Tone, I have followed (a little) these measurements, they are tests that the testing institute did in the 60-70s in Sweden, if I were to follow these I would have under 50mm restriction.
The length of the nozzle is probably quite similar to mp.
I’m afraid of getting more tar in if the restriction is bigger, there will be less mixture on the gas.
One thing I thought about, we talk a lot about how the nozzles should be, slanted upwards or straight etc.
What does the airflow actually look like, when we pull air through, isn’t it different than when we push air through?
K = constant , gasoline 0.0024 and diesel gas 0.0025 for idle.
K= constant, petrol 0.0094 and diesel gas 0.0082 full throttle.
N = engine speed rpm.
V = engine volume in liters.
In the drawing, if the yellow zone outside super hot lobe center area, is at 1200 plus °f or 648.9 plus °C, you will not have any tar getting pass the restriction opening at that temperatures. This drawing is the simple straight wall inside of the WK Firetube. With all the mathematics the restriction opening size and height adjustments of the opening is for tuning for the engine size that is going to be used. To large you can make tar, to small the flow of charcoal will back up and ash up and a voiled will happen right below the restriction opening at that point. Losing any charcoal reserve above the grate.
The charcoal that’s just above the nozzles on down is white/orange a very hot lobe of hell where if any brands of wood will quickly pyrolysis and tar will changing to Carbon Monoxide, Methane, with H20 vapor changing to Hydrogen gases.
If the charcoal slows down and plugs up with ashes the gas production slows down. plugging up will cause a cool down like at idling periods. This is where tar can get by on any gasifier. Keep it hot and flowing making charcoal and passing ashes on through the system pass the grate. Slipping charcoal pass the grate is good. It can be reused back into the hopper after the smaller charcoal is removed with the ashes.
Bridging in the hopper is bad. But right below the restriction opening is not good ether.
According to my books, the highest heat is in restriction, well it should be because the highest speed of the gas is there, unless the reduction has taken the heat down.
The temperature is highest where oxidation (burning) takes place, this theoretically means where oxygen is a combustible substance and the temperature is high enough, here the charcoal is glowing white. In our case, this happens between the nozzles and the flow of gases pushes this hot zone down towards the restriction opening. But if we imagine a case where the gasifier works with a small load, only a little oxygen arrives, which is consumed near the nozzles, and cold tar gases descend against the wall towards the restriction opening,… the gas will be very strong but will contain tar. When we compare this to WK with short nozzles, we can imagine that the cold pyrolysis gas mixes with air at the wall, which can prevent oxidation near the nozzles, but the oxidation happens lower in the hot pipe, … looks the same, but a big difference.
Ok, then I probably didn’t understand this correctly, but what happens if I shorten the nozzle, won’t there be a hole in the middle that becomes cold then that the tar can drain into?
Hmm, I think that in the middle of the area there is always a great heat of hot gases that force upwards, with a small load oxygen does not penetrate here, only the effect of hot oxidation surrounds the middle, which overheats the material in the middle, and buoyancy drives the hot gases upwards… you can imagine the circulation of gases, condensation of water vapor, cooling of the tar gases along the wall and going down, so they can escape the reduction by passing the long nozzles and reach the restriction opening, if there is not enough hot activated carbon in this area, they have a free path to the engine.
Jan, I think the long nozzles will have teething issues until a hardened layer of ash forms behind them like with Joni’s gasifier. If you don’t want to cut the nozzles you could do as Steve suggested with mineral or kaowool either hardened with water glass, or skinned with a layer of stainless steel to prevent the wool from falling apart when using the poker. The wool would effectively do the same as shortening the nozzles by not allowing tar gasses to creep between.
The wool would give an aided benefit of holding in heat where it is most desired, and let it reflect upwards to drive more moisture out of the fuel.
Or you could follow Tone’s suggestion which I think would be the least expensive fix.
Are you sure that the highest temperature in an imbert like gasifier is highest at the nozzles?
I checked in the library here, and it says that the smaler the throttle is, the higher the temperature, up to a certain limit.
It is usually the throttling that causes damage in my unit, the nozzles usually manage.
