A crossdraft gasifier injects air through a nozzle on one side and draws gas out of the opposite side, so the reaction is confined to a small, very hot hearth between them, which makes it compact and quick to respond. Because the gas has no long hot zone to pass through, it is only clean enough for engines when the fuel is charcoal, and making that charcoal costs 50-70% of the wood’s energy.
Part of Biomass Gasification
The crossdraft gasifier moves air and gas horizontally through the fuel bed, suited to charcoal and small, fast-responding engines.
One Hot Spot In A Sideways Flow
Air enters through a nozzle on one wall, burns charcoal directly in front of it, and the gas crosses the bed and leaves through the opposite wall. The oxidation and reduction happen in the same small region, with the fuel stored above as a reservoir. Compared with the downdraft there is no separate narrowing throat and no thick char bed to filter and reduce the gas, so the vessel is small and light and the hot zone reaches temperature quickly. That is the “very fast response time, compact design” the parent describes.
Why It Wants Charcoal
Wood in the hopper would pyrolyse and send its tar straight across the bed to the outlet with only a short, hot path to crack it. Charcoal has already lost its tar-forming volatiles, so the gas has little tar to carry. That is why the parent says it works best with charcoal and is suited to small engines and low power.
The cost is in the fuel. The parent says making charcoal loses 50-70% of the wood’s energy. A cross-check with the corpus figures: charcoal yields of 15-25% by weight from a pit (Charcoal Making) at about 30 MJ/kg (recalled) against 18.5 MJ/kg for the wood retain 24% (0.15 × 30 ÷ 18.5) to 41% (0.25 × 30 ÷ 18.5) of the energy, so 59-76% is lost, which overlaps the parent’s 50-70% and is a little higher at the top. That makes crossdraft fuel dear in wood terms, and the land that grows the wood is not growing food or anything else in the meantime.
Where It Fits
- A small engine and charcoal on hand. A workshop engine run in short periods suits the quick start.
- Not for heavy or sustained loads. The small hearth concentrates heat at the nozzle and the opposite wall, so the metal there wears fastest.
- Not for mixed or wet fuel. It has no stage that destroys tar.
Method: Matching Fuel And Hearth
- Use charcoal broken to uniform pieces; fines pack and starve the hearth.
- Keep the charcoal dry: wet charcoal also gives off steam and tar, and cools the hearth.
- Check the nozzle and the facing wall regularly for erosion or burn-through, because a leak there is a gas leak.
Common Mistakes
- Fuelling it with raw wood. The tar is not cracked and the filters clog.
- Ignoring the nozzle wall. It is the hottest and most stressed point.
- Counting the charcoal as free. It is wood that has already been spent.
Safety
Charcoal gas is rich in carbon monoxide
Gas from charcoal is richer in carbon monoxide than gas from wood, plausibly toward a third by volume, and it is odourless and colourless. Never run a crossdraft gasifier indoors, in a shed or in an enclosed yard, and never light or refuel it in a closed space. A small hearth with a fire in it and a gas-filled vessel behind it can flash back on restart. See CO Risk and Carbon Monoxide Risks.
What’s Next
- Learn next: TLUD (Top-Lit Updraft) — a design that burns its own gas
- Combine with: Charcoal Production — making the fuel it needs