What is it?
A wood gasifier is a reactor that turns solid biomass — wood chips, chunks, or pellets — into a combustible gas mixture, commonly called producer gas, syngas, or wood gas. Unlike an open fire, which burns wood directly for heat and light, a gasifier deliberately starves and stages the combustion so that the wood is converted into a mix of gases — mainly carbon monoxide (CO), hydrogen (H₂), and some methane (CH₄), diluted with nitrogen and CO₂ — that can be piped away, filtered, cooled, and then burned separately in an engine cylinder or a burner nozzle.
What is it good for?
- Running internal combustion engines on wood instead of gasoline or diesel — the best-known application, from WWII "gasogene" cars to modern off-grid generators.
- Heating and cooking — wood gas burns hotter and cleaner at the point of combustion than raw wood, since the messy solid-fuel chemistry has already happened upstream in the reactor.
- Combined heat and power — small gasifier-generator sets can power a site while the leftover heat is captured for drying or space heating.
- Waste biomass utilization — turns wood chips, shells, and agricultural residues that are awkward to burn directly into a uniform, pipeable fuel gas.
The physics behind it
A downdraft gasifier is a vertical reactor with four distinct reaction zones stacked from top to bottom, through which the fuel descends as air is drawn (or pushed) downward through it:
| Zone | Approx. temperature | What happens |
|---|---|---|
| Drying | 100–200 °C | Moisture in the wood evaporates; no chemical breakdown yet |
| Pyrolysis | 200–600 °C | Wood decomposes into char, tar vapors, and light gases — the same chemistry as charcoal burning |
| Combustion / oxidation | 700–1400 °C | A small, controlled volume of air burns part of the char and tar, generating the heat that drives the whole process and forming CO₂ and steam |
| Reduction | 600–900 °C | The hot CO₂ and steam pass back through the incandescent char bed and are chemically reduced to CO and H₂ — the actual combustible product |
The key difference from charcoal burning is what happens to the air supply. Charcoal burning (see the companion article) tries to exclude oxygen almost entirely so that pyrolysis stops at the char stage and nothing burns further — the goal is to preserve carbon as solid charcoal. Gasification does the opposite in its lower zones: it deliberately lets a small, tightly metered amount of air into the oxidation zone, then routes the hot combustion products back through the char bed in the reduction zone, where reactions like C + CO₂ → 2CO and C + H₂O → CO + H₂ convert solid carbon into combustible gas. Gasification is, in effect, charcoal burning with an extra, controlled reduction stage bolted underneath it.
Downdraft (versus updraft) design matters because it forces the tar-laden pyrolysis gases from the upper zones to pass through the white-hot combustion zone before exiting, cracking most of the tar into lighter, cleaner-burning gases. This is why nearly all engine-fuel gasifiers are downdraft: an updraft unit produces more tar, which fouls engine intakes and valves quickly.
History
Wood gas is not new chemistry — coal and wood gasification for lighting and industrial gas predates the internal combustion engine. Its most famous chapter is the wood-gas vehicle era: during WWII, when petroleum was rationed or unavailable across occupied and blockaded Europe, hundreds of thousands of cars, trucks, and buses were fitted with rooftop or trailer-mounted gasifiers ("gasogenes," "Holzgasgeneratoren," "gengas" units) burning wood or charcoal to run ordinary gasoline engines with minor carburetion changes. Performance dropped substantially (often 20–50% less power, plus a 10–20 minute startup lag to get the reactor up to temperature), but it kept essential transport running. Interest resurfaced during the 1970s oil shocks and again from the 2000s onward in the DIY and biomass-energy community, driven by open-source small-gasifier designs intended for backup power generation.
Simple version
A basic single-drum downdraft gasifier: an outer steel drum forms the body, a hopper at the top holds wood chunks, a narrow brick or steel throat partway down concentrates the air injected there to create the hot oxidation zone, and a grate near the base supports the char bed above a gas outlet. No blower — natural draft pulls air in through a bottom vent, drawn by the suction of a burner or engine intake downstream. Suitable for direct-fired heat (a burner flame) but too dirty in tar and particulates for an engine without further cleanup.
Advanced version
Adding a cyclone separator to spin out heavier tar droplets and ash particles, followed by a cooling coil or radiator to condense remaining tar and drop the gas temperature (which also increases its energy density per volume), and a final filter stage (wood-chip, cloth, or fine mesh) before the gas reaches an engine. A forced-draft blower replaces natural draft, giving much better control over the air-to-fuel ratio and faster startup. This is roughly the level of a functional WWII-style vehicle gasifier or a serious DIY generator setup.
