Carbon footprint of hydrogen from gas, electrolysis and biomethane: comparison and EU thresholds

The carbon footprint of hydrogen depends on how it is produced and can differ by an order of magnitude: hydrogen from natural gas reforming without CO₂ capture emits about 9–10 kg CO₂ per kg H₂, with capture several times less, electrolysis powered by electricity from a high-emission grid can give even more than 30 kg CO₂/kg H₂, and hydrogen from biomethane from slurry can have values close to zero or negative. In EU law, renewable fuels of non-biological origin (RFNBO) and low-carbon hydrogen are subject to a 70% emission reduction threshold relative to the fossil fuel comparator of 94 gCO₂e/MJ, i.e. 28.2 gCO₂e/MJ, which corresponds to about 3.38 kg CO₂e/kg H₂.

Unit conversion

The calorific value of hydrogen is 120 MJ/kg. Hence 1 gCO₂e/MJ corresponds to 0.12 kg CO₂e/kg H₂. The threshold of 28.2 gCO₂e/MJ is 3.38 kg CO₂e/kg H₂. The comparator of 94 gCO₂e/MJ corresponds to 11.3 kg CO₂e/kg H₂.

Comparison of pathways (worked examples)

For biomethane reforming it was assumed that the determining factor is the emission intensity of the biomethane (which already includes supply chain emissions); the CO₂ from reforming is biogenic. Additional emissions from auxiliary energy may raise the result. The values in the table are illustrative.

Why a coal-based grid ruins the electrolysis balance

Electrolysis consumes about 52 kWh/kg H₂. With a grid intensity of 0.6 tCO₂/MWh this gives 31 kg CO₂ per kg of hydrogen – almost three times more than gas reforming. That is why EU law requires additionality and temporal and geographical correlation for RFNBO, so that the hydrogen is produced from genuinely additional renewable energy, and not from energy that comes from the existing mix.

Hydrogen from biomass in the regulations

Hydrogen produced from biomass (biomethane reforming, gasification) is subject to the sustainability and GHG emission reduction criteria for biomass fuels. For biofuels in transport from plants commissioned after 2020, the required reduction is at least 65% relative to 94 gCO₂e/MJ, i.e. a maximum of 32.9 gCO₂e/MJ (about 3.95 kg CO₂e/kg H₂). The exact requirements depend on the application and the current version of the regulations. Low-carbon hydrogen from fossil fuels with CO₂ capture has a separate calculation methodology (taking into account methane leaks and capture efficiency).

The role of methane leaks

In fossil gas reforming, methane leaks in the supply chain can significantly raise the footprint of hydrogen. If, for example, we assume a chain with an intensity of 15 gCO₂e/MJ of gas, and gas consumption is 170 MJ/kg H₂, the additional emissions are 2.55 kg CO₂e/kg H₂ – almost as much as the entire threshold of 3.38. This explains the importance of the methane regulation and of controlling methane losses.

Example: hydrogen from biomethane and the EU threshold

Biomethane with an intensity of 25 gCO₂e/MJ gives hydrogen with 170 × 25 = 4250 g, i.e. 4.25 kg CO₂e/kg H₂ – above the threshold of 3.38. To fit within the 70% threshold (3.38 kg CO₂e/kg H₂), the biomethane must have an intensity below about 20 gCO₂e/MJ (3380 g / 170 MJ = 19.9); for the 65% threshold applied to biomass fuels in transport (3.95 kg CO₂e/kg H₂) it would be about 23 gCO₂e/MJ (3950 g / 170 MJ = 23.2). This requires closed digestate storage, low methane slip and good logistics, and is easiest to achieve with slurry and waste.

Conclusions

1. The footprint of hydrogen does not result from its “colour”, but from data: the energy source, methane losses and CO₂ capture.
2. Electrolysis is clean only when the electricity is clean.
3. Biomethane from waste and slurry can be a feedstock for hydrogen with a low or negative footprint.
4. When assessing projects, demand a balance stating the system boundaries and methodology.

Summary

Comparisons of the carbon footprint of hydrogen require consistent units and explicit assumptions. Hydrogen from biomethane can meet EU requirements if the biomethane has a low footprint, but the details of the production chain are decisive.