In 2018, ACS Energy researchers studied cashew nut shell liquid as a potential diesel alternative; the biofuel may be used in compression-ignition engines after blending or processing

Not many people who purchase a bag of cashews consider the shell that encloses each nut. Understandably so: raw cashew shells contain highly caustic anacardic acids, related to the same compounds found in poison ivy, so the nuts almost never reach a store shelf still in the shell; the shell is removed and dealt with long before that point, usually at the processing plant. But that discarded shell may hold more value, in the aggregate, than the industry currently gets out of it. But that wasted shell could be worth more in the long run than the nut itself. According to a 2018 study titled “Cashew Nut Shell Liquid as a Fuel for Compression Ignition Engines: A Comprehensive Review,” published in the American Chemical Society journal Energy & Fuels, the thick dark oil that occurs in the cashew nut shell shares enough properties with petroleum diesel that researchers have studied it as a potential fuel for the same kind of compression-ignition engines that power trucks, tractors, and back-up generators.

Researchers Shiva Kumar, P. Dinesha and Marc A. Rosen analyzed years of laboratory work and engine testing of this material, called "cashew nut shell liquid" or CNSL, and found it to have real potential as an alternative to petroleum-based diesel. This is based on a synthesis of existing research on the subject, and it may be an eye-catching discovery for a waste product of food, which usually ends up in the trash.

Meet the shell nobody wanted


The shell is not what you're paying for most of the time at the grocery store. The kernel, or edible portion of the nut, is a small fraction of the nut itself. According to a TechnoServe report on cashew shells for energy, roughly 70% of a raw cashew nut's weight is its shell, which is typically discarded after processing or burned as a rough, inefficient fuel.

Beneath this shell, nestled in a honeycomb-like layer of resin, lies CNSL, a caustic resinous liquid extracted from the shell by roasting or heating it out of the shell in various ways, some of which are simpler, like hot oil baths, and some of which are industrial-scale pyrolysis. It's not some sort of new-age extraction wizardry. Cashew processors in India, Vietnam and West Africa have been extracting this material for decades, primarily to use in the paint, varnish and brake-lining industries.

How trash becomes torque

More recent, however, is the continuing scientific interest in the burning of this substance. CNSL is rich in anacardic acid, cardanol and cardol, which, after separation from the shell, give the raw liquid a calorific value broadly in the same range as diesel — though published figures vary by study and by how the CNSL was processed, generally somewhere between the low 30s and upper 40s in MJ/kg, versus roughly 42 MJ/kg for diesel. A 2019 review titled “Cashew nut shell: a potential bio-resource for the production of bio-sourced chemicals, materials and fuels,” published in the journal Green Chemistry. Fuel is actually the branch of a much larger tree.

The catch you ought to know about

But there's a problem that doesn't let people get too excited about pumping cashew juices at gas stations. A disadvantage of Neat CNSL is that it has a higher viscosity than diesel, which will be a disadvantage to engines designed around the viscosity of diesel. The Energy & Fuels review makes clear that raw CNSL must be mixed with regular diesel, or chemically treated to become biodiesel, before it is reliable, and even then, the combustion characteristics and emissions vary according to how the raw material is processed. This is not a drop-in replacement fuel. An exciting feedstock, but requires engineering to make it behave itself.

Why this should matter

The U.S. biodiesel and renewable diesel market has grown dramatically over the past few years, and is fueled by a voracious demand for soybean oil, a food crop. A 2025 report by S&P Global Commodity Insights found that biofuel mandates are driving the up-to-20-fold increase in imports of waste cooking oils and fats from the US and Europe since 2020, with much of it coming from China and Indonesia. That same dynamic: countries importing waste feedstock across oceans to meet biofuel mandates, is what makes the cashew shell's story worth a second look: it's a nonfood, energy-dense agricultural waste product that is already concentrated in India, Vietnam, and West Africa, regions that are themselves being courted as biofuel feedstock suppliers, and much of that shell is still being burned for low-value heat or simply discarded rather than exported or refined into fuel.

CNSL won't be substituting the diesel pump at your local gas station next year, and no one's saying so. At a time when biofuel policy is quietly battling with the world's food supply, and waste oil is being shipped thousands of miles across the ocean to meet demand, a shell that's currently thrown away or burned for scrap heat looks like an underused resource worth a closer look.