July 26, 2009, Jim Welch from Troy NY built a backyard barrel burner on my concrete patio. His homemade biochar baker is a hybrid design that combines a rocket stove combustion chamber underneath a 55/30-gallon nested 2-barrel biochar burner (a scaled-up Folke Gunther design). The rocket stove's primary purpose is to produce a very hot, smokeless fire that quickly kickstarts biomass gasification to initiate the pyrolysis phase of making biochar.
I met Jim at an Albany permaculture meeting. We discussed ideas for controlled combustion, and I suggested a hybrid design of rocket stove added to a 2-barrel biochar retort.
I had never built a rocket stove, but Jim built a 5-gallon burner. Jim took initiative to think through a design and assemble materials to build a larger one for a 55/30-gallon nested retort. July 26, I was teaching a biochar workshop and being videotaped when Jim showed up with his equipment. I gave Jim space on my patio, and went on with my event while Jim assembled and fired up his creation.
The rocket stove consisted of three dozen firebricks arranged in a circle to support the 30 & 55 gallon barrels of the biochar retort & kiln. On one side of this ring of bricks, a long, narrow throat extends outward as a path to feed split kindling wood into the firebox. This ring of bricks had glacial sand dug out of my garden heaped all around it to seal and insulate the combustion chamber.
The 30-gallon barrel is the retort, and was loaded with moist, recently cut lengths of red pine poles and limbs. The barrel lid was locked on, and the barrel inverted on top of the brick ring, carefully positioned so it is nested symmetrically inside the other barrel.
The 55-gallon biochar barrel is installed upside down above the combustion chamber and over the 30-gallon retort, and insulated with fiberglass. A hole cut in the barrel bottom—now on the top—allows a stovepipe chimney to be attached. Exhaust from the rocket stove under this kiln barrel flows up and around the 30-gallon inner retort, then up out the short chimney on top.
This arrangement separates the combustion chamber, fuel and air
intake from the biochar barrels. The firebox is fixed in place, while retort barrels are loaded on & off this rocket stove base. Hopefully, the rocket stove will achieve higher temperatures, faster combustion and better fuelwood efficiency than the TLUD-style 2-barrel retort. Also, all air flow, fuel and other combustion controls are in a single, fixed unit, isolated from the biochar retort & kiln. This makes fabrication easier and simpler, and
operation cleaner and more convenient. Ultimately, this can allow close control of the retort temperature and the gasification process.
The lid of the 30-gallon retort has a short length of threaded iron pipe installed as a gas tap. This short pipe ends at a "T". A pipe from one end of the "T" extends forward toward the fuelwood feed throat. The other end extends out the rear, sticking out beyond the rocket stove bricks and sand. This back tap is fitted with a screw-in plug.
Jim fired up the rocket stove with 2-foot pieces of split lumber, and kept the rocket stove roaring with a hot, vigorous flame. After perhaps 30 minutes, the back tap began to emit dense, white, pine-scented
steam. But even after nearly an hour of steady heating, Jim was never able to ignite these
exhausts and flare off the gases. It seemed the pine wood was too moist, and the rocket stove too small for the 30-gallon retort.
Jim went home at dark, leaving the biochar burner undisturbed with its load of feedstock. The next morning, I left for Saratoga Apple, then Vermont to begin a 3-week trek that eventually landed me in Boulder Colorado at the North American Biochar Conference.
A month later, Jim returned to fire up the rocket stove. After fiddling and diddling a long time feeding split kindling into the rocket stove, jim again began making lots of sweet, white steam. Gradually, in over 30 minutes, this white steam became blue smoke, then more yellowish-brown with an acrid odor.
Finally, the exhaust from the retort was able to sustain an 18-inch long flame. The water was mostly distilled out of the pine wood, and enough hydrogen, methane and carbon monoxide were produced to support combustion.
Jim screwed the plug into the back external gas tap to force the gases to flare out the front toward the feed throat. Soon the gas jet overwhelmed the narrow
feed throat and blasted out nearly four feet.
We were impressed by the
volume of energy that flamed out of the rocket stove, and by steady pressure of this energy release. Slowly this exhaust flare subsided, and eventually the retort temperature dropped below the gasification threshhold and pyrolysis shut down.
