Assessment of Ammonia as an Energy Carrier from the Perspective of Carbon and Nitrogen Footprints

Ammonia has been proposed as a promising energy carrier and is expected to play a resilient and sustainable role in future energy scenarios. Energy systems critically impact biogeological carbon and nitrogen cycles. Thus, carbon and nitrogen footprints are two important indicators of sustainability for energy systems. In the present study, we explored the optimal supply pathway and identified impact hotspots by investigating the carbon footprint associated with greenhouse gas emissions and the nitrogen footprint associated with reactive nitrogen emissions from the ammonia energy system. Four scenarios (Japan to Japan, JP JP; Australia to Japan, AU JP; Chile to Japan, CL JP; Saudi Arabia to Japan, SA JP) were modeled based on international relations and energy distribution between these countries. Compared with the Japan electricity mix, it is a win win situation under scenario JP JP from the perspective of carbon and nitrogen footprints, while trade-offs were identified under the scenarios AU JP and CL JP. SA JP performed worse in both carbon and nitrogen footprints. Improvement of key processes is critical to mitigate greenhouse gas and reactive nitrogen emissions. When the efficiency of partial oxidation increased by 25% in SA JP, the carbon and nitrogen footprint decreased by 17% and 8%, respectively. This evaluation relayed information on the sustainable use of ammonia as an energy carrier by examining the relative impacts on both carbon and nitrogen footprints.