs. Here, we explore how a connection to the electricity grid may reduce these additional infrastructure costs. While previous authors have considered green ammonia plant optimisation without a grid connection (islanded production), or a grid connection with a fixed electricity price, this work considers the possibility of using both variable renewable energy and variably priced grid electricity for a large number of locations. Using Australia as a case study, we use a MILP model to optimise the production cost of green ammonia at a 1° spatial resolution, where the model can both buy and sell electricity from the grid if the connection costs are economically justified. The minimum Levelised Cost of Ammonia (LCOA) achievable when a grid connection is possible is almost 11% lower than if no grid connection is used, which amounts to savings of 2.5 USD GJ⁻¹. Benefits from the grid are most significant in the state of Tasmania; although it does not have the cheapest power on average, it provides the best opportunity for green ammonia plants to exploit low prices when they occur. We demonstrate that where an ammonia plant is both a consumer from and supplier to the national electricity network, it is robust to price fluctuation in the grid. Although no Australian electricity market is yet decarbonised, only a small fraction of total power supply for green ammonia production (<15%) comes from the grid, meaning that production using a semi-islanded approach can still be considered green in most Australian locations; in some cases, where power is sold back to the grid, the emissions avoided from electricity sale may be larger than the emissions generated from electricity purchase.