July 26, 2009, Jim Welch from Troy, NY built a hybrid biochar burner on my patio. His design grew from my suggestion to put a rocket stove under a 55/30-gallon 2-barrel nested retort.
Results exceeded our expectations. We were impressed by the energy expelled by 30 gallons of biomass. Our test had many troubles, but taught us much to scale and control this novel approach to pyrolysis.
Our first hybrid burner experience was promising, but we need to control the pyrolysis process, and a bigger rocket stove to start pyrolysis quicker. We agreed to continue this path of investigation, and collaborate to build more test models.
Over winter, I toyed with ways to build a better 55/30 barrel burner and a bigger rocket stove. Jim decided to build a burner for his backyard, scaled down to a 5-gallon retort and 18-gallon kiln. A distinct advantage of this small size is a light-weight, portable unit that can be taken to workshops for demonstrations. Folks always get a thrill watching pyrolysis gases flare off.
Saturday, June 19, Jim brought his new burner to Saratoga Apple and assembled it for a biochar workshop which was to feature my own new rocket stove and 55/30-gallon nested retort. But Jim's smaller 5-gallon model captivated our audience, and I continue to be impressed with Jim's creative and funky fabrication skills.
Jim's new rocket stove is quite small—smaller than a shoebox. Actually, half of it is a beer can. This device seems tiny after the earlier experiment—hardly big enough to heat a 5-gallon retort. This modest little burner is attached to a much larger steel frame that is the base to support the 18-gallon kiln and 5-gallon retort.
Jim squeezes optimum thermal efficiency from this tiny burner box by carefully insulating the firebox and kiln—even the chimney is tightly insulated to hold heat, boost combustion temperatures and increase the up-draft.
Jim needed nearly 30 minutes to assemble his apparatus and get it ready for a test burn. But within 15 minutes his little firebox was drawing a steady, vigorous, hot flame and cooking lots of blue-white steam out the rear vent.
The 5-gallon bucket that serves as a retort has a one-inch hole drilled through the bottom and lid. A pipe is fitted through this that is both gas vent and support column for the retort & kiln. This pipe allows Jim to flare the pyrolysis gases off inside the firebox a few inches below the retort.
This pipe has a horizontal extension that exits the back of the rocket stove firebox, and allows Jim to vent steam from the early phase of biomass heating. Once flammable gases are being generated, Jim caps this external vent out the rear of the firebox, and re-directs the gases inside the firebox to be ignited by the flames from the rocket stove and burned.
Ron Slabaugh of Middlebury VT and Saratoga Apple owner Nate Darrow stuffed perhaps three dozen sticks of hardwood—some over an inch in diameter—inside the 5-gallon retort. They were careful to pack the branches tightly with a minimum of air space between the sticks. The more densely biomass is packed in the retort, the longer and stronger the pyrolysis gases will flare off.
This loaded retort was then sealed with a lid, and positioned inside the 18-gallon kiln, which is well-insulated with a 1-inch sheet of refractory insulation. The lid to this kiln is also well insulated, as is the first length of attached stovepipe. With this lid & chimney sealed into place, the rocket stove was ready to be fired up.
Jim started a fire in the front of the rocket stove firebox with half a dozen small sticks—nearly twigs. I had doubts such a small fire can generate enough heat to kick off gasification in a 5-gallon bucket six inches above the firebox. My own 20-inch square firebox is a monster compared to Jim's mini-burner in a breadbox & beercan.
Workshop participants were impressed by the strong updraft created inside the tiny rocket stove firebox. The flames generated by a few small sticks didn't rise up out of the firebox.
Instead, flames were sucked vigorously down and forward into the firebox, then up the chimney. The draft was a strong, steady wind that fanned flickering flames to a bright yellow hot.
The effect is like a fast-spinning fan blowing air into and through the firebox, whipping flames into furious action—burning with a very hot, smokeless fire. This boosted air flow increases with more insulation around the rocket stove, kiln and chimney, and a taller, wider chimney.
Within 10 minutes, a thick plume of fragrant white steam streamed out the rocket stove rear vent. Water boiled out of seemingly very dry wood. This gradually shifted from blue-white to yellowish. In 10 more minutes, this exhaust could support a flame if ignited by Nate Darrow's lighter.
Jim screwed a cap on the back vent to force pyrolysis gases out the front and up the chimney. Burning gas made a low whooshing as it ignited and rushed up the rocket stove chimney and around the retort.
After a few minutes, Jim pulled the burning sticks out of the rocket stove firebox, and pyroysis flames continued whooshing up the chimney unabated, with only glowing coals in the rocket stove to ignite them. The fire was no longer wood-fueled, but completely sustained by the hydrogen, methane and other flammables exiting the retort.
This soft whoosh slowly rose in volume over the next 15 minutes and became quite loud. This outburst of pyrolysis gas truly put the "rocket" in rocket stove. We couldn't see the pyrolysis flames except where they gushed into the firebox, but the sound was impressive and entrancing. Every workshop participant was excited by this audible indication of a large, strong release of flammable gases.
To highlight the self-sustaining nature of this burning, I announced, "Now we're cooking with gas." I went on to explain that only 10% of the gas is needed to keep gasification going in the retort. Thus, up to 90% of the energy released can be stored and burned later for more useful purposes. Or it can be burned in a generator or fuel cell to produce electricity. One ton of biomass can equal as much energy as 5.5 barrels of petroleum.
This gasification flare continued for about 30 minutes, and then rather rapidy subsided. After a few more minutes, Jim removed the chimney and lid from the 18-gallon kiln. A bit of pyrolysis gas was leaking out of the lid of the 5-gallon bucket, and flared with a weak, transparent red flame.
Jim unscrewed the screws holding the lid and opened the retort. Inside, the hardwood sticks were thoroughly charred. The tough sticks were now quite brittle, and easy to crack and crumble in my hand. Every stick was charred all the way through. The burn was a complete success!