Biochar not only captures carbon dioxide but when spread on top soil can promote healthy, nutritious soil

Defining Biochar

Biochar is thermally decomposed plant matter produced by pyrolysis - the exposure to elevated temperatures (400-500ºC) and a low oxygen environment.

A pile of processed biochar

Biochar as a Carbon Sink

Plants are made out of biomass consisting of about 50% carbon (C). They get the C from carbon dioxide (CO₂) in the air by performing photosynthesis in which CO₂, water (H₂O) and light energy is converted into organic molecules (glucose, cellulose and lignin) and oxygen (O₂) - this is how plants bind CO₂.

If the plant is burned or decomposes, the biomass will be converted back into CO₂ which is released back into the atmosphere. To remove CO₂ permanently from air, the biomass must not burn or decompose and thus remain in the form of the living plant, wood for applications like buildings or in the form of biochar.

If plant biomass undergoes pyrolysis about half of the carbon escapes in the form of gas and the other half is transformed into a stable solid form of carbon - biochar - that degrades extremely slowly under natural conditions. Biochar only becomes a long-term carbon sink when it reaches the topsoil - often it will reach the topsoil after or due to agricultural applications like soil conditioning, livestock bedding, compost additive and feed additive - or when it becomes part of concrete or similar long-lasting composite materials.

Rate of Degradation of Biochar

Given that the biochar is not burned the carbon in the biochar remains in the system for several centuries. Introduced into soils, an average degradation rate of 0.3% per year can be assumed1. This means that after 100 years 74% of the carbon will still be sequestered.

Biomass source

Biochar is only a carbon sink if it is produced from biomass, which is either regrown or would be burned or let to rot otherwise like residual materials. In other words the biomass can’t be taken from existing carbon sinks and reduce them in the process of harvesting it. Otherwise the production of biochar in total will produce more emissions than it removes.

Life-cycle analysis

Greenhouse gas emissions produced in the life cycle of biochar have to be deducted from the produced negative emissions. This includes energy and fuel emissions for harvesting, transportation to pyrolysis plant and preparation of biomass; external energy for operating the pyrolysis plant and methane emissions during pyrolysis; and post-pyrolysis treatment and transport to long-term storage location of biochar2.

  1. Experimental evidence for sequestering C with biochar by avoidance of CO2 emissions from original feedstock and protection of native soil organic matter ↩︎

  2. European Biochar Certificate ↩︎

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