Producer gas from a well-run downdraft gasifier burning dry wood is about 50-55% nitrogen, 18-22% carbon monoxide, 12-18% hydrogen, 8-12% carbon dioxide and 2-4% methane by volume on a dry basis, and these are ranges, not a recipe. Wetter fuel, more air, a colder bed, charcoal instead of wood or a change of load all shift the figures, so a composition quoted without its fuel moisture and operating conditions is not a specification.
Part of Biomass Gasification
The mix of gases leaving a gasifier, the range each component occupies under stated conditions, and the operating variables that move it.
The Ranges Describe One Condition, Not Every Gasifier
The table gives the parent article’s ranges for a well-run downdraft (Imbert) gasifier on dry wood chunks, as volume percent of the dry gas. The gas leaves carrying water vapour as well, which is not counted here (Moisture Separation).
| Component | Volume % (dry gas) | Burns? |
|---|---|---|
| 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 ends of the ranges cannot all hold at once. The lows sum to 90.5% and the highs to 112%; the midpoints sum to 101.25%, which shows the ranges are bracketing values, not a single analysis. The methane figure of 2-4% is at the upper end of what I would expect from a downdraft unit and I could not confirm it from a source; the energy at stake is bounded in Energy Content of Wood Gas.
What Moves The Numbers
- Fuel moisture. Steam is a reactant in the reduction zone, so wetter fuel raises hydrogen and carbon dioxide and lowers carbon monoxide, but it also cools the bed, and a cooler bed converts less and leaves more tar. Every moisture figure here is on a wet basis: 20% wet basis is 25% on a dry basis, and the parent’s target of 10-15% wet basis is 11-17.6% dry basis. See Seasoning & Drying.
- Air rate. More air burns more of the fuel, which raises carbon dioxide and nitrogen and lowers everything that burns. Less air lowers the bed temperature and leaves tar.
- Bed temperature. Reduction to carbon monoxide needs the bed above roughly 800 °C (Pyrolysis, Reduction, & Oxidation Zones). A cold bed raises carbon dioxide and passes tar.
- Fuel type. Charcoal has already lost its volatiles, so its gas runs richer in carbon monoxide and poorer in hydrogen, methane and tar than wood gas (recalled, not in the carrier’s figures); this is why charcoal is the cleanest fuel and the most expensive in wood (the parent says making it loses 50-70% of the wood’s energy).
- Load. A gasifier run well below its design gas rate tends to run cold and tarry, and the composition drifts. A vehicle spends most of its time at part load, so this matters more than the table suggests.
Carbon Monoxide Is Roughly A Fifth To A Third
The carrier range for wood is 18-22%. Charcoal-fed gas can be richer, and the reader should plan for “a fifth to a third” in the air around any leak.
You Cannot Measure It By Eye Or Nose
Without an analyser the only checks are indirect: whether the gas burns steadily at a flare, the flame colour, the engine’s behaviour and the amount of tar collected. A steady pale blue to yellow flame suggests combustible gas in quantity; a smoky, sooty yellow flame suggests tar. None of this measures carbon monoxide, and nothing a person can sense in the air does.
Common Mistakes
- Quoting the table as a single analysis. The ranges are brackets and sum to more or less than 100%.
- Mixing wet and dry basis. The percentages are dry gas; the wet gas also carries steam.
- Judging toxicity by smell. The odour is tar and smoke; carbon monoxide has none.
Safety
A fifth to a third of the gas is carbon monoxide
Whatever the exact analysis, the gas is poison at any plausible composition. Carbon monoxide (28 g/mol, against about 29 for air) mixes evenly through a space rather than pooling, so there is no safe height to stand at. Never light, run, refuel or purge a gasifier indoors, in a shed or in an enclosed yard. Cold start-up gas is rich and almost all carbon monoxide: do not lean over it. See Gas Hazards (Methane, CO).
What’s Next
- Learn next: Energy Content of Wood Gas — how much a cubic metre of this gas is worth
- Combine with: Moisture Separation — the water the table leaves out