woodgas stove

Posted by: Heber

woodgas stove - 11/30/08 05:35 PM

I think I have mentioned before that I like to mess with woodgas stoves. I've been doing it for several months and finally settled on a design that is easy to make and reliably boils 2 cups of water (and keeps it boiling for a few minutes). I've posted the instructions here.

There is no real original work here on my part. I tried several designs, both my own and others, and I settled on a Garlington-type design. Mine weighs 1.4 oz. The potstand/windscreen resembles Jim Wood's fire bucket and weighs 2.2 oz. I designed mine so that I could disassemble and roll it up to save space. The windscreen is quite important in the design. Besides blocking the wind it also pre-warms the secondary air which I have found is important to optimal functioning.

If you've never made a woodgas stove they are really fun to use. It's super cool to watch it work and see the flames shoot in from the secondary air holes where the air mixes with the woodgas rising up from below.
Posted by: hikerduane

Re: woodgas stove - 11/30/08 06:01 PM

Years ago, "The Mother Earth News" rigged up a pickup with a stove in the back and ran it around. I'll have to study this more, trying to figure out how the tendency for heat and the smoke to rise are over come.
Posted by: thecook

Re: woodgas stove - 11/30/08 07:02 PM

Nice looking stove and very clear instructions. Think I'll have to try it out. I use a similar windscreen for my alcohol stove but instead of wing nuts, I permanently fixed the screews and cut slots in one side of the screen so that the circle can be narrowed. By planning the stove-windscreen-pot combination properly, I can get the wind screen to reduce diameter just enough to fit in the pot. Those small slts let me visually monitor the stove too. Don't know if this would work with a woodgas stove as the soup can is a lot taller than most alcohol stoves.
Posted by: JAK

Re: woodgas stove - 12/01/08 05:45 AM

I gotta say that looks really really good. I've been doing some tinkering, but mostly just thinkering, on a hobbo stove for awhile now, and yours looks way better than any ideas I've come up with so far. Very nice balance between the simplicity of a hobbo stove and some extra gasification action without overthinking or overdoing it and adding too much weight. I think your single wall burner / separate windscreen/potstand is definitely the way to go, and your dimensions and holes and gap between pot/screen all seem right on. I'm going to build some like this for sure, just play around with the dimensions some, just to know for sure. Maybe a bigger burner for winter. Easy change.
Posted by: rootball

Re: woodgas stove - 12/01/08 06:03 AM

I like the size. But how do you feed it with wood? I know that I have to almost constantly feed my little monster with twigs to keep it going long enough to do my cooking. I do pancakes in the morning (for two) and coffee and wash water -- and the little wood burner I made uses a lot of fuel.
Posted by: Heber

Re: woodgas stove - 12/01/08 07:16 AM

Yeah there is a subtle difference between a woodgas stove like mine and something like a bush buddy which you feed continuously. Both have primary and secondary air intakes. The difference is that a woodgas stove depends on having the charcoal on top of the unburned wood. When you feed wood in from the top you reverse this.

Nothing wrong with a stove that you can feed. In fact that is probably just the thing when you don't know how long you might need the stove to run for. My woodgas stove must be pre-filled with all the fuel it will use. It's a bit like an alcohol stove that way. Once it's running you can't really add any fuel. But since I only ever boil water it's not really an issue for me.
Posted by: finallyME

Re: woodgas stove - 12/01/08 07:32 AM

I like it. I think you could combine it with what Jim Woods has done with his windscreen and cat stove and maybe make a stove that is an alcohol, esbit, wood stove. Or, at least have the same windscreen for all three. Maybe use the wood stove can as a stand for the esbit and cat stove. <img src="/forums/images/graemlins/grin.gif" alt="" /> <img src="/forums/images/graemlins/grin.gif" alt="" /> <img src="/forums/images/graemlins/grin.gif" alt="" />
Posted by: JAK

Re: woodgas stove - 12/01/08 07:57 AM

I like the idea of a batch feed also. You get a sense for how much fuel you need. With the Kelly Kettle I can easily drop more sticks in the top but I try and get it right from the start. In cold wet weather I often get it wrong and have to start over.

I was thinking with your stove you could have a small round hole, maybe 1/2" diameter in the wind screen and in the stove, just above the grate, but it might not be easy to jam a stick in sideways with the other sticks vertical. Rocket stoves work that way. Maybe you would need 1" diameter and do 3 sticks at once and do it either continuous horizontal feed or batch vertical feed but not both. A metal tube for horizontal feed tube can help limit the primary air.

I think there is really a continuum between a conventional wood stove and a gasification wood stove. What varies is the amount of secondary air. You can sometimes minimizes primary air to get higher combustion temperatures, but you still need a little or it will stall. It is also possible to get high enough combustion temperatures without restricting primary air and using secondary air, but this can get hard with very small wood stoves, especially when the fuel is moist and you want to start heating up a cold pot as soon as possible. Success still depends largely on the fuel, and increasing the size of the combustion chamber for a little more volume to surface area is helpful for small stove.

