The reactor of a small downdraft gasifier is a vertical steel firetube of 25-40 cm inside diameter and 80-120 cm tall, with walls of at least 3 mm, usually mounted inside a 200-litre drum, and every seam must be gas-tight. Salvaged extinguisher and gas cylinders make good firetubes, but a closed cylinder must be vented and filled with water before it is cut, and galvanised or chemical-contaminated steel must not be heated.
Part of Building a Downdraft Gasifier
The vessel in which a downdraft gasifier’s fuel column sits: its dimensions, materials and seals.
Sizes And Materials From The Parent
- Firetube: steel pipe or drum, 25-40 cm inside diameter, 80-120 cm tall, at least 3 mm wall. The 3 mm minimum applies where the oxidation zone reaches 1,400 °C at the constriction; see Hearth Constriction.
- Shell: a 55-gallon (200-litre) steel drum around the firetube works well.
- Inner firetube candidates: a fire extinguisher tank, a gas bottle or thick pipe.
Plain steel is adequate for the shell and the fuel column. The hot parts are different: structural steel loses roughly half its strength by about 600 °C (recalled, from building-fire design tables), and the oxidation zone sits at 800-1,400 °C. The throat and nozzles therefore need a thick sacrificial ring, stainless steel or a ceramic lining, and the 3 mm wall is a floor for the cool parts, not a promise about the hot ones.
How Much Fuel The Column Holds
Fuel capacity sets how long it runs unattended. Wood chunks of 3-5 cm pack at a bulk density of about 250-350 kg/m³ (an assumption; weigh a full bucket of your own fuel).
- Firetube 30 cm across: area = π × 0.15² = 0.0707 m².
- An 80 cm fuel column: 0.0707 × 0.8 = 0.0566 m³.
- Mass: 0.0566 × 250 = 14 kg to 0.0566 × 350 = 20 kg.
- At the parent’s 5-8 kg/h for a 5 kW set: 14 ÷ 8 = 1.8 h to 20 ÷ 5 = 4 h per fill.
Method
- Prepare the steel. Use bare steel. Zinc from galvanised steel gives off toxic fume when heated or welded, and a drum that held oil, solvent or chemicals can still hold flammable vapour or residue.
- Cut a salvaged cylinder safely. A fire extinguisher must be fully discharged and its valve removed. A gas bottle must have its valve removed and be filled completely with water before any cutting or welding; with propane left inside, a cut spark ignites the vapour.
- Set the throat and nozzle ring in the tube before closing the vessel; see Air Nozzle (Tuyere) Design.
- Weld every seam continuously and fit a hopper lid that seals on a heat-resistant rope or sand channel. Hinge the lid and point it where no one stands. It should lift before the vessel does if gas inside ignites.
- Test for leaks with air at the pressure a person can blow, with soapy water on every joint.
Common Mistakes
- Assuming 3 mm survives the hot zone. It does not at the throat without protection.
- Cutting a sealed cylinder. Pressure, residue or fuel vapour makes the cut dangerous.
- Leaving seams tack-welded. A leak under suction lets air in, and a flammable mixture forms inside; a leak under blower pressure releases carbon monoxide.
Safety
The vessel holds a fire, combustible gas and carbon monoxide
The gas inside is about 18-22% carbon monoxide by volume. Never light or run the finished unit indoors, in a shed or in an enclosed yard, and do not weld or cut near it with fuel or gas inside. A hot bed full of gas and air can flash when the lid is opened; open it only when cold. See Carbon Monoxide Risks.
What’s Next
- Learn next: Air Nozzle (Tuyere) Design — the ring that feeds the hot zone
- Combine with: Grate & Ash Removal — the bottom of the vessel