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!