Pyrolysis
Heat without oxygen
Pyrolysis is the reaction that turns plant residue into carbon that lasts. Take away the oxygen, add heat, and biomass cannot burn — it rearranges. What comes out is decided by how you run the reactor.
The reaction
Combustion releases carbon. Pyrolysis traps it.
Burn wood and its carbon meets oxygen, becomes CO₂ and leaves. Heat that same wood in a sealed reactor with almost no oxygen and the carbon has nowhere to go. The long cellulose and lignin chains break apart; hydrogen and oxygen escape as gas and vapour; and the carbon left behind reorganises into fused aromatic rings — flat, stacked, and remarkably hard for soil microbes to digest.
That structural change is the whole basis of biochar's permanence. It is also why the reactor conditions are not an operational detail but the product specification: temperature and residence time decide how aromatic the carbon becomes, and therefore how long it lasts.
Inside the reactor
What happens, and at what temperature
100–200 °C
Drying
Free water leaves the biomass. Nothing chemical has happened yet — but the energy this costs is why feedstock moisture dominates the energy balance of the whole process.
220–315 °C
Devolatilisation begins
Hemicellulose breaks down first. Volatile organic compounds, water vapour and CO₂ leave as gas; the solid darkens and begins losing mass.
315–400 °C
Carbonisation
Cellulose pyrolyses in this window, and lignin decomposes slowly across a much broader range. Oxygen and hydrogen are stripped away and the remaining carbon begins reorganising into fused aromatic rings. This is where biochar is actually made.
400–700 °C
Aromatisation
Rings condense further into stacked sheets. Carbon yield falls, but what remains is more aromatic, more stable — a lower H/C ratio and longer permanence.
The European Biochar Certificate requires a molar H/C ratio below 0.7 for material to be called biochar at all — the threshold that separates carbonised material from merely toasted biomass. Where a batch lands below that line is set here, in the reactor.
Regimes
Same chemistry, different product
Pyrolysis is a family of processes, and the settings decide which product dominates. Only one of them is optimised for durable carbon.
| Regime | Temperature | Residence time | Main product | Used for |
|---|---|---|---|---|
| Slow pyrolysisours | 450–700 °C | Minutes to hours | Biochar (25–35% of dry mass) | Carbon removal — the regime we operate |
| Fast pyrolysis | 450–600 °C | Seconds | Bio-oil (60–75%) | Liquid biofuel and chemicals |
| Gasification | 700–1,000 °C+ | Seconds | Syngas — little solid carbon | Energy; a poor route to durable carbon |
| Torrefaction | 200–300 °C | Tens of minutes | Energy-dense solid, not biochar | Fuel pellets — H/C stays far above 0.7 |
Yields are indicative ranges from the process literature; actual output depends on feedstock, moisture, particle size and reactor design.
The other two thirds
Nothing should leave the reactor unused
Solid — biochar
The carbon that stays. Roughly a quarter to a third of the dry input mass in slow pyrolysis, and the only fraction that carries the removal claim.
Gas — pyrolysis gas
A combustible mixture of hydrogen, methane and carbon monoxide, alongside a substantial non-combustible CO₂ fraction. (Syngas proper is a gasification product.) Burned to sustain the reaction and supply process heat, which is what makes a well-run plant largely self-fuelling.
Liquid — bio-oil and water
Condensable vapours. Minor in slow pyrolysis, dominant in fast. Usable as fuel or feedstock; either way, not vented.
This matters for the carbon accounting, not just the economics. Every joule of fossil energy a plant imports, and every gram of methane it vents, is deducted from the removal — which is why the energy design of the reactor is part of the climate claim, not separate from it.
Other uses
Pyrolysis is not only for carbon
The same reaction underlies several industries. Charcoal production is the oldest. Fast pyrolysis is pursued as a route to liquid biofuels and bio-based chemicals. Plastic pyrolysis breaks polymers back down into hydrocarbon feedstock. And engineered carbons — activated carbon for filtration, hard carbon for batteries — start from the same carbonisation step, then go further.
What separates carbon-removal pyrolysis from all of these is intent and accounting. The others optimise for a product; we optimise for carbon that stays put, and then prove it. Same reactor physics, entirely different success criterion.
FAQ
Questions people ask
The reactor is where permanence is decided
Which is why we measure every batch instead of trusting the recipe.