Biomass · Wood gasification
A gasifier turns wood into a gas an ordinary engine can burn. It worked at national scale during the Second World War, it works today, and the thing that makes the gas useful is the same thing that makes it lethal. Carbon monoxide is the fuel.
This page explains the system, what it demands, and when it makes sense. It does not publish plans or operating procedures, and the reason is set out below rather than assumed.
What it is
Give wood all the air it wants and it burns completely to carbon dioxide and water, releasing heat and nothing you can put in a fuel tank. Give it far less air than that and it cannot finish the job. What comes off instead is a mixture of gases that still hold most of the wood's chemical energy, and those gases will burn in an engine.
Inside the vessel the wood passes through four regimes as it descends. It dries. It breaks down thermally and gives off volatile compounds. Some of it burns, which supplies the heat driving everything else. And the hot gases pass across glowing char, where carbon dioxide and water are reduced back into carbon monoxide and hydrogen. That last stage is the one that makes fuel rather than exhaust.
The arrangement of those zones, and the path the gas takes through them, is the whole engineering problem. It determines whether what leaves the vessel can run an engine or will destroy one.
What comes out
Producer gas from air-blown biomass gasification runs broadly 15 to 22 percent carbon monoxide, with hydrogen in a similar band, a few percent methane, some carbon dioxide, and the balance nitrogen[1].
Close to half of it is nitrogen, which came in with the combustion air, burns in nothing and carries no energy. That dilution is the reason the gas has perhaps a tenth to a seventh of the energy per unit volume of natural gas, and it is the direct cause of the engine derating discussed further down. It is a property of using air rather than oxygen, not a fault in any particular machine.
Why the risk is structural
This is the arithmetic that decides how this subject is treated on this site, so it is worth setting out plainly rather than gesturing at.
Set that against every other fuel a household handles. Natural gas and propane are odorized precisely so that a leak announces itself. Gasoline announces itself. Wood smoke announces itself, stings, and drives people out of a room. Producer gas does none of this, and the concentration involved leaves very little margin between a small leak and a serious one.
There is a further turn worth understanding. Raw gas carries tars and particulates that do have a smell. The entire purpose of gas conditioning is to remove them before the gas reaches an engine. So the better the gas is cleaned, the closer it comes to the odorless description, while the carbon monoxide content is untouched by any of it. The cleanup that protects the engine removes what little warning the gas had.
Producer gas is also flammable across a wide range. NIOSH gives carbon monoxide a lower explosive limit of 12.5 percent and an upper limit of 74 percent, and its density is close enough to air that it neither reliably rises nor reliably settles. An accumulation does not conveniently gather anywhere you would think to check.
New World Survival covers wood gasification because leaving it out would misrepresent what property-scale generation involves, and because a reader who wants to know will find a worse source in under a minute.
What this page publishes is what the system is, what it genuinely demands, why the risk sits in the design rather than in carelessness, what it costs in money and attention, and when it makes sense at all. What it does not publish is dimensioned plans, fabrication steps, gas handling arrangements or operating procedures.
That is not squeamishness about a difficult subject. It is that a page cannot supervise a first light-off, cannot inspect a weld, and cannot be present when something does not smell like anything. The same standard applies to biogas digesters and small-scale steam.
The failure mode
When wood breaks down thermally it releases heavy organic compounds. If they leave the reactor intact and the gas subsequently cools inside an engine's intake, they condense into a sticky resin that coats passages and valves, causes valves to stick, and in time seizes the engine.
The important insight is where this gets solved. It is solved inside the reactor, by forcing the volatiles through a high temperature region and across a bed of glowing char, which breaks most of those heavy molecules into permanent gases that will not condense later. Reactor geometry does this work. Filtration afterwards is a secondary measure, and no amount of it rescues a design that lets tar through in the first place.
This is why gasifier architectures are not interchangeable. Arrangements that draw gas off before it crosses the hot char bed produce a gas that is fine for burning directly as heat and unsuitable for an engine without heavy cleanup. It also explains why a design that runs well at steady output can produce tar during starting, low-load running and shutdown, when the reactor is not hot enough to do its job.