But Jim felt the 30 gallons of pine wood had more gas to expell, so he fired up the rocket stove again. As dusk deepened to dark, Jim stoked up a hot crackling fire under the retort.
To our surprise, the retort began blasting gas again. In a few minutes, a gas flare gushed fiercely out the fuelwood feed throat, making a soft, low "whoosh-sh-sh." This gas flare kept building in intensity and pressure, and soon exeeded the earlier outburst in both size and density. The "whoosh" slowly increased very slowly in intensity as the gasification reaction increased, forcing more and more gas out the narrow pipe under the retort.
Bursts of flame occasionally rocketed up the barrel and flickered out the chimney, but most of the flames shot out the feed throat. In the deepening darkess, these flames had greater luminosity and intensity compared to their pale transparence in full sunlight. The 55-gallon barrel got redhot, the fiberglass insulation began to melt. Eventually, the insulation's paper backing began to burn.
Jim scraped away sand to allow the barrel burner to suck up more air as the gas flare kept growing. The low "whoosh" got insistent, and almost a dull roar. Insulation in the rear also caught fire, and we had to douse it with water. This fiery display went on for a long time while I dragged a garden hose in the dark to the barrel—just in case the flare continued building and started to consume the entire insulation and overwhelm the barrel.
Eventually, this second, even more impressive outburst subsided as the gas release subsided and flames receded, dopping the retrt temperature. But Jim thought there might be more energy in those red pine limbs, so he fired up the rocket stove a third time. To our surprise, he was able to initiate even more gasification, which went on more sedately for quite a few minutes.
By a few orders of magnitude, this was the most impressive, inspiring performance we have ever seen by a 2-barrel retort. We were astonished by the volume and intensity of energy released from 30 gallons of moist biomass. The sound effects emitted certainly put the "rocket" in the rocket stove. It can't get much more exciting than watching this fire-breathing dragon of pyrolytic discharge! But how do we tame this dragon to bring it indoors?
The next morning, Jim returned to open the burner to see what kind of biochar he had cooked up with all that gas and fire. At the bottom of the retort (top of the 30-gallon barrel), even large split logs of red pine were thoroughly charred. Log slices over three inches thick were black and brittle to their core, attesting to the high temperatures achieved by Jim's apparatus.
However, we discovered a few large pieces at the top of the retort that were incompletely charred. One uncharred log had a large knot; the added wood density may have retarded heat penetration into the wood.
It seemed the very center of the top of the retort never reached gasification temperatures, and carbonization never occurred. Getting heat to the top of the retort is a weakness of this rocket stove design, especially since Jim's gas tap blasted most of the gas and flames out the feed throat rather than up between the barrels and out the chimney.
We also observed thousands of small black, shiny beads on the inside of the retort, and also on some of the charred feedstock. It seemed that at lower temperatures, pine resins were boiled out of the feedstock, then condensed inside the retort to form liquid drops. These beads were the carbonized into this hard crust on the retort and feedstock. Jim and I discussed the extra energy unleashed by the pine resins compared to hardwoods.
As we worked dismantling the barrels and sorting the biochar, we
discussed how to tame this pyrolysis dragon to harness this outburst of flame to heat a food-growing greenhouse. Clearly we will need significant refinements and controls of this crude experiment before we can consider installing a pyrolytic heater & biochar burner in a house. First safely design and operate one in a greenhouse, then—with learning from that experience—attempt to build a burner safe and reliable enough to fit in a human home.
While the burn had several difficulties, it was eventually successful, and we learned a lot in the lengthy process of getting gasification started—and then watching flames gusher without control. Fiberglass insulation must be replaced by a sturdy material for the extreme high temperatures of pyrolysis. A larger rocket stove seems necessary. Pyrolysis gases must be directed up the chimney, not out the feedthroat. Some way to boost air intake during the outburst of gasification flares.
We understand better how to restrain and control the huge, hot, noisy gas flare—maybe install an "on/off" switch—even a variable control of temperature and rate of gasification. Jim will revise his rocket stove design and operating procedures, and be sure to use dry feedstock. I will build a permanent, in-the-ground rocket stove and barrel burner in my garden for my gardens soils.
In any event, Jim and I are committed to further explore this strategy and construct more equipment to experiment with this hybrid burner.
And next time we will have cold beer to celebrate the appearance of
the fire dragon!