Cashew Nut Shell Liquid (CNSL) is gaining traction in the marine fuels landscape as the shipping industry searches for greener alternatives that don’t compromise vessel reliability or operational performance. With tightening emissions targets and mounting regulatory pressure on conventional bunker fuels, CNSL has emerged as a promising, sustainable blend component for residual fuel (HFO) and Marine Gas Oil (MGO). However, its transition from agricultural by-product to viable marine fuel is not without significant technical uncertainties and commercial hurdles that stakeholders must address.
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From Agricultural Waste to Fuel Component
For decades, the maritime sector has been exploring how to optimise fuel mixes to reduce carbon emissions. CNSL is not a new discovery, but it represents an innovative twist on this challenge because it is sourced from what is traditionally considered waste. Cashew processing results in vast quantities of discarded shells, from which CNSL is extracted using heat or mechanical pressing. This oil, a dark and sticky liquid rich in naturally occurring phenols, has the potential to be blended with marine fuels.
A Sustainable Edge Over Food-Based Biofuels
Unlike many biofuel blends produced from food crops, CNSL avoids the contentious food-versus-fuel debate. Instead, it leverages an agricultural by-product that is abundant in cashew-growing regions, including parts of West Africa, India, Vietnam, and Latin America. The potential repurposing of this waste on a large scale could contribute to reducing both greenhouse gas emissions and overall fuel costs. If processing methods are refined, CNSL might emerge as a low-cost complement to traditional bunker fuels.
Chemical Complexity: CNSL’s Performance Issues
Despite its green credentials on paper, CNSL is not without its challenges. Its unique chemical composition provides potential benefits like enhanced energy and lubricity but also introduces complications. Chief among these is CNSL’s inherent acidity and propensity to oxidise, which can lead to issues if the oil is not carefully processed and blended.
Real-world tests with CNSL have produced mixed outcomes for shipping operators. Trials on various vessels showed that while blending CNSL into marine fuel could help reduce emissions, its performance under actual operating conditions is unpredictable. Operators observed that blends containing up to about 20% CNSL often lead to significant sludge formation. The reactive compounds in CNSL can form gums and sediments when exposed to the high-temperature, high-pressure environment inside an engine. These sediments can then accumulate and block fuel filters, restricting fuel flow to critical components. This results in reduced engine performance, unexpected shutdowns, and a likely increase in maintenance requirements.
Moreover, the high acidity of CNSL has been linked to accelerated corrosion of essential parts such as fuel pumps, injectors, and turbocharger nozzle rings. These issues not only elevate the operational risks but also contribute to unplanned downtime, posing serious challenges for an efficiency-driven shipping market.
Expert Insight: Early Days and Inconsistent Results
David Browbank, Fuel Chemist at Brookes Bell, the global maritime consultancy renowned for its forensic analysis and technical expertise in fuel cargo disputes and marine engineering analysis, noted that the use of cashew nuts as a blend option with traditional bunker is still in its early days.
“There’s limited evidence and few published studies confirming that CNSL can be safely used in bunker fuels, especially when it’s in a raw or only lightly processed state. At Brookes Bell, we’ve noticed that even slight variations in CNSL blend ratios or inconsistencies in its quality can trigger a wide spectrum of issues, from blocked fuel filters to accelerated wear on critical engine parts. These differences, observed even between samples from the same batch, underscore the urgent need to define precise performance thresholds.”
Regulatory Pressure and the Search for Alternatives
The International Maritime Organization (IMO) has set ambitious targets for reducing greenhouse gas emissions, aiming for net-zero emissions by 2050. Similarly, the European Union’s FuelEU Maritime regulation enforces increasingly stringent limits on the carbon intensity of marine fuel. These measures are forcing ship operators to explore options that reduce reliance on high-carbon fossil fuels.
When CNSL is compared to more established alternative fuel components like FAME, the contrast is clear. Fuels containing up to 100% FAME (B24 up to B100) have become a mainstay in the marine sector due to their established reliability and well-defined production processes. Derived from vegetable oils or animal fats, FAME biodiesel closely mimics conventional diesel and adheres to rigorous industry standards. In contrast, CNSL is still considered a less mature biofuel; while both aim to reduce reliance on fossil fuels, CNSL must overcome additional technical challenges before it can match the dependable, drop-in performance of FAME fuels.
“In the context of decarbonisation, CNSL could offer significant appeal. When blended appropriately, it could help lower the overall carbon footprint of bunker fuels. Its status as a renewable, waste-derived resource means that, on paper, it qualifies as an environmentally friendly alternative. However, the journey to regulatory acceptance is not straightforward. Current fuel quality standards, such as ISO 8217, are designed for conventional fuels and established biofuels like FAME and HVO, leaving CNSL without a clear specification,” David added.
