Biochar carbon removal

Photosynthesis takes carbon from the atmosphere in one growing season, and decomposition hands it back almost as fast. Biochar removal interrupts the return path: biomass is heated without oxygen until part of its carbon becomes a solid that microbes break down orders of magnitude more slowly than the original wood or straw. Nothing is stored in a tank or a well. Durability is a property of the carbon’s own chemical structure, and everything else in the route follows from it.

What pyrolysis does to carbon

Biomass is cellulose, hemicellulose and lignin, each with its own decomposition window — hemicellulose first, cellulose near 300–400 °C, lignin across the broadest range and contributing most of the char. As temperature rises, water, carbon dioxide, carbon monoxide, methane and tars leave the solid. What remains loses hydrogen and oxygen faster than carbon: dehydration, demethylation and ring closure reorganise the residue into clusters of fused aromatic rings — small, disordered graphene-like sheets stacked imperfectly. The persistence follows from this structure. Soil microbes dismantle polysaccharides with hydrolases acting on hydrolysable bonds; condensed aromatic carbon offers neither substrate nor bond type. Its carbon–carbon bonds are among the strongest in organic chemistry, and no common soil enzyme cleaves them at meaningful rates. Char is therefore preserved not by anoxic conditions, as peat is, but by its own structure: Amazonian dark earth, enriched with charcoal centuries to millennia ago, is still black and carbon-rich today.

Hydrogen as the telltale

Aromatisation is invisible, but it leaves a trace in elemental composition: as clusters fuse, hydrogen is stripped faster than carbon and the molar H:C ratio falls. Fresh wood sits near 1.5; well-carbonised char drops below roughly 0.4; certification methodologies treat values above about 0.7 as insufficiently persistent to count as durable carbon. The ratio is a proxy, not a half-life. Field persistence depends on soil, climate and char properties, and pool models consistently show a small labile fraction decaying within years to decades alongside the aromatic majority with mean residence times of centuries. A low H:C ratio says the labile fraction is small, not that it is absent — which is why permanence is issued as classes with probabilities, not certainties.

Why permanence sets the credit value

The cost is that the same conditions which aromatise carbon also destroy yield. Hotter, longer pyrolysis converts more of the remaining carbon to gas: mass yield falls from roughly a third of dry biomass toward a fifth or less, even as the char’s carbon content climbs toward 90 percent. Each tonne of durable carbon is bought with tonnes of carbon sent up the flue. Accounting follows: only the fraction diverted from the fast biological cycle into the slow one is removal, and carbon that returns to the atmosphere — unconverted or burned — is not removal. Registries and buyers therefore price durability, not tonnage. Soil-applied char carries the residual risk of its labile fraction and of land disturbance; bio-oil injected into geologic formations approaches permanent storage and clears accordingly. The limiting quantity of the route is the share of feedstock carbon the condensation chemistry can hold onto — and raising it costs energy, yield, or both.