Chain of custody and blockchain in the bioeconomy and the circular economy – Mass balance model (1) – Overview and types
Series: Chain of custody and blockchain in the bioeconomy and the circular economy
- Mass balance model (1) – Overview and types
Introduction
The chemical industry uses a small number of feedstocks to generate tens of thousands of different products. A large part of this production starts in the steam cracker in the conventional petrochemical industry, where steam is used to split naphtha (a mix of long-chain hydrocarbons) into smaller molecules (hydrogen, methane, ethylene and propylene). These molecules then serve as the building blocks for downstream production (plastics, coatings, solvents…).
There is an arising availability of renewable naphtha coming from biobased and recycled feedstocks. Deploying new dedicated biorefineries and circular refineries as well as the application of identity preservation and segregation models in existing facilities entail a challenge due to the huge capital investment required. Conventional petrochemical sites are already processing these biobased and recycled feedstocks. However, they operate at scales far beyond the possible current supply of renewable feedstocks. Diluted amounts of renewable feedstocks are used alongside non-renewable raw material, leading to small amounts of renewable content in the vast array of final materials. Once a material or chemical is recycled into simpler building blocks, it cannot be distinguished from identical building blocks of other origins, making the traceability of recycled feedstock a key challenge.
Mass balance method offers a workable set of rules to ensure the traceability of renewable feedstock into new products. The second post of the series defines the main characteristics of this Chain of Custody model and shortly describes the different levels or variations of the model.
Key characteristics
- Biobased or recycled feedstock replaces an equivalent amount of fossil feedstock at the beginning of the value chain (input) to be allocated to the product (output) in such a manner that the input and output match. Certified physical product is not separated from and may be mixed with non-certified physical product at any stage in the production process, provided that the quantities are controlled.
- The products can be processed exactly like conventionally produced materials. There is no need to adapt formulations, plants or processes. Customers who buy mass-balanced products can use them as they would traditional products, while benefiting from the same level of quality.
- The balancing task must be met in a proven and reliable manner by considering boundary conditions in the calculation: (1) amount of fossil feedstock necessary for the manufacturing of each product; (2) amount of renewable feedstock can replace a certain amount of fossil feedstock; (3) system boundaries in space and time.
- It is based on reconciliation of inputs and outputs of the certified product through the manufacturing process, including all product variants or derivatives resulting from the original certified product. The reconciliation period is the pre-defined timeline within which this process should happen (to prevent indefinite prolongation).
- Independent institutes audit the allocation. At the moment, experts are still using different methods for auditing and certification.
- There are different approaches to quantify and to compare the feedstock sources: mass determination; energetic determination (LHV); carbon counting (determination based on chemical reaction); 12C/14C analysis (measurement of sustainable share).
Figure 1. The mass balance approach along the supply chain (taken from the Reference [3])
How it works? (videos of relevant companies)
Perstorp – Explaining mass balance
Basf – Biomass Balance Approach
Levels (see specific examples on the Reference [1])
The volumes can be balanced at: the batch level, the site level and the group level. These options are significantly different and the ability to audit is stronger with batch, then decreases for site, then decreases further for group level.
Batch-level (also called: percentage blending, batch blending)
Segregation until the final point of blending or mixing for a specific batch of a product.
Mixing with non-certified product is controlled and recorded, so the proportion of certified content in each final product is known.
The end-product contains at least a proportion of certified product, which allows specific end-use claims to be made.
Site-level (also called: controlled blending, factory gate mass balance)
Segregation until the manufacturing or processing stage in the supply chain, when the certified product can then be mixed with non-certified product.
The proportions of certified and non-certified product at the overall site level are recorded and reconciled.
The proportion of certified content entering and leaving the site is known, though the proportion in each final product may not be known.
Group-level (also called: multi-site)
"Group" may refer to a company with several sites, a country or any other combination of more than one site where volumes are tracked.
Physical mixing or volume reconciliation of certified and non-certified product is allowed at any stage in the production process provided that the quantities are controlled in documentation.
The sustainability data or certified claim can be allocated to any physical product leaving the system as long as the volumes are appropriately balanced.
REFERENCES
[1] “Chain of custody models and definitions”. ISEAL Alliance, September 2016. [2] Whitepaper "Enabling a circular economy for chemicals with the mass balance approach", CE100 network of the Ellen MacArthur Foundation, 2020.
[3] ISCC website – “The Mass Balance Approach”.
[4] Perstorp website – “Mass balance - a necessary strategy towards renewables”.
[5] BASF website – “The Mass Balance Approach”.
