What is a TLUD?

How Do Top-Lit Updraft Gasifiers Work?


Top-lit updraft (TLUD) gasifiers automatically make char in the process of cooking. The way TLUDs work is very simple.


Reaction cycle

  • A TLUD is a canister filled with a small pieces of fuel such as broken twigs, wood chips, broken rice-hull briquettes, paper cakes, or vertical bundles of jute sticks.   This is called a fuel bed.  Air can enter the canister through holes in a grate at the bottom and at the top of the cylinder, above the fuel bed.
  • Kindling, or fuel pieces wetted with kerosene, are added to the top of the fuel bed and ignited.
  • An ignition front (also called a “migratory pyrolytic front” and “flaming pyrolysis“) is formed that moves down through the fuel producing white smoke, (“producer gas” or “wood gas”) and leaving unburned char above.  The ignition front is supported by the air (primary air) that enters the bottom of the cylinder.
  • The white smoke is flammable, and burns at the top of the TLUD for cooking.  This gas flame is supported with air (secondary air) entering through side-holes or a gap near the top of the cylinder.
  • When the ignition front reaches the bottom of the cylinder, the smoke-producing reaction is complete, and the gas fire goes out.   At this point we can remove the char from the cylinder, and use it for biochar, as charcoal fuel, or sell it.  If we don’t remove the char, it will burn slowly from the bottom up, producing only a warming heat at the top of the cylinder.
  • The primary and secondary air can move by natural draft, relying on the buoyancy of hot gasses to push air through the system, or can be forced draft by using a small electric fan to push the air.

Primary air flow rate is critical to controlling the reaction rate.  Since wood already contains oxygen, it does not take much primary air to support the flaming pyrolysis.  The aperture that regulates primary air can be quite small.   For more information, we have a two-page summary of Basic Operating Parameters of a Natural Draft TLUD.

For a comprehensive collection of articles on TLUD gasifiers, see Dr. Paul Anderson’s website, www.drtlud.com.

For a forum for discussing TLUD gasifiers and other biomass stoves, see “Stoves” at bioenergylists.org.

Next: [TLUD-Biochar Ecology]