Post by pigbuttons on Jan 23, 2021 16:13:27 GMT -8
The cage will certainly prevent a catastrophic feed tube fire where the stove runs backwards. You still may have some issues because the temperature of the stove is preheating the wood just above the combustion zone through radiant heat. It is probably less of a problem in a cold environment like the pit yours sits down in and the primary heat generated by the stove is being channeled away and the brick wall between the feed and exhaust tubes.
The scale of this build is amazing. Thanks for keeping us informed.
Hmm, I've been thinking about this, here's what I've come up with.
You've solved the air problem by capping the feed. The problem is, wood can provide some of its own air when burning. You're producing woodgas by pyrolyzing (may not be the right word) the wood in the feed, just by heating it.
This is one of the less talked about things about the J tube design. The intake air cools the wood and the feed, keeping it from igniting and burning creating woodgas (a mixture of CO and H2) which is what happens if you cap the wood feed.
This is why pellet stoves feed with an auger and why gravity feed pellet stoves do not (that I've seen) use a vertical feed hopper. They feed on an inclined chute which keeps the fuel away from the heat for as long as possible. They mix the air with the fuel and heat (the burn triangle) as late as possible.
Another idea I've just had, a constriction below the wood feed, between the air holes and the combustion chamber on your diagram. An inverted cone that creates an air space between the wood and the combustion chamber that is cooled by the intake air.
If the wood starts to gasify and vent carbon monoxide into the room, that would be a problem.
But would temperatures high enough to create woodgas be present at the bottom of a closed-end feed tube? If there was insulation between the metal tube and the primary burn chamber (not the riser), could the tube and the wood reach the temperature required for gasification?
The cage will certainly prevent a catastrophic feed tube fire where the stove runs backwards.
That's why it's capped. If I leave it uncapped it burns everything in a spectacular and very quick way, which I might actually do on purpose occasionally to burn off tar that seems to slowly build up on the inner walls of feed tube
Solomon There's certainly some wood gasification going on, but it seems to all get burned up as the furnace draws a lot of air. I've yet to fill the feed tube all the way to the top, and I think that once that's done the wood itself will serve to isolate wood above it from radiant heat and prevent excess gaisification. The main idea behind the cylindrical feed tube is that it's the only shape that has the least chance of wood getting stuck and stop falling down to feed the fire. Any kind of cones and irregularities would increase the chances of wood getting stuck.
joseph All exhaust gasses will be led thru piping outside.
My main goals with this project was a furnace that fulfills these conditions:
-can burn all night without the need to add wood manually (seriously, who wants to get up in the middle of the night!) -burns efficiently (complete combustion + burns only a small portion of total fuel at any time) -can work with variety of different wood shapes and sizes
Now that last one is a big one. It would be very easy for me to make a stove that uses wood pellets as there are plenty of designs online to look at and self-feeding is a non-issue as pellets easily drop through gravity assisted shapes because they're small. Wood pellets where I live, cost more per unit of weight than coal, and I can always easily modify mine to work on pellets, and I'm thinking in the future, even coal.
"All exhaust gasses will be led thru piping outside."
Of course the exhaust will be directed outside.
I am addressing the possibility of woodgas produced in the feed tube as mentioned by Solomon. Since CO is lighter than air and cannot be seen or smelled it may be difficult to contain.
Post by roaringembers on Jan 27, 2021 10:53:01 GMT -8
Okay so today I added a barrel and a long tube made from thin aluminium sheet metal above exhaust pipe, it goes through the whole length of the GH, exchanging heat with the air. When exhaust gasses reach outside, they're only slightly warm, meaning almost all heat has been transferred to GH.
I was almost about to give up on the whole thing when I saw a bunch of smoke coming out the back, I thought this was an efficient furnace and that there would be no excess smoke...
Than I noticed the "smoke" kinda disappears into thin air once it expands a bit. When I put my hand through it, it would get wet, this was water vapor. I didn't even know that wood decomposes into so much water, I thought it was mainly CO2, but turns out a large portion of decomposition gasses is H2O. Since the gasses get cooled down so much before exiting GH, water would condense into visible "vapor" (which is not really vapor, it's really tiny water droplets)
Post by pigbuttons on Jan 27, 2021 15:13:16 GMT -8
So glad to hear it's working out. That is a good sized GH you have there. How long did you run the stove? Were you able to get any idea of the change of temps in the GH over outside temps?
The whole water thing is quite amazing even with "dry" wood. It can be a problem if it collects somewhere in the path so be careful, It'll steal heat and also can breed bacteria and molds.
So glad to hear it's working out. That is a good sized GH you have there. How long did you run the stove? Were you able to get any idea of the change of temps in the GH over outside temps?
The whole water thing is quite amazing even with "dry" wood. It can be a problem if it collects somewhere in the path so be careful, It'll steal heat and also can breed bacteria and molds.
Over half of the weight of dry wood gets converted to water once it's burned up, how amazing is that? Total weight of CO2 ends up being almost twice the weight of wood but that's because of all the oxygen that it takes up from the air. Now I also realize why you can't just burn smoke from the exhaust, it's supposed to be made from flammable gasses (CO and H2), all that water needs to be condensed and taken out first.
When it's burning, the inside is a couple of degrees (C) warmer than outside, but I haven't measured exact numbers. My GH doesn't have any isolation, only 1 layer of foil, but considering it gets warm inside even with just that, I imagine it will get quite toasty once I put another layer and seal everything up.
