Steve Bee, Group Marketing and Strategic Projects Director of marine fuels testing company VPS on Thursday (24 April) highlighted the potential and the pitfalls of Cashew Nut Shell Liquid (CNSL) in marine biofuels in view of the maritime industry turning to CNSL as an alternative low carbon bio-option for Fatty Acid Methyl Esters (FAME):

Cashew Nut Shell Liquid – Background

As demand grows from all modes of transportation for low-to-zero carbon fuels, to support efforts in complying and achieving numerous environmental legislation leading to global decarbonisation, many alternative fuel sources are being considered. One of the most common and in demand sources of such fuels, is Fatty Acid Methyl Esters (FAME), as either a 100% fuel-source, or as part of a bio-fossil blend. But with road transportation, aviation and shipping, all seeking to use FAME in their respective biofuels, demand is outweighing supply. Therefore, other bio-materials are being considered as alternatives to FAME.

One such material is Cashew Nut Shell Liquid (CNSL), which is the oil extracted from the shells of the cashew nut. This by-product of the cashew industry is a naturally occurring substituted phenol, which is abundantly available and a waste product, with a lower demand than FAME. The composition, properties and quality of CNSL depend upon the specific manufacturing production process used to extract the oil from the shell. These vary from, mechanical pressing to solvent extraction, vacuum pyrolysis, vacuum distillation or solvent extraction.

The industrial applications where CNSL is a key component are wide ranging and include, the production of polymers, plastics, resins, adhesives, surface coatings, insecticides, fungicides, anti-termite products and even pharmaceutical products.

There are three main components of CNSL, these are Cardanol, (also known as Ginkgol), Cardol and Anacardic Acid:

These substituted phenols tend to exhibit high acid number values (>3mgKOH/g). They also show high iodine values (>300gI2/100g), indicating an elevated level of unsaturation and hence increased degrees of reactivity and instability. Then also, high potassium levels leading to potential post-combustion deposits and corrosion of turbocharger nozzle rings.

As monomers, these chemicals are also prone to polymerisation at temperatures, >200ºC. As a consequence CNSL is potentially a highly reactive, very corrosive material.

However, the levels of acidity and reactivity can be reduced during the production and refining process by converting Cardol and Anacardic Acid to Cardanol. If the CNSL is >98% Cardanol, then the reactivity is significantly reduced.

VPS Testing of CNSL as a Biofuel

Over the past 3 years VPS have tested various CNSL compounds and fuel-blends to assess the potential of CNSL to be a viable biofuel.

Firstly, the use of CNSL blends can significantly reduce HC, CO/CO2 and smoke emissions, although they can raise NOx emissions slightly. However, VPS would advise never to use 100% CNSL as a fuel, as it is far too reactive and corrosive. Further advice is to always check with the OEM regarding the compatibility of CNSL-based biodiesel blended products, with their machinery. Traditional marine fuels when blended with CNSL, may reduce the high acid number, reactivity and potassium levels of 100% CNSL, but also increase the energy content, sulphur content, cold-flow and sediment potential issues.

Fuel Combustion Analysis (FCA) of CNSL/Fossil Fuel Blends

In the recent past, VPS have tested CNSL products, blended with marine gas oil (MGO), very low sulphur fuel oils (VLSFO) and high sulphur fuel oils (HSFO). When undertaking fuel combustion analysis (FCA) of CNSL blended at varying percentages with MGO, VLSFO and HSFO, a wide range of results were produced in relation to, estimated cetane number, ignition delay and rate of heat release (ROHR), examples are given in the table below:

The CNSL blends with HSFO which undertook FCA, were particularly poor, with low Estimated Cetane Number (ECN), long Ignition Delay and low ROHR. CNSL blended with VLSFO gave better results, with acceptable ECN, shorter ignition delay and improved ROHR. Blending CNSL with MGO, gave better results than those obtained by HSFO/CNSL and VLSFO/CNSL blends.

Whether the blends were 80/20, 70/30 or 50/50 Fossil/CNSL, the blends using HSFO consistently gave the poorest FCA results. This may be due to a negative interaction between the asphaltenic content of the HSFO and the acidic nature of the CNSL.

Each of the CNSL blends gave poorer FCA results, when compared with the 100% fossil fuels, HSFO, VLSFO, MGO and 100% FAME.

Please note, to VPS knowledge, the above highlighted CNSL blends were not burnt onboard a vessel.

Burning CNSL/Fossil Blends

CNSL-blended fuels with MGO, VLSFO, or HSFO, have shown mixed reactions to vessel operations, where some CNSL-blends have been stored and burnt without issue, whilst, other CNSL-blends have given rise to operational problems such as:

• Fuel sludging
• Fuel injector failure
• Corrosion of engine parts
• Filter clogging
• Fuel system deposits
• Corrosion of turbocharger nozzle rings
• Damage to Selective Catalytic Reactor (SCR) units.

The quality of the CNSL, through its production and refining processes, will of course be significantly influential in relation to the quality of the fuel, but also the quality of the fossil fuel with which it is blended, will also have an influence.

Note: The full article on CNSL can be viewed here.

Photo credit: VPS
Published: 25 April, 2025