The 2-barrel nested retort developed largely from Folke Gunther's "simplest of the simple" biochar burner. My retort, however, is not only much larger, but is intended to be sealed to fully control the release of pyrolysis gases and vapors, since one goal for this unit is to tap off the gases produced to provide flammable fuels for accessory combustion, or to store gases produced for later use.

The 2-barrel nested retort is much like the ordinary household oven, which is a closed box that is heated from the outside to bake the bread inside. My retort, however, must achieve far higher temperatures—at least 500 degrees C, and as high as 1000 degrees C. The purpose of this "oven" is to intentionally "burn" the biomass "bread" into charcoal, not just bake out the water.

CARBON-NEGATIVE Biochar Demonstrations 9am - noon 3rd Saturday monthly Saratoga Apple Schuylerville, NY

My retort is designed to be self-sustaining. Once it reaches "ignition" temperatures (well-above 250 degrees C), it will produce flammable gases that is burned in the rocket stove to continue heating the retort. After the retort enters this second pyrolysis phase of gasification, it not only produces enough gas to keep gasification going, but unleashed a huge outburst of energy to be harvested and harnessed for other purposes.

Second Test Burn 55-gallon barrel with full fire

Kiln Barrel

The outer 55-gallon barrel is the kiln. In my current equipment, this barrel sits upside down on top of the rocket stove. The diameter of the barrel's open top (now on the bottom) matches the width of the rocket stove.

The kiln must contain flames blazing up from the rocket stove, and direct this heat all around the retort—as much heat as possible—and concentrate that energy on the retort. My former 2-barrel retort allowed too much heat to radiate away, and thus lower retort temperature, decrease thermal efficiency, and make it uncomfortable and hazardous to work close around. Thus, insulation on the kiln is a must.

Spacing between kiln and retort is tight—little more than two inches. This spacing is critical at the bottom, where flames exit the rocket stove through a narrow gap between the barrels. If this space is too tight, it restricts updraft, accumulates heat in the rocket stove, and retards performance.

Similarly, vertical spacing between barrels must be uniform all around to evenly distribute flames and heat. If barrels are off-center, or improperly aligned, fire flows up one side of the retort to cause uneven heating. Also, spacing between top of the retort and upper inside of the kiln must be carefully fixed. Too tight, and it chokes the updraft. Too wide, and pyrolysis flames slow down to retard the updraft.

The only fabrication needed for the kiln barrel is to cut a hole in the bottom to attach a chimney. The size, height and placement of the chimney determine the updraft. I used three lengths of 6-inch diameter, heavy duty stovepipe. Jim Welch did excellent handiwork to cut and flange the chimney hole and lip.

Retort Barrel

The inner 30-gallon barrel is the retort—the oven in which the biomass is sealed for baking. Using a curled-edge lid and a clamp ring, I achieve a reasonable seal and a minimum of pressure for the pyrolysis gas.

The only retort fabrication is to provide exits for pyrolysis gas. The primary vent is downward out of the lid into the rocket stove, where gas ignites to flare upward and heat the retort further, and sustain pyrolysis. However, convection drives hot steam and gas upward in the retort, where it's trapped at the top. So, a gas tap can be attached at the retort top to vent steam, and later to tap gas for accessory combustion or storage.

First time I fired a loaded retort, I drilled four quarter-inch holes in the lid to vent pyrolysis gas. This proved inadequate, and gas pressure inside the retort got high enough to significantly stretch and bulge both retort bottom and lid, and the retort emitted a strong and long "whoosh" similar to Jim's 5-gallon retort.

Astonished by such high pressure, I drilled a dozen half-inch holes in a 5-inch cluster in the lid center. The next burn was quieter, as a 6-inch wide gas jet flared down from the lid, with no evidence of retort pressure.

Satisfied, I had Jim Welch install a 2-inch pipe in a lid to provide a more controlled exit path for the gas flare. Later, I will add a "T" to this flare pipe, and tap off excess gas.

click for 39 second video

For optimum operation, kiln and retort must be aligned with vertical, so their spacing is equal all around the retort. This alignment affects gas distribution both in and around the retort. Barrels too far off vertical, or tilted relative to each other, will distort the flow of flames to create hot and cold spots in the retort to cause uneven or incomplete charring.

Because the retort is vertical, with the fire under its bottom, heating will be uneven between bottom and top. Pyrolysis and charring will proceed gradually from bottom to top. This admits the possibility that a "cold" spot at the top of the retort may leave some biomass uncharred, or partially charred.

A support structure is needed to hold the retort in position inside the kiln so the retort bottom is open for pyrolysis gas to exit and jet down into the rocket stove firebox. Rebar was cut and inserted between the top-level blocks of the rocket stove to form a grid for the retort to sit on. After three hot burns, the rebar—heated to red hot—has softened, stretched, and now sags, making it difficult to properly position and align the barrel.

Sturdier, heat resistant steel is needed for this support bracket. Ideally, the retort will attach to this support.

