Biomass Gasification

Gasification converts solid biomass (wood, charcoal, agricultural waste) into a combustible gas mixture called producer gas or syngas. This gas can run internal combustion engines, replacing gasoline or diesel with wood. During World War II, over one million vehicles in Europe ran on wood gas when petroleum was unavailable. The technology is proven, buildable from salvaged materials, and could power generators, vehicles, and workshop equipment in a post-collapse world.

Gasification Theory

How It Works

Gasification is NOT burning. Instead of allowing complete combustion, you restrict air supply so the biomass undergoes a series of chemical reactions:

1. Drying zone (100-200°C): Moisture evaporates from the fuel
2. Pyrolysis zone (200-500°C): Wood breaks down into charcoal, tar vapor, and volatile gases. No air needed — pure thermal decomposition
3. Oxidation zone (800-1,400°C): A controlled amount of air enters. Some charcoal burns, generating extreme heat and CO₂
4. Reduction zone (600-1,000°C): The hot CO₂ and steam pass over a bed of hot charcoal, which strips the oxygen: CO₂ + C → 2CO and H₂O + C → H₂ + CO. These reactions produce the combustible gases

Syngas Composition

Typical producer gas from a well-run downdraft gasifier:

Component	Percentage	Combustible?
Nitrogen (N₂)	50-55%	No
Carbon monoxide (CO)	18-22%	Yes
Hydrogen (H₂)	12-18%	Yes
Carbon dioxide (CO₂)	8-12%	No
Methane (CH₄)	2-4%	Yes
Other hydrocarbons	0.5-1%	Yes

The high nitrogen content (from the air used in the oxidation zone) means producer gas has roughly 1/6 the energy density of natural gas. An engine running on wood gas produces about 50-60% of its rated gasoline power.

Energy Balance

Approximately 1 kg of dry wood produces enough gas to generate 1 kWh of electricity through a generator engine, or to drive a vehicle 1-1.5 km. This is roughly 70-75% of the energy theoretically available in the wood.

Gasifier Types

Downdraft (Imbert) Gasifier

The preferred design for engine fuel because it produces the cleanest gas with the least tar:

• Fuel enters from the top
• Air enters through nozzles (tuyeres) at a constriction point partway down
• Gas flows downward through the hot charcoal bed, cracking tar as it passes
• Clean gas exits at the bottom
• The hot oxidation zone sits above the reduction zone, so all tars must pass through both — most tar is destroyed

Advantages: Low tar (10-100 mg/Nm³ vs 10,000-50,000 for updraft). Gas can go directly to an engine with moderate filtering. Disadvantages: Needs relatively uniform, dry fuel (moisture below 20%). Does not handle fine or dusty fuel well.

Updraft Gasifier

• Fuel enters from the top, air from the bottom
• Gas exits at the top — it passes through the pyrolysis zone picking up tar
• Produces very tarry gas (up to 150 g/Nm³) — unsuitable for engines without extensive cleaning
• Simple to build and tolerates wet, non-uniform fuel
• Best for heating applications where gas is burned immediately (kiln, boiler, forge)

Crossdraft Gasifier

• Air enters from one side, gas exits the opposite side
• Very fast response time, compact design
• Works best with charcoal (already de-tarred fuel)
• Suitable for small engines and low-power applications

Building a Downdraft Gasifier

This section covers a practical Imbert-style gasifier suitable for powering a 5-15 kW generator engine.

Reactor Chamber

The reactor is a vertical cylinder (the “firetube”) where gasification occurs:

• Material: Steel pipe or drum, 25-40 cm inside diameter, 80-120 cm tall
• Wall thickness: Minimum 3 mm steel (the oxidation zone reaches 1,400°C but is concentrated at the constriction)
• A 55-gallon / 200-liter steel drum works well for the outer shell
• Inner firetube: A smaller steel cylinder (fire extinguisher tank, gas bottle, or thick pipe) mounted inside the outer shell

Air Nozzle (Tuyere) Design

• 3-5 nozzles equally spaced around the circumference at the constriction point
• Each nozzle: 8-12 mm inside diameter steel or stainless tube
• Angled slightly downward (15-30°) to direct air into the oxidation zone
• Connected to an air manifold ring outside the reactor
• Air supply is naturally aspirated (engine vacuum pulls air through) or fan-assisted for startup

Hearth Constriction

The critical design element — a narrowing of the reactor where air nozzles inject:

• Throat diameter = approximately 1/3 of the firetube diameter
• This concentrates the oxidation zone, ensuring high temperatures that crack tar
• Too large: tar passes through uncracked. Too small: fuel bridging and choking
• Typical dimensions for a 5 kW system: firetube 30 cm, throat 9-10 cm
• Use a thick steel plate or ceramic ring as the constriction

Grate & Ash Removal

Below the reduction zone:

• A grate (perforated steel plate or grid of bars) supports the charcoal bed
• Below the grate, an ash collection chamber
• Ash must be removed periodically (every 4-8 hours of operation)
• A shaker mechanism helps break up clinker (fused ash) that forms at high temperatures

Gas Cleaning & Filtering

Even a good downdraft gasifier produces gas that will damage an engine if used directly. Cleaning is essential.

