A downdraft (Imbert) gasifier feeds fuel from the top and air through nozzles at a narrow throat partway down, so the tar vapour from the fuel above has to pass through the hottest zone and the charcoal bed below before it leaves, which cracks most of it. It produces gas at about 10-100 mg/Nm³ of tar, the lowest of the common designs, but it demands dry, uniform fuel at 20% moisture or below on a wet basis and makes less gas than its size suggests when run at low load.
Part of Biomass Gasification
The downdraft gasifier is the standard design for engine fuel: gas and fuel both move downward and the gas exits below the throat.
The Gas Must Cross The Hottest Zone To Leave
Follow a piece of wood down the vessel. It dries, then pyrolyses and releases tar vapour, and then it reaches the throat where air enters through the nozzles and the temperature is highest, at about 800-1,400 °C by the parent’s ranges. Because the gas exit is below, everything the wood gave off on the way down is drawn through the same narrow hot zone, then through the glowing charcoal bed where reduction makes carbon monoxide and hydrogen (Pyrolysis, Reduction, & Oxidation Zones). Tar that crosses that zone is largely cracked into lighter gas.
This also explains the limits. The cracking happens only if the throat is hot and every vapour molecule actually passes through it. A cold throat, a channel of fuel that lets vapour skirt the hot zone, or wet fuel that cools the bed all let tar through.
The Parts, From Top To Bottom
- Hopper and drying zone. Holds fuel and lets it dry using heat conducted upward from the throat.
- Pyrolysis zone. Fuel breaks down here with no air.
- Nozzle ring and throat. Air is admitted at the narrowest point. The parent sizes the throat at about one third of the firetube diameter, for example 9-10 cm in a 30 cm tube for 5 kW. See Hearth Constriction and Air Nozzle (Tuyere) Design.
- Reduction bed. Glowing charcoal where carbon dioxide and steam become fuel gas.
- Grate and ash pit. Supports the bed and collects ash (Grate & Ash Removal).
- Gas outlet. At the bottom, at 300-400 °C according to the parent.
What The Design Costs
- Fuel. Dry, uniform chunks of about 3-5 cm at 20% moisture or below on a wet basis (25% dry basis). It does not handle sawdust, fines or very wet fuel.
- Load range. The throat is sized for one gas rate. Below it the hot zone cools and tar rises; above it the bed channels. A vehicle engine that idles and climbs hills spends much of its time outside that range.
- Heat loss. The gas leaves hot, so energy leaves with it and must be removed again in cooling (Tar Removal (Cyclone & Cooling)).
- Residual tar. 10-100 mg/Nm³ is low for a gasifier, but the tar limit commonly quoted for engines is of the same order (about 100 mg/Nm³, recalled), so the parent’s “moderate filtering” is optimistic. Plan a full clean-up train.
Method: Judge Whether It Is Running Right
- After the unit is lit and fan-drawn, the gas should burn at a flare with a steady flame that stays lit when the igniter is removed.
- A smoky, sooty yellow flame, or tarry streaks in the pipework, means the throat is cold or the fuel is wet. Do not connect an engine.
- Check the hopper level and the bed regularly; a hollow under a bridged load of fuel sends gas uncracked past the throat.
Common Mistakes
- Letting the hopper run low. A short fuel column leaves the pyrolysis zone thin and the throat exposed.
- Using fines or mixed sizes. They pack, bridge and channel.
- Treating idling as free. Low gas flow cools the throat and makes tar.
Safety
Carbon monoxide at the lighting port, hopper lid and every joint
The gas is about 18-22% carbon monoxide by volume. Never light or run a downdraft gasifier indoors, in a shed, in a lean-to or in an enclosed yard, and keep the lighting port under suction so gas is not pushed out at your face. Opening the hopper lid on a hot, gas-filled unit can flash the gas in your face: the vessel holds combustible gas with a fire inside it, and flashback on restart with a hot bed is a recognised failure. See Carbon Monoxide Risks.
What’s Next
- Learn next: Building a Downdraft Gasifier — dimensions and construction
- Combine with: Updraft Gasifier — the design this one corrects