Wood gas is a fuel gas made by heating wood in a deliberately limited supply of air, so that it burns to carbon monoxide and hydrogen instead of to carbon dioxide and water; the carrier gives about 2.5 cubic metres of gas per kilogram of dry wood. The gas can run an internal-combustion engine, and its main component, carbon monoxide, is odourless and poisonous. Never test it with a flame, and run every gasifier outdoors.
Part of Organic Chemistry
The reactions by which wood becomes a combustible gas, and the carbon-monoxide hazard that comes with it.
Four Zones, Four Reactions
In a downdraft gasifier fuel passes through four stages, as set out in Pyrolysis, Reduction and Oxidation Zones and Gasification Theory.
- Drying: water leaves the wood.
- Pyrolysis: heat breaks wood into charcoal, tar and gases without oxygen.
- Oxidation: a little air burns part of the carbon and gives the heat that drives the other zones: C + O2 → CO2.
- Reduction: the hot carbon strips oxygen from the CO2 and from steam, giving the fuel gases: C + CO2 → 2 CO and C + H2O → CO + H2. The second is the water-gas reaction, which the carrier also uses in its ammonia section.
The composition that results is on Syngas Composition, which says the figures move with moisture, air rate and fuel type. This article gives no composition table so as not to duplicate it.
What Matters For The Reaction
- Dry fuel. Water must be driven off before the gas zone; the cooler the reduction zone, the poorer the gas. Moisture limits are on Downdraft (Imbert) Gasifier.
- A hot throat to crack tar vapour.
- Cleaning and cooling before the engine (Gas Cleaning and Filtering).
Worked Example: Energy In A Kilogram Of Wood
- The carrier gives about 2.5 m³ of gas per kg of dry wood.
- Energy Content of Wood Gas gives 4.3 to 6.2 MJ per cubic metre of producer gas.
- Energy per kilogram of wood: 2.5 × 4.3 = 10.75 MJ to 2.5 × 6.2 = 15.5 MJ.
- Petrol holds about 32 MJ per litre (a recalled value), so 10.75 ÷ 32 = 0.34 L to 15.5 ÷ 32 = 0.48 L of petrol equivalent per kilogram of wood.
The carrier’s “about 0.4 litres of gasoline” per kilogram sits inside that range if read as the gas from one kilogram of wood; its wording is loose, and this working shows the reading that makes it consistent. These are energy equivalents before engine losses, and the gas yield falls with wet fuel.
Common Mistakes
- Sitting next to a running gasifier in a shed. Carbon monoxide builds up.
- Judging the gas by its smell. Carbon monoxide has none; the smell is tar and smoke.
- Using wet wood. The gas turns tarry and weak.
- Testing the gas with a lit match or candle.
Safety
Carbon monoxide kills silently; never use a flame as the detector
Wood gas is a flammable gas whose main hazard is carbon monoxide, which cannot be smelled and poisons without warning. Light a flare at a distance, before gas is expected, and let the flare, not your nose, tell you gas is present. Never lower a candle or flame into a vessel or pipe to test it. Run gasifiers outdoors, upwind, and shut down cold. See Gas Hazards and Emergency Procedures.
What’s Next
- Learn next: Gasifier Design — turning this chemistry into hardware
- Combine with: Charcoal Chemistry — the other side of the same pyrolysis