An updraft gasifier admits air at the bottom and takes gas off the top, so the gas rises through the drying and pyrolysis zones and picks up their tar and steam on the way out. The tar load is the highest of any design, the parent gives up to 150 g/Nm³, which makes it unfit for an engine without extensive cleaning but well suited to burning the gas at once in a kiln, boiler or forge, and it is simple and tolerant of wet, uneven fuel.
Part of Biomass Gasification
The updraft gasifier passes gas upward through the fuel column, trading gas cleanliness for simplicity and fuel tolerance.
The Gas Leaves Through The Tar It Just Made
Air comes in under the grate and burns the charcoal at the bottom of the column. The hot gas rises through the reduction zone, then through the pyrolysis zone above it, where the heat releases tar vapour from the fuel, then through the drying zone, where it picks up steam. Nothing downstream is hot enough to destroy the tar, so it leaves in the gas. This is the mirror image of the downdraft, where the tar must cross the throat (Downdraft (Imbert) Gasifier).
The tar quantities in the parent are inconsistent: 10,000-50,000 mg/Nm³ in one place and up to 150 g/Nm³ in another, which is 150,000 mg. Taking the lower range, 10-50 g/Nm³, and the roughly 2.5 Nm³ of gas per kilogram of dry wood derived in Energy Content of Wood Gas:
- Tar per kg of dry wood: 10 × 2.5 = 25 g and 50 × 2.5 = 125 g.
- As a share of the fuel mass, that is 2.5% to 12.5%.
A meaningful fraction of the wood leaves as tar rather than as fuel gas, and it condenses in any pipe that cools below its dew point. That is why the gas is burned where it is made.
What It Is Good For
- Heat. A short, insulated, hot gas line to a burner, kiln or forge burns the tar with the gas. The hot, tar-bearing gas is a fuel rather than a contaminant there.
- Wet and uneven fuel. The gas passes through the fuel and dries it ahead of the pyrolysis zone, so the design tolerates damp, ragged material that would defeat a downdraft. This is why it is simple to build.
- A tar supply. The condensate is a mixed wood tar; see Wood Tar Production. It is not the controlled product of a retort, and it holds acids and phenols that burn skin.
Method: Using It As A Burner Feed
- Keep the pipe from the gasifier to the burner short, sloped back toward the gasifier or toward a drain, and hot. A cold pipe fills with tar and blocks.
- Light the burner only after the gas is flowing steadily, and never let the gasifier stop feeding gas to a lit burner without shutting the burner down first. A burner that runs out of gas can draw air back up the pipe.
- Keep a tar trap at the lowest point and empty it when the unit is cold.
Common Mistakes
- Connecting an engine. The tar glues valves and rings in minutes to hours.
- Long cold gas lines. The tar condenses and blocks them, and a blocked line holds a plug of tar and combustible gas.
- Leaving tar traps unemptied. They become a fire load.
Safety
Tarry gas that is also about a fifth carbon monoxide
The gas is roughly 18-22% carbon monoxide by volume plus tar vapour, and a gasifier next to a burner holds a combustible gas mixture with a fire in it. Never run an updraft gasifier indoors, in a shed or under a lean-to, and never vent or purge the gas into a building. Flame running back from a burner into a starved gas pipe, or a lid opened on a hot, gas-filled bed, can explode or flash. The tar and condensate are skin-irritating and contain compounds that may cause cancer, so wear gloves and keep them out of wells and streams. See Carbon Monoxide Risks.
What’s Next
- Learn next: Crossdraft Gasifier — the third layout
- Combine with: Wood Tar Production — what to do with the tar this design makes