Introduction: Backyard BBQ Biochar

Picture of Backyard BBQ Biochar

Your beautiful and well maintained lawn can be one of the best carbon sequestration devices in existance. Lawn grass, because of its abundance, captures more ambient CO2 than any existing or proposed technology. The plants in home landscaping are well watered, well fed and well cared for (generally) and grow at optimum rates, which means optimum carbon dioxide uptake. If this captured carbon dioxide can be diverted and sequestered the power of this resource can be realized. Well now it can.

How? By converting your weekly yard waste into biochar using your backyard BBQ. Once your yard waste has been reduced to its native carbon it can be bagged and disposed of ease with the trash, added to your compost heap or buried in that old family Cold War era bomb shelter.

Now that summer is here most of will be mowing the grass and firing up the BBQ. For those of us with a charcoal BBQ the leftover coals can be put to good use. I haven't tried with a gas grill yet, but I will. What we're going to do is make a simple biochar reactor and use the BBQ, some charcoal and a small fan to turn it into biochar.

Here's some helpful hints:
hint #1: Don't be an idiot. Make sure you produce more charcoal than you burn.
hint #2: Check your local burning regulations
hint #3: This stuff is hot, really hot, be careful

Step 1: Bill of Materials

Picture of Bill of Materials

Biochar is produced by combustion of organic material, normally plant material, in the abscence of oxygen. To do that we going to take a steel can and drill some holes in the side. Then we're going to fill the can up with grass trimmings and seal it up. Then we're going to heat it up and see what happens....

Okay, you're going to need a lawn...or yard waste from a lawn...

Also you'll need a backyard charcoal BBQ with a cover. I'm using a small Weber brand for the proof concept. The cover may be optional, I remember a traditional style BBQ with a wind shield. That could work as well since the wind shield will capture the fan breeze.

Some charcoal

A method for lighting the charcoal. I'm using petroleum based charcoal lighter fluid. I've seen the starters cans that use newspaper and they work pretty well. I may get one....

A small fan. I'm using a standard electric fan but I recommend a small battery powered fan with a solar recharger.

A steel can with a lid. I'm using a 1 gallon paint can I picked up at my local paint store for $2. It will do for this purpose and fits inside the small Weber. The can must have a lid which can be secured in a way that will withstand heat. I've got an idea for another reactor design I got from I will update this if that works out. To scale up use a full size Weber kettle and a 5 gallon paint can from the paint store.

A drill and drill bit for drilling ventilation holes in the steel can. This will vent the methane and other gasses produced into the fire where it will be consumed.

Okay with all of that we're ready to start....

Step 2: Assemble the Biochar Reactor

Picture of Assemble the Biochar Reactor

That's a fancy way to say drill some holes in the side of the can, fill it with grass and hammer down the lid. Make sure it's secure.

I picked a drill bit at random and started drilling. Not the best plan as it turned out. You want to make a reasonable vent for gasses that develop during the creation of the biochar. The metal walls aren't that thick and if one goes drilling around at random the grid becomes too weak. Drill from the center out and don't go crazy.

I drilled on the opposite side from the seam for no particular reason at all...

Once you've drilled the vent remove the lid and fill it with grass clippings. I have no good advice on how to fill it, I'm new here myself. I didn't pack it down tightly and have no idea what the yields will be, I will update this as I progress.

Now put the lid back on and press it down tightly. I originally pressed mine in by hand after a little bit in the fire the metal expanded and the lid fell off. I hammered it back down ( lightly ) and it seems to be secure.

Step 3: We Didn't Start the Fire...

Picture of We Didn't Start the Fire...

It was always burning since the world was turning...Sorry these Billy Joel moments come and go. Okay with the lid securely on the can we're ready to start making our biochar.

At this point I'm not sure how much charcoal to use or what the yield will be. Your mileage may vary so experiment.

If you've started the coals in the metal gizmos with newspaper arrange them on the fire grill in circle with enough room in the center for the can. Otherwise just put some charcoal briquets in a circle with enough room for the can.

If you want to be really, really, really clever you've prepared your biochar reactor in advance. Once you've finished the hamburgers for dinner you can carefully remove the food grill, push the hot coals around with a spatula and place the biochar reactor into the coal bed.

Place the can on its side in the center of the circle. Push the coals as close to the edge of the can as you can.

Place additional coals so you've got two levels. If your using the leftover coals make sure you've got a solid coal bed against the sides and ends of the cans.

If you're charcoal lighter spray it liberally on the charcoal and light.

Once the coals have begun to show some gray ash position the fan so that it blows air onto the coals.

Once the coals are firmly established place the lid partially over the grill to bank the heat. Leave a gap turned towards the fan and place the hood vent holes opposite for smooth air flow.

NOTE: You have now turned your BBQ into a mini-blast furnace for making biochar. It will get hotter than you think. Use a pair of heavy garden gloves once things heat up.

Step 4: Cooking - the Final Frontier

Picture of Cooking - the Final Frontier

Once you've got things underway there's not much too it. Periodically it needs to be checked the coals pressed close into the can. Don't be an idiot use a firestick.

I stirred the coals twice pushing increasingly smaller chunks of coal against the sides and ends of the can.

I also added briquets at key point to create hot spots that looked weak. They burn nicely if well placed and will ignite from a small contact with a burning (gray area) coal.

Due to the forced air feed the fire burns much hotter and faster than normal. Experimentation will determine the correct placement and number of coals.

My fire has burned for just over two hours and seems about burned out. Remember that the contents of the can are also combusting (in the abscence of oxygen).


Step 5: Disposal and Sequestration

Picture of Disposal and Sequestration

Obviously some of the biochar you've created can be preserved and used in the next round.