Commercial Potential vs Operational Risk
Without a recognised standard, any use of CNSL is treated as experimental rather than a normal practice. Ship owners and suppliers must carefully negotiate and monitor CNSL’s inclusion in fuel blends—a risky proposition given its currently unpredictable performance. For CNSL to gain regulatory and market acceptance, industry players need to establish standardised quality guidelines, which will only come after extensive testing and data sharing.
On the commercial front, CNSL stands out due to its low-cost feedstock. Cashew shells, once discarded as waste, are both abundant and inexpensive, particularly in regions with strong cashew industries that can have good annual harvests. This gives CNSL an edge in terms of raw material costs, which is appealing when operating costs are under constant scrutiny.
However, the economic benefits of CNSL are balanced by concerns over its operational risks, as increased maintenance from potential issues could offset initial fuel cost savings. For ship operators, reliability is paramount, and any new fuel must prove its worth, not just in environmental gains but also in consistent, trouble-free performance. For now, the market remains cautious until CNSL demonstrates a stable, standardised performance in diverse operational conditions.
The Path Forward: Collaboration and Standardisation
The future integration of CNSL into mainstream marine fuels depends on addressing its technical and commercial challenges head-on. Improving CNSL’s processing is key, as refining the oil to reduce its acidity and stabilise its reactive compounds can help mitigate many of the issues observed in early trials. Adapting chemical treatments or optimising blend ratios could lead to CNSL formulations that are not only environmentally beneficial but also engine friendly.
Equally important is developing clear and consistent quality standards. Collaboration across the industry—linking fuel producers, engine manufacturers, researchers, and regulatory bodies—will be essential in establishing these benchmarks.
“It’s critical that all stakeholders come together to share insights and develop consistent testing methodologies. Our objective is to determine the exact conditions under which CNSL can be incorporated without compromising engine integrity. By collectively refining blend ratios and processing techniques, we can build the data needed to safely introduce CNSL on a broader scale,” David noted.
Conclusion: Innovation with Caution
This coordinated effort should also extend to regulatory adaptations. As the data on CNSL grows, standards bodies like ISO can update fuel specifications to include provisions for novel biofuel components. In the meantime, industry-led initiatives—such as pilot projects, joint field trials, and detailed reporting—will be critical for demonstrating CNSL’s viability under real-world conditions.
CNSL embodies both the innovation and the inherent challenges of transitioning to renewable marine fuels. With the shipping industry under increasing pressure to reduce emissions while maintaining operational reliability, CNSL offers a compelling, if experimental, route forward. Its status as a waste-derived fuel carries economic and environmental benefits, yet its technical hurdles—fuel sludging, filter blockages, and corrosive tendencies—mean that much work remains before it can achieve mainstream acceptance.
“Looking ahead, our industry’s commitment to rigorous testing and cross-disciplinary collaboration will be crucial in overcoming the challenges inherent in CNSL. By establishing clear, data-driven standards, we can pave the way for a fuel solution that not only meets stringent environmental targets but also guarantees the reliability required for modern shipping. This is not just about CNSL, but about all alternative fuels. It’s about building a future where innovation and real-world performance go hand in hand, helping create a greener future for the maritime industry,” David added.
The path ahead will depend on comprehensive research, improved processing techniques, and, most importantly, a collaborative approach among all stakeholders. Unlocking CNSL’s full potential is contingent on establishing clear performance standards and regulatory pathways. Until then, CNSL should be approached with both optimism and caution—a promising prospect that must prove its reliability in the demanding conditions of marine operations.
As the sector continues to explore sustainable marine fuel alternatives, CNSL’s journey from agricultural by-product to integral component of the bunker fuel mix will be one to watch closely, offering lessons that could pave the way for a greener, more resilient shipping future.
About the Author – David Browbank
David Browbank is a Fuel Chemist with a deep background in analytical chemistry and over two decades of experience. He began his career in 1998 at Pattinson Scientific Services in Newcastle Upon Tyne, where he progressed from Food and Water Analyst to Section Head of the Water Testing division. During his tenure, he specialised in techniques such as HPLC, GC-FID, IC, and AAS, while managing quality control, staff training, and regulatory compliance within a UKAS-accredited laboratory.
About Brookes Bell
Established in 1903, Brookes Bell is a leading global technical and scientific consultancy serving the maritime, energy, and industrial sectors. Renowned for our expertise in casualty investigation, forensic analysis, dispute resolution, and expert witness services, we are the trusted partner for many of the world’s top shipping and insurance companies. Our capabilities are enhanced by The Lab, a world-class scientific laboratory, and 3D Marine, our full-service consulting division in the Americas, offering marine engineering, surveying, and specialist advisory services.
Source Brookes Bell | David Browbank