Life cycle assessments of ammonia production reveal significant variations in environmental impact depending on the hydrogen source used in the synthesis process. Ammonia, primarily produced via the Haber-Bosch process, relies heavily on hydrogen as a feedstock, making the sustainability of its production closely tied to the methods used for hydrogen generation. The three main hydrogen sources—gray, blue, and green—each present distinct profiles in terms of greenhouse gas emissions, water consumption, and energy requirements across the value chain.
Greenhouse gas emissions are a critical metric in ammonia LCAs. Gray hydrogen, derived from steam methane reforming of natural gas without carbon capture, results in the highest emissions, typically ranging between 8 to 12 kg CO2-equivalent per kg of hydrogen. When this hydrogen is used for ammonia synthesis, the total emissions for gray ammonia can reach 1.6 to 2.4 kg CO2-equivalent per kg of ammonia, accounting for both hydrogen production and the Haber-Bosch process. Blue hydrogen, which incorporates carbon capture and storage (CCS) during SMR, reduces emissions substantially, with reported values of 1.5 to 3 kg CO2-equivalent per kg of hydrogen. Consequently, blue ammonia emissions fall between 0.6 to 1.2 kg CO2-equivalent per kg of ammonia, depending on CCS efficiency. Green hydrogen, produced via electrolysis using renewable electricity, offers near-zero emissions during hydrogen production, leading to ammonia emissions as low as 0.2 to 0.5 kg CO2-equivalent per kg, primarily from the Haber-Bosch process and upstream renewable infrastructure.
Water usage is another key factor in ammonia LCAs. Gray and blue hydrogen production consume significant amounts of water, primarily for steam generation and cooling in SMR plants, with estimates ranging from 10 to 20 liters per kg of hydrogen. Green hydrogen, while free of process emissions, requires substantial water inputs for electrolysis, averaging around 9 to 12 liters per kg of hydrogen. However, the source of water matters—electrolysis using desalinated seawater increases energy demand, whereas renewable-powered electrolysis in water-rich regions minimizes additional burdens. The water footprint of ammonia synthesis itself is relatively small, but the cumulative water use across the hydrogen production stage dominates the LCA results.
Energy inputs vary widely across hydrogen sources. Gray hydrogen is energy-intensive due to the high-temperature SMR process, with energy demands of approximately 50 to 55 kWh per kg of hydrogen. Blue hydrogen has similar energy requirements but includes additional energy for CCS, adding 10 to 20% to the total. Green hydrogen demands the highest energy input, with electrolysis requiring 50 to 60 kWh per kg of hydrogen, though this is offset by the use of renewable electricity. The Haber-Bosch process itself is energy-intensive, consuming 8 to 12 kWh per kg of ammonia, regardless of hydrogen source.
Methodological challenges complicate LCA comparisons. System boundaries differ across studies, with some including upstream natural gas extraction or renewable infrastructure manufacturing, while others focus solely on hydrogen production and ammonia synthesis. Allocation methods for co-products, such as oxygen from electrolysis or sulfur from natural gas processing, also influence results. Temporal and geographical variability further affect LCA outcomes—green hydrogen’s emissions depend on regional grid carbon intensity, while blue hydrogen’s effectiveness hinges on CCS infrastructure availability. For example, ammonia produced with green hydrogen in a region with high solar irradiance will show better performance than in areas reliant on grid electricity with fossil fuel dominance.
Regional variability is pronounced. In regions with abundant natural gas reserves, such as the Middle East or North America, gray and blue ammonia dominate due to low feedstock costs and existing infrastructure. Conversely, countries with strong renewable energy policies, like those in Northern Europe, are pivoting toward green ammonia. The carbon intensity of grid electricity also plays a role—green ammonia in Norway, with its hydropower-based grid, has a lower footprint than in regions where renewables are less established.
A comparative summary of key LCA metrics is presented below:
Hydrogen Source | GHG Emissions (kg CO2-eq/kg NH3) | Water Usage (liters/kg NH3) | Energy Input (kWh/kg NH3)
Gray | 1.6 - 2.4 | 15 - 25 | 30 - 35
Blue | 0.6 - 1.2 | 12 - 22 | 32 - 38
Green | 0.2 - 0.5 | 10 - 18 | 40 - 50
The table highlights trade-offs between emissions, water, and energy. While green ammonia excels in emissions reduction, it demands higher energy inputs and careful water resource management. Blue ammonia offers a transitional solution with moderate emissions and lower energy penalties compared to green, but CCS dependency introduces logistical and cost barriers. Gray ammonia remains the most polluting but is currently the most economically viable in many regions.
Future LCA research should address gaps in data quality, particularly for emerging technologies like advanced electrolysis or novel ammonia synthesis methods. Standardized methodologies and transparent reporting will enhance comparability across studies. As the ammonia industry evolves toward decarbonization, LCAs will play a pivotal role in guiding policy and investment decisions, ensuring that sustainability metrics are prioritized alongside economic and technical feasibility.
The environmental performance of ammonia is inextricably linked to its hydrogen feedstock, making the choice between gray, blue, and green hydrogen a defining factor for the sector’s sustainability. While green ammonia holds promise for deep decarbonization, regional infrastructure, resource availability, and technological advancements will shape its adoption trajectory. Blue ammonia serves as a bridge, but its long-term viability depends on CCS scalability and cost reductions. Gray ammonia’s dominance underscores the urgency of transitioning to cleaner alternatives to meet global climate targets. Life cycle assessments provide the necessary framework to evaluate these pathways rigorously, ensuring informed decision-making for a sustainable ammonia economy.