biochar is charcoal made from biomass, from agricultural waste or other plant material. If left to rot or burned, the carbon trapped in this plant material would return to the atmosphere. By turning it into charcoal, a large percentage of the carbon is fixed into a stable form for decades.
Turning plant material into charcoal is a straightforward process: heat without sufficient oxygen to burn. This process is called pyrolysis (from the Greek pyro meaning fire and lysis meaning separating). In ancient times this was accomplished by burying smoldering wood under a layer of dirt, cutting it off from air. More recently, a kiln provided a more efficient way to produce charcoal by heating wood without burning it. Modern methods generally use sealed heating chambers in order to capture all of the produced gases.
Pyrolysis produces three outputs:
the solid char, which has a much higher concentration of carbon than the original plant material.
a thick tar referred to as bio-oil, which is much higher in oxygen than petroleum but otherwise similar.
a carbon-rich gas called syngas. It is flammable, though it contains only about half the energy density of methane. In earlier times the gas generally just escaped, while modern processes capture and usually burn it as heat to continue the pyrolysis process.
The temperature and length of pyrolysis determines the relative quantity of char, bio-oil, and syngas. Baking for longer time at lower temperature emphasizes char, shorter times at higher temperature produces more gas and oil.
The idea of biochar for carbon capture is to intercept carbon about to return to the atmosphere, primarily agricultural waste, and turn it into a form which both sequesters carbon and improves the soil into which it is tilled. The very fine char produced from agricultural waste is quite porous and makes soil retain water more effectively. It can also improve the soil health of acidic soils, balancing the pH and making the soil more productive.