One of biochar's strongest arguments as a carbon removal method is permanence. Unlike a forest that can be logged or burned, high-quality biochar can lock carbon away for hundreds to thousands of years. But "high-quality" is doing a lot of work in that sentence - biochar permanence varies significantly with feedstock, production temperature, and pyrolysis process. Standards quantify this variation through laboratory testing, and the results directly affect how many credits your project generates.
To see that effect numerically, use this guide alongside the biochar carbon calculator. The permanence discount in the tool shows how durability assumptions translate into a lower or higher net tCO₂e outcome.
What is permanence in biochar?
Permanence refers to the fraction of sequestered carbon that remains locked in the biochar over a 100-year timeframe - the standard assessment period used in carbon crediting. Biochar carbon exists in a complex mixture of aromatic and aliphatic organic structures. The aromatic structures (the graphite-like carbon rings formed at high pyrolysis temperatures) are highly resistant to decomposition. The aliphatic structures are more reactive and will decompose more quickