Equipment for making biochar can be as simple as a primitive campfire or as complex as a modern bio-refinery. The basic process is called pyrolysis.
Pyrolysis is the breaking down (lysis) of a material by heat (pyro). As the material is broken down, it releases gas. This is the first step in the combustion or gasification of biomass.
All the processes involved in pyrolysis, gasification, and combustion can be seen in the flaming match. The flame provides heat for pyrolysis, and the resulting gases and vapors burn in the luminous zone in a process called flaming combustion, leaving behind char. After the flame passes a given point, the char may or may not continue to burn. When the match is put out, the remaining wood continues to bake, or pyrolyze, releasing a smoke composed of condensed tar droplets as it cools.
Read the article Campfire Lessons: Breaking Down the Combustion Process to Understand Biochar Production by Mark R. Fuchs, M. Garcia-Perez, P. Small and G. Flora published in the Biochar Journal, 2014 for a good step by step explanation of how biochar is formed.
Pyrolysis systems use kilns and retorts and other specialized equipment to contain the baking biomass while excluding oxygen. The reaction vessel is vented, to allow pyrolysis gases to escape. Pyrolysis gases are often called “syngas”. The process becomes self-sustaining as the syngas produced is combusted, and heat is released.
There are two types of pyrolysis systems in use today: fast pyrolysis and slow pyrolysis. Fast pyrolysis tends to produce more oils and liquids while slow pyrolysis produces more syngas.
Gasification systems produce smaller quantities of biochar in a directly-heated reaction vessel with introduced air.
The more oxygen a production unit can exclude, the more biochar it can produce. Biochar production is optimized in the absence of oxygen.
Gasification and pyrolysis production systems can be developed as mobile or stationary units.
At the local or regional level, pyrolysis and gasification units can be operated by co-operatives or larger industries, and can process up to 4,000 kg of biomass per hour. Small scale gasification and pyrolysis systems that can be used on farm or by small industries are commercially available with biomass inputs of 50 kg/hr to 1,000 kg/hr.
Biochar ovens are low tech biochar production units with a primary design function of producing biochar. This category of biochar production unit can be suitable for clean, healthy, distributed low tech biochar production (DLT) of by developing country smallholders and micro-entrepreneurs; “backyard” producers utilizing yard waste; small and urban farmers; nurseries; communal gardens; etc to convert the thinly distributed feedstock (TDF) available to them. The feedstock ‘chamber’ of these units will usually be in the range of very small to 4-500 liters. The primary functional design of these units is to produce biochar. To date, the primary technologies used in units that fall within this category are retorts; Top Lit Up Draft (TLUD) units and TLUD/retort hybrids; and Top Fed Open Draft (TFOD) units such as cones and pyramids (metal & pit) and rings. Other functional designs may fall within this category (based on size, primary design function, and technology level) as they are developed. The name of this category is based on the metaphor that, as a bread oven is a unit used to bake dough to produce bread, a biochar oven is unit used to bake feedstock to produce biochar. Open-source designs for ovens can make them a very economical choice.
Charcoal-making stoves show promise of bringing low-cost biochar to rural areas. Biochar production can help build soils and provide households with new opportunities to earn income.
There is also potential to develop stoves and furnaces for urban and suburban use that gasify biomass and leave behind charcoal. Such stoves could cook food and heat water while they make biochar for gardens and landscaping.
For a detailed look at pyrolysis and gasification technologies, see Chapter 8: Biochar Production Technology, by Robert Brown (Iowa State University, Ames) of Biochar for Environmental Management.
Dr. Brown outlines some specific goals for advanced biochar manufacture:
Some technologies that hold promise for helping achieve these goals include drum pyrolyzers, rotary kilns, screw pyrolyzers, the flash carbonizer, fast pyrolysis reactors, gasifiers, hydrothermal processing reactors, and wood-gas stoves, all of which produce varying quantities of gas and liquids along with biochar.
Since 2007, the Washington Department of Ecology and the Center for Sustaining Agriculture and Natural Resources (CSANR) at Washington State University have produced a series of in-depth reports on biochar production, use, and economics. The interest in biochar grew out of a state solid waste management plan called Beyond Waste that created the Organic Waste to Resources project, charged with examining ways to use nearly 17 million tons of organic waste identified in Washington State. A large portion of this waste is ligno-cellulosic waste from wood and straw, where pyrolysis is an attractive option for recovering energy and producing stable carbon that can benefit soils and climate.