Otherwise, if you've done it right, you've created an extremely high quality carbon suitable for long term sequestration. Pour it into a plastic bottle and throw it into the regular trash where it will be buried in a landfill for thousands of years. If you put it in the recycle bin the carbon will get recycled into mulch and re-released.

Now you've got something to talk about over the water cooler on Monday morning. "What did you do this weekend" "Oh, I sequestered 10 lbs of CO2 from my lawn...and you?"

To increase the carbon recovery from this process there are two very simple steps you can.

Dump your power mower and get a reel mower. Yes, they still make them. They're quiet, clean and you get exercise while you use them. Simply by switching from fuel powered to human power maintenance you have substantially increased your carbon recovery and improved your health.

Instead of the electric fan pictured use a battery powered fan with solar recharged batteries.

That's it. I hope you find this useful. I am starting work on a follow up instructable for a biochar burner.


jimvandamme (author)2012-03-27

According to what I've read about biochar tossed in the garden, the carbon in the soil won't get released for hundreds or thousands of years. Meanwhile, it loosens soil, and soaks up water and nutrients. So it's better than throwing it in a landfill, and more lasting than composting it where it creates methane and complex carbohydrates which will turn into CO2 in a few years.

I agree its useful in a ground but buried is buried...

Ronyon (author)2011-03-07

Your use of a paint can was very instructive.
Thank you for your Instructible.

egbertfitzwilly (author)2011-02-14
  • Learn to spell and punctuate the English language
  • Learn to organize your thoughts
  • Develop a coherent point to make
  • Come back and make it.
JohnBonitz (author)2010-03-01

Thanks for making this Instructable.  And good for you, for emphasizing the importance of the "energy input vs output" question.

However, please note that there is an additional problem here and that is production of VOCs. This method of charcoal production might NOT be climate-friendly.  Any combustion process that releases un-burned gases will actually exacerbate the greenhouse-effect.  The simple pyrolysis/gasification effect created in this paint-can-on-the-BBQ will release gases called "volatile organic compounds" or VOCs, including methane.  This looks like smoke or fumes, and may be gray or yellowish in color. 

Methane is a gas that is 20 to 25 times more potent than CO2 in trapping heat in the atmosphere.  In other words, the effect of the gases you create while making biochar could exceed the carbon-capture benefit of biochar in soils.

To fix this, you have two options:
1) make certain that any gases you create (i.e., "smoke" or "fumes") are burned or flared, thus reducing the VOCs to CO and CO2 (less potent GHGs than methane).
2) even better, engineer a system to make use of these gases for thermal energy.  Waste-not-want-not!


Actually this is a common misconception. Since any materials released by the biomass (either the raw biomass or the fuel) must have come originally from ambient the absolute worst case would be a zero sum game IF all consumed carbon were released back into the atmosphere. However only a tiny percentage of consumed carbon gets re-released (much of which is consumed by the carbon fabrication process), most is retained as complex organic or inorganic compounds.

With all due respect, this is not a misconception, it is good science.  And science I have just this morning re-confirmed with Dr Julie Major of the International Biochar Initiative.  Here is confirmation from another source:
"If 3% Methane-CH4 is emitted, then there is more damage done to the atmosphere by increasing the Methane-CH4 content than could ever be recovered, even if all of the carbon from the original biomass could be captured and sequestered."

What you say about 'zero-sum' would be the true IF all VOCs are reduced to CO2 or CO through flaring or total combustion.  Pyrolysis is inherently incomplete combustion, because you are starving the process of oxygen in order to produce char. 

As GHGs come, methane is 20 to 25 times more potent than CO2.  There is no escaping the atmospheric physics.  Incomplete combustion is worse for the climate than any potential benefit from storing carbon in the soil as biochar. 

If you want to help the climate, don't make biochar unless you can be sure to flare the resulting VOCs.

I hear this claim made a great deal but I no one presents any science to support it. Bad science repeated on the internet is still bad science.

Secondly you will note that the gases vent into fire, flaring the methane so that CO2 is emitted (as noted in your references).

Lastly methane breaks down, explosively, in the presence of lightning producing 1M of CO2 for each mole of methane (and H2O). So how 1M of the methane is 20x worse than 1M of CO2 escapes me completely.

dchall8 (author)2009-06-22

I can't be overly positive on this. Sorry. You're burning off carbon brickettes to supposedly capture CO2 from...where again? CO2 is a gas at room temperature. It never will become a black solid at any temperature. What you are doing with this is burning away minerals (including carbon) that would otherwise have been used by the microbes in your soil had you left the clippings in the lawn. You end up with a net loss of carbon to the atmosphere. On the other hand, live grass will absorb CO2 from the air, convert the carbon to sugar (which does not evaporate), and release the oxygen to the air for animals to breathe. To my thinking the best use of grass is to let it get as tall as you can stand it and grow it on every inch of bare ground you have for as much of the year as you can. When you mow it, leave the clippings in place so the soil microbes can decompose the clippings and produce plant food. There's nitrogen in those clippings.

steveo_mcg (author)dchall82009-06-23

Carbon Dioxide is a gas yes, but carbon its self need not be a gas. The Black matter at the bottom is largely carbon, what is being burned off is largely water. Though I'm dubious about how much you'll sequester using briquettes as fuel.

I agree about the need to make sure one produces more charcoal by weight than is consumed, otherwise there's not much point. Although that's not actually hard. The Mark II model is significantly improved, I'll put that up this week...maybe..

egbertfitzwilly (author)dchall82009-06-22

You are correct, the whole point is to create a net loss of carbon to the atmosphere. That would be whole point of this instructable. By taking in CO2 and isolating the carbon for long term storage your lawn becomes a carbon sequestration engine. When the grass clippings are reduced to carbon the carbon portion of the CO2 is isolated and may be permanently sequestered.