A further method that uses the CO2 uptake of plants for the long-term sequestration of carbon dioxide is the production of a type of charcoal known as biochar. In a process called pyrolysis, vegetable biomass or waste plant matter is heated to several hundred degrees Celsius in the absence of oxygen to leave a solid, coal-like substance. Biogas and bio-oil are produced as by products and can be used as a substitute for fossil fuels. Materials that can be subjected to pyrolysis include crop residues, cuttings, livestock manure, slurry, sewage sludge and other organic waste. Unlike charcoal, biochar is not meant for combustion. Instead, it can be worked into arable soils, where it remains in the ground for a long time. This is because its stable structure means it breaks down very slowly. The carbon contained in it thus remains sequestered for long periods. Biochar also improves soil properties, in that water and nutrients are better retained in the ground. This enables more biomass to be produced, which could additionally counteract climate change.
An advantage of biochar over BECCS is that it does not necessarily require crops to be grown specifically for the purpose as it can also be produced from waste plant matter. This avoids direct competition with food production. A slightly different picture nevertheless emerges if biochar is to be used on a large scale as a CDR method. To remove several billion tonnes of CO2 from the atmosphere each year, several hundred million hectares would have to be planted with miscanthus or similarly fast-growing species. These hardly produce any food, however, and there would consequently again be competition with food production in a similar way to BECCS. One basic advantage of biochar is that the process is technically easy to implement, so in many countries – and notably in emerging and developing countries – it can be produced locally using small installations.
Potential and scale
According to various scientific studies, if the biochar process were to be developed in the years ahead, between 0.5 and two billion tonnes of CO2 a year could be removed from the atmosphere in future worldwide. A decisive factor, however, is whether biochar is produced from waste plant matter alone or if fast-growing crops have to be cultivated specifically for the purpose. For CO2 to be sequestered on the scale of billions of tonnes, that would require a very large area of land. But biochar could still contribute to a certain extent even if it were to be produced from waste plant matter alone. There are estimates that about ten percent of annual European CO2 emissions could be offset by turning biomass waste into biochar throughout Europe. This would not need additional land.
Application readiness and research needs
Biochar is not only good for improving soil quality. Research is also being conducted into the use of the waste heat from biochar facilities and the use of biochar as a fertiliser carrier, as a substitute for sand in building materials, and as carbon for medical applications. With regard to the bio-oil component, research is being done on using it to make bioplastics. The technology for producing biochar is mature and has already been developed to an industrial scale in large pilot plants. However, it is unclear how much biochar can be spread on agricultural land or in the environment in general. While it has the potential to sequester carbon for the long term, there is still a lack of detailed studies on how it behaves in large quantities in the environment.