David Browbank, Fuel Chemist at global technical and scientific consultancy Brookes Bell and scientific analyst, recently dived into the complexities of using Cashew Nutshell Liquid (CNSL) in marine fuel blends.

He noted that inconsistent engine performance and varying CNSL grades point to the need for more comprehensive testing and better data to ensure safe and efficient operations:

As industry stakeholders look for quick and available ways of embracing more sustainable bunker fuel sources, driven by an urgent need to reduce carbon emissions and comply with stringent IMO regulations, many are turning to Cashew Nut Shell Liquid (CNSL) as a biofuel blend option. 

Despite its growing popularity, ongoing trials continue to reveal that integrating CNSL into marine fuels as a blend component may pose operational challenges that could potentially disrupt vessel performance. 

However, due to its plentiful supply, green credentials, and the requirement to achieve short-term emission gains, many are overlooking the potential long-term operational risks associated with CSNL.

“Since CNSL comes from the cashew nut process, it is an easily accessible renewable resource.  Its extraction is efficient, and it has a smaller environmental footprint compared to traditional fossil fuels, making it a viable candidate for reducing carbon emissions.  Moreover, a robust cashew crop can ensure a plentiful and affordable supply for biofuel production,” said David Browbank, Fuel Chemist at Brookes Bell and a respected scientific analyst.   

Despite these benefits, the use of CNSL in marine fuel blends has not been without complications.  Early field reports indicate a host of operational issues that have raised concerns among maritime engineers and fuel chemists.  Many vessels using CNSL-enriched fuels have experienced problems such as fuel sludging, filter clogging, and deposits within the fuel system.  More alarmingly, there have been cases of corrosion affecting critical components like fuel pumps, injectors, and turbocharger nozzle rings.  These issues have led to increased maintenance requirements and unplanned operational disruptions for ship owners globally.

David noted that these issues are primarily due to CNSL still being in early development. 

“There’s limited research on using CNSL directly as a bunker fuel blend.  We’ve seen that some engines struggle with the raw blend, while one trial using a highly refined CNSL mix with a large proportion of Marine Gas Oil (MGO) produced good results.  However, performance varied greatly between engines, suggesting that differences in CNSL concentration or quality in various blends could affect outcomes.  Ship operators should be cautious given these potential issues,” he added.

He further emphasised the inherent uncertainties when working with new fuel blends.  “Every fuel possesses its own unique chemical signature, so predicting how each blend will interact within different engine systems remains challenging.  This is precisely why expertise from fuel chemists, marine engineers, and metallurgists is indispensable.  The current uncertainty around CNSL, along with other novel biofuel components, is posing significant hurdles for vessels trying to meet environmental standards.”

The existing fuel standards are further complicating the situation.  The latest edition of ISO 8217 permits the use of biofuel blends, but these are generally limited to Fatty Acid Methyl Ester (FAME) blends, which have clearly defined characteristics and performance limits.  CNSL, in its current form, falls outside these established parameters.  As a result, its adoption without rigorous and standardised testing protocols could lead to an increased number of disputes over fuel quality and performance between ship owners and fuel suppliers. 

Maritime operators are facing a dual challenge: balancing the environmental benefits of renewable fuels with the practical realities of fuel performance and engine safety.  While CNSL offers a pathway to reducing carbon emissions, the operational risks—such as increased maintenance costs and potential engine damage—need greater consideration.  The situation calls for comprehensive research and extensive field trials to establish reliable data on CNSL’s performance under various operating conditions.

David noted that Brookes Bell has seen an increase in the number of CNSL-related cases it handles.  As a result, the company’s team of fuel scientists have had to deepen their understanding of the commercial and operational risks associated with cashew nuts as a biofuel blend.

 “Our team is really putting in the effort to understand how CNSL reacts in different fuel blends.  By understanding the chemical fingerprint of CNSL and how it interacts with different fuel blends and engine configurations, we aim to build a robust data foundation that not only confirms its potential but also guides safe adoption.  This isn’t just about CNSL – it’s about creating a solid groundwork for new biofuels we might use, so we can meet environmental goals without risking engine performance.”

Looking ahead, industry stakeholders understand that a cautious and methodical approach is essential for CNSL use in bunker fuels.  The increasing availability of biofuel blends will require ship owners to invest in regular fuel testing and quality assurance measures.  Enhanced collaboration among fuel suppliers, researchers, and maritime operators will also be vital to ensuring that renewable fuels can be integrated into the global shipping fleet without compromising operational reliability.

Whilst CNSL represents a promising step toward a more sustainable maritime industry, its integration into marine fuel blends is fraught with challenges.  The initial setbacks, ranging from fuel system blockages to component corrosion, highlight the importance of rigorous testing and the development of standardised fuel quality parameters.  As the industry continues to pursue renewable alternatives, the path forward may be complex, but the lessons learned from CNSL trials will be instrumental in guiding future innovations. 

Photo credit: Brookes Bell
Published: 6 May, 2025