Carbon footprint of biomethane, natural gas and LNG: comparing chains in gCO₂e/MJ

The carbon footprint of biomethane can be much lower than that of natural gas, but the spread of results is large: from negative values for biomethane from slurry with closed digestate and low methane slip to several dozen gCO₂e/MJ for installations based on energy crops with high losses. Burning natural gas causes emissions of about 56 gCO₂/MJ (according to the IPCC default value), to which emissions from extraction, transmission and possible leaks are added; in the RED II methodology the fossil fuel comparator for transport has a value of 94 gCO₂e/MJ. The comparison below serves to understand the components, not to give a single "true" figure.

Components of the balance

In the RED II methodology (Directive (EU) 2018/2001, Annexes V and VI), CO₂ from burning biomethane is counted as zero, and the balance covers the remaining components.

Orders of magnitude (illustration)

The values below are illustrative and result from assumptions, not from the results of specific installations.

The ranges are wide because they depend on the assumptions adopted; exact results for a chain must be calculated or read from the current RED annexes.

Conversion to MWh

1 MWh = 3600 MJ. Example: biomethane with an intensity of 25 gCO₂e/MJ gives 90 kgCO₂e/MWh, and the fossil fuel comparator (94 g/MJ) – 338 kgCO₂e/MWh. The difference is 248 kgCO₂e for every MWh replaced by biomethane. For an installation selling 30 GWh per year this is 7440 t CO₂e per year (30,000 × 0.248). For biomethane with an intensity of −20 gCO₂e/MJ, the difference rises to 114 g/MJ, i.e. 410 kg/MWh and about 12,300 t CO₂e per year.

What changes the result most

1. Digestate management. An open tank can add a dozen to several dozen gCO₂e/MJ.
2. Methane slip. An increase from 0.5% to 2% slip adds a few gCO₂e/MJ (a 1% methane loss is about 0.2 g CH₄/MJ, i.e. 5.6 gCO₂e/MJ at GWP₁₀₀ = 28; in the RED II methodology a GWP of 25 is assumed, which gives about 5.0 gCO₂e/MJ).
3. Feedstock. Wastes and slurry have low or negative emissions, crops – positive.
4. Energy. Grid electricity with high emission intensity raises the result.
5. Transport. Long hauls of substrates and digestate increase emissions.

Natural gas: leak emissions

Methane leaks in the fossil gas chain can account for a significant share of whole-cycle emissions. Estimates differ depending on the region, source and measurement method; hence the politically significant EU methane regulation. For this reason comparisons of fossil gas should clearly state the assumptions about leaks.

How to compare fairly

• use the same unit (gCO₂e/MJ) and the same GWP,
• include all gases (CO₂, CH₄, N₂O),
• define the boundaries (well-to-tank, tank-to-wheel, well-to-wheel),
• indicate the data source and uncertainty,
• separate default values from actual calculations.

The importance of primary data

The most reliable are calculations based on measurement data from the installation: energy consumption, methane losses, actual quantities of substrates and transport. Default values from the annexes simplify the calculation, but do not reflect the merits of well-run projects. It is therefore worth building a data collection system from the start of operation and storing the data in a way that allows audit. In the case of fossil gas, the variability of leak data is a large uncertainty, so comparisons are best presented as ranges.

Summary

Biomethane can significantly lower the carbon footprint of gas, but the result depends on the feedstock and the quality of plant operation. Instead of a single figure, it is worth presenting a range and indicating the factors that change it.