Here is a thought, I build a fire in my fire pit or barrel. There are holes in the bottom for drafting air upwards along the side walls. I do not have to worry if the center of the wood will burn or Charcoal brands will not be completely converted to Charcoal. It just takes heat and time. This is at atmospheric pressures not in a vacuum which is completely a different environment.
In the firetube with nozzles with limited air supply being introduced and traveling downwards increasing in vacuum pressures as it passing through the high heat of the charcoal that is much more hotter and intense than a open fire at atmospheric pressures.
Think about it raw wood into the gasifier hopper and in 30 to 40 minutes it is now charcoal. It takes a longer time to do this in a open fire pit at atmospheric pressures with the same amount of wood. And it does not have air restriction on top.
Where are the highest temperatures? Where coal oxidation takes place. It is very good to observe this in the blacksmith’s hearth. If we transfer this knowledge to the gasifier, we can conclude that your above statements are true, a small inflow of air causes oxidation near the nozzle, but if the amount of air is greater, it penetrates further between the glowing coals up to the limit, if there is a void below the limit, you can it oxidizes coal into CO2 at a very high temperature, which melts metals, the oxidation can be so pronounced that the coal is consumed in front of the restriction opening and a void is created in this area as well. Imbert had an air intake in the narrowest part, and the area below expanded, so the coal from the upper part could fill the lower part without interruption, where there was a large reserve of it for conversion into combustible gas.
Jan, I’m not urging you to modify the gasifier, I’m just thinking out loud and giving my opinion, I want this system to work well and for a long time without any inconvenience with the engine.
Jan, you flush out a lot of wood. Are you also the one chopping all this wood? I also harvest and flush out wood from our forest at this time of year. But I’m jealous of your equipment I only have a trailer and a wheeled tractor
Yes, I chop the wood, but this is not much, maybe 1/2 of what I burn in my cabin next winter.
I bought the machine for about 2100 dollars, but it has been rebuilt quite a lot, I made the crane and carriage myself, but I’m quite bad at welding, so it usually falls off a bit from time to time.
Yes, as you can see in the table, the nozzle usually sits 100-170mm above the restriction on the imbert.
The expansion, is it to slow down the gas so that the conversion have longer time?
One more question, I saw like ash in the restriction when I cleaned it last, saw something similar last year, why does it happen, haven’t seen this on the s10, do I have too big nozzles?
In my opinion, in this case there is a lot of dead space between the nozzles, especially if I shortened them, so it would be desirable for the air to be blown into the layers, thus creating a continuous glowing layer of charcoal in the shape of a saucer, which would prevent an unobstructed passage tar gas.
Jan, it’s true, the air intake in the Imbert gasifier is made approx. 100 mm before the narrowing, if you look at the posts of our friend Joni, he also uses a similar distance (120 mm) from the restriction opening, in his case the long nozzles are surrounded by ash, which it protects against overheating and fills the dead space between them, and by extracting water vapor and part of the tar gases, it achieves that the pyrolysis spreads upwards and thus obtains a large supply of charcoal and relatively dry tar gases,…
Ash is produced when wood burns or completely gasifies.
Jan, I think you and I have to accept the fact that our 12-13" firetubes are too wide for our small 4" restrictions. In the Volvo gasifier I’ve also seen ash accumulate in the restriction area. Our gasifiers don’t breathe well enough and pull air deep enough, most of the time, to properly vent the ash. Soulutions - narrow down our firetubes or introduce some oxygen further down.
Also, I believe in order to get the desired increase in temp when passing the restriction, like your books are showing, there has to be trace amounts of oxygen still present. With the residence time our wide oxidation zones provide at low demand, I would guess we’re close to zero and our restrictions will act mostly tight filters.
I know @KristijanL talked about that he had also seen ash, but in a coal aggregate.
I don’t think he gave any explanation though.
Do you think it really matters that the fire tube is wide, if the measurements are correct on the choke and the nozzles, then a tube is formed in the coal that is the right size?
I have too big a distance between the nozzles and too big a distance between the nozzles and the throttle, that could be why it’s causing problems.
Why does the ash collect at the throttling, it should be during the throttling where the gas loses speed that it would collect, unless there is too much oxygen left so it burns down at the throttling?
