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Wood Gas Camp Stove

Make a simple tin-can stove that costs 99 cents, runs for free, and sequesters carbon as you cook.

wood_gas_camp_stove

Nearly half the world’s population cooks and heats using solid fuel, much of it burning up in pits that have seen no improvement since Homo erectus first tamed fire.

This is not a small problem: inefficient cooking fires waste fuel, impoverishing both the planet and the person burning it; they inject startling quantities of soot, carbon dioxide, and worse greenhouse gases into the atmosphere; and they injure and kill the families who use them to cook and stay warm.

You can build a simple example of an appropriate technology that addresses all these problems: a biomass gasifier stove. It sounds more complicated than it is. Charring wood or other natural solid fuels releases gases that burn quite nicely. If you’ve ever watched a campfire closely, you’ve seen little jets of smoke erupt from the wood ahead of the flame. If conditions are perfect, a smoke jet catches fire and turns, briefly, into a tiny geyser of flame. A good gasification stove recreates these conditions reliably, generating smoke and moving air to produce these little fire geysers on demand.

There are many designs for efficient stoves, and gasification is only one way to boost the efficiency of a cooking fire. The wood gas stove in this article is an elegantly simple gasifier design called a TLUD stove (for top-lit updraft), also known as an inverted downdraft stove. If you don’t care how it looks, you can build it with a can opener, a punch, and a big rock. This design, which I’ve adapted from one I first saw on Instructables, is built around a 1-quart paint can. It easily boils enough water for a small pot of tea or a bowl of noodles, using nothing more than a fist-sized charge of scrap wood.

How It Works

This TLUD stove works in batches: fill it up with twigs and woody bits, and light it from the top. As the fire burns, it makes a layer of hot coals, and as this burning mass descends into the can, it becomes starved for air. Enough oxygen comes up from below to keep the embers alight, but not enough to sustain a flame.

This charring layer slowly descends, releasing flammable gases as it goes — a process called pyrolysis. The hot gases rise to the top of the stove, where they are met by an inrush of preheated air and, if all goes well, combine completely with this air in a clean secondary burn that consumes the methane, soot, and carbon monoxide produced by the primary combustion.

When a TLUD is dialed in, it’s lovely: a layer of smoke hangs over the coal bed like a fog, and appears imprisoned by a gauntlet of inward-facing flame jets that rush in from the edges of the stove. While the stove is gasifying, it’s remarkably clean: there’s little or no smoke and only a faint odor, reminiscent of diesel or creosote. It leaves a trace of soot on your pot, compared to the heavy blackening an open fire imparts. And when your stove is done gasifying and the flame jets go out, clean charcoal remains in the stove.

This residual charcoal is a fringe benefit: not only can you use it for gunpowder (see MAKE Volume 13, page 54), but you can cook with it a second time in a clean-burning charcoal fire. You can also throw it into your compost and bury it. Called biochar, this buried charcoal enriches the soil and actually makes your carbon-neutral biomass fuel carbon-negative.

WARNINGS!

  • This stove produces and consumes carbon monoxide, a deadly poison. DO NOT EXPERIMENT WITH THIS STOVE INDOORS! Use it only outdoors.
  • Your stove will get hot. Using it on a wooden bench will leave a scorch mark. Use it on concrete, a tile you don’t care for (thermal shock may crack it), or on dirt you don’t mind scorching.

Steps

Step #1: Prepare the backup block.

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  • Open the paint can and drop the block of wood upright in the can. Mark the top edge of the can on the block.
  • Withdraw the block and make a parallel mark 3/8" below the first.
  • Cut a 5/8" deep slot between these lines. Precision is not critical: anything wider than 3/8" will do. A table saw is easiest, but a router or a handsaw and chisel will work too.

Step #2: Mark the 1qt paint can.

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  • Use marking dye or a Sharpie to lay down a stripe 3/4" from the bottom of the paint can.
  • Then, using either a surface gauge or a marking gauge, circumscribe a line 3/4" from the bottom of the can.

Step #3: Punch 12 o'clock mark.

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  • Clamp the backup block to the edge of your workbench with its slot facing up. Slip the paint can over the end of the block and drop its rim into the slot.
  • Find the can seam, and punch a point on the scribed line about 1/2" away from the seam. This is your 12 o’clock mark.

Step #4: Divide, scribe, and punch bottom of can.

