Sunday, November 30, 2008

Building a Woodgas Backpacking Stove

In previous posts I've mentioned that I generally use an alcohol stove or Esbit tablets to boil water for rehydrating meals while backpacking. However the concept of using a wood stove is beginning to catch on among backpackers. The reason is that the fuel is available in the forest so you don't have to carry any fuel. I've been experimenting with this for a few months and I now have a design I like and am confident enough with to use on backpacking trips. In this post I'll show you how to make it.

Now in general I am opposed to collecting firewood in the wilderness because dead wood plays an important part in the forest ecosystem and if every hiker made a fire then the forest would suffer. However I am going to show you how to build a stove that can cook a pretty big meal with literally a handful of twigs (what most people would use as kindling to start a regular fire) so the impact is very small.

There are two keys to this efficiency. First we will cook with flames rather than coals so we don't need to wait around until coals have formed. Second we will design the stove for more efficient combustion. In fact what we will burn is woodgas. Woodgas is the gas that is formed when wood is heated. It's an excellent fuel. In fact during WWII there were tractors with internal combustion engines that used woodgas for fuel. Our stove will be a downdraft gasifier, meaning that the wood will burn from the top down. The stove design I will be following is basically the Garlington woodgas stove with a few modifications in the windscreen and potstand.

To begin with take a normal soup can and cut the top and bottom off. It's nicest to do this with something like a Good Cook can opener which leaves no sharp edges.
Next decide which end of the can will be the bottom of the stove. Cut some tabs in the side of the can and bend them toward the middle. Now cut a circle of hardware cloth (or wire mesh) to fit inside the can. The tabs will hold this up.
What you are looking at in the picture above is how we get primary air to the stove. Air comes in the holes at the bottom that we cut the tabs out of and then flows up through the hardware cloth to the wood.

Now we need to make some holes for secondary air. This is the air that will mix with the woodgas to produce the flame we will cook on. Flip the can over and cut some slits in the side of the can near the top. Then insert a scredriver into each hole and bend the can so that the slits turn into vents leading from the outside of the can up and into the can.

At this point the stove itself is done. We fill the stove with twigs about the diameter of a pencil broken into pieces. I've found that it works best if I make the pieces about 3 inches long and put them in the stove the long way. The other approach is the make shorter pieces and just pour them into the stove. I find standing the twigs upright seems to help with airflow.

Now we can put a little kindling on top and start a fire. This is counter-intuitive because we are used to starting a fire the other way, with kindling on the bottom igniting wood above. But in this case it is very important that the fire burn from the top down. What happens is that the top layer quickly forms into a layer of charcoal. The wood below is heated and woodgas and air rise through this charcoal and a reaction called pyrolysis takes place. A main product of this reaction is hydrogen gas and that is what will burn when it meets the secondary air coming in from the holes at the top. In fact the way you know your stove is working correctly is that a few minutes after starting you will see that the only flames appear to be shooting into the stove from the secondary air holes. The flame will be quite steady and fairly blue in color (perfect combustion would yield a blue flame, yellow indicates some lack of efficiency).

Now in order to make this a good system for heating water I needed to rig some way to suspend a pot above the stove. I also needed some way to block the wind so it doesn't carry all the heat of the flame away before it can heat the pot. Actually a wind screen serves a secondary purpose in this case. The air around the stove inside the windscreen will become quite warm and so the secondary air entering the stove will be preheated, contributing to more efficient burning.

I built my windscreen/pot-stand out of a sheet of aluminum roof flashing. I drilled two holes and put wing nuts in them to hold the flashing in a cylindrical shape. Then I cut triangular holes in the bottom for air to enter. Last I drilled four holes about 1 1/4 inches from the top into which I can insert two tent stakes. The pot will sit on these stakes about 1 1/2 inches above the top of the stove. Using tent stakes is a perfect example of a dual use item. My total pack weight is reduced by carrying items I can use for more than one purpose. I always have to carry tent stakes for my tarp. So I might as well put them to good use.

The pot I use is an AntiGravityGear 3-cup aluminum pot. It's a great little pot. About the right size, very light, inexpensive, and the outside is already black so the black residue from the fire won't make it look bad. I sized the windscreen to allow a gap of less than an inch all the way around the pot for fumes to escape.

In the pot is 16 ounces of cold water (I kept it outside overnight and the temperature was in the high 30s). After a few minutes I have a nice rolling boil. That's not smoke in the picture, it's steam. A good woodgas stove produces almost no smoke. After a few minutes of boiling the flame begins to die down. Inside the stove is a pile of glowing charoal. After a few minutes this has been reduces to a fine ash, probably about 1 tablespoon full. So in this way also the impact on the forest is very small.

