Making a Top-Lit Updraft (TLUD) is a great way to create biochar and a clean-burning heat source from biomass. The key principle is creating a fire that burns from the top down in a low-oxygen environment, which pyrolyzes the fuel below it.
Tools and Materials
For a simple, small-scale TLUD, you’ll need:
- Two steel cans: One smaller can (e.g., a coffee can) and one larger can (e.g., a gallon paint can or a large food service can). The smaller can will be the fuel chamber, and the larger will be the outer casing.
- A metal lid or plate to serve as a grate inside the smaller can.
- Basic hand tools: A can opener, a hammer, a nail or awl for punching holes, and tin snips.
- Safety gear: Heavy-duty gloves and safety glasses.
Step-by-Step Instructions
1. Prepare the Inner Can (Fuel Chamber)
This can holds the biomass you will pyrolyze.
- First, use a can opener to completely remove the top and bottom of the smaller can, creating an open cylinder.
- Next, create a grate. Take a flat piece of metal (like a can lid) and punch several holes in it. This grate will sit at the bottom of the fuel chamber to allow primary air to enter. You can either secure it with small screws or simply let it rest on a few small tabs you create by folding in the bottom of the can.
- Finally, make a series of small holes around the bottom of the inner can. These holes, along with the grate, are for the primary air intake.
2. Prepare the Outer Can (Air Channel)
This larger can creates an air channel for secondary combustion.
- Cut a series of holes around the bottom rim of the outer can. These are the air inlets.
- Punch a few larger holes near the top of the outer can, about one inch below where the inner can will end. These holes, along with the air channel, will feed preheated secondary air to the top of the stove to burn the combustible gases.
3. Assemble the TLUD
- Place the smaller, inner can inside the larger, outer can. There should be a gap between the two cans, which serves as your air channel.
- You may need to add a “crown” or a smaller can without a bottom to create a lid and funnel for the flame. This helps direct the secondary air to the top.
How it Works and Important Safety Tips
The TLUD works by filling the inner can with biomass (like wood chips or pellets) and lighting it from the top. As the fire burns down, it pyrolyzes the biomass below it. The gases released during this process rise up and are mixed with the secondary air that is heated and drawn up through the channel between the two cans. This secondary combustion creates a clean, smokeless flame.
Safety is crucial!
- Do work in a well-ventilated area, preferably outdoors.
- Don’t use treated or painted metal, as it can release toxic fumes when heated.
- Always wear gloves and safety glasses when cutting and handling metal.
- Do monitor the stove while it is operating and have a fire extinguisher or water source nearby.