Tar Removal

Cyclone separator: Gas enters a cylindrical chamber tangentially, spins, and heavy tar droplets are flung to the walls and drain out. Removes particles above 10 microns.

Gas cooler: Pass the gas through a long pipe (radiator, coiled tubing) to cool it from 300-400°C to below 40°C. Tar condenses and drains off. A car radiator works excellently.

Particulate Filtration

After cooling:

• Fabric filter: Gas passes through a bag or tube of closely woven cotton or synthetic fabric
• Sawdust filter: A drum packed with dry sawdust or wood shavings — gas percolates through, particles stick
• Sand filter: A vertical tube filled with sand — effective but creates more pressure drop

Replace or clean filters regularly. A clogged filter starves the engine of gas.

Moisture Separation

Cooling the gas condenses water vapor. Include a drain point (condensation trap) after the cooler and before the engine. Even small amounts of water in the gas cause engine misfires.

Engine Connection

Mixing Valve

The engine needs a mixture of producer gas and air:

• Build a simple butterfly valve or sliding plate valve where the gas line meets the air intake
• This “mixer” controls the gas-to-air ratio
• Start the engine on gasoline, then gradually open the gas valve and close the gasoline supply
• Adjust the gas valve for smooth running — too rich (too much gas) causes misfires, too lean (too much air) causes overheating

Engine Modifications

Gasoline engines (spark ignition) require minimal modification:

• Increase compression ratio if possible (from ~8:1 to 10-12:1) for better efficiency with low-energy gas
• Advance ignition timing by 10-15 degrees (producer gas burns slower than gasoline)
• Install a mixer valve on the intake

Diesel engines require a small pilot injection of diesel (5-15% of normal consumption) to ignite the gas, since producer gas does not compression-ignite reliably. This “dual-fuel” mode works well.

Power Derating

Expect 40-60% of rated power when running on producer gas compared to gasoline. A 10 kW gasoline generator will produce 4-6 kW on wood gas. Size your engine accordingly.

Safety

Carbon monoxide warning: Producer gas is 20% CO — odorless, colorless, and lethal. Never operate a gasifier indoors or in an enclosed space. All connections must be airtight. A small leak can kill. Treat the entire gas path with the same respect you would give to natural gas plumbing.

Feedstock Preparation

The gasifier is only as good as its fuel:

• Wood chunks: 3-5 cm cubes or irregular pieces. Uniformity matters — mixed sizes cause bridging and channeling
• Moisture content: Below 20% (ideally 10-15%). Wet wood produces excess tar and reduces gas quality dramatically
• Hardwood vs softwood: Hardwood (oak, beech, ash) produces cleaner gas with less tar. Softwood (pine, spruce) works but needs better filtering
• Charcoal: The ideal gasifier fuel — already de-tarred, produces very clean gas. But making charcoal loses 50-70% of the wood’s energy
• Agricultural waste: Nut shells, corn cobs, dried fruit pits all work. Size and moisture are the key factors
• Do NOT use: Treated wood (toxic fumes), green wood (excessive tar), sawdust or fine chips (blocks airflow)

Processing wood for a gasifier:

1. Split logs into rough 5 cm pieces
2. Air-dry under cover for at least 6 months (hardwood) or 3 months (softwood)
3. Store in a dry location — re-wetting ruins the fuel quality
4. Test moisture: a piece should snap cleanly when bent, not flex

A 5 kW gasifier-generator system consumes roughly 5-8 kg of dry wood per hour of operation. For 4 hours of daily electricity generation, you need 20-32 kg of prepared fuel per day, or roughly 7-12 tonnes per year. This is well within the output of a small coppiced woodland.

Maintenance Schedule

Interval	Task
Every 4-8 hours	Shake grate, remove ash
Daily (after shutdown)	Drain condensation traps
Every 50 hours	Clean or replace fabric filter
Every 100 hours	Inspect air nozzles for erosion
Every 200 hours	Check hearth constriction for erosion, clean cyclone
Every 500 hours	Full inspection — check all welds, replace worn parts

See Also

• wood-gas-vehicles — Mobile gasifier systems for vehicles
• coppicing-fuel-management — Sustainable fuel supply for your gasifier
• ethanol-fuel-production — Alternative liquid biofuel