July 26, 2009, Jim Welch built a hybrid biochar burner in my backyard. His design emerged from my suggestion to put a rocket stove under a 2-barrel nested retort. This separates the burner from the retort, so fuelwood doesn't get packed in a tight space between two barrels.
Instead, fuel, fire and all air and burner controls are under the barrels in a separate, stable structure—the rocket stove firebox. A rocket stove delivers a fast, hot fire to quickly, reliably initiate gasification. Retort barrels are easy to flip on and off the fixed burner base for simpler operation.
Results exceeded our expectations, and astonished us how much energy is released from 30 gallons of biomass. Our first effort to apply this new idea was quite promising, but we faced serious challenges to harness this pyrolysis dragon.
Over winter, we developed ideas to upgrade Jim's creative contraption of spare parts and scrap pieces. We need to control the pyrolysis process—ways to slow the rate of gasification and lengthen its time. We must use all the heat released. My priority is to heat a farm production greenhouse.
Jim scaled down to a smaller 5-gallon retort design for his backyard. His unit is portable, so Jim took it to Saratoga Apple, the Permaculture Convergence and other places for some enchanting demos in our biochar workshops.
I stuck with a 30-gallon nested retort, since large volumes of char are needed for field-size test plots. And my goal is a unit big enough to heat a farm production greenhouse. This means a bigger rocket stove firebox to crank out more heat in a fast, furious fire to kick off pyrolysis quickly, reliably. Once a flammable gas jet is initiated, only a small fire is needed to sustain ignition—a pyrolysis pilot light.
I chose to build this bigger burner underground, first to insulate the stove and raise combustion temperatures. But also to minimize its wind profile and reduce effects of gusts such as backfire, uneven burn—even burner snuff-out. Working outdoors, I can minimize—but not eliminate—wind disturbances, while our goal is to build a unit to burn indoors, safe from tricky winds.
This meant digging a hole. I planned to build this bigger, better rocket stove here at Turtle EyeLand Sanctuary in East Greenbush, NY. But circumstance conspired to convince me Saratoga Apple in Schuylerville, NY is the safer, more secure and useful place to install this experiment.
Saratoga Apple has an abundance of 4x8x16-inch blocks to build the rocket stove. Saratoga Apple also has an abundance of black locust and apple wood—and apple is as dense as oak, maple and black locust. When I added up layers of block and number of inches, my hole was 24 inches deep, perhaps 30 inches square.
Rocket Stove Construction
Digging commenced Sunday, May 25. I hadn't done serious mattock and shovel work for five years. I quickly discovered I was chopping out heavy clay. Quite hard physical therapy for my broken back and toothpick muscles, plus bending over into a 2-foot hole for four days—very therapeutic lower back stretch. And all that sunshine on my back. I sweat so much I almost stopped peeing.
The clay is dense, essentially impermeable. I found 12 earthworms in 10 cubic feet. I realized my hole needed a sump, in case it rains 8 inches, or someone dumps gallons of water in my firebox. So, one air port became a sump to pump out any water, and I packed pea gravel under and around the two blocks at the bottom of the burn chamber. However, this caused me to twist my base blocks 45 degrees to accomodate the 6-inch sump and vertical access pipe.
Another challenge is to fashion a firebox that generates a spiral vortex flow of air, gas and flames. First, this increases the mixing of air and gas for more thorough combustion with a rapid, hot, smokeless burn. But also, a longer flame path holds more heat in the kiln to boost temperatures and improve thermal efficiency. A circular flame path around the firebox and retort is better than bottom-to-top vertical combustion outflow.
A whirlwind in a barrel.
To create this circular flow, I designed a 4-sided firebox with a 4-inch space between each right-angled block. Air enters the firebox from these four ports at the corners, in a tangent to the radius. This encourages a clockwise circular flow from periphery into the center, rather than an immediate upward linear flow.
Since this firebox is 20 inches underground, stovepipe lines four slanted tunnels to deliver air to the four ports at the corners of the firebox. Combustion can be adjusted by opening and closing these air ports.
During gasification, a small fire is needed as a "pilot light" to ignite pyrolysis gases and vapors expelled from the retort. So, a narrow throat is needed to steadily feed wood to the combustion chamber.
Thus, one air tunnel is a throat to feed fuelwood to the firebox, and is wide enough—6 inches—to accept large sticks. The sump—at opposite corner of the firebox—is also 6-inch stovepipe. The other two air ports are 4-inch pipe; a second pipe was added to increase volume and control flow.
Above the 4-sided firebox is a larger, 5-sided "swirl chamber"—8 inches high, formed by five blocks on edge. The larger space encourages more swirling and turbulence in the expanding flow of gas and air. Five is a natural symmetry to generate exponential spirals—best known as PHI, the Divine Ratio, more popular today as The Davinci Code.
A small test fire at this stage of construction demonstrated effective performance. A pile of crumpled newspaper and dry, split kindling wood burned fiercely with the kiln barrel and one length of chimney. Even with 4-inch airports on the sides closed, air supply to flames two feet underground was more than generous.
The next ring of blocks are laid face-up rather than on their long edge. These blocks are contact points to support the 55-gallon kiln barrel, so their placement is fixed by the barrel diameter.
This layer is the work deck for loading and unloading barrels and fueling the rocket stove, so it must be very solid to remain stable
under heavy use.
This layer also has two metal bars to support the 30-gallon retort. Spacing between the two barrels and the inner edges of the blocks is tight—three or four inches—and requires careful, precise placement. This task is complicated by fitting these blocks around the stovepipe for the two side air ports.
The sticky, dense clay soil was useful as mortar to lock these blocks in position, and seal the combustion chamber from leaks of air and water. Clay was screened into a bucket, then mixed gradually with water until a thick paste formed. This was plastered under and around all the blocks, and in all joints and gaps in the construction. This heavy paste was also plastered all around the stovepipe air ports to reduce rainwater penetration into the firebox. The edges of this clay sealed area was filled with coarse sand to assure water drainage away from the stove.
Two more low temperature test fires again showed excellent combustion. The third test included closing the sump air port, and adding a few branches of dense applewood to the pile of fuel, which burned rapidly in a crackling hot fire as a cone of red flame shot out the top of the kiln barrel. Clearly the rocket stove will generate the needed volume of heat and high temperature to kick the 30-gallon barrel into pyrolysis.
However, the rocket stove may burn too fast to get pyrolysis started. At least 20 minutes—perhaps 30—of heating are usually required to initiate gasification, and all three burns were shorter. The principal variable affecting the time required for the onset of gas flaring is the amount of moisture in the feedstock. The drier the feedstock, the fast gasification begins. I expect adding a few inches of applewood on top of the kindling will yield a longer, adequate burn, with lots of coals. But other methods may be needed to prolong the fuelwood burn time.
With kiln barrel in place, a ring of 1-inch thick cinderbrick paving tiles were positioned radially around the bottom of the barrel to clasp this lower edge and help seal the air gap between barrel and blocks. The entire work zone around these paving tiles will be filled with coarse sand for firm footing and easy drainage.
Rocket Stove + Retort Operation