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What you will need
- A hoe
- A sheet of corrugated roofing metal, the longer the better. NOTE: It does not need to be new. Any old piece will work as long as it does not have holes so big that you cannot block them with dirt.
- A long stick. Two meters is great.
How to make your trench
Lay the sheet of roofing metal on flat ground where you want to make your trench.
Using a stick, outline it by drawing all around its edges.
Set the roofing sheet aside.
Inside the rectangular outline of the roofing sheet you made in the dust, draw a second, smaller rectangle. Make the sides of the second rectangle at least a full hands’ breadth inside the sides of the original outline.
With your hoe, dig out the area inside the inner, smaller rectangle. Dig down approximately 70 centimeters (about the length of your arm). Make the sides of the trench slope in toward the bottom. Not much, maybe 10 cm.
Scrape the dirt you dig from the trench outside of the original outline of the roofing sheet. Leave it piled around all four sides.
When you are finished digging, test your trench by laying the roofing sheet over it. You have a good fit it:
- The roofing sheet covers the trench with no gaps around the edges.
- If the trench is too wide or too long and you cannot cover it completely with the roofing sheet, dig a new trench. The gaps will allow air to enter. Your biochar will burn up!
- The roofing metal should lie flat on the ground. Scrape all the loose dirt from the hole back so that the roofing sheet lies flat on the ground all the way around. No bumps!
- If the roofing metal does not lie flat, scrape back any loose dirt. If that is not enough, use your hoe to level the ground around the trench.
How to use your trench
- When you are satisfied that you have a good trench, set the roofing sheet aside again. You will not need it until the end.
- Start a small fire at the bottom of the trench and feed it. When it is going well, spread the fire the length of the trench.
- When you have a good fire going along the bottom of the trench, begin to add feedstock.
Good trench use practice
Anybody can make a fire. You are not making a fire. You are making biochar. You are making biochar (1) to stop the smoke that just burning the feedstock would produce and (2) because you want as much biochar as possible from the work you are doing. To succeed at (1) and (2) takes practice and know-how.
- Fires smoke when they are not hot enough.
- If you add feedstock too slowly and your fire begins to die down, it will smoke unless you feed it better.
- If your feedstock is wet, it will not char until the moisture has evaporated. This cools the fire and your trench will smoke. Do not use wet or green feedstock.
- If your feedstock is straw or anything else that comes in bundles, it will smoke because it partially smothers the fire. Do not throw bundles of stuff into the trench. “Sprinkle” them in.
- Really big fires do not smoke, but they burn up a lot of your biochar, too.
- It is very satisfying to build a bonfire in your trench. Unfortunately, bonfires burn lots of carbon leaving you with less biochar. Restrain yourself.
- You want balance when using a trench and balance takes practice. You have to watch your fire and think about your two goals. You want a fire big enough not to smoke but small enough not to waste carbon.
- Here are three other tips.
- If you are using a low-density feedstock – say, millet or maize stalk, rice or wheat straw – poke the burn regularly with your long stick. The char will crumble and you will be able to make much more char per trench.
- If you are using tree branches or bamboo, always use pieces that are roughly the same diameter. If you do not, either the small stuff will be consumed by the time the big stuff is ready or the big stuff will be only half done when the small stuff is ready.
- If you have a bit of clay handy, especially nice reddish clay, sprinkling it onto your fire will improve the quality of your char. Just toss a handful or so on every few minutes.
Finishing your burn
- You can stop adding feedstock at any time; you must stop when the trench is completely full of biochar.
- When you want to stop, simply stop adding feedstock.
- Watch the flames. For a minute or two, the surface of the char will be covered with dancing orange flames. When these die down to almost nothing, your char is ready.
- Place the roofing sheet over the trench.
- Scrap the loose dirt over the edges of the roofing metal and
pack it down with your feet.
- If you see any smoke escaping from gaps under the roofing sheet, put dirt over them and pack it down. Your object is to seal the trench so that no air can get in.
- Now wait – a long time. Biochar is stubborn stuff. A trench can take five hours to go out.
- When you open your trench, use your hoe or long stick to turn
the biochar. Watch it carefully. If it begins to go grey – anywhere – or to
smoke – even just little wisps – close the trench up again and wait some more.
- If you do not, you biochar will all burn up.
There are great stories about guys who left their truck overnight with a load of new char in the back only to return to a gutted, melted ruin in the morning.
The best thing about using a smothered trench is that the char is bone dry. If you are charring anything soft/flexible (not bamboo or wood), the char also does not require grinding, but will powder under foot. If you are charring wood or bamboo and need a fine, regular grain, you may need a grinder. If you do not, spread the char on hard ground and stomp on it. Kids on bikes, motorbikes and even pickups also do the job. Warning: The dry char will produce a lot of fine dust that is very bad to breathe. Unless you need bone dry char, consider spraying some water about. If you are in a confined space (a shed or barn), absolutely spray some water about to avoid an explosion risk.
- How much char you get from a trench depends on your feedstock. We assume about 5 kg of dry char from a trench fed corn stalk, more for tree branches, less for rice straw.
- If you have a ready supply of feedstock (straw, stalk), it should take no more than 10-15 minutes to fill a trench.
- We, therefore, recommend that you prepare as many trenches as possible during or just after the rains when the ground is soft. If your feedstock is naturally centralized (for example, straw piles left by threshers), dig there. If you feedstock is on a steep slope (for example, mountain corn), dig a row of trenches across the bottom of the field.
- Consider your only real limit to be the number of roofing sheets you can afford. After all, if you have 30 sheets, two of you can probably do 15 trenches in the morning, 15 in the afternoon and then refill the 15 from the morning to leave to smother overnight with the 15 from early afternoon.
- We have found the easiest way to move char is to shovel it into old rice bags. A full bag of dry char should weigh no more than about 10 kg, which makes it easy to carry out of the field.
- If possible, wear an N95 rated mask. We do not recommend the basic N95 without a side vent because you will find it hard to breathe. An N95 mask with one or two side filters should do fine and will allow you to pant.
What if the people you are working with are too poor to be able to afford roofing sheet? (This is not a joke. Warm Heart’s partner in Malawi wrote to say that the first draft of this set of instructions was very nice, but useless in Malawi – because farmers could not afford roofing sheet.)
Panic not. There is a good solution.
If you cannot afford roofing sheet, do not want to use it or simply want to make more trenches than you have sheets for, here is an elegant solution from southern Africa.
Just add dirt.
Fill your trench to the very, very top with biochar and beat the char down with your long stick. When the trench is absolutely full, push the dirt you dug out of the trench back on top of the hot char. Spread it out carefully to leave no char exposed. Look for any smoke. If you see any smoke, put dirt on it. Pack the dirt down with your feet. (Be careful!) Add more dirt. Look for smoke. If you see any…. And so on.
Leave for at least five hours before you open the trench up.
Remember, if it turns gray or smokes, you must cover it up again or else…!
Don’t worry if you get a bit of dirt mixed in with your char. It will make no difference.
Best biochar tech for the poor
©️ D. Michael Shafer 2019
#biochar #biochartrench #makingbiochar