I'm not concerned about water, even if it builds up the tube has a downward position and goes under the GH door and than out, so any liquid water would just flow down on the ground.
The plan for the future is to have 3-4 pipes of a lesser diameter instead of this large one, my budget got tight so this big one will have to do for now.
Last Edit: Jan 28, 2021 8:11:23 GMT -8 by roaringembers
That's not precisely accurate. CO and H2 are products of gasification. They are not visible, and therefore I'd say not smoke, though they are components of smoke. In a wood gasifier, they can reach nearly 30% together, which can burn in free air. What happens is when temperatures are high enough and with low oxygen content, carbon atoms steal oxygen atoms from water molecules resulting in the equation H2O + C = CO + H2. This costs heat to do and in the presence of the right amount of oxygen and heat, oxygen will react with carbon producing CO2 preferentially, releasing heat.
But in a free oxygen burn we try to get the temperature high enough in the combustion chamber to spontaneously burn the CO and H2 (as well as the smoke) releasing more heat.
That's the reburn that happens in the hottest section of the combustion chamber or the reburn flames in an efficient stove.
We want to burn everything hot enough and with enough oxygen so the maximum amount of unburned smoke as well as CO and H2 are burned spontaneously and keep overall temperatures high enough so that water does not condense in the system. If we have condensing water, then we need stainless steel components, which most of us can't afford.
Smoke is wasted fuel. And even in clear exhaust there can be quite a bit of CO and a lesser amount of H2, but not nearly high enough concentration to burn in free air.
I made a video with my gasifier and explain some of how the chemistry works:
Cut a hole the size of your feed tube magazine. Dig a bit the earth on the side of it, so you can lower the top edge of the tray at ground level. Put some aluminium tape between your feed tube and the tray. Fill the tray up to ground level, with sand or earth. This will act as a dampening mass. For the moment, your free standing tube acts as a tuning fork! Amplifying the vibrations produced by the fire.
Cut a hole the size of your feed tube magazine. Dig a bit the earth on the side of it, so you can lower the top edge of the tray at ground level. Put some aluminium tape between your feed tube and the tray. Fill the tray up to ground level, with sand or earth. This will act as a dampening mass. For the moment, your free standing tube acts as a tuning fork! Amplifying the vibrations produced by the fire.
Damn don't tell me metal i doomed now lol! What materials would one use to build something like this to have any hope of it being long-lasting then!? I hope it lasts at least one winter...
I've increased the size of air perforations and the sound is gone. Air going in through small holes tends to increase in speed so I guess that was the problem.
I tried it today with 1/4 of feed tube full and it burned like hell, GH got pretty warm. One annoying problem I still have is starting it. Until it heats up, it smokes a bunch and takes quite a bit of time to start burning properly. I am going to build a blower that forces air through it which I'm going to use when starting it.
Damn don't tell me metal i doomed now lol! What materials would one use to build something like this to have any hope of it being long-lasting then!? I hope it lasts at least one winter...
I've increased the size of air perforations and the sound is gone. Air going in through small holes tends to increase in speed so I guess that was the problem.
I tried it today with 1/4 of feed tube full and it burned like hell, GH got pretty warm. One annoying problem I still have is starting it. Until it heats up, it smokes a bunch and takes quite a bit of time to start burning properly. I am going to build a blower that forces air through it which I'm going to use when starting it.
Glad to hear that it works at warming up the GH. Always nice to see a successful project get into the useful phase.
We tried to warn you early on that metal won't last. LOL If you want something that will last you only have three choices for the feed / firebox area: 1) fire brick, you can cut it with a diamond cutoff wheel with or without water. With water is preferred because the blade lasts longer and there is little dust but a lot of slurry. 2) refractory cement, it can be found online as either a powdered product much like Portland Cement, or if you have any large industry nearby there will be a steam boiler repair business close by. I get mine from such a company. There are also premixed products that come in a bucket and you just scoop it out and trowel it into the mold. 3) geopolymers, but to date I don't know of anyone that can make them in a repeatable fashion, so my advice is use 1) or 2) above.
For the burn tunnel and riser, easiest and cheapest is ceramic fiber mat or ceramic fiber board. Look up "5 minute riser". You might be able to retro fit it to your current build if you can give up some CSA to improve efficiency.
As far as starting goes, I use charcoal that is saturated in charcoal starting fluid. As big as your stove is it may take twenty briquets or more, but it is quick and easy to get up to speed. Just make sure to put the briquets at the opening of the burn tunnel or even in the burn tunnel, and not at the feed end of the firebox.
You were told twice last week. You said, and I quote "Exhaust is made from 4mm thick metal pipe, so should last imo."
If you're not going to pay attention to advice, you're going to go through a lot of trial and error before getting to something useful. No judgment, but it really is up to you to do the research.
I see this on YouTube, people build over and over again when they could have just followed instructions and had a workable model. One guy went through like four heat risers until he finally built one out of fire brick, which was still unnecessarily complex and still contained metal supports.
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Donkey: rockinon, place them on some other web hosting service and link them here.
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atrii: How can I see these photos Donkey?
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Donkey: atrii When the images are properly linked, they will be visible.
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dvawolk: For images i use "Greenshot" app - i can print screen part or whole of my screen and upload them directly to imgur throught the context menu. Works very well and fast for me...
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martinm: Hi there , looking for info on hot water heat exchanger for integrating in the bell of masonry rocket stove.
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lightworker: Hi beppe:
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