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A carbon-free, circular economy is required to decrease greenhouse gas emissions. A commonly named alternative to the carbon-based economy is the hydrogen economy. However, storing and transporting hydrogen is difficult. Therefore, the ammonia economy is proposed. Ammonia (NH3) is a carbon-free hydrogen carrier, which can mediate the hydrogen economy. Especially for long-term storage (above 1 day), ammonia is more economically stored than hydrogen.
Transportation costs are greatly reduced by adopting a decentralized energy economy. Furthermore, political-economic factors influence energy prices less in a decentralized energy economy. With small-scale ammonia production gaining momentum, business models for the decentralized ammonia economy are currently under development.
Within the current research, current technological advances are reviewed, as well as their feasibility for long-term, industrial application. A process flow diagram was developed, based on the conceptual process design method by Douglas. An example case of 3.06*107 kWh y-1 electricity requirement is used. The solution constitutes of wind and solar power, combined with a battery for short-term storage (up to 1 day) and a Power-to-Ammonia-to-Power process for long-term storage (above 1 day).
Technology pushes within the process design are the development of new materials (the Battolyser for a combined battery function and hydrogen generation, Ru/Ba-Ca(NH2)2 for ammonia synthesis, and CaCl2/SiO2 for ammonia separation and storage), enabling the decentralized production of ammonia from renewables at a low pressure (16 bar). Furthermore, the development of ammonia-fueled solid oxide fuel cells (SOFCs) enables electricity generation from ammonia directly, rather than from a cracked hydrogen and nitrogen feed stream. This simplifies the process design significantly (i.e., no ammonia cracker is required), and temperature swings within the process are decreased.
The combined ammonia separation and storage by CaCl2/SiO2 (a form of process intensification) is heat integrated with the SOFC, giving an electrical round-trip efficiency of ~30% for P2A2P (Power-to-Ammonia-to-Power). For comparison, the electrical round-trip efficiency of the battery function is ~80% for the Battolyser.
The total capital investment of such a system (3.06*107 kWh y-1) is about 30-40 M€, including wind and solar power, a battery for short-term storage, and the Power-to-Ammonia-to-Power process. By modular design, the installation time is decreased and the working efficiency increased. Depending on the electricity price (in the range 0.10-0.20 € kWh-1), the payback period varies from 10 to 30 years.
Ammonia synthesis has been termed the bellwether reaction, and technological advancements go hand in hand with new complications. Therefore, a roadmap is presented with potential improvements on current research and technologies, with a focus on mechanistic understanding of the processes involved. In case the desired technologies are not feasible, energy consumption penalties are indicated for alternative technologies.
In the upcoming years, it becomes technologically feasible to perform decentralized electricity storage in the form of ammonia. Ammonia as an energy vector is currently widely researched. Various technologies are competing (and transmutable), making the implementation of an ammonia economy technologically feasible. Furthermore, technological developments in the upcoming years can provide even better solutions for the decentralized ammonia economy.