Biochar: the pathway

Biochar is what you get when you cook biomass — crop residues, forestry waste, sewage sludge — in a low-oxygen environment at several hundred degrees Celsius. The carbon that the plant pulled out of the atmosphere ends up locked in a stable, ring-structured solid that resists microbial decay for centuries when applied to soil or used as a filler in concrete and asphalt. It is, by volume, the largest delivered carbon dioxide removal (CDR) pathway today: biochar accounts for the majority of tonnes actually issued on registries like Puro.earth and the European Biochar Certificate (EBC), even as direct air capture attracts more capital per tonne announced.

It matters for durable CDR because it sits in an unusual sweet spot — co-products (heat, syngas, fertiliser benefits) generate revenue that lowers the net cost per tonne, the technology is off-the-shelf, and the resulting carbon is meaningfully more permanent than soil organic carbon or afforestation, though less so than mineralised or geologically injected CO₂.

How it works

The science is pyrolysis: heat biomass to roughly 350–750 °C in the absence of oxygen and the lignocellulose fractures into volatile gases (the “syngas” and bio-oil) and a carbon-rich solid. Higher peak temperature, longer residence time, and slower heating rates increase the fraction of aromatic, condensed carbon — the part that survives in soil. A widely cited synthesis by Lehmann et al. lays out the climate-mitigation accounting and the conditions under which biochar systems are net-negative (Lehmann et al., 2021). A practical engineering review of feedstock-process interactions and how blending feedstocks affects yield is available in Sakhiya et al., 2022.

Once produced, biochar is most commonly applied to agricultural soil, where it can also raise crop yields modestly and reduce nutrient leaching — effects that vary widely by soil type and feedstock, as summarised across 26 meta-analyses by Schmidt et al., 2021 and in the broader review by Joseph et al., 2021. The biochar is the climate product; the agronomic benefit is what makes farmers accept it.

Hydrothermal carbonisation (hydrochar) is an adjacent process for wet feedstocks, with different durability profiles (Khan et al., 2021).

Who’s doing it

The supplier base is unusually geographically distributed because feedstock is local and shipping biochar is uneconomic.

  • NetZero (France, ~207 FTE) runs decentralised >600 °C pyrolysis plants in Cameroon, Brazil and elsewhere, processing coffee husks and sugarcane residues.
  • Pyreg (Germany) sells modular PX500/PX1500 reactors designed for sewage sludge and biomass; the company has been a default hardware vendor for European EBC-certified projects since 2009.
  • MASH Makes (Denmark/India) operates containerised plants converting cashew-industry waste in Karnataka, with biofuel as a co-product.
  • Aymium (US) targets a different market entirely — high-purity “biocarbon” as a drop-in replacement for metallurgical coke, with a joint venture (TerraForge) announced in late 2025.
  • Bio-Logical (Kenya) claims Africa’s largest biochar plant at Kabati, blending output into an organic fertiliser distributed to smallholders.
  • Syncraft (Austria) has deployed 45+ wood-gasification “reverse power plants” where electricity and heat are the primary products and biochar is the co-product.
  • Carbo Culture (Finland) operates its Carbolysis reactors, with offtake deals that have included Microsoft.
  • AquaGreen (Denmark) focuses on the wet-feedstock problem — superheated-steam drying integrated with 650 °C pyrolysis of sewage sludge, a feedstock most kilns can’t handle.

Around these are dozens of smaller operators (Carbon Remove, Dark Earth Carbon, Husk, ARTi, BluSky Carbon, AgroCCS) running single-site projects from a few hundred to a few thousand tonnes per year.

The durability question

The honest answer is: it depends, and the field is still arguing about the right discount factors. Biochar’s stability in soil is a function of its H/C-organic ratio (lower = more aromatic = more stable), the soil environment, and temperature. Puro.earth’s methodology and the EBC use H/C thresholds (typically <0.7) and apply a default 100-year permanence factor in the 60–80% range depending on production temperature. A reasonable upper-bound estimate from the literature is centuries to a millennium for the recalcitrant fraction, with a labile fraction (often 10–30%) decomposing within years (Lehmann et al., 2021).

The harder questions for buyers:

  • Counterfactual. If the feedstock would otherwise have been burned in the open or composted, the net removal is real. If it would have stayed in a forest, the accounting is far weaker.
  • Leakage and life-cycle emissions. Transport, drying energy, methane from pyrolysis off-gas if not flared or used — these can erode the net removal by 10–30%.
  • Application-side measurement. Once biochar is in soil, nobody actually measures its decomposition in situ at the project level; durability is inferred from lab-derived kinetic models. Effects on the surrounding soil organic carbon pool are genuinely uncertain (Singh et al., 2021).

measurement, reporting, and verification (MRV) is consequently production-side: feedstock mass, pyrolysis temperature, H/C ratio of output, and application records. It is more auditable than soil-carbon credits, less so than CO₂ flowing into a Class VI well.

What to watch over the next year

  • Whether buyers continue to accept production-side MRV or push for field validation, especially as larger volumes flow.
  • Standardisation between Puro.earth, the EBC C-Sink, and Verra’s emerging methodology — currently the same tonne can carry different permanence factors.
  • Whether the metallurgical biocarbon route (Aymium, CHAR Technologies, Airex) scales meaningfully — it could pull biochar economics away from agronomic markets entirely.
  • Sewage-sludge biochar regulation in the EU, where phosphorus recovery and PFAS destruction are becoming co-arguments.
  • The fate of price: durable biochar credits have transacted between roughly $130 and $600/tonne in 2023–2025. Whether that band tight