I like your design because it allows you to keep the combustion burner small enough to fit in a bottle holder, perhaps nesting a water bottle into it. It's easier to push a water bottle back into the pocket if there is a can or mug there. Maybe mug on one side and burner on the other. I like the fact that the wind screen can be rolled out flat or around the sleeping pad or whatever, and does all the work of holding the pot up, but is far enough away from flames to be light aluminum. It might be prone to burning where the tent pegs go through. Titanium or aluminum pegs might act as less of a catalyst than the steel nail. You could also push the envelop on height maybe, for better draft. Maybe adjust your holes so it is always the same diameter on the bottom but becomes more conical if you want to narrow the top for a smaller diamether mug or canteen. Or maybe it could be rectangular when folded flat, and be shorter and wider for your pot, and taller and narrower for your mug or canteen.
Posted by: JAK

Re: woodgas stove - 12/01/08 08:04 AM

I would also like to see how small you could push this concept for just heating up a small mug at a time, like 500ml. I haven't had much success with very small hobbos in cold wet weather, but I think this could work and still be very compact. Also, once you've heated up your first mug of coffee, you could maybe drop in a taller pot of the same diameter, once you have coals, even if it sticks out the top of the windscreen some.
Posted by: Heber

Re: woodgas stove - 12/01/08 09:15 AM

Yes I've thought about this as well. Jim talks about the possibility in his Fire Bucket article. I think the way you could adapt my design would be make the stove simply a can without a bottom and with secondary air holes near the top. The screen bottom of Jim's Fire Bucket system would take the place of the hardware cloth in my stove. Height may be an issue of course since you want the pot to be at least an inch above the stove.
Posted by: Heber

Re: woodgas stove - 12/01/08 09:25 AM

Yeah I've seen the rocket stoves with the fuel feed tube. It's an interesting thought. I might have to try that at some point. I think that would be a feature that you might want in a larger stove than this one. Something you would use for group cooking or for heating a winter tent like Phat's.
Posted by: JAK

Re: woodgas stove - 12/01/08 09:27 AM

That's it. I gotta go burn something. Oops. I mean build something.
Posted by: leadfoot

Re: woodgas stove - 12/01/08 09:29 AM

Jim Falk's Bushwhacker stove is similar, except it has another can around the inner can. If you use the inner can and outer can together, you don't need to feed it with sticks. Just fill it up once and you're good. If you use just the outer can, then it's like the Bushbuddy and needs to be fed. I have this stove and it really is a lot of fun to use. Very little soot, too. There's a learning curve in getting it lit, because you need for it to burn from the top down to create the gases. But, I'm happy with it.
Posted by: JAK

Re: woodgas stove - 12/01/08 05:16 PM

I've bookmarked your site heber. Looking forward to your december blog. Your October and November might be hard acts to follow, so just keep having fun and I'll take what I can get.
Posted by: JAK

Re: woodgas stove - 12/01/08 06:15 PM

Regarding gasification and smoke and mirrors and all that...

Bark and sticks and stuff, the good stuff, is a combination of 3 things:
1. Moisture, which needs to be boiled off, which sucks energy and reduces temperatures.
2. Pitch and other hydrocarbons that burns as volatile gasses, and more moisture.
3. Cellulose that burns initially mostly as volatile gas, later mostly as charcoal with no moisture.

As I understand it you always will get a combustion cycle that begins as mostly #1, then a combination of #1 and #2, then a combination of #2 and #3, and then finally mostly #3, which is the hotest and cleanest but perhaps only 25% of the total heat released.

The gasification strategy is to reduce the primary air to below what is required for complete combustion, and then provide secondary air later, enough for complete combustion. This has at least three desirable effects that are relevant to small wood stoves.
1. Most of the combustion happens above the fuel when the secondary air is introduced, which increases the temperatures locally to complete combustion of the volatile gases released.
2. The fuel bundle burns more like a continuous feed, from the top down, so that the volatile gases and charcoal of each level of the bundle only has to deal mostly with boiling off its share of moisture, and not so much the moisture from the lower levels until it burns down to there.
3. Even though #1 and #2 are spread more evenly across the fuel cycle, you also get the benefit of more charcoal being formed and carbon monoxide gas being generated at lower levels with the primary air. So you get more even combustion, but also a hotter fire at the end, to help bring your water to boil, and perhaps clean some soot off your pot.
4. The lower level also acts to insulate the fire in the upper levels from the cold ground.

Still works better with good fuel though. Spruce sticks like we have here would work very well I think, and some birch bark as well to get things going. It takes some practice to figure our how much fuel you need, and how much tinder to mix in with the top layer, and lower levels, to get it burning and have it progress well, and breath right. Really dry fuel makes it very easy, and wet fuel and a cold pot on a cold day can make things really hard, especially if your in a hurry. With this type of stove I would simply use a bigger combustion can in winter, and always carry my fuel for the next fire in it.

I've never really thought out or gotten the secondary air thing working right.
This stove design has inspired me to do so. Great stove, and nice pictures also.

p.s. I finally ready the last paragraph of your description and I agree now that I think this stove would perform better in a more pure top down and batch feed mode rather than some sort of continous side feed mode. Just gotta match the size of the combustion can to the size of your pot, with some adjustment for the quality of fuel in you area, and climatic conditions.