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.
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