The hearth constriction, or throat, is a narrowing of about one third of the firetube diameter at the level of the air nozzles, and it forces all the gas through the one zone hot enough to crack tar. A throat that is too wide lets tar through uncracked, one that is too narrow makes the fuel bridge and the unit choke, and its area, not the vessel size, sets how much gas the gasifier can make.
Part of Building a Downdraft Gasifier
The constriction is the narrowest part of a downdraft gasifier’s fuel column, where the nozzles inject air and the hottest reactions take place.
One Hot Zone, Sized By Its Area
Everything the fuel gave off above the throat has to pass through it. The throat concentrates the air from the nozzles into a small cross-section so the temperature there is high enough to crack tar, and it fixes the gas rate the bed can serve. The parent sizes it at about one third of the firetube diameter: a 30 cm tube takes a 9-10 cm throat for a 5 kW system. The vehicle table in the second carrier keeps the same ratio (60-75 mm in 200-250 mm, up to 100-130 mm in 300-400 mm).
Gas flow is the figure to design for. Taking the parent’s 5-8 kg/h of dry wood at the 2.5 Nm³/kg derived in Energy Content of Wood Gas:
- Gas flow: 5 × 2.5 = 12.5 and 8 × 2.5 = 20 Nm³/h.
- Throat area for 9-10 cm: π × 4.5² = 63.6 cm² and π × 5² = 78.5 cm².
- Hearth load, gas per throat area: 12.5 ÷ 78.5 = 0.16 and 20 ÷ 63.6 = 0.31 Nm³/h per cm².
The vehicle table asks for far more per cm² of throat; see Downdraft Gasifier Sizing for Vehicles, where the two are compared. I could not find a sourced maximum hearth load, so treat the 0.16-0.31 figure as the loading the parent’s 5 kW example implies, not as a rule.
What Each Error Looks Like
- Too wide: the hot zone is diffuse and cooler, some vapour passes round it, and tar appears in the cooler and filter. This is the parent’s “tar passes through uncracked”.
- Too narrow: the fuel column bridges above the throat and starves it, and the gas the throat can pass falls below the engine’s demand, so the engine loses power while the vessel still looks as if it is running.
- Throat eroded: after hundreds of hours the metal wears and the opening grows, so a gasifier that once ran clean drifts to a tarry one. The parent’s schedule checks the constriction every 200 hours.
Method: Build And Keep It
- Make the throat ring from a thick steel plate or a ceramic ring, as the parent says, and make it replaceable rather than welded irreversibly.
- Set it level with the nozzle tips or just below them so the jets fire into the ring.
- Fit the fuel column above with no ledges where chunks can hang.
- Measure the finished opening and check it against one third of the firetube diameter.
- Re-check it for wear on schedule. Replace it when the diameter has grown noticeably.
Common Mistakes
- Tuning tar by widening the throat. Opening up the throat makes tar worse, not better.
- A welded-in throat that cannot be replaced. It is the first part to wear out.
- Using the vessel diameter as the capacity. The throat sets it.
Safety
The throat is the hottest point of a carbon-monoxide-producing fire
The gas that passes it is about 18-22% carbon monoxide by volume and the zone runs at 800-1,400 °C. Never run or test a gasifier indoors, in a shed or in an enclosed yard, and do not open the vessel to inspect a hot throat: a hot bed full of gas can flash when air is admitted. Inspect only when cold and fully cooled. See Carbon Monoxide Risks.
What’s Next
- Learn next: Grate & Ash Removal — the bottom of the bed
- Combine with: Air Nozzle (Tuyere) Design — the air that fires into the throat