Producers may use a range of different feedstocks to generate their biomethane. They must however agree with Ofgem via a Fuel and Measurement Sampling Questionnaire which fuel classification each of their feedstocks falls into. These classifications are:
Products – where the primary reason for a feedstocks production was for use as a fuel in the anaerobic digestion process it will be classified as a product. Examples include Maize, Rye, Wheat and Sugar Beet crops.
Co-products – when a feedstock considered a product, such as Maize, is produced there may be other useful products created as part of that process, which are then classified as co-products.
Residues - when a feedstock is “not the end product that a production process directly seeks to produce, [is] not a primary aim of the production process and the process has not been deliberately modified to produce it” (more details here), then it may be considered a residue. Examples include cereal straw, unsalable vegetables, vegetable peelings and whey permeate.
Wastes – feedstocks that are substances or objects that would otherwise be discarded, or the holder would be required to discard, are considered wastes. Examples include food waste from domestic properties, food waste from commercial catering operations, sewage sludge and animal slurry/manures.
Further information on the process of classifying feedstocks can be found in Section 4. Fuel Classification of the Non-domestic RHI Sustainability Self-Reporting Guidance (version 2).
Where a biomethane producer is using a feedstock or feedstocks that fall into a single fuel classification all of the RGGOs related to that plant will have the same technology/feedstock identifier.
Where a biomethane producer is using feedstocks that are from more than one fuel classification then they will provide evidence to the GGCS of their feedstock-to-biomethane apportionment and their RGGOs will be given different technology/feedstock identifiers.
For example the producer calculates that feedstocks which are classified as products were responsible for 60% of their biomethane production and that feedstocks classified as wastes were responsible for 40% of their biomethane production. Of the total kWh of biomethane injected in that quarter, RGGOs accounting for 60% of that total would contain “Biomethane from Product or Co-Product” identifiers. RGGOs accounting for 40% of that total would contain “Biomethane from Wastes“ identifiers.
Biogas, and therefore biomethane, yields vary between feedstocks and producers make feedstock-to-biomethane apportionment calculations based on the best available information. Producers calculation methodologies are assessed during an annual audit and this audit may be available from the producer on request. More information can be found in the Non-domestic RHI Sustainability Self-Reporting Guidance (version 2).
More information on the fuel classification of GGCS producers can be found here