Osa Biochar Project Kiln

The kiln built for the Osa Biochar Project was designed by Nikolaus Foidl, through association

with the International Biochar Initiative (IBI). It is a moderate size retort kiln (4M3 capacity) with a removable lid and basket for loading/unloading by means of an overhead hoist. It's a clever design, with a central chimney that passes through the retort from the wood-fired primary combustion chamber below. Once pyrolysis is underway, secondary air inlets promote combustion around the sides of the retort as well, for more even heating.

A unique feature of this kiln design is a water jacket for condensing pyrolignous acids (wood vinegar) and lower temperature pyrolysis gases. Water is circulated through the jacket while temperatures climb through the 150-280C range, and the condensate is routed to an exit spout with a dip tube immersed in water to further condense the smoke. These "smoke chemicals" have been demonstrated to stimulate seed germination and promote plant growth, and are now considered the main actors in fire-response vegetation such as chaparral. (Back in the day when I was studying this stuff, heat of the fire was held to be the trigger that stimulated germination and sprouting.)

Once temperatures in the load have reached 300C, the reaction became self-sustaining (no more need to stoke the firebox with wood), and the chimney outlet holds a strong flame. A few hours later, pyrolysis burns out, and the load needs to cool before exposing to air. The char is then crushed and screened. First production from the new kiln is being used in plant growth field trials conducted by CATIE, the Costa Rican agricultural research institution.

The kiln will eventually be set up at the site of the Sustainable Agricultural Center at La Palma, where diverse feedstocks can be processed into biochar for local farmers. It may be fitted with additional apparatus to utilize waste heat for crop drying and pre-drying on-deck kiln loads. A specific need that has already been identified is powering a drying kiln for timber bamboo grown by the local "Amigos de Bamboo" agricultural cooperative--a key step in promoting commercialization of locally cultured bamboo as an alternative to harvesting rainforest trees for construction.