After about 2 months building & testing small woodstoves of various configurations, I stumbled on an easy to build, lightweight stove (3.5 - 4 oz) that will boil 1 quart of water and hold the boil for about 10 minutes, using only about 2 oz of wood as fuel. The stove is batch loaded, fun to use, and nearly smokeless when properly fired.
On the negative side, this stove requires a starter fluid and will blacken your pots. Also, the current version gets hot enough at the end of the burn to ignite newspaper under the burner, so the stove should not be used on flammable surfaces.
The design attempts to exploit the "batch-loaded, inverted down-draft gassifier" wood-burning technique and manages about 1/3 - 1/2 "blue flame" at peak output. As the gas-burning stage winds down and while the stove is still quite hot, the flame is mostly blue. Typically, the stove is burning wood gas shortly after ignition and has a stable yellow/blue flame within about 1 minute. After about 10 minutes the wood gas is depleted and the stove transitions to charcoal burning. Charcoal burning continues for about 20 minutes after this transition.
The stove comprises a steel can, fire grate, stove windscreen, pot stand and pot windscreen. The steel can is the combustion chamber and is 3" in diameter and 4 1/2" tall. Primary air holes are punched at the bottom edge of the can, and secondary air slits are cut about 3" up from the bottom. A fire grate (wire screen) is fitted to the bottom of the can to allow even distribution of the primary air to the bottom of the fuel supply. The stove windscreen is made of light aluminum (disposable baking pan) about 3 1/2" in diameter and 4 1/2" tall. Slots are cut in the bottom of the stove windscreen to allow primary/secondary air to enter. The pot stand is fashioned by bending a coathanger into a clip that slips onto the rim of the steel can and holds the pot about 1 1/4" above the rim. The pot windscreen is a piece of doubled aluminum foil that goes from the ground to at least 1/2 way up the pot.
As of the fall of 2003, I had been using an alcohol-burning, soda-can stove for several years. This stove had met my rather limited requirements for cooking on my annual AT section hikes; however, after reading about Rick's (aka geoflyfisher's) successful work building a simple, light, forced-draft stove, I inexplicably became inspired to try building a wood stove that would not require a battery and fan. I decided, more or less arbitrarily, that the stove would have to be very light (5 oz or less), would have to boil 1 quart of water, hold a simmer for 10 minutes, and be fun to use.
In researching what had already been done in this area, I was very suprised at the amount of information about wood burning stoves available on the internet. Soon, I had many weeks worth of reading to do. It turns out that some pivotal work was done on this as late as the mid 1990s. In May, 1996 T.B. Reed and Ronal Larson of the Biomass Energy Foundation in Golden, Colorado presented a paper titled "A WOOD-GAS STOVE FOR DEVELOPING COUNTRIES" at the “Developments in Thermochemical Biomass Conversion” Conference, in Banff, Canada. In this paper they describe a wood stove which uses a component they describe as the “inverted downdraft gasifier, which operated using only natural convection. I read this paper early on in my development process, but put it aside because it was "too complicated". Instead, I began countless iterations which employed the more intuitive Winarski Rocket style burner. Eventually, I became frustrated by the typical smokey & slow starting of the Rocket. (Note: for those 'Rocket' fans, my problems were mostly due to the very small scale of the stoves I was working on. These little stoves have almost no draft. Also note, the rocket burner worked well if the wood was very dry and the initial wood charge was small; however, this required too much baby-sitting and failed my "fun to use" design parameter.) I was about to call it quits, when I decided to just throw together a pure "inverted downdraft gasifier". I quickly fabricated a much simplified and smaller version of the Reed & Larson stove. The first lighting of this little stove was smoke free and delightfully easy. The metaphorical clouds dispersed, and I spent several days refining this design.
The 'inverted downdraft gasifier' is NOT intuitive and goes against the way most of us learned to build a fire. It works because the fire is started ON TOP of the wood supply. The flames consume the top layer of fuel and drive gas from the wood. As sufficient heat is generated, the layer below the current layer is consumed in a similar manner. This flaming region is called the flaming pyrolysis zone, and proceeds downward through the stack of fuel at a speed controlled roughly by the amount of air made available to it. The gas that is driven off from the flaming pyrolysis zone flows up through the charcoal stack where additional air is added resulting in nearly complete combustion and no smoke generation. Well, OK, it is more complicated than that, but you get the idea. It works.
Find a supply of dry sticks about the diameter of a #2 pencil and smaller. The amount of fuel required is approximately 2 oz by weight. Dump out the ash from the previous firing, and position the stove windscreen and pot stand on the stove body. Break up the sticks into about 1" lengths and throw them into the burner. As the stove fills, periodically shake/tap the stove to settle the fuel. When you get close to the secondary air slits, use only the smallest of the sticks you have collected. Shake/tap down the stove once again.
Spray a small amount of starter fluid (charcoal starter fluid, kerosene, alcohol, etc.) on the top surface of the wood supply. Do not use too much, because you only want to ignite the top layer of wood. (Igniting the lower layers will result in a smokey mess.) Light the starter fluid. After about 1 minute you should have a good flame going. Postion the pot on the stand and place the pot windscreen around the pot.
Caution: If the stove fails to ignite, do not spray additional starter fluid. There will be hot embers present from your previous attempt which are capable of igniting the fluid as you spray it on. This could result in SEVERE injury to you and others. The safe thing to do is unload the stove, reload it, and try again.
Note on Starter Fluids: I have had the most reliable results with charcoal starter and kerosene. They seem to soak into the wood somewhat. Alcohol works ok, but you have to use a little more for a reliable start. (However, alcohol trumps charcoal starter & kerosene in that it has good secondary uses on the trail.) For some reason, I have not had consistent, reliable starts with white gas or gasolene. Rick suggested using lamp oil which apparently works well, but I have not tried that yet.
|The combustion chamber is a 3 inch steel can 4 1/2 inches high. Four holes are punched at the lower edge and a screen is inserted into the can to assure the even distribution of primary combusion air across the bottom of the fuel charge. The secondary air inlets are 4 slots about 3 inches up the can. The stove windscreen is a piece of heavy aluminum from a disposable baking tray that has been paperclipped into a cylinder.|
|| The fuel must be broken up into small pieces and loaded into the combustion chamber.
|| Top view of the stove ready for lighting. The fuel has been added and shaken down tightly and the stove windscreen placed around the combustion chamber.
|| The pot stand has been fitted, the fuel ignited, and the pot windscreen positioned.
|Here, the wood gas generation is in full swing. There is little to no smoke being produced.|
|At the end of the burn, all that remains is a small pile of fine ash.|
|In it's current configuration, the bottom of the stove gets too hot to place directly on combustible materials. Here, newspaper placed under the stove has smoldered.|
|If you can deal with black pots, you will probably like this little 4 oz. stove|
|1 min||Water on Stove|
|5 min||Water temp 130F|
|1 min||Water on|
|31 min||boiling has subsided|
I noticed in the plans you mentioned somewhere about using a little fabric dipped in wax as a fire starter. Here is what I do. I save lint from our clothes drier and collect old candles. Ask friends or even buy some parafin from the grocery store. Melt wax over low heat. Mix drier lint and wax into a paste. I put it in an old plastic ice cube tray then let harden, then pop out. Makes great fire starters cubes. You could shave off a little to start the wood in your stove. I also use pine shaveings(son had some left over from when he had a pet gerbil) I think if you used clean parafin(canning type) you could cook over it because a cube burns atleast 15 minutes... MIKE