The science behind wood gas
How Gasification Works
Gasification is a century-old process that converts solid biomass into a combustible gas. Here's everything you need to understand it — from the chemistry to the components.
From Wood to Fuel Gas in Four Steps
Inside a gasifier, solid biomass goes through a series of chemical reactions driven by heat and limited oxygen. The result is syngas — a mixture of hydrogen, carbon monoxide, and methane that can power engines, generators, and heating systems. No petroleum. No grid. Just wood and fire.
Inside the reactor
The four reaction zones
Drying Zone
As biomass enters the top of the gasifier, heat from below drives off moisture. Fuel with less than 20% moisture content works best — wet wood robs energy and produces more tar.
This zone doesn't produce gas — it simply prepares the fuel. Properly dried wood chips or pellets pass through quickly and efficiently.
Pyrolysis Zone
In the absence of oxygen, heat breaks down the biomass into charcoal, volatile gases, and tars. This is where the solid fuel begins its transformation into gas.
Pyrolysis produces a mix of useful gases and unwanted tars. The design of the gasifier determines how much tar makes it into the output — a well-designed downdraft gasifier cracks most tars before they exit.
Combustion Zone
A controlled amount of air is introduced here. The charcoal and volatile gases combust partially, generating the intense heat that drives the entire process.
This is the hottest zone. The partial combustion provides the energy for pyrolysis above and reduction below. Air-to-fuel ratio is critical — too much air burns everything; too little starves the reaction.
Reduction Zone
Hot combustion gases pass through a bed of glowing charcoal. Carbon dioxide and steam react with the charcoal to produce hydrogen and carbon monoxide — the useful components of syngas.
This is where the magic happens. The reduction reactions are endothermic (they absorb heat), which is why maintaining the right temperature balance is key to a high-quality gas output.
The output
What Is Syngas?
Syngas (synthesis gas or wood gas) is the output of gasification. It's a mixture of combustible gases that can be burned directly or used to power internal combustion engines.
| Gas | Typical % |
|---|---|
| Carbon Monoxide (CO) | 18–22% |
| Hydrogen (H₂) | 15–21% |
| Methane (CH₄) | 1–4% |
| Carbon Dioxide (CO₂) | 9–12% |
| Nitrogen (N₂) | 45–55% |
The full system
Key components explained
A complete gasifier system is more than just the reactor. Here's what each part does and why it matters.
Reactor Vessel
The main chamber where gasification occurs. Usually made from steel pipe or a repurposed pressure vessel. Must withstand temperatures up to 2200°F (1200°C).
Air Nozzles (Tuyeres)
Introduce controlled amounts of air into the combustion zone. Placement and sizing directly affect gas quality and tar production.
Grate & Ash Pit
Supports the char bed and allows ash to fall away. Regular ash removal keeps the gasifier running efficiently.
Gas Outlet
Hot syngas exits here and travels to the cleaning system. Temperature at the outlet is a key indicator of gasifier health.
Cyclone Separator
Removes coarse particulates (char dust, ash) from the gas stream using centrifugal force. First stage of gas cleaning.
Filter / Scrubber
Removes fine tars and particulates. Can be a packed bed of wood chips, a wet scrubber, or a fabric filter depending on the application.
Which design is right for you?
Types of gasifiers
Downdraft (Imbert) Gasifier
Updraft Gasifier
FEMA Emergency Gasifier
Common questions
Quick answers
Is syngas dangerous?
Yes — carbon monoxide (CO) is a significant component of syngas and is odorless and toxic. Always operate gasifiers outdoors or with proper ventilation, and install CO detectors. Treat syngas with the same respect as natural gas or propane.
How much wood does it take to run a generator?
A rough rule of thumb: 2.2 lbs (1 kg) of dry wood produces about 70–90 cubic feet (2–2.5 m³) of syngas, which can generate roughly 1 kWh of electricity in a gasifier-engine system. A 5 kW generator running for an hour needs about 4.5–6.5 lbs (2–3 kg) of dry wood chips.
Can I use any type of wood?
Most hardwoods and softwoods work well. The key factors are moisture content (under 20%) and particle size (consistent 1–2 inch chunks for downdraft gasifiers). Avoid treated wood, painted wood, or materials with high ash content like straw.
What is tar and why does it matter?
Tar is a mixture of condensable organic compounds produced during pyrolysis. In small amounts it's manageable, but heavy tar buildup clogs filters, damages engines, and reduces efficiency. Downdraft gasifiers minimize tar by routing gases through the hot combustion zone.
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