Heating biomass in a zero-oxygen environment to temperatures of 250°C or greater yields energy-rich gases and liquids, and a solid charcoal, or char.
When this char has been produced specifically to have beneficial effects - for example as a soil improver or to store carbon - we call this material biochar.
The thermal process used to produce biochar is known as pyrolysis, and by altering the pyrolysis conditions, it's possible to change the character of the biochar. In general higher pyrolysis temperatures mean a smaller amount of char, but containing a greater proportion of highly stable carbon.
This carbon seems to remain sequestered in biochar for centuries, and so sustainable biochar production could be a powerful tool in the fight against anthropogenic climate change.
There is strong evidence that biochar can also have some beneficial effects when added to soils. Its highly porous structure can act like a slow-release 'sponge' for water and useful soil nutrients.
Biochar can be made from almost any type of dry biomass - including waste materials. Therefore, biochar production could be an enormous opportunity for 'closed-loop' type resource management, with numerous valuable benefits.
Biochar, or more accurately other forms of plant and animal char, have been used to enhance soils for millennia.
The strongest evidence of the beneficial effects of char additions to soils are in the terra preta soils of the northern Amazon, where - through native land management - dark, highly fertile soils with very high levels of both stable (char) carbon and organic carbon were established and remain today, thousands of years later.
Char was also added historically to soils in parts of northern Europe (including Netherlands, NW Germany and Belgium). Chars have been, and still are being, used today as soil amendments, for example in Japan and West Africa.