Can Decentralized Electrosynthesis Surpass Centralized Production in Cutting Ammonia's CO2 Emissions?

Decarbonization of current centralized ammonia production, methane steam reforming followed by Haber-Bosch, is important for global efforts against climate changes. Electrochemical nitrate reduction (NO3RR) and metal-mediated nitrogen reduction (M-eNRR) offer decentralized alternatives, though their decarbonization potential remains unclear. This study evaluates the energy demands of decentralized ammonia production from wastewater nitrate (≤2000 ppm), showing that direct NO3RR with steam stripping or air stripping requires more energy than the centralized method. Therefore, a “concentrate-convert-separate” strategy is proposed to reduce energy use. Among six process combinations, only three reduce the total energy cost, with reverse osmosis coupled with NO3RR and air stripping (RO-NO3RR-AS) matching the centralized method if NO3RR operates above 91.5% efficiency. Interestingly, RO-NO3RR-AS energy use decreases with nitrate concentration but rises above 17,000 ± 6% ppm. A wind-powered, distributed RO-NO3RR-AS system may achieve lower emissions than centralized productions of green ammonia but is limited by nitrate availability. Conversely, M-eNRR using N2 offers broader feedstock flexibility. However, Li- or Ca-mediated M-eNRR shows higher emissions than the centralized method, while transition metals (e.g., Fe, Mo and their clusters mimicking nitrogenases) could theoretically rival the centralized method in energy and emissions. With ultra-low-cost carbon-free electricity, high-efficiency, low-cost NO3RR and M-eNRR could enable viable decentralized ammonia production with emission reductions.