Gasification turns solid biomass into a combustible gas by supplying too little air for the fuel to burn completely: heat drives the volatile parts out of the wood, and the hot charcoal bed then converts what is left into a gas of carbon monoxide, hydrogen and methane diluted with nitrogen. The product is a fuel because it is about 18-22% carbon monoxide, which also means it is odourless, colourless and lethal, so a gasifier is only ever run outdoors.
Part of Biomass Gasification
Gasification is the partial oxidation of solid fuel in restricted air to make producer gas, a low-energy fuel gas used in engines, burners and kilns.
A Gasifier Is A Fire That Is Deliberately Not Allowed To Finish
An ordinary fire has plenty of air, so the carbon in wood leaves as carbon dioxide and the hydrogen as water. Nothing is left that can burn. In a gasifier the air is rationed: only part of the fuel is burned, and the heat from that part does the work of converting the rest into gas that can still burn somewhere else, in an engine or a burner.
That is why the same chemistry that makes a good fuel makes a dangerous atmosphere. The combustible part of producer gas is mostly carbon monoxide and hydrogen, and the carbon monoxide is also the poison. Fuel value and lethality cannot be separated.
Four Things Happen To The Fuel In Order
In a downdraft gasifier, wood moving down the vessel meets four conditions, described in full in Pyrolysis, Reduction, & Oxidation Zones:
- Drying. Water leaves the wood as steam, at roughly 100-200 °C.
- Pyrolysis. At about 200-500 °C the wood breaks down, with no air needed, into charcoal, tar vapour and gases.
- Oxidation. Air enters at a narrow point and burns part of the charcoal and vapours at about 800-1,400 °C. This is the heat source.
- Reduction. The hot carbon dioxide and steam pass over glowing charcoal at about 600-1,000 °C and are converted to carbon monoxide and hydrogen, the gases that carry the fuel value.
The temperatures are the carrier article’s ranges. The point for a beginner is the order: the heat is made in the narrow burning zone, and the gas is made in the charcoal bed below it.
What Comes Out, And What Does Not Burn
The gas is not pure fuel. Typical ranges for a downdraft unit on dry wood are in Syngas Composition: roughly half is nitrogen from the air, which does nothing but dilute it, and about a tenth is carbon dioxide. That dilution is why producer gas holds only a fraction of the energy of natural gas, and why an engine on it makes less power than on petrol (see Power Output Derating).
What You Need To Make It Work
- Dry fuel. Wood at or below 20% moisture on a wet basis, which is 25% on a dry basis. Wet fuel makes tar and weak gas.
- Uniform pieces. Chunks of about 3-5 cm that let gas and fuel move through the bed.
- A design that forces the tar through the hot zone. That is the entire reason for the downdraft layout in Gasifier Types.
- Gas cleaning. Raw gas carries tar, dust and water that wreck an engine; see Gas Cleaning & Filtering.
Common Mistakes
- Treating it as a stove. A stove tolerates a leak and an upset; a gasifier is a closed vessel of combustible gas with a fire in it.
- Judging the gas by its smell. The smell is tar and smoke, which carry no information about carbon monoxide. The carbon monoxide itself has no smell.
- Ignoring fuel moisture. It is the largest single variable in whether the machine works.
Safety
Producer gas is about a fifth carbon monoxide and gives no warning
The carbon monoxide in producer gas is 18-22% by volume in a well-run downdraft unit. One cubic metre escaping into a closed 30 m³ garage puts 0.2 m³ of carbon monoxide into it, which is about 6,700 ppm, over five times the 1,200 ppm level that is immediately dangerous to life. The early signs are headache, nausea, clumsiness and confusion, and they remove the judgement needed to leave. Never run, light, refuel, purge or vent a gasifier indoors, in a shed or lean-to, or in an enclosed yard. See Carbon Monoxide Risks for exposure limits. A gasifier is also a vessel holding combustible gas and a fire, so a flashback or explosion on restart is a real failure mode.
What’s Next
- Learn next: Gasifier Types — updraft, downdraft, crossdraft and top-lit designs and what each does to tar
- Combine with: Seasoning & Drying — getting fuel to 20% wet basis or below