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    Tuesday, January 08, 2008

    Homemade Wood Gasifier Stove Revisited

    Nothing beats a campfire when out in the woods. Unfortunately there are times/places where such a primitive joy is not allowed or just unsafe. This means if you want some hot food you need to use a stove of some sort. Being infatuated by camping gear and fire has resulted in a good variety of stoves cluttering my shelves. The quest for a perfect campstove has led me to the realization that, like many other tools, there's not one perfect stove for all situations (though the Swedish Trangia is close, especially when lightened up some).

    If I just want to boil water and not carry any fuel I like the wood gasifier stove I made from assorted cans.

    woodgas5.jpg
    Batch-loaded inverted down-draft wood gasifier stove.

    This is a neat little stove. You throw a handful of 2" twigs in the burner unit, light the pile at the top, give it a few minutes to get burning well then place the water bottle on top of this little volcano. The science behind it is really neat. The fire burns from the top down but air is drawn from the bottom. The air passes by the fire and on it's way to the bottom of the stove and get heated up very hot by the fire. This hot air volitilizes the flammable compounds in the wood (methanol and assorted other molecules) which then burn at the top of burner unit. The twigs are converted to charcoal by this process and once all the flammable components have been burned this charcoal ignites and continues to supply extremely high heat. The charcoal burns down to a fine ash which is easy to hide.

    woodgas
    Bad artist's representation of the system. The air is the blue/purple/red lines being drawn down through the gap between the burner unit (dark gray) and the outer wall (medium gray). Most of the heat travels up through the chimney (light gray) over the surface of the aluminum water battle (orange).

    As I mentioned earlier, this was made from assorted cans. The biggest was from the largest sized Hi-C drink can, the chimney was made from a large can of baked beans and the burner unit was made (IIRC) from a tall asparagus can. The water boiling can was from an aluminum sport's drink bottle and the mesh shelf in the burner unit was just a peice of "expanded metal" screen.

    woodgas1.jpg
    Parts

    The burner unit holds the twigs. To make it cut four 1"x1" vents in the bottom of the asparagus can. Fold the tabs inward to make ledges for the screen shelf to rest. A set of four 1/4" vent holes are also drilled in the can about 1.5" from the top.

    woodgasburner1.jpg
    Burner unit, side view.

    woodgasburner2.jpg
    Burner unit looking down from top.

    Next comes the outer shell. This is made from the Hi-C can which is just about an inch larger in diameter than the asparagus can. The top is removed but the bottom of the can is left intact. Cut four larger (2.5" tall x 1" wide) and four smaller (1.5" tall x 1" wide) vents around the top of this can. The burner sits down inside the outer shell. When the burner is inside the shell air can only eneter the burner after it's been drawn down between the burner and the outer shell.

    woodgas2.jpg
    Burner (with sticks) and outer shell, separated for picture.

    For this to work properly you also need a high chimney. The chimney increases the draft produced by the stove which makes it burn hotter. It also forces the produced heat into more contact with the boiling container. The chimney is made from the large baked beans can with both ends removed. This can should be around 1/4" or so smaller in diameter than the Hi-C can but larger than the asparagus can. Three wire skewers peirce the chimney about 1.5" inches from the bottom. These skewers form both the rack the water boiler sits on and also keep the chimney from sliding down over the burner can.

    woodgaschimney.jpg
    The chimney.

    The chimney is placed on the stove after the top of the twigs have begun burning well. I like using a cotton ball rubbed with a bit of petroleum jelly as the tinder to get the twigs lit. After the chimney is on the water boiler gets placed down it. Less than ten minutes later you'll have vigorously boiling water.

    woodgas4.jpg
    Fire!

    woodgascoals.jpg
    Burning charcoal.

    I'm not sure of the weight of this stove, maybe someday I'll bring it into my lab to weigh. The benefits of this stove are fuel (twigs) are usually easy to find, it produces just a small amount of ash and almost no smoke when burning properly, and heats water suprisingly fast. On the downside it is rather large/bulky in shape, lighting the twigs can be tricky the first few times, it isn't nearly as effective at cooking food in a pot as it is at boiling water, and you have to let everything burn completely before reloading it with twigs. The last isn't ever a problem if you are just boiling water.

    Safety note: The bottom of this stove gets pretty hot so don't set it on anything flammable or use the stove inside your tent.

    Adventure! Excitement! Piping Hot Cup Of Tea!

    13 comments:

    David said...

    Hi Blast - thanks for posting this. I'd be interested in replicating what you've made, and the photos are great...any chance of adding photos of the cans and bottles you used with the labels on, and/or sizes (what size can in ounces) so I know what to add to the next grocery shopping list?

    Thanks

    Dave

    Anonymous said...

    Neat stove.

    Have you tried the Pocket Cooker? I did a mini review of it on Outdoor Magazine Forums. I know you are a member there. Check it out. Would the pocket cooker be allowed in places where you can't have a fire?

    ziptieisthenewducttape said...

    I love your blog, can I link to it on mine?

    Merriwether the Adventurer said...

    David,
    I'll swing by the store tomorrow morning and figure out sizes/brands of cans I used and then post a follow-up. I did bring the stove into my lab and weigh it. The total weight is 13.6oz broken down as follows:
    1. Water bottle with cork - 2.6oz
    2. Burner with grate - 2.8oz
    3. Wire skewers - 0.5oz total for all three
    4. Chimney - 3.7oz
    5. Outer shell - 4.0oz

    Anonymous,
    Are you "Wildcat" over on the Outdoor Magazine Forums? I'm "Scavanger" on them. I've drooled over that Pocket Cooker stove. It looks really neat and I've seen a lot of good reviews, but it's weighs 2 lbs which is too much for me to drag into the woods. I'm also not sure if it could be used in places like Big Bend...

    ziptieisthenewducttape,
    Oh wow, Yes! Send me a link and I'll add you to my "Mutual Admiration" blogroll.

    Anonymous said...

    yes, i am wildcat.

    Darren in Oz said...

    Hi,
    I am having a go at this trying to cut the tin using handtools but finding it difficult to do cuts anywhere as neat as yours!! what did you use to make the cuts??
    Also I will try a mod...a slotted sliding sleeve around the air intakes on the base tin from cutting down another tin, to see if the burn rate can be controlled at all.

    Merriwether said...

    Darren,

    I used a Dremel tool with a small cutting wheel. It slices through the cans with ease.

    The slotted sleeve to control airflow should work. I've seen similar devices on other stoves like this, I just haven't tried making one. I'm curious to see how well the air-flow controller works and how hard it is to make. If you could, please e-mail me pictures of it when you finish yours.

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