The term “biochar” is an abbreviation for “bio-charcoal” and could more accurately be called “agrichar”. This plant-based carbon is, in fact, traditionally used to improve soil aeration, limit GHG emissions, reduce nutrient leaching and acidity, while potentially increasing water retention. Its application can increase agricultural productivity, provided it is dosed correctly and adapted to the type of soil. Both a carbon sink and a fertilizer, biochar has attracted growing interest, especially since 2018, when the IPCC classified it as one of the “negative emissions” technologies essential for capturing atmospheric CO₂.
Biochar is mostly produced from plant biomass (see List of biomasses allowing certifiable biochar) by pyrolysis, a heating process in the absence of oxygen. Wood residues (residues from forest maintenance, agriculture or the wood industry) or from dry crops are the most suitable because they have few recovery options (incineration, composting, amendment) to store CO2 over the long term. The IPCC thus estimates that 2.6 MdtCO2/year could be sequestered via biochar. These productions can be part of a circular economy dynamic but have difficulty benefiting from a scale effect. Plantations dedicated to the conversion into biochar have thus been created and make it possible to increase their gains and their quality, but may compete with other uses of biomass or land.
Figure 1: Biochar production (Source: SpaceBakery 2021)
The steps for producing biochar by pyrolysis are detailed in FIG. 1. The biomass is dried then heated to high temperature without oxygen and then reduced. The products of these steps are a solid (biochar) composed of 80-90% carbon, synthesis gas (some of which is consumed on site to dry, heat and reduce biomass) and a biooil resulting from pyrolysis. There are multiple ways of carrying out this transformation by pyrolysis (refer to QUALICHAR report from ADEME) affecting its yield, energy consumption, the types of co-products, the quality of biochar and ultimately the overall carbon balance of the process. 1 ton of biochar would thus make it possible to store the equivalent of 1.4 to 3 tons of CO2 (more than charcoal, less rich in carbon) when about 3 tons of biomass are needed to produce it.
Biochar can be added to agricultural soils (at a rate of 1-10 t/ha) and improve their yields, but these gains are only significant for poor and acidic soils (i.e. in developing, tropical countries, and particularly in sub-Saharan Africa). By helping to improve these soils, biochar thus promotes the natural capture of CO2 by plants. Depending on the conditions, biochar will degrade over a period of about a hundred years. Other applications are emerging to reduce emissions: food additive (reduction of enteric methane) and binder based on biochar (low-carbon concrete).
Figure 2: Estimation of the evolution of the costs of technological and natural solutions for the sequestration of atmospheric carbon (source: Carbon removal: how to scale a gigaton industry, McKinsey Sustainability, 2023)
While biochar seems an interesting solution for permanently trapping carbon with a low or even positive footprint, it is little studied in France (in comparison, China publishes 30x more on the subject, the US 10x more and the UK 3x more). A few projects and emerging players for an economy largely focused on the Great Green Wall. The lack of large-scale validated tests and the question of the availability of biomass are holding back investors. The cost of biochar remains high (figure 2) but could become competitive, boosted by the development of the carbon credit market. There is still time for the development of a French sector biochar.
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