Short answer: we tested biochar made from cashew nut shell and from palm kernel shell. Palm kernel shell passed. Cashew shell failed, well above the limit. The cause is the oil in cashew shell, which turns into tar during heating and sticks to the char. It can be fixed by changing how the machine handles smoke and cooling.
First, what is biochar?
Biochar is a kind of charcoal. It is made by heating farm waste, like shells or husks, with very little air.
It locks carbon away for hundreds of years. That is why companies pay for it as carbon credits. It is also used to improve soil.
For a factory, that means waste you pay to get rid of can become a new source of income.
What are PAHs?
PAHs are tar-like compounds. They can form when biomass is heated, the same way smoke and soot form when you burn wood.
Some PAHs are harmful to people and soil. So the amount left in the char has to be very low.
Why do buyers test for them?
Biochar that goes into soil has to be safe. The main quality standard is the European Biochar Certificate (EBC). It sets a strict limit on PAHs.
Carbon credit buyers rely on that standard. If your char is over the limit, it cannot be sold for soil use or earn those credits.
What does pass or fail mean for your factory?
Pass: your waste can become certified biochar. It can earn carbon credit income.
Fail: the char cannot earn credits yet. The process has to change, and then it has to be tested again. Each test takes weeks and costs money, so it pays to get it right.
What our tests showed
We had char from two kinds of shell tested at an accredited lab, using the same method.
| Feedstock | Result | What it means |
|---|---|---|
| Cashew nut shell | Failed, well above the limit | Not ready for carbon credits yet. The process needs to change first. |
| Palm kernel shell | Passed | Safe for soil use and ready to earn carbon credits. |
One simple comparison: the worst cashew sample had many times more of the harmful PAHs than the limit allows. The palm kernel shell samples were comfortably under it.
The good news: the cashew char passed on everything else, including carbon quality and metals. The carbon is good. The char is just dirty.
Why cashew shell failed
Cashew shell is full of oil. About a quarter of its weight is a thick, dark liquid.
When the shell is heated, that oil turns into thick tar smoke. If the smoke cools down while it is still touching the char, the tar sticks to it. Think of soot coating the inside of a chimney.
That stuck-on tar is what the lab measures as PAHs. Palm kernel shell has much less oil, so it burns cleaner.
How to fix it
The fix is mostly about the smoke, not the shell.
- Get the smoke out fast. Don't let tar smoke sit over the char inside the machine.
- Burn the smoke hot and clean. Send it to a proper burner. The heat can even dry shells or run a boiler.
- Run hotter. A higher temperature drives more tar out of the char, as long as the smoke is removed quickly.
- Cool the char the clean way. Cool it in a closed, dry system, or with clean water. Never reuse dirty quench water, because it puts tar back on the char.
Then test again, trying a few settings in one round so you only pay for shipping once.
What to do next
If you run a cashew or palm factory and already have a lab result, even a failing one, send it to us. We'll tell you what is most likely to fix it, at no cost. If you haven't tested yet, see what we look for in a waste stream, how our pyrolysis units handle gas, or our guide to cashew shell uses and prices.
FAQ
What are PAHs?
PAHs are tar-like compounds that can form when biomass is heated. Some are harmful, so biochar sold for soil or carbon credits has to stay under a strict limit.
Why did cashew shell fail?
Cashew shell is full of oil. When heated, the oil turns into thick tar smoke. If that smoke cools on the char before it leaves the machine, the tar sticks to the char and the test picks it up.
Can it be fixed?
Usually, yes. Burn the smoke hotter and cleaner, get it away from the char quickly, and cool the char in a closed, clean way. Then test again.
Can I earn carbon credits if my char fails?
Not for soil use. You need a passing test first. Once you pass, the char can qualify for credits.