Co-processed Biofuels

As the shipping industry seeks solutions to lower greenhouse gas (GHG) emissions, co-processed biofuels are emerging as a scalable, drop-in alternative. However, it is essential to understand their characteristics and content as they differ from a standard biofuel.

Co-processed fuel is in its infancy in the maritime sector. While it provides operational benefits it raises concerns about traceability, carbon accounting, and fuel transparency.

What are co-processed biofuels?

Co-processing is the integration of bio-derived feedstocks - such as used cooking oil, animal fats, or less conventional materials like cashew nutshell liquid (CNSL) or tyre-derived oils - into established refinery operations. These materials are processed in conjunction with traditional fossil fuel streams within existing refinery infrastructure to produce a final fuel that delivers greenhouse gas (GHG) reductions across its full life cycle. This approach offers advantages for refineries, enabling their continued operation while supporting decarbonisation efforts.

During refining, the renewable and fossil components are chemically transformed together. The process is designed to remove oxygen and reshape other molecules, producing a final fuel that is chemically very similar to conventional petroleum products.

As a result, co-processed fuels:

• Broadly align with ISO 8217 specifications

• Are compatible with existing engines and infrastructure

• Avoid the handling issues associated with traditional biofuel blends

These fuels are appealing to ship operators as convenient options, but they are classified as standard grade fuels instead of biofuels like those listed in ISO 8217:2024.

How do they differ from conventional biofuels?

The key difference lies in physical traceability and composition.

Conventional marine biofuels, such as FAME blends or hydrotreated vegetable oil (HVO), are produced in dedicated processes. Their renewable content is physically present and measurable, and blend ratios (e.g. B30) are clearly defined.

In contrast, co-processed fuels are:

• Chemically indistinguishable from fossil fuels.

• Produced through integrated refinery processes.

• Not physically separable into renewable and fossil components.

This means the renewable share cannot be verified through standard laboratory testing. Instead, it is allocated through accounting methods, primarily mass balance.

Mass balance and carbon accounting

The renewable content in co-processed fuels is assigned using a mass balance system, consistent with frameworks such as the Renewable Energy Directive (RED II) and certification schemes like International Sustainability and Carbon Certification (ISCC).

In simple terms:

• A refinery inputs a known quantity of renewable feedstock.

• This quantity is tracked through the system.

• The renewable share is then allocated across final products.

However, this allocation is not necessarily uniform.

For example:

• A refinery processing 10% renewable feedstock may allocate:

  • 20–30% renewable content to one batch.

  • Lower or zero content to another.

In summary:

The fuel delivered to a vessel may be documented as containing a certain percentage of biofuel, even though the physical molecules may differ.

From a carbon accounting perspective, GHG reductions are calculated based on:

• The lifecycle emissions of the renewable feedstock.

• The allocated share in the fuel.

This method is acknowledged by regulatory bodies, such as the FuelEU Maritime Regulation and the IMO Life Cycle Analysis (LCA). However, it predominantly depends on documentation rather than direct physical verification. Should any aspect of the documentation process fail, there will be limited means to authenticate or assess the bio components of the fuel, resulting in it potentially being classified as a conventional fuel with reduced GHG advantages.

Innovative techniques like C-14 carbon analysis, commonly used by archaeologists to date organic materials, offer important insights into bio content. These approaches can differentiate between modern (biogenic) carbon and fossil carbon, although their use in marine fuels remains relatively new and not yet widespread. Suppliers apply this testing at the refinery level to ensure compliance with carbon accounting regulations. However, few independent laboratories perform this analysis, and when available, it is expensive and requires multiple days to complete.

Operational characteristics and fuel behaviour

From an operational standpoint, co-processing addresses many of the challenges associated with traditional biofuels.

The refining process significantly reduces:

• Oxygen content

• Hygroscopic behaviour (water absorption)

• Risk of microbial growth

• Storage instability

Cold flow characteristics are comparable to traditional fuels, assisting onboard management making them more suitable alternatives to biofuels in colder climates. However, like all fuels, cold flow properties should be verified before use.

Co-processed biofuels have energy content comparable to traditional fuels, leading to greater CO₂ reduction since equal volumes yield higher output than standard biofuels with lower energy content.

However, some uncertainty remains. Depending on feedstock and process severity:

• Trace amounts of reactive by-products may persist if unconventional materials are used in the refinery process.

• Certain fuel behaviours may not be captured by routine ISO 8217 parameters.

In such cases, fuels may meet specification but still raise concerns under Clause 5 of ISO 8217 standards, where a fuel is required to be fit for purpose.

While refining may remove harmful effects from unconventional feedstocks, traces of these products may still impact fuel performance by causing oxidation, polymerisation, or sludge. Since testing of co-processed biofuels is in early stages, it is suggested to use advanced methods such as GC-MS alongside ISO 8217 tests to better evaluate fuel quality and reduce risks.

Conclusion

Co-processed biofuels provide a near-term, scalable way to reduce maritime emissions. Their compatibility with existing ship infrastructure and similarity to conventional fuels makes them attractive to shipowners.

Despite the benefits, relying on mass balance accounting and certification instead of physical traceability may in certain circumstances cause issues. Furthermore, the presence of trace levels of specific bio feedstocks utilised in the process may present risks related to fuel quality.