Chimney & Draft

The chimney provides a exit path for combustion gas. Properly sized and attached, a chimney creates a negative pressure that sucks gases rapidly up out of the firebox and around the retort. This effect can be as strong as attaching a fan to blow gas through the equipment.

My current chimney is two lengths of 6-inch, heavy gauge stovepipe. Given the high temperatures of pyrolysis, heavy gauge metal is essential to stand up to the extreme thermal stress. While my chimney performs adequately, larger diameter and longer length could improve the updraft.

Another way to improve performance is to insulate the chimney, creating higher temperature, and thus stronger draft. My current chimney isn't insulated because it's constantly handled to remove and attach it to the kiln.

Often, in full operation, pyrolysis gas volume is so great it overwhelms the rocket stove capacity to suck in air and thoroughly burn all the gas, which results in smoke (unburned carbons) exiting the chimney.

To avoid this pollution, one solution is to add air above the retort with holes in the chimney base. Added oxygen allows unburned pyrolysis gas to ignite and combust up the chimney. A better solution is to tap off excess gas, rather than burn it in the firebox, and thus avoid firebox overload.

Insulation

The most critical factor in performance is retort temperature. Pyrolysis of flammable gas—the "ignition point" of a self-sustained burn—is somwhat above 250 degrees C. Every effort must be made to retain heat in the kiln, minimize heat loss and cooling, and keep retort temperature above the ignition point. The basic strategy is to insulate the kiln so heat can't simply radiate away.

A simple insulation is to enclose the kiln in a larger barrel. This can be enhanced by filling space between barrels with inert, non-flammable, heat-resistant material such as ash, clay, vermiculite, dirt.

A masonry kiln enclosure is another strategy: clay, firebrick, stone or other mineral materials. A masonry construction could replace the metal kiln barrel, but for now, the metal barrel assures air-tight containment of the flame path and retort.

My first effort was fiberglass I found in a Saratoga Apple storage room. It was hard to attach to the barrel, but worked well in my first retort test burn to boost retort temperature and successfully kick off char-making pyrolysis. However, fiberglass on the hottest kiln areas (the top) melted, and paper backing blackened. A material more heat sturdy was needed.

Jim Welch generously donated a 2x25-foot roll of 1-inch ceramic fiber insulation. This cost as much as all the rest of my equipment together, but was a wise investment. When the kiln is at peak heat, I can comfortably place a bare hand on the insulation. Very impressive heat blocking and retention.

My seventh burn on October 11 ended with the entire kiln glowing cherry red with heat, yet ceramic fiber insulation was unchanged in color or physical condition.

All I could scavenge to fix insulation to the kiln was very stiff fence wire. A better solution is a 3x6.5-foot metal sheet; this will improve overall insulation and protect insulation from damage. In my situation—working outdoors—wire worked well, because when rain and dew wet the ceramic fiber, a hot fire quickly steamed the heat blanket dry.nd.

Crane

The kiln must be hoisted up to position a retort on or off the rebar support. The kiln is then lowered around the retort and onto the rocket stove. This procedure is awkward, and requires re-creating an air-tight seal between kiln and rocket stove with every burn—a chore I accomplish with mud from the site's sticky clay. This stove-to-kiln seal is crucial to prevent cool air entering the kiln-retort space and lowering retort temperature.

My next kiln will have a top that opens to allow a retort to be placed and extracted—a simpler, more sensible procedure, except a retort loaded with feedstock will generally be heavier than my insulated kiln.

In either design, a machine is essential to lift heavy equipment—especially given my physical condition. And on removal, equipment may be quite hot.

So, for this task, I assembled a simple 4-legged crane to suspend a pulley with wire rope and counterweight. I scavenged pipe and small parts at Saratoga Apple, and spent $27 at a hardware store, mostly for a pulley, chain and wire rope. A 56-pound tractor weight—the counterweight—allows me to raise and lower the fully insulated kiln with one hand.

Heat Exchangers

The next equipment I will install is a heat exchanger to tap off surplus heat—with associated plumbing and hot water storage tank. To heat a greenhouse, hot water is more useful that hot air. For plant growth, soil temperature is more critical than air temperature, so hot water is useful to pipe under the growing beds to warm the soil.

The optimum area to extract heat is the top of the kiln and base of the chimney. First, because this area is above the retort, and heat extraction will minimally affect retort performance. Also, evidence from test burns confirms this area is a hot spot in the flame path.

A heat exchanger can also be installed under the kiln in the rocket stove, since surplus heat is released from combustion of fuelwood and pyrolysis gas. The advantage is to use natural thermosiphon to circulate heated water upward into a storage tank. As long as this rocket stove heat extraction doesn't excessively retard retort temperature, this is a workable strategy to harvest the thermal energy.

In an advanced design, a heat exchanger can be built around the retort and used to regulate retort temperature, lowering it to restrain (or throttle down) the rate of pyrolysis. Being able to raise and lower retort temperature is a valuable, direct way to regulate the rate of gasification and fully harness this pyrolysis dragon.

Rocket Stove Construction
Rocket Stove + Retort Operation