The downside of a downdraft woodgas stove is that all the fuel has to be in before you start the fire. You can't add wood later. Another way to build a backpacking wood stove would be to make a very similar system to the one I have built but also allow a space for wood to be fed in. You also get fairly efficient burning (better than a camp fire) because of the secondary air supply but it's not a true woodgas stove. However in some instances that might be better because it would allow you more control over how long a burn you need. You could keep such a fire going indefinitely if needed. However I like this system better. Mostly just because it's so cool!

10 comments:

Jacqueline said...

Wow. Your blog is a never-ending source of good information, clearly presented.

Grant said...

Dude, this is a really good idea. I have always thought campfires were the least efficient thing ever. How's the weight of this system? Obviously you don't have to bring fuel but it was pretty light if I recall, and it looks like the system is a bit heavier. At least bulkier. Actually, though, this has the advantage that you don't have to plan perfectly how long you are going to be out, how many meals you are going to make, etc. Pretty neat. When the wood burns out is it easy to refill and start up again or is it hard to fill in place (and hot)?

From the looks of it the windscreen you use doesn't compact down, right? So that's the size of the stove apparatus? I've got to say that soup cans are more aesthetically pleasing. I like this stove a lot, basically.

Using your titanium tent stakes to hold up a pot? Doooood. I guess they aren't tempered, then?

I really want to see one of these working. The fact that they have to be filled in advance is quite a downside, but I still like it. With a little practice you can know how much is necessary, just as you do with alcohol. Much better than the huge roaring campfire that scouts use (or used to, back when fire was a thing).

This is a super interesting post. Have you taken it on any hikes yet?

Grant said...

Another question: do you have to put this on a rock or something? It seems like hot ash would fall down on the ground and if things are like they were when you and I were up there, it seems like there's a potential to start a fire.

Heber Farnsworth said...

Ah, what is the weight? Good question. We'll turn you into an UL backpacker yet. It turns out that this system is quite lightweight. The stove weighs 1.4 oz, the windscreen weighs 2.2 oz, and the pot (with lid) weighs 4 oz.

Actually the windscreen does compact down. The wing-nuts that hold it in place can be loosened and you can wrap it into quite a small cylinder.

Don't know about whether the stakes are tempered. Not sure what difference that would make?

It might be good to operate on some non-flammable surface. Not so much because of the ash but because of the radiant heat from the bottom. Whatever is underneath will get scorched.

Anonymous said...

Rem: I copied the E-Mail into this comments.

Dear Ray, Richard, Heber!

Thank you for your Homepages based on The Garlington WoodGas Stove.
I have sucessfully built my own stove, based on your hints, and it worked from the beginning. I have made a webpage with the description and with links to your pages:

http://hannes-birnbacher.de/interessen/outdoor/woodgas-stove.html

It's in German, of course, but I am sure that you, as the original inventors, won't have any problems to understand it ;-).

What's new or different to your ideas?

1. I had a jumbo soup can of approx. 1.5 ltr as a pot stand and wind shelter. It's working fine.
2. I had no wire mesh, so I used the bottom of the inner can and made holes into it. Works very well.
3. For the first experiments, I used fine spruce twigs, with a lot of resin still in it, on top. I have the impression that the fire would have started well even if I did not use alcohole on it.
4. For starting the fire, not only alcohole (german "Spiritus") was usable, but simple salad oil.
5. I recommend to everybody an old pair of plyers like that which is on my homepage. It is great for handling the hot pots (e.g. for re-fill and heating water for dish cleaning) immediately, and for breaking twigs in one-inch-sizes.

Thanks again for your great web pages,

Sincerely yours,

Hannes Birnbacher, Germany

Anonymous said...

Update: I improved my construction very much. I made bigger the secondary air inlets and changed the Garlington Wood Stove into a real double-wall construction. Furthermore it is much easier to build than any real double-wall Wood Gas Stove I found, and it is easy to use. Look at http://hannes-birnbacher.de/interessen/outdoor/woodgas-stove.html.
It is not commercial, it is not for sale, and I made the website to honor the people who published the first instructions, like you, Heber, and Ray Garlington:-).

Regards, Hannes

Anonymous said...

BTW: The webpage is in german and english side-by-side :-).
Hannes

Heber said...

Hannes,

I've been looking at your site where you describe the building of the Mark II. Nice stove! I like the way you incorporated the pot stand into the design.

kimberly said...
This comment has been removed by a blog administrator.
Michael O'Hara said...

Nicely done and superbly descriptive. Although I just ordered the boilerworks stove I'm going to attempt to make one of these stoves and give it a run this weekend.

Again; excellent blog!