Mass Balance
Mass balance is a specific Chain of Custody model.
What is Chain of Custody?
Chain of custody is the ‘process by which inputs and outputs and associated information are transferred, monitored and controlled as they move through each step in the relevant supply chain’ [ISO 22095:2020 Chain of Custody - General terminology and models accessed 08.09/2022].
What is Mass balance?
Mass balance is a chain of custody model where material or products with a set of specific characteristics (in this case material from a chemically recycled source) are mixed according to defined criteria with material or products without that set of characteristics (in this case virgin material) in a continuous operating production. The mass balance model ensures that the incoming material balances the outgoing materal within the mass balance period (minus losses). The recycled feedstock can then attributed to a selected product.
The diagram below explains this further.
Application of Mass Balance to Chemical Recycling
Why is mass balance so important?
Chemical recycling processes break down plastic into oils and gases which can be fed into a petrochemical plant as recycled content (there are different types of chemical / non-mechanical recycling which take the material back to different stages in the process - see Technologies). Petrochemical plants are large industrial operations dealing with thousands of tonnes of material. They are complex operations and therefore it is not possible to physically trace the exact product the chemically recycled material has gone into as it is mixed in a continous operating production process and the material is identical. Mass balance provides an alternative approach which still documents the material and ensure only the amount of recycled plastic feed into the process is in the final product (minus losses).
Mass balance can enable material to be allocated to a product or group of products. Assuming an even split across all products produced would mean the amount per product would be so minimal there would not be an incentive to invest in this technology as it offers no benefit for the product. Allocating material provides the incentive as companies can meet their sustainability targets or requirements, for example, under the plastic packaging tax. Companies therefore want to invest further in chemical recycling so there is more material available to be allocated.
In the Autumn Budget 2024, the government confirmed that to support use and investment in advanced chemical recycling technologies, businesses will be permited to use a mass balance approach to evidence recycled content in chemically recycled plastic for the Plastic Packaging Tax.
Where else is mass balance used?
Types of mass balance allocation
There are different mass balance approaches available:
- Free allocation
- Fuel exempt
- Polymer only
- Proportional allocation
Free allocation allows all the chemical recycled input material to be allocated except any losses in the system.
Fuel exempt means you can only allocate the percentage of the input material which does not go towards fuel and losses in the system also have to be removed.
Polymer only will only allow you to allocate the percentage of the input material which goes for polymer production.
Proportional allocation means each output is equally allocated chemically recycled content based on the proportion of that output which is produced.
In response to the 2023 mass balance consultation, the Autumn Budget 2024 revealed an important development: businesses will be permitted to utilise a mass balance approach to verify recycled content in chemically recycled plastics for the Plastic Packaging Tax (PPT). The government also confirmed that a fuel use exempt allocation method could be used.
Overview of chemical recycling and mass balance
Watch our short 2-minute video on chemical recycling below and discover the crucial role of mass balance in evidencing chemically recycled plastic.
