Every now and then in histories of the 20th’s century’s earlier years, you will see pictures of cars and commercial vehicles equipped with bulky drums, contraptions to make their fuel from waste wood. These are portable gas generators known as gasifiers, and to show how they work there’s [Greenhill Forge] with a build video.
When you burn a piece of wood, you expect to see flame. But what you are looking at in that flame are the gaseous products of the wood breaking down under the heat of combustion. The gasifier carefully regulates a burn to avoid that final flame, with the flammable gasses instead being drawn off for use as fuel.
The chemistry is straightforward enough, with exothermic combustion producing heat, water vapour, and carbon dioxide, before a further endothermic reduction stage produces carbon monoxide and hydrogen. He’s running his system from charcoal which is close to pure carbon presumably to avoid dealing with tar, and at this stage he’s not adding any steam, so we’re a little mystified as to where the hydrogen comes from unless there is enough water vapour in the air.
His retort is fabricated from sheets steel, and is followed by a cyclone and a filter drum to remove particulates from the gas. It relies on a forced air draft from a fan or a small internal combustion engine, and we’re surprised both how quickly it ignites and how relatively low a temperature the output gas settles at. The engine runs with a surprisingly simple gas mixer in place of a carburetor, and seems to be quite smooth in operation.
This is one of those devices that has fascinated us for a long time, and we’re grateful for the chance to see it up close. The video is below the break, and we’re promised a series of follow-ups as the design is refined.
I’ve gone down the gassifier rabbit hole a few times over the years, though I have yet to build one myself.
They were apparently fairly common in England during WWII, especially for things like tractors, as gasoline was obviously highly rationed for war use.
The biggest downside from what I see is you either need to run the gassifier and use the fuel it produces immediately (I have seen one example from the WWII era of a car pulling a trailer with a gassifier running on it), or store the results in low pressure gaseous form, which is obviously extremely space inefficient. Because of the wide variety of molecules present in the gas, liquification is not really achievable, and even pressurization has major challenges.
Tar buildup is another major issue, as mentioned. But still, very cool tech. If I had more time and funds to mess around with this stuff I’d love to get a setup going someday.
The Germans tried it with tanks at one point towards the end…
How many trees per minute?
If I remember correctly, carbon monoxide starts to polymerize under pressure in the presence of iron.
This has lead to some accidents when people have tried to store syngas in steel containers under pressure, because the result will be fine carbon dust and oxygen gas, and hydrogen and the other stuff. It can go boom.
There’s a quite funny Swedish facebook group with some enthusiasts building an ecosystem around this. Incidentally, Sweden has a lot of wood, not to mention growing petrol and electricity prices.
First, they make their own coal, including some very large “cookers” or whatever they call it to make a lot of it, optimizing the burn. The coal apparently makes less of a messy/cloggy experience in the engine than pure wood. Then, they adjust cars to run on said coal, trying to optimize the design. Last time I checked they were making electricity from a generator powered by coal, trying to optimize that process . Haven’t checked in for a while, maybe they’re onto the next thing, whatever that would be.
Yeah, running gasifiers on charcoal is underrated; saves a lot of complexity because you don’t have to deal with water vapor or tar. You lose a significant chunk of the energy in the wood, but it’s a much cleaner process overall.
On my To Do: Build a search light with syngas, an ancient style blowtorch and limelight.
If I wanted to routinely remove the heads of my engine to have it media blasted I’d have bought a direct injected BMW.
It would be rare to have completely dry ambient air and completely dry fuel once it’s been sitting around for a while. As long as there is moisture there will be some hydrogen. You might also just sprinkle some water on top to get moisture in the fuel.
There’s also some tar left in the charcoal which would also contain some water. The tar is converted to gases in the combustion zone, then break down in the combustion and reduction zones and also produce other stuff like methane, but then the gases pass through a cooler area and the charcoal there will off-gas tar and water vapor, and some air will be bypassing the combustion zone through the upper section of the box. A better burner design would limit this. In more advanced designs there’s also a shaker that keeps the charcoal bed and the ashes moving, so you don’t form channels or voids and bridges in the burning fuel.
Tar is a problem in the entire fuel system, not just the cylinders.
It will condense inside the fuel lines when the gas cools down as it heads toward the cylinder.
It clogs everything.
The fuel lines.
The gas mixer.
The valves.
Burning straight wood is a valuable option if you get into a “we literally cannot get a better fuel and we need to run this NOW” situation, like running a tractor during wartime.
But it’s almost always a bad idea otherwise because you are introducing an order of magnitude more maintenance.
If you need to rebuild your engine once a week to stop starvation it is a great option.
But otherwise? Now way. Better to spend that effort to make and store cleaner fuel.
Charcoal in this case.
A gasifier tar trap is a component designed to remove tar, a sticky, high-molecular-weight hydrocarbon contaminant, from the syngas (producer gas) produced during biomass gasification. These traps use methods like mechanical filtering, wet scrubbing, char bed adsorption, or catalytic processes to prevent tar from damaging downstream equipment or contaminating the syngas
Charcoal is not a single, pure chemical compound. It’s a porous, carbon-rich material left over from heating organic matter without oxygen. Its exact composition, including the precise amounts of hydrogen, oxygen, and other impurities, varies depending on the source material and the charring process.
C₇H₄O is sometimes used as an empirical or approximate formula for charcoal in specific contexts, such as for gunpowder, because it accounts for the small amounts of hydrogen and oxygen that are present as impurities. The true composition varies depending on the source material and how it was charred
That sounds like an interesting way to capture excess CO2.
The actual answer is that pressurized CO in steel tanks carburizes the steel, making it brittle, and corrodes the tank from the inside by rusting, then the pressure ruptures the tank. Apparently no free oxygen is produced since it would be consumed by the iron.
The comments go jumping all over the place again…
The actual effect is that CO makes steel brittle and causes it to rust, which ruptures the tank from the pressure.
I always see this used with vehicles, but have thought a much better use would be with an emergency generator. Any disaster big enough to require its use would leave plenty of wood lying around.