Biochar as Durable CDR
Core pathway
- Photosynthetic capture — Biomass stores atmospheric CO₂ (~45–50% C dry matter, depending on feedstock).
- Thermochemical conversion — Pyrolysis or gasification (typically >350 °C, limited/no O₂) converts a substantial fraction into an aromatic carbon matrix (biochar); gases/oils must be accounted for in LCA.
- Durable storage — Soil application, burial, or materials (concrete, asphalt, composites) so that a large fraction of organic C does not return to the atmosphere on multi-decadal to millennial timescales.
This is carbon dioxide removal (CDR) / biochar carbon removal (BCR) when the credited outcome is net atmospheric CO₂ stored in a durable reservoir, after subtracting lifecycle emissions and counterfactual storage.
CDR vs avoidance
Avoidance / reduction claims (displacing fossil fuels with pyrolysis heat; avoiding methane from waste) are a different claim. Most modern biochar removal methodologies deliberately credit durable storage of biochar carbon, not fossil displacement. Mixing removal and avoidance without clear boundaries is a common greenwashing critique—see Risks and Controversies.
Why treated as durable
- Higher pyrolysis severity → lower H/C_org (and often O/C_org) → slower mineralization.
- IPCC 2019 · estimates soil C stock change from biochar F_perm ≈ 0.65 / 0.80 / 0.89 by temperature class at ~20 °C baseline.
- models soil GHG from biochar refined with H/C_org + soil temperature. DOI: 10.1021/acs.est.1c02425
- Petrographic approaches (Ro / inertinite) argue millennial persistence for highly condensed fractions (inertinite permanence benchmark; DOI: 10.1016/j.coal.2023.104409).
Important: Standards do not typically credit 100% of biochar C—they apply permanence factors, LCA deductions, leakage, and (in some programs) mandatory offsetting of process emissions.
Soil / materials pathways
Dominant voluntary-market pathway for ag operators: soil application (direct spreading, compost/manure/digestate blends, or bedding/feed routes where allowed). Non-soil pathways (concrete, asphalt) also generate credits under Verra, Puro, Isometric, and CSI with different end-use evidence.