The cheap, easy, surefire way to make biochar is stuff any size metal container with any kind of biomass, close the container with a tight lid and toss it in a big fire. To avoid deforming the container, it's wise to put at least one hole in it to allow pyrolysis gases to escape. The gas pressure generated may necessitate adding a few screws, clamps or other devices to hold the lid on.
After my Carbon-Negative Biochar workshop in July 2009 at the Greene County Cooperative Extension Agroforestry Center in the northern Catskill Montains of eastern New York, grass-fed beef farmer Lonnie Avery was excited to start making char. He chose a straight and simple path to cook up his first batch of char. His burner was the outdoor water jacket wood furnace that heats his farm' His retort was a 10-gallon galvanized trash can. His biomass was woody debris gathered from the edge of his woodlot.
He tossed his trash can of sticks and bark into the furnace and closed the door. After a few minutes smoke exiting the furnace chimney thickened and lightened as water was evaporated out of the moist wood. Soon this smoke thinned, and curious Lonnie opened the furnace firedoor to see what was occuring. What confronted his eyes ignited a real thrill to his pyromania.
Gasification was underway at full force. A fiery halo encircled the top of the trash can. A ring of flames flickered and danced in long tongues several inches long. The sight of this outburst of energy was equal to the memory of a midsummer bonfire.
As Lonnie snapped photos, this flaming display gradually increased in intensity and size, and the thin metal of the trash can crumple and deform. A few spots even became plastic and began to melt, and Lonnie became concerned he had unleashed a chain reaction he did not know how to control.
Uncertain what to do, he seized a pole and knocked the can over on its side. fortunately, the lid stuck on the can, and the pyrolysis fireworks continued. Slowly at first, then rather rapidly, the flames subsided and shrunk, until only a few flares flickered around the rim of the now-warped lid.
When even those flares ceased, Lonnie put on heavy leather gloves, reached in the wood furnace to remove the trash can. He set it down on the flagstone in front of the furnace, believing the fireworks were over. He was startled when the can erupted with a new halo of flames flickering out from beneath he lid, although much more subdued than the extreme outburst in the furnace.
This afterburn declined quickly. When it was obvious no more flames were likely, Lonnie pried the lid of the can, and discovered his biomass was nicely, thoroughly charred. He had made his first batch of biochar. He was impressed how easy it was, and his mind immediately set off designing more functional equipment and operating procedures. He hadn't had this much fun with fire in quite a few years.
But he was going to need a new—and sturdier—retort.
Fired up with inspiration, Lonnie searched around his farm and found a 20-gallon half barrel—blue, with a tight lid and ring clamp. This was loaded with a new batch of biomass, closed, clamped, and tossed into the woodfurnace firebox.
In a few minutes, white smoke again indicated steam boiling out of the biomass. A few more minutes and this smoke ignited and began to burn with flickering orange and yellow flames. With rapt fascination and frequent photo shutters, Lonnie watched the fiery halo again grow in a ring around the barrel lid.