Sunday, January 19, 2014

Chicken-Flavored Biochar

On Thursday we wrote about our brand-spankin' new pocket rocket stove and how excited we were that it was working.  This weekend we put it to the test with a task we'd been saving for a long time: turning the leftovers from chicken butchering (or more precisely, from chicken roasting and shredding so they'd all fit in our freezer) into calcium- and phosphorus-rich biochar.

Today's objective: get rid of this without throwing it in the trash or stinking up the neighborhood.  Katie says, "You've been saving that since November?!  Eww!"  The smell is...less offensive than expected.  There's also some bonus biomass to process, and fun colors, too!

Step 1: Get a roaring fire going in the pocket rocket on top of a good bed of coals.  Notice we added the concrete blocks around the sides for extra stability this time.

One foolproof way to see if the stove is drafting correctly is the old marshmallow test.  If you can roast a marshmallow above the chimney, it's drafting correctly.  We recommend this test be performed during every burn.

Step 2: When the fire is burning really hot, add a little bit of the chicken waste.  It's hard to set a hard ratio of wood-to-chicken, but if you try it, you'll get the hang of it pretty quickly.

There's something oddly poetic and at the same time Terminator-esque about pyrolyzing chicken bones.

When things are burning well, there's no smoke.

If the fire gets too cold, say, from adding too much wet chicken material, the smoke is kind of white, but not really acrid like one might expect.  We found that adding a handful of loose, very dry leaves usually fixed this situation since the leaves would ignite and burn hot for a few seconds, which was enough to get the wood back on track.   Hopefully no one thinks there's a new pope--we kind of like this Francis guy.

After about five hours, the bucket was full enough of char and ashes that it became difficult to scrape the coals away from the chimney opening to keep it drafting properly.  We let it burn for a little while without adding any fuel, which allowed the chimney to cool down enough that we could pick it up with insulated leather gloves.  The top of the bucket came right with it.

Then we dumped the hot biochar into another steel bucket.  You can tell this one's different because it still has paint.  Then we started over with re-lighting the stove.  Probably we could have also scooped some of the hot coals from this bucket back into the first one to light the next fire and saved a few minutes.

And there you have it: chicken-flavored biochar.  You may use that as the name of your garage band, if you like.  After about 10 hours, we had all of the chicken waste taken care of, and got a little less than one five-gallon bucket worth of char and ashes.  That works out to about one chicken per hour at this scale. Also, a word of caution: even 24 hours later, combining the ashes into one bucket, they were still hot and had some glowing embers.  Don't store them too close to combustibles!

Some of the bones are just charred, others have had all the carbon burned out of them, leaving a fragile ash matrix.  Both will be very good for the garden.  None of the nearby houses started on fire, and none of the neighbors came over to complain (which would have certainly followed had we failed on the first goal), which means we went two-for-two on the day.  Success!

Have you made biochar before?  What's your setup?  What do you do with chicken leftovers?  Let us know in the comments section below!


  1. I'm almost half wondering if a two-chamber rocket stove would be good for this. The first chamber would be the actual combustion chamber, and you'd burn your wood there. The rocket exhaust would then go through/around a second chamber where you'd put the biomass. Basically kind of a sealed gasification chamber? You'd have to find some way to prevent a lot of oxygen from getting into the biomass part. That way, you could char it without actually burning it, if that makes sense. Kind of like how back during WWII, people converted old cars to run on gasified woodchips by building a fire underneath an oil drum full of chips. I'm not sure if there would be much advantage to such an idea except for possibly having a higher carbon content left over, though.


    1. I was actually surprised how much char this setup made, since it draws air so well. Maybe it happened because I was trying to keep the fire roaring the whole time and didn't give the charcoal long enough to burn down before adding more wood.

      I was considering building something like Shaun from Shaun's backyard , but didn't want to go to that big of scale. The premise in both cases is to make sure the wood gas (or chicken gases) get totally burned--to avoid stinking up the neighborhood--but I think folks trying to run their cars on wood gas would want to leave much less charcoal.

      In this case it didn't matter because we want biochar, too, but I'm working on another project that will hopefully allow us to reach much higher temperatures because of a more complete burn. (To get an idea, search for 'Dasifier.')

    2. Looks like the link didn't take. This is Shaun's setup: