A downdraft gasifier for a small generator is built from a steel firetube of 25-40 cm inside diameter with a throat about one third as wide, a ring of air nozzles at the throat, a grate below and a gas outlet beneath it, and its output scales with the throat area. The parent’s example is a 30 cm firetube with a 9-10 cm throat for about 5 kW, so a 40 cm tube at the same loading gives roughly 9-11 kW, not the 15 kW the parent also claims.
Part of Biomass Gasification
The overall build of an Imbert-type downdraft gasifier: vessel, throat, nozzles, grate, and the order in which they are made and tested.
Size It From The Throat
The one dimension that fixes the output is the throat. The parent’s worked example is a firetube of 30 cm with a throat of 9-10 cm for a 5 kW generator, throat diameter about one third of the firetube. Throat area scales with the square of its diameter, and gas flow follows area at a given loading.
- Throat area at 9 cm: π × 4.5² = 63.6 cm². At 10 cm: π × 5² = 78.5 cm².
- A 40 cm firetube takes a throat of about 13.3 cm: π × 6.65² = 139 cm².
- Ratio to the 5 kW example: 139 ÷ 78.5 = 1.8 and 139 ÷ 63.6 = 2.2, so about 8.8-10.9 kW.
- A 25 cm firetube takes a throat of 8.3 cm: 54 cm², which is 0.69-0.86 of the example, or 3.5-4.3 kW.
So the parent’s 25-40 cm range covers roughly 3.5-11 kW at the example’s hearth loading, and 15 kW would need a throat of about 16-17 cm in a firetube of about 50 cm. Either the 15 kW figure or the diameter range is wrong; this page works from the diameters. The same arithmetic applied to the vehicle table is in Downdraft Gasifier Sizing for Vehicles.
What You Need
- Steel vessel of 3 mm or thicker plate or pipe for the hot parts (Reactor Chamber Construction).
- Air nozzles (Air Nozzle (Tuyere) Design) and a throat ring (Hearth Constriction).
- Grate and ash pit (Grate & Ash Removal).
- A way to test for leaks before any fire: soapy water and a hand pump, or a smoke source, used at low pressure.
- A cleaning train and a flare ready before the first fire (Gas Cleaning & Filtering).
Method: Build In This Order
- Make the throat first. Everything else is set around it.
- Fit the nozzle ring at the throat, angled slightly downward, with an air manifold outside.
- Build the firetube and shell, sealing every weld.
- Add the grate, ash pit and gas outlet.
- Leak-test the whole vessel with air before any fuel goes in. Keep the test pressure low, what a person can blow, not a compressor’s.
- Light a first batch outdoors with the blower running, flaring the gas and watching for tar and flame stability before an engine is ever connected.
Common Mistakes
- Scaling the throat by feel. Output follows the square of its diameter.
- Building the engine side first. A gasifier that does not make clean gas is a bad engine feed whatever the engine is.
- Skipping the leak test. A leak in suction lets air in and makes a flammable mix inside the vessel; a leak under the start-up blower pushes carbon monoxide out.
Safety
A fire, a combustible gas and carbon monoxide in one vessel
Producer gas is about 18-22% carbon monoxide by volume. Never light, run, test or purge the finished gasifier indoors, in a garage or in an enclosed yard. Before lighting, think through where the gas goes when the blower is on and when the engine is off. A hot bed full of gas and air can flash or explode when opened or restarted. Cutting a closed cylinder that once held fuel gas or extinguisher charge is an explosion risk; see Reactor Chamber Construction. See also Carbon Monoxide Risks.
What’s Next
- Learn next: Reactor Chamber Construction — the vessel and the materials
- Combine with: Engine Connection — what the gas is for