The air nozzles of a small downdraft gasifier are 3-5 steel tubes of 8-12 mm bore, equally spaced round the throat, angled 15-30° downward and fed from a manifold ring, and together they must pass about 8.3-13.3 normal cubic metres of air an hour for a fuel rate of 5-8 kg/h. That fixes the jet speed between roughly 4.4 and 26 m/s depending on how many nozzles of what bore are chosen, so the number and size of the nozzles are the parameters to tune, not fixed details.
Part of Building a Downdraft Gasifier
The nozzle ring delivers the air that burns part of the fuel at the throat; its count, bore, angle and supply decide whether the hot zone is even.
Size The Nozzles From The Air The Fuel Needs
Gasification burns only part of the fuel, and the air required follows from the nitrogen in the gas. The derivation in Pyrolysis, Reduction, & Oxidation Zones gives about 1.66 m³ of air per kg of dry wood. For the parent’s 5 kW set at 5-8 kg/h:
- Air at 0 °C: 1.66 × 5 = 8.3 and 1.66 × 8 = 13.3 Nm³/h.
- At ambient 20 °C the volume is larger by 293 ÷ 273 = 1.073: 8.9 and 14.3 m³/h.
- In m³/s: 8.9 ÷ 3600 = 2.47 × 10⁻³ and 14.3 ÷ 3600 = 3.96 × 10⁻³.
The parent’s nozzles span a wide area range: 3 nozzles of 8 mm have π/4 × 8² × 3 = 151 mm² and 5 nozzles of 12 mm have π/4 × 12² × 5 = 566 mm².
- Fastest jet: 3.96 × 10⁻³ ÷ 151 × 10⁻⁶ = 26 m/s.
- Slowest: 2.47 × 10⁻³ ÷ 566 × 10⁻⁶ = 4.4 m/s.
- A middle case, 4 nozzles of 10 mm (314 mm²) at 6.5 kg/h (air 11.6 m³/h, 3.2 × 10⁻³ m³/s): 10 m/s.
A six-fold spread in jet speed across the parent’s own ranges means the dimensions are not a specification. I have no sourced target velocity, so I do not give one; the nozzle area is a variable to adjust by observation of the flare and of tar, as described below. The corpus internal-combustion article gives 4-6 holes of 10-15 mm for a larger hearth, a different range again.
The Supply Pressure Is Small
A suction gasifier draws its air through the nozzles with the engine’s vacuum. The pressure needed to accelerate air to speed v is ½ρv²:
- At 26 m/s: 0.5 × 1.2 × 26² = 405 Pa.
- At 10 m/s: 0.5 × 1.2 × 10² = 60 Pa.
Each 1 kPa of suction spent costs the engine about 1% of its intake charge (1 kPa ÷ 101 kPa), so even the fast case costs about 0.4% and nozzle sizing is not an efficiency question. It is a mixing and hot-zone question.
What Goes Wrong And How It Shows
- Too few or too small nozzles: the jets are fast, penetrate and overheat a small central core, wearing the nozzle tips and the opposite wall, with cold fuel at the edges where tar escapes.
- Too many or too large: the jets are slow and do not reach the centre, so the hot zone is a ring at the wall and the middle of the bed stays cool.
- One nozzle blocked: the hot zone goes lopsided; tar appears in the flare, and the bed bridges or slags on one side.
- Uneven manifold feed: the nozzle nearest the air inlet takes most of the air. Feed the ring from several points or make the manifold larger than the nozzles’ combined area.
Method
- Start with 4 nozzles of 10 mm, angled 15-30° down, evenly spaced, from a ring larger than their total area.
- Make the tips of stainless tube or replaceable, and recessed slightly from the hot bed.
- Run a batch outdoors with the flare lit. A steady flame that survives igniter removal is good; a sooty yellow flame or tarry pipes means too cold or uneven a hot zone.
- Change one thing at a time (close a nozzle with a plug, enlarge another) and check again. Inspect for erosion every 100 hours, as the parent’s schedule says.
Common Mistakes
- Copying nozzle dimensions without the throat. The nozzles and the throat are sized together; see Hearth Constriction.
- A single air inlet to the ring. It starves the far nozzles.
- Letting ash plug the tips. Clean them when cold.
Safety
The nozzles are an opening into a burning bed of carbon monoxide
Producer gas is about 18-22% carbon monoxide by volume. A nozzle ring that is blocked, burned through or under blower pressure can push gas out around the vessel. Never light, run or test the unit indoors, in a shed or in an enclosed yard, and never put your face over a lighting port or manifold. Hot gas-filled vessels can flash back when opened or restarted. See Carbon Monoxide Risks.
What’s Next
- Learn next: Hearth Constriction — the throat the nozzles feed
- Combine with: Reactor Chamber Construction — the vessel that holds them