Water in the fuel absorbs heat that the reactor needs to reach cracking temperature. Wood that heats a room acceptably at higher moisture is not adequate here, and drying is not a refinement.
The bed has to settle evenly and let gas through predictably. Mixed sizes bridge and channel, air finds a shortcut, temperatures fall in the zone that most needs to stay hot.
Cooling the gas drops out water and tar as a liquid that has to be collected and disposed of properly. It is not something to pour on the ground, and its handling belongs in the plan.
What it demands
Published derating for spark ignition engines on producer gas spans roughly 15 to 50 percent depending on compression ratio, the larger losses falling on the low compression ratios typical of an engine taken as it comes[3]. Size the generator against the derated figure, not the plate.
On the screening arithmetic used across this hub, a cord of air-dried hardwood yields somewhere near 800 to 1,000 kilowatt-hours of electricity through gasification and an engine. A managed acre gives about half a cord a year, and the house has first claim on it.
This is not an appliance. Fuel goes in on the machine's schedule, ash comes out on the machine's schedule, condensate accumulates, and the whole thing wants watching while it runs. Capital cost is the easy part of the decision.
Steady running is the well-behaved condition. Lighting off, running at low load and shutting down are when a reactor is coolest, when gas quality is worst, and when the gas has to go somewhere other than into an engine.
A great deal of heat, going spare
Cooling the gas is a required step, and the engine has a jacket and an exhaust as well. That is three separate heat streams, and on the electrical efficiencies involved they carry most of the energy in the wood.
Whether any of it lands somewhere useful decides whether the whole exercise beats simply burning the wood for heat, which is covered in the combined heat and power guide.
When it makes sense
Wood gasification earns its place where liquid fuel is genuinely unavailable rather than merely expensive, where large volumes of dry woody material already exist as a byproduct of something else, and where somebody on the property has the time and inclination to run a machine that needs running. Historically it appeared exactly when petroleum did not.
Against solar and batteries at present prices, on a property that has ordinary fuel access, it does not compete on cost or on effort. That is a plain assessment rather than a discouragement, and anybody proposing this ought to be able to say which of the conditions above applies to them.
If the conditions do apply and you intend to proceed, the competences involved are specific and worth naming, because they are what separates a working installation from a hazard.
Where a manufactured system exists and is supported, it resolves most of the above at once, and that is worth a great deal more than it appears to cost.
Common misconceptions
It proves it is possible under extreme necessity, in an era with a very different tolerance for risk, and with widespread mechanical skill and shared operating knowledge. It does not prove the machine is forgiving.
Tar is dealt with by cracking it inside the reactor. Filtration catches what the design already handled. A gasifier that makes tar will keep making it faster than anything downstream removes it.
Carbon monoxide is odorless. The smell in raw gas comes from what the cleanup exists to remove, so a well-conditioned gas is the most dangerous kind to be near and the least noticeable.
Carbon monoxide is close to the density of air, so it neither rises away nor pools somewhere predictable. Sheds, lean-tos, partial enclosures and still air on a cold night are not the open air people picture.
Residential alarms are designed around slow accumulation from a faulty appliance. They are not instruments for working near a gas that is a large multiple of the immediately dangerous concentration.
It has to be cut, split to a consistent size, dried properly, stored dry and carried to the machine on its schedule. That labour is the actual price, and it recurs every day the thing runs.
Next
Biogas digesters produce methane rather than carbon monoxide, and they store it in a sealed vessel by design. The hazard is different in character and comparable in seriousness, and the guide is written the same way this one is.
It is next in the sequence and is being written now.
Whether the woodlot yields enough to feed this at all, worked from acres to kilowatt-hours before any equipment question.
Read the guideThe eight guides in this hub, and the standard governing this page and the two that follow it.
Back to the hubSources
Composition and derating figures vary widely with feedstock, moisture, reactor design and operating condition, and the ranges above are for understanding the system rather than for designing one. This page is written under the standard described in its third section: it explains what the system is and what it demands, and it does not provide plans or operating procedures. Confirm fuel gas, electrical and structural requirements with your authority having jurisdiction, and your coverage position with your insurer, before any of this becomes a project.
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