Ammonia is increasingly recognized as an important, sustainable fuel for global use in the future. Applications of ammonia in heavy transport, power generation, and distributed energy storage are being actively developed. Produced at scale, ammonia could replace a substantial fraction of current-day liquid fuel consumption. This ammonia-based economy will emerge through multiple generations of technology development and scale-up. The pathways forward in regard to current-day technology (generation 1) and immediate future approaches (generation 2) that rely on Haber-Bosch process are discussed. Generation 3 technology breaks this nexus with the Haber-Bosch process and enables direct reduction of dinitrogen to ammonia electrochemically. However, the roadmap toward scale in this technology has become obscured by recent research missteps. Nevertheless, alternative generation 3 approaches are becoming viable. We conclude with perspectives on the broader scale sustainability of an ammonia economy and the need for further understanding of the planetary nitrogen cycles of which ammonia is an important part. It is increasingly clear that there is massive global potential to generate renewable energy at costs already competitive with fossil fuels. However, a means of storing and transporting this energy at a very large scale is a roadblock to further development and investment. Ammonia produced from renewables is widely seen as viable liquid fuel replacement for many current-day uses of fossil fuels, including as a shipping bunker fuel, as a diesel substitute in transportation, as a replacement fuel in power turbines, and even as a potential jet fuel. The global transportation of ammonia by pipeline and bulk carrier is already a well-developed technology. In this roadmap, we envisage renewable ammonia being produced in the future at a scale that is significant in terms of global fossil fuel use. This will emerge via three overlapping technology generations. Generation 1 is based on an expansion of current-day Haber-Bosch ammonia production using CO2 sequestration or offsets. Generation 2 moves the Haber-Bosch process to renewable sources of hydrogen, while generation 3 avoids the need for the Haber-Bosch process entirely by direct electrochemical conversion of N2 to NH3. One of the attractive features of generation 3 technology is that it can be implemented at any level of scale, from kW to GW, and in a highly distributed fashion. Ammonia produced sustainably and at sufficient scale could become one of the important liquid fuels and energy stores of the future. This roadmap article surveys the state of development of the production technologies and the many developing modes of direct use of ammonia as a liquid fuel, including as a shipping bunker fuel, as a diesel substitute in transportation, as a replacement fuel in power turbines, and even as a potential jet fuel. Economic, safety, and sustainability factors impacting on this roadmap are also discussed.
- nitrogen fixation