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In Ground Biochar Retort

 
Brendan Edwards
Posts: 35
Location: Hiroshima-shi, Japan
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Just looking for any pointers or thoughts on my need for quick and dirty demolition debris reduction. I'm rebuilding an old house and have so much wood I need to reduce. I am very busy with the construction and don't have time to mess too much with it. It's also difficult to move it much, so I was thinking I would do a trench or pit biochar cook. It does not need to be terribly efficient, just simple. I thought I might try to grab some junk metal roof panels to cover with .. I also considered using a 55 gallon drum as a sort of riser to drive the heat since I can access them.. I have way too much to load drum can retort and do not want to waste time cutting timber. Anyway, if anyone has any notes on how to get this working easily, I'm all ears!

Thanks!
 
R Scott
Posts: 3305
Location: Kansas Zone 6a
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http://www.fao.org/docrep/x5328e/x5328e06.htm

here is the best writing I have found on the subject. I haven't tried it yet, but bookmarked it because I plan to.
 
Brendan Edwards
Posts: 35
Location: Hiroshima-shi, Japan
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K I read it, thanks! I've got an idea that I think will work using the ideas from the paper you linked and a drumcan retort which I already have... Imagine a drum can maybe mounted on a few stones sitting at the edge of the trench... so there will be space for air feed and wood material continuous from the trench up into the can which sits over it at the end but partially dug in to the ground... The thought is that the can will quickly add useful volume to the cook. So, Fill the trench with all this long uncut stuff, pack the can, get the whole thing cooking then lid the can, toss the junk metal roof panels over the trench, add some soil to seal the gaps a bit and wait... I can quench it with some water to stop the process after a few hours if I think its not sealed well enough.. Quick and dirty, but will give me some usable material, more than just incinerating the whole lot... It'll leave useful soil amendment and some stuff to roast meat with...


If anyone else has any thoughts I'm definitely interested... I hope to do something with a significant portion of my debris in the next few days..
 
Marco Banks
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Location: Los Angeles, CA
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Give careful attention to where you locate your trench (out of the way where you will not get people coming by and stumbling into it) and then how you build the pit/trench and lay the wood down in it. If you can put thick pieces of wood along the bottom of the trench running along the length of it, and leave gaps between those thick pieces, you can force the air to move along the bottom of the trench from one end to the other. If you are using scrap lumber, consider nailing a few of the thicker boards (like 2 x 10's or 2 x 12's from floor joists or rafters) together to make thicker beams.

Thus, if your trench is 3 feet wide and 12 feet long and 4 feet deep, you would lay the 12 foot beams down in the bottom of the trench first, spacing them so that there is room between them. Then throw some small stuff between them to burn as the fire moves along the bottom of the trench from one end to the other. This small stuff should be kindling -- you'll need it to burn backward from the vent pipe toward the intake pipe. You'll want it to be dry. You could even put crumpled newspaper along with it --- it's important for the fire to get burning along the length of the trench on the bottom if you are going to get an even burn.

The next layer of wood would go perpendicular to those bottom beams. In the example dimensions above, these pieces would be 3 feet long, and would lay across the top of your base beams. Pack them tightly. You don't want to leave much space between them. You may wish to then add a thin layer of dirt to seal that layer a bit -- not airtight, but enough to keep too much air from moving upward too quickly.

Then the main body of your trench would be filled with the rest of your scrap wood. Pack it in tightly. It can be stacked above the level of the ground, but not too much -- maybe a foot or so.

On one end of the trench, you'll place a metal chimney. Any metal chimney will work -- six inches is a good size. Put the bottom of the chimney on bricks or rocks, so that it will not slide down and block the air flow. It should sit above a pile of kindling that connects directly to the wood in the bottom of the trench --- the kindling that fills the space between the beams. Your vent should stand upright, to encourage heated air/smoke/steam to vent straight up. A similar air feed pipe goes on the other end, laying it horizontally (or diagonally out of the bottom of the trench). Your intake pipe can be much smaller -- a 3 inch pipe works well.

Once your pipes are set into place, you can backfill around them, and then cover the entire mound of wood with a thick layer of 8 inches (or so) of the excavated soil.

Starting in the morning, light the trench from the vent pipe. Start by dropping burning pieces of wood down the vent pipe. Once its burning pretty good, you can add a few more boards down the vent pipe to increase the draw. The fire will slowly work it's way back along the length of the trench toward your inlet pipe. You want to start the fire in the morning, because it'll take hours to really get going, and you don't want to leave it unattended all night. Bring beer and a lawn chair.

If it gets too hot and you've got flames coming out of your vent pipe like a rocket ship, you may need to close off your intake vent a bit. Keep an old paint can handy. Punch a few holes in it, and then use it to slow down the air intake by placing it over your intake vent. After 5 hours or so, dramatically slow the air intake. The goal now is to keep that heat in the trench and let the wood char with minimal air getting to the fire. As evening comes, really limit the airflow. Perhaps put a second bucket over your vent pipe is needed as well, with a few holes to allow exhaust outward.

It's important that you keep people back from the trench, particularly after it's burned for a day or so. The crust of soil on the top will harden and look solid, but under it is a deep trench filled with hot coals. If someone were to stumble onto it, they could break through and that would be bad. Hell on earth.

Cracks will form in your soil cap as it heats up. You can seal them up if they get too big and the fire is getting too much air. You want a little bit of smoke to escape, but if it really starts to crack and too much air is getting in, shovel more dirt on to keep it sealed. Depending upon the moisture content of your wood, the construction of your trench, and how much air you are letting into the trench, it can burn for a week. For dry wood/construction waste, it'll probably burn pretty quickly -- perhaps as quick as 2 or 3 days.

The dirt cap will sink as the wood is consumed by fire. When you feel that the burn is done, you can pull the vent pipes out and seal it up with dirt. If there are parts that haven't burned as much as you want, you can stick a steel bar down through the dirt cap and create new vents. Completely seal it and wait 3 days to assure that your fire is completely out. You may even wish punch a hole through the dirt caps to soak it with a hose for a couple of hours.

It's a lot of work. In the end, you'll get a nice pile of charcoal but it can go wrong and when it does, you'll have no choice but to either let it burn up, or stick a hose on it and put the fire out.
 
Michael Cox
Posts: 1574
Location: Kent, UK - Zone 8
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I've had good results with a simple bonfire quenched with water. It doesn't generate quite a much char per unit of wood burned, but is quick, easy to manage and if done well isn't too smokey. As you keep putting more layers of wood on top, the top layers protect the bottom layers from burning, as oxygen cannot reach them. As long as you keep adding more fuel on top the char will keep building up.

Just be ready to quench it with LOTS of water.
 
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