Default and actual emission values in RED II: when it pays to calculate your own

Under the RED II directive the GHG emissions of biomethane can be demonstrated in three ways: by adopting default values from the annexes, by calculating actual emissions on the basis of data from the installation, or by combining the two approaches through so-called disaggregated values (for individual stages of the chain). Default values are conservative, that is, higher than typical, to motivate producers to carry out more precise calculations. For this reason a well-run biogas plant often gains from actual calculations, provided it bears the cost of measurements and audit.

Three levels of accuracy

In the annexes the disaggregated values are given for the components e_ec, e_p, e_td, so one can, for example, use the default value for transport and calculate the actual emissions from processing.

What the default value for biogas depends on

The RED II annexes distinguish biogas and biomethane chains according to:

• type of feedstock: wet slurry, manure, maize silage (whole plant), biowaste;
• method of digestate management: open or closed tank (so-called "open digestate" and "closed digestate");
• off-gas management: whether the waste gas from upgrading is burned or released to the atmosphere;
• form of biomethane: biogas purified to compressed form or in the grid.

The differences between the "open digestate, no off-gas combustion" variant and "closed digestate, with afterburning" can reach several dozen gCO₂e/MJ. The exact values must be read in the current version of the annexes; we do not repeat them in the text so as not to mislead.

When it pays to calculate your own

The decision calculation can be built simply. Let us assume (illustratively) that the default value gives E_def = 45 gCO₂e/MJ, and the actual calculation with a closed digestate store and afterburning gives E_act = 25 gCO₂e/MJ. Annual production of 30 GWh is 30,000 MWh × 3600 MJ/MWh = 108 million MJ. Emission difference: 20 gCO₂e/MJ × 108 million MJ = 2160 t CO₂e a year. If the market values emission reduction (e.g. under premiums for low-emission fuels or the value of certificates), the additional reduction has real monetary value. If this value is, say, PLN 100/t CO₂e (an assumption), the benefit is PLN 216 thousand a year, against the cost of an audit and measurements of the order of several dozen thousand PLN. The figures are assumed; the scheme is what matters.

Data needed for actual calculations

1. Quantities and types of substrates (delivery records, mass balance).
2. Emission factors from cultivation or extraction (for crops – farm data, for wastes – usually zero).
3. Electricity and heat consumption and the source.
4. Methane slip in upgrading and the way the off-gas is treated.
5. The type of digestate storage and any biogas recovery from the store.
6. Transport of substrates and product.
7. Measurement documentation (meters, analyses, calibrations).

The role of certification bodies

Actual values are verified by an independent auditor under a voluntary scheme recognised by the European Commission (e.g. ISCC). Verification covers the consistency of data, the credibility of sources and the conformity of the calculations with the methodology. Measurements carried out by the producer itself are accepted if they meet the scheme's requirements; in case of doubt the auditor may demand additional tests.

Example of technical scenarios

Technical decisions that affect the result include: covering or equipping the digestate tank with gas recovery, afterburning off-gas from upgrading in a system with heat recovery, powering the installation with energy from its own sources, and shortening substrate delivery routes. Each of these decisions reduces one of the components e_p or e_td, and this shows up in actual calculations but not in default values. Investments in gas-tightness and afterburning can therefore pay back not only through environmental protection but also through a higher value of the product.

A common mistake

Assuming that a low carbon footprint results solely from the feedstock. In reality two installations on the same slurry can differ by several dozen gCO₂e/MJ because of the way the digestate is managed and the methane loss.

Summary

Using default values is the simplest, but often the most expensive in terms of lost benefits. If the installation has a closed digestate store, low methane slip and good documentation, calculating actual emissions usually pays off. Take care of the data from the first day of operation.