The four common gasifier layouts differ mainly in where the gas leaves relative to the fire, because that decides how much tar it carries: an updraft gasifier gives very tarry gas for burners, a downdraft (Imbert) gasifier gives the cleanest gas for engines, a crossdraft suits charcoal, and a top-lit updraft burns its own gas as a clean stove. Choose by the job: engines need low tar, heat does not.
Part of Biomass Gasification
Gasifier types are classified by the direction of fuel and gas flow through the vessel, which sets tar content, fuel tolerance and use.
Tar Is The Difference
Tar is the sticky mixture of heavy vapours that wood gives off when it is heated. In an engine it condenses on valves and rings and glues them. Every gasifier type is a different answer to one question: does the tar-laden vapour pass through a hot zone that destroys it, or does it leave the vessel untouched?
| Type | Gas path | Tar in the gas (parent’s figures) | Fuel it tolerates | Best use |
|---|---|---|---|---|
| Updraft | Up through the fuel, out at the top | Very tarry; 10-50 g/Nm³ in one place, “up to 150 g/Nm³” in another | Wet, non-uniform, bulky | Burning the gas at once: kiln, boiler, forge |
| Downdraft (Imbert) | Down through the hot throat and charcoal bed | 10-100 mg/Nm³ | Dry, uniform chunks, below 20% moisture | Engines |
| Crossdraft | Sideways, air one side, gas the other | Low only on charcoal | Charcoal | Small engines, fast response |
| Top-lit updraft (TLUD) | Up through the fuel, burned at the top | Burned in place | Dry, uniform | Cookstoves, biochar |
Moisture here and throughout is on a wet basis: 20% wet basis is 25% on a dry basis.
The parent article gives two different tar ranges for the updraft, 10,000-50,000 mg/Nm³ in its Imbert comparison and up to 150 g/Nm³ in the updraft section. 150 g is 150,000 mg, three times the top of the first range. Even comparing the lowest updraft figure (10,000 mg) with the highest downdraft one (100 mg), the updraft is at least 100 times tarrier; see Updraft Gasifier.
Choose By What You Need To Run
- An engine or generator: downdraft, and be prepared to clean the gas anyway. See Downdraft (Imbert) Gasifier.
- A kiln, boiler or forge: updraft. The tar burns with the gas, and the design takes damp, ragged fuel. See Updraft Gasifier.
- A small engine with charcoal available: crossdraft. See Crossdraft Gasifier.
- Cooking or making charcoal for soil: TLUD. See TLUD (Top-Lit Updraft).
What You Give Up With The Downdraft
The downdraft is the engine choice, but its cost is real. It needs dry, uniform fuel, it does not handle fines or dust, and the throat limits how much gas it can make, so turndown is poor. The gas still has to be cooled and filtered (Gas Cleaning & Filtering).
Common Mistakes
- Putting updraft gas into an engine. It will clog it in minutes to hours.
- Calling any downdraft gas “clean”. 10-100 mg/Nm³ is low for a gasifier but not zero, and a cold or overloaded unit makes far more.
- Choosing the design before the fuel. Damp or bulky fuel suits an updraft burner, not an engine.
Safety
Every type makes carbon monoxide
Updraft, downdraft and crossdraft gas is roughly a fifth carbon monoxide by volume; even a clean-burning TLUD makes it while it runs and while the char smoulders afterwards. It is odourless and colourless, and the tar smell tells you nothing about it. Never run any gasifier indoors, in a shed or in an enclosed yard. See Carbon Monoxide Risks.
What’s Next
- Learn next: Downdraft (Imbert) Gasifier — the layout used for engines
- Combine with: Gasification Theory — why the layouts behave differently