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  • If you have a caliper, set your dividers to 1.075" (1/12th division of the can in chord sections); if you don’t, set your dividers to a hair more than 1-1/16". Following the scribed line, use the dividers to scribe marks to the right and left of the initial punch mark, marking as you go, to the far side.
  • When you reach the far side, your scribe marks may meet perfectly at 6 o’clock, but will likely over- or under-shoot each other. If you’ve “nailed it,” punch it down; otherwise, split the difference and punch between the two marks.
  • If the cumulative error is not excessive, punch the other marks back to the first punch mark; otherwise, adjust your dividers, re-scribe the 3/4" line, and use the same method from 6 and 12 o’clock to mark and punch the 9 and 3 o’clock positions. When you like what you see, punch down the 8 remaining marks. Perfect spacing is not critical.
  • To speed things up, you can approximate this method by wrapping a tape measure around the circumference, or you can avoid measuring entirely by downloading the drilling templates from makeprojects.com/v/27 and gluing them to the cans.

Step #5: Drill the 1qt paint can.

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  • With the can resting on the backup, drill 1/2" holes at the punch marks. On your first hole, push only the tip of your step drill through the can and into the wood.
  • Remove the can from the backup block and put the drill tip back into the hole you just made. Drill straight into the wood past the 1/2" mark, creating a step-bit-shaped recess. (If you skip this step and drill straight through the can into the wood, your first hole will be egg-shaped.)
  • Next, steady the can with your hand, and allow it to “float” a little as the drill heads down its hole. Don’t force the drill: the step bit will cut effortlessly and cleanly.
  • When you’re done drilling, wash off your marking dye or any glue. (Steel marking dye comes off with brake cleaner, Sharpie with rubbing alcohol, and most glues with lighter fluid.) When this can gets hot, any remaining goo will be cooked on forever.
  • Using the can opener, remove the bottom of the paint can.

Step #6: Mark and drill the 19oz food can.

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  • Using the methods described in Steps 2-5, apply your drilling template, or mark and scribe a line 1/2" from the open end of the 19oz can. Set your dividers to the chord division of 0.431" (a little less than 7/16"), and make 24 divisions at the 1/2" line.
  • Punch your marks, and drill twenty-four 1/4" holes. Remember to create a 1/4" step-drill recess in the wood before you drill all the holes.
  • Clamp the wood block vertically (a vise comes in handy here) and slide the 19oz can over the end.

Step #7: Drill the bottom and snap cans together.

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  • Drill at least thirty 1/4" holes in the bottom of the can. You can do a fair job by eyeball if you follow the rings stamped into the bottom of the can. The important thing is that you create an open enough bottom for good airflow while not letting all your fuel fall out. Clean off any dye or glue remaining.
  • Snap the 19oz can open-side first into the paint can. They should make a satisfying press fit.

Step #8: Mark and drill the standoff.

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  • Apply the template, or mark and scribe a line 3/4" from the top of the 12oz can, and set your dividers to the chord division of 0.767" (a hair past 49/64"). Scribe 16 divisions at the 3/4" line. Start anywhere: these cans are seamless.
  • Punch your marks and drill sixteen 1/2" holes. Clean off any dye or glue.
  • Turn the can upside down, and drill a ½" hole through the bottom of the can.

Step #9: Enlarge the hole and smooth edges.

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  • Dig in one jaw of your tinsnips and cut around the hole, spiraling out to make a hole about 2-1/2" in diameter. Use the ridges and valleys pressed into the bottom of the can as landmarks.
  • When you’re satisfied, turn the wood block up to vertical and hammer down any rough edges , finishing the job with a half-round file.

Step #10: Start the Wood Gas Stove

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  • Fill your stove with wood chips, acorns, eucalyptus pods, small pinecones, or dried pelletized dung. Your fuel must allow some airflow through it, so don’t use loose sawdust or a single big chunk.
  • Starting these stoves is not easy — you may have to cheat with a little charcoal lighter, Sterno, or other fire-starting material (my favorite is old denim strips soaked in wax).
  • TIP: Your gasifier will burn most of the soot, but not all of it! For easier cleanup, coat your pot with a thin layer of dishwashing liquid before you start. The soot will rinse off without scrubbing, and there’ll be a small patch of burnt soap that comes off with a light scrub.

Step #11: Add the standoff, and get your ramen on!

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  • As the fire burns down past the side jets, blow down on the coals to start pyrolysis. Try to get a uniform coal bed all the way around the can. Once you see good coals with fire floating on top, put the standoff on the stove, rim up.
  • If you see an orderly column of flame rising with no smoke, you’re gasifying! Put a pot of water on top and get your ramen on.
  • When the fire goes out, you’re done with this charge. You can pile another charge on top, but you may need to relight it. To save the charcoal for reuse or sequestration, pour in some water. To reduce it to ash, leave it for another half hour and it’ll burn away. When you’re done, the standoff nests neatly between the other 2 cans for easy storage.
  • You can experiment with changing the intensity and duration of the flame by changing the size of the hole on the top of the stove. The standoff lets you choose between a 2-1/2" hole or a wide-open top simply by flipping it over. You can add old can lids with different-sized holes cut in the middle to “throttle” the flame higher or lower.

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