A top-lit updraft (TLUD) gasifier is lit at the top of a loaded column of dry fuel, so the fire front moves slowly downward while air rises from below, and the gas driven off the fuel ahead of the front burns in a clean flame above it. It makes a cookstove or a char-maker rather than an engine fuel, because the gas is burned on the spot in a batch and never collected or cleaned.
Part of Biomass Gasification
The TLUD is a batch gasifier lit from the top in which air rises through the fuel and a secondary flame burns the gas; it leaves charcoal behind.
The Fire Burns Downward Into Fresh Fuel
In an ordinary updraft the fire is at the bottom and the gas escapes through cold fuel carrying its tar. In a TLUD the fire is lit at the top instead. The fuel below the fire front is heated and releases its gas, which rises into the flame zone and burns with the air that comes up from the bottom. As the front moves down, the fuel behind it is left as charcoal. The gas is consumed where it is made, so there is no gas pipe to foul.
Burning the gas in the open is why the flame is clean. Smoke is unburned tar and gas, and a TLUD burns them with preheated secondary air.
What You Need
- Dry, uniform fuel, loaded to within about 30 cm of the top in the corpus’s barrel design. Moisture is on a wet basis; the same rule as the other designs, 20% wet basis or below (25% dry basis).
- A 200-litre steel drum with 16-24 evenly spaced holes of 20-25 mm around the bottom for primary air.
- A secondary air ring near the top with 12-16 holes of 10-15 mm pointing inward and upward, and a chimney 30-40 cm tall and 15-20 cm across. These are the corpus barrel dimensions from Biochar Production, where the construction and operation are described in full.
Method: Run A Batch
- Load the dry fuel evenly.
- Light kindling on top, fit the chimney and do not seal the drum.
- Watch the flame settle to a steady, mostly smokeless burn as the front descends.
- When the flame dies, the fuel has been converted to charcoal. Shut the air off and let it cool before opening: hot charcoal exposed to air re-ignites.
The batch nature is the main difference from an engine gasifier: it cannot hold a steady gas flow for hours, so it cannot run a vehicle or generator.
Why It Matters Beyond Cooking
The charcoal left behind is biochar when it is made at the right temperatures and quenched correctly; see Biochar for its use as a soil amendment. The wood that becomes char is also fuel not burned for heat, so each batch trades heat for soil benefit.
Common Mistakes
- Sealing the drum to “improve” it. It starves the flame and makes smoke and carbon monoxide.
- Using wet or oversized fuel. The front stalls and the flame goes sooty.
- Opening it while hot. The charcoal re-ignites.
Safety
A TLUD still makes carbon monoxide
A clean flame does not mean clean air. The unit produces carbon monoxide whenever the flame is poor, at start-up when the gas is still cool, and for hours afterward from smouldering charcoal, and carbon monoxide has no smell. Never run, light, quench or store a hot TLUD indoors, in a shed or in an enclosed yard. The drum, chimney and char are hot enough to burn, and water on hot charcoal makes steam and more fuel gas. See CO Risk and Safety Practices.
What’s Next
- Learn next: Downdraft (Imbert) Gasifier — the layout used when the gas has to be collected for an engine
- Combine with: Biochar — putting the charcoal to use