The carbon footprint of hydrogen production varies significantly depending on the method used, with green and blue hydrogen representing two prominent pathways. Green hydrogen is produced via electrolysis powered by renewable energy, while blue hydrogen is derived from fossil fuels, typically natural gas, with carbon capture and storage (CCS) mitigating emissions. A life cycle assessment (LCA) approach reveals critical differences in their CO2-equivalent emissions, spanning upstream, process, and downstream stages.
Upstream emissions encompass all greenhouse gas (GHG) releases associated with raw material extraction, energy generation, and infrastructure development. For green hydrogen, upstream emissions are tied to the manufacturing and installation of renewable energy systems, such as wind turbines or solar panels. These emissions are relatively low, often ranging between 1 to 5 kg CO2-equivalent per kg of hydrogen, depending on the energy source and location. In contrast, blue hydrogen’s upstream emissions include methane leakage during natural gas extraction and transportation. Methane, with a global warming potential 28-36 times higher than CO2 over a 100-year horizon, significantly impacts the total footprint. Estimates suggest upstream emissions for blue hydrogen can reach 5 to 10 kg CO2-equivalent per kg of hydrogen, heavily influenced by the methane leakage rate, which varies by region and extraction practices.
Process emissions arise directly from hydrogen production. Green hydrogen relies on electrolysis, which emits no CO2 during operation if powered entirely by renewables. However, the carbon intensity of the electricity grid can affect emissions if renewable penetration is incomplete. Assuming 100% renewable energy, process emissions are negligible. Blue hydrogen, however, depends on steam methane reforming (SMR) or autothermal reforming (ATR), both of which emit CO2. SMR produces approximately 9 to 12 kg CO2 per kg of hydrogen without CCS. With CCS, emissions can be reduced by 50-90%, depending on capture efficiency. ATR with CCS may achieve higher capture rates, around 90-95%, yielding process emissions of 1 to 3 kg CO2 per kg of hydrogen. The variability in CCS performance is a critical factor; imperfect capture or system failures can substantially increase net emissions.
Downstream emissions include those from hydrogen storage, transportation, and end-use. Both green and blue hydrogen share similar downstream challenges, such as energy losses during compression or liquefaction and potential leakage. Hydrogen leakage is a concern due to its indirect global warming effects, though current estimates suggest rates are low, below 1% in well-managed systems. The downstream footprint for both pathways typically adds 0.5 to 2 kg CO2-equivalent per kg of hydrogen, depending on the distribution method and distance.
The net emissions of blue hydrogen hinge on CCS efficiency. A system capturing 90% of CO2 will emit significantly less than one capturing only 60%. For example, SMR with 60% CCS may result in 4 to 6 kg CO2 per kg of hydrogen, while 90% CCS reduces this to 1 to 3 kg. However, even with high CCS rates, residual emissions and methane leaks prevent blue hydrogen from being entirely carbon-neutral. In contrast, green hydrogen’s emissions are front-loaded in the renewable infrastructure phase and approach near-zero during operation.
Renewable energy sourcing also plays a pivotal role in green hydrogen’s footprint. Solar PV-based electrolysis tends to have a lower carbon footprint than wind-based systems due to differences in material intensity and capacity factors. However, wind-powered electrolysis benefits from higher operational efficiency in certain regions. The choice of renewable technology and its location-specific performance can alter emissions by 1 to 3 kg CO2-equivalent per kg of hydrogen.
When comparing the two pathways, green hydrogen consistently demonstrates a lower carbon footprint under optimal conditions. Blue hydrogen can reduce emissions compared to conventional fossil-based hydrogen but remains dependent on CCS performance and upstream methane management. The following table summarizes key emission ranges:
| Pathway | Upstream Emissions (kg CO2-eq/kg H2) | Process Emissions (kg CO2-eq/kg H2) | Downstream Emissions (kg CO2-eq/kg H2) | Total Emissions (kg CO2-eq/kg H2) |
|------------------|--------------------------------------|--------------------------------------|----------------------------------------|-----------------------------------|
| Green Hydrogen | 1-5 | 0-0.5 | 0.5-2 | 1.5-7.5 |
| Blue Hydrogen | 5-10 | 1-6 | 0.5-2 | 6.5-18 |
The data highlights that blue hydrogen’s total emissions can overlap with the upper range of green hydrogen, particularly when CCS efficiency is low or methane leakage is high. Green hydrogen’s advantage grows as renewable energy systems become cleaner and more efficient.
In conclusion, LCA reveals stark differences between green and blue hydrogen. Green hydrogen, powered by renewables, offers a path to near-zero emissions, while blue hydrogen’s footprint is contingent on CCS efficacy and methane control. Policymakers and industry must prioritize CCS improvements and methane reduction strategies to make blue hydrogen a viable transitional solution. Meanwhile, scaling renewable capacity remains essential for minimizing the carbon footprint of hydrogen production in the long term.