Industrial version
Fluidized-bed and multi-stage fixed-bed gasifiers running continuously at biomass power plants, feeding gas turbines or large stationary engines. These add automated fuel metering, continuous ash removal, multi-stage gas cleanup trains (cyclones, wet scrubbers, catalytic tar cracking), and gas conditioning to pipeline or engine-grade quality. Combined-cycle biomass gasification plants can reach substantially higher electrical efficiency than direct wood combustion in a boiler, because the gas can drive a turbine rather than only raising steam.
Building your own
- Reactor body: a 200-liter steel drum works well as the main vessel — the same starting stock used for a charcoal-burning box kiln, but reworked internally.
- Hopper: a smaller drum or cone welded/bolted to the top, sized to hold several hours of wood chunks (2–5 cm pieces gasify most evenly) and refillable without shutting the unit down if built with a sealed lid.
- Throat/nozzle: the narrowest point of the reactor, lined with fire brick or refractory clay, where air nozzles inject a metered air stream to sustain the oxidation zone. This is the single most important dimension to get right — too wide and temperatures drop too low for good reduction; too narrow and it chokes flow.
- Grate and ash zone: a grate below the throat supports the char bed, lets ash fall through and be removed, and lets gas exit sideways or downward.
- Cleanup train: route the raw gas through a cyclone, then a cooling coil, then a filter before any engine or precision burner. Skipping this step is the single most common reason DIY gasifier engines fail within hours.
- Startup: gasifiers need several minutes to reach operating temperature (often flaring the raw gas off to a torch until it burns clean blue rather than smoky yellow) before switching it to the engine or burner.
Common mistakes
| Mistake | Consequence / fix |
|---|---|
| Treating it like a simple wood stove | Uncontrolled air floods the whole reactor, and you get combustion (heat, ash) instead of staged gasification — the air path must be metered and directed at the throat only |
| Skipping tar/particulate cleanup | Tar gums up engine valves and intake tracts within hours; always cyclone, cool, and filter before an engine |
| Feeding wet wood | Excess energy goes into evaporating moisture instead of pyrolysis and reduction, and gas quality (heating value) drops sharply — keep fuel moisture under ~20% |
| Undersized or misaligned throat/nozzle | Poor reduction-zone temperature, more tar carryover, unstable gas composition |
| Running the engine or burner during cold startup | The first minutes of gas are tar-heavy and low-quality — flare it off separately until it burns clean |
| No bridging/agitation in the hopper | Wood chunks arch over the throat and starve the reduction zone of fresh char — a simple internal poker or shaker bar prevents this |
| Ignoring carbon monoxide risk | Wood gas is a significant fraction CO by volume — treated as casually as wood smoke, it can kill |
Safety note: wood gas typically contains 15–25% carbon monoxide by volume. It is colorless, odorless (in this mixture), and lethal in an enclosed or poorly ventilated space. A wood gasifier must always be operated, stored, and vented outdoors or with dedicated, leak-tested piping straight to the point of combustion — never run or left idling in a garage, basement, or any enclosed structure.
How to measure
- Flame test: gas piped to a torch tip should burn with a clean, mostly blue flame once the reactor is up to temperature. A smoky yellow-orange flame means excess tar or an air-starved reduction zone.
- Temperature monitoring: a thermocouple at the throat should read roughly 900–1100 °C in steady operation; the reduction zone below it should sit lower, around 600–800 °C. Falling throat temperature signals fuel bridging or air-supply problems.
- Gas composition (if instrumented): a rough target for good producer gas is around 18–22% CO, 15–20% H₂, 1–3% CH₄, the rest CO₂ and nitrogen, with a heating value of roughly 4–6 MJ/m³ — far lower energy density than natural gas, which is why gasifier engines need larger intake volumes and lose power output.
- Tar carryover check: run gas through a clean white filter paper or cloth for a fixed time; a heavily browned or sticky sample indicates the cleanup train is undersized or the reactor is running too cool.
- Engine behavior: rough idling, power loss, or soot buildup on plugs/valves after short runs is a reliable downstream signal of inadequate gas cleaning upstream.
Videos
(TODO)
Downloadable PDF
(TODO)
Sources
- General gasification-engineering literature (downdraft and updraft gasifier design principles)
- WWII-era wood-gas ('gasogene'/'Holzgas') vehicle historical documentation and postwar civilian conversion manuals
- Small-scale/DIY gasifier project literature (e.g. FEMA and university open-source gasifier plans from the 2000s–2010s biomass-energy revival)