BioChar (sometimes called AgriChar) is short for Bio-Charcoal and refers to processes that take the carbon that is captured by living plants and turns the biomass into the solid form of charcoal. Such processes appear to offer one of the most promising technologies for carbon capture and sequestration (storing it permanently away).
Although plants are nature's primary way of absorbing carbon dioxide from the atmosphere, they only store the carbon temporarily. When the plant dies and decomposes, the carbon is released back into the atmosphere. Or if the plant is burnt, the carbon is immediately released. BioChar processes take the waste material from food crops, forest debris, and other plant material, and turn it into a stable form that can be buried away permanently as charcoal. In other words, they turn it back into coal.
The processes involved also produce energy as a by-product, creating an alternative energy source, which reduces the need for further fossil fuel burning. Moreover, some of the processes under development use the carbon to create a fertilizer that is plowed back into the land, promoting the growth of further crops.
Details of the BioChar Process
A typical process starts with the plant material undergoing pyrolysis. It is heated in a closed chamber, in the absence of oxygen, breaking down the complex organic molecules into simpler compounds that are released as gases. This is what charcoal makers have done for centuries; but they allowed the gases to escape. With BioChar, the gases are drawn off and put to use. A small proportion of the gases released are used to drive the pyrolysis process.
(The same process occurs with a wood fire. The wood itself does not burn, but as it heats up, inflammable gases are driven off and it is these that burn, further heating the wood, and driving off more gas. This understanding lies behind the building of a good fire. See: The Art of Fire.)
Under the right conditions, the pyrolysis produces hydrogen, which can be used in fuel cells, or used to create ethanol and biodiesel, two valuable alternative fuels. To get an idea of the energy potential of such processes, if (and it is a big "if") the waste from all world's agricultural processes were used in BioChar processes, the alternative fuels produced would be equivalent to the world's total consumption of diesel oil.
Some of the hydrogen is also fed into the Haber-Bosch Process which combines hydrogen with nitrogen from the atmosphere to produce ammonia. The ammonia is then combined with carbon dioxide (from flue gases or from the atmosphere) to produce ammonium bicarbonate, a widely used low-tech fertilizer, which is absorbed into the charcoal. (Note this capture of carbon dioxide is in addition to that already achieved by the plants). The activated charcoal is a good host for beneficial microbes such as mycorrhizal fungi adding to the nutritional value.
Three big advantages of BioChar processes are:
1. They mimic the natural carbon cycle.
2. They are local. They work best on farms or forest areas where the waste plant material is gathered locally, the energy produced is used locally, and carbon fertilizer created can be returned to promote the growth of local crops.
2. The technologies involved are all old-tech, and would be familiar to any chemist or engineer in the early 1900s. They are tried and tested, small-scale, and cheap to manufacture.
If (again that big "if") the process were used on all the world's agricultural waste, the total carbon absorbed and buried away safely as charcoal would equal our current global carbon emissions.