Biochar Production Units

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.

Biochar production systems are generally classified as either pyrolysis or gasification systems.

Pyrolysis Systems

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

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.

Scale and Variety of Units

Gasification and pyrolysis production systems can be developed as mobile or stationary units.

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.

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.

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

Opportunities for Advanced Biochar Production

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:

1. Continuous feed pyrolyzers to improve energy efficiency and reduce pollution emissions associated with batch kilns.
2. Exothermic operation without air infiltration to improve energy efficiency and biochar yields.
3. Recovery of co-products to reduce pollution emissions and improve process economics.
4. Control of operating conditions to improve biochar properties and allow changes in co-product yields.
5. Feedstock flexibility allowing both woody and herbaceous biomass (like crop residues or grasses) to be converted to biochar.

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.