At Four Oaks Farm in north Topeka, we're learning how to apply a new carbon-negative climate strategy to the practical reality of growing nutrient-dense food for the local sustainabilty community. This means making charcoal from organic waste and debris, blending it with compost, sea minerals and rock powders for farm soils.
This learning journey began January 26 with a 55-gallon drum and a length of 8-inch stovepipe. Our barrel didn't have a removable lid, so we opted for the simplest approach. Brad cut an 8-inch hole on the barrel bottom, and removed 1 and 2-inch plugs from bungholes in the lid.
Next, a purification ceremony. The upside-down barrel was loaded halfway with broken brush, and set up on bricks. We started a fire inside, and installed the chimney. It was smoky, nasty, toxic, as oily residue and paint burned out of the barrel and off the metal. For once, the persistent Kansas wind was our friend as it blew this toxic smoke away.
fully loaded barrel
The next day, we were ready for a test burn. We loaded the barrel with twigs and stems from the huge brush pile. We broke branches up as much as we could by hand into small short piecesŃmost under 8 inches to fit through the chimney hole. By various means, we packed them tightly in the barrel.
chimney cap for wind deflection
Wind is a capricious disturbance that affects barrel burners by changing air intake and exhaust updraft. Wind can change the shape of flames, and the burn geometry. In extreme gusts, wind creates chimney downdraft that can snuff out a fire. In Kansas, wind is a persistent pest.
So, we added a few features to protect our burn barrel. We put the barrel on a leeward downslope, with air intakes below ground, fed by a trench in the downwind direction. Brad made a skirt of cinderblocks around the windward side of the barrel as a deflector.
I found a ventilator shaft turbineŃa slotted dome in a frame, suspended by a pulley to rotate freely in the wind. This made an excellent cap to place on our chimney to shield the gas flare from winds.
up-draft lifts flame above chimney
Brad soaked a cup of wood pellets in mineral spirits, then scattered them on top of the packed biomass. One match, and a vigorous fire was ignited. As flames danced over a foot high, Brad slid the stovepipe in position, with a noticable increase in updraft. In few minutes, flames were leaping above the chimney before Brad slid the cap in place.
The burn went very well for about 20 minutes. Fire steadily intensified as flames flew furiously up the chimney. Eventually, flames began to flare out beyond the cap. All we could see of the mass of flames dancing in the barrel was through a narrow crack between the chimney and barrel top (photo at right).
Brad used a wax crayon to mark a line down the side of the barrel. As the fire descended, we watched the crayon mark slowly change from color to white. Thus, we could follow to progress of the burn in the barrel from top to bottom (photo at right).
smoke from smoldering biomass
Suddenly, without much warning, chimney flames collapsed as the fire choked, and soon went out completely. The chimney began boiling out a dense plume of thick, smelly smoke.
Several attempts failed to get the dense smoke & gas mixture billowing up the chimney to ignite and flare off. We removed the chimney as smoke continued to pour out of the barrel. An oxygen-starved fire smoldered in the barrel. Eventually we doused the fire with water and quench it.
The simple diagnosis for smoke is insufficient oxygen or low temperature. Smoke is unburned carbon, and carbon burns last and burns hottest. The biomass fire in the barrel generated excess volatile gases and carbon, which can't ignite in the oxygen-starved interior. The one and 2-inch bungholes are far too inadequate to deliver enough air to the huge flames in the burn zone.
smoking barrel of smoldering biomass
Later, we examined the barrel contents for more clues about what went wrong. Our hope was to shed more light on the sudden failure of the burn, and learn to improve performance.
Some biomass was well charredŃperhaps as much as a third. But very little wood at the barrel center was charred, or lighly scorched by flames. Fire had burned rapidly down the outer edges of the barrel, following the two sources of oxygen, and avoided the center, leaving it to smolder and smoke in the oxygen-starved interior.
The ideal, of course, is to have a slowly descending burn zone that remains a flat flame front, that is symmetrial and even as it proceeds down through the biomass. In effect, like a cigar that burns gradually from end to end.
Obviously, achieving this uniform burn won't be easy. Aside from wind, air flow up through the barrel must be well distributed. A single larger air intake at the center would serve better that our two small bungholes, but likely a should be supplied around the edges of the barrel, too.
Another limit is the size of feedstock a TLUD can char. The moving flame front only burns a short time in an area, and can't fully char interiors of thicker, denser pieces. Our current TLUD maybe can char up to 2-inch thick sticks. But higher temperature and slower burn time are needed to char up to 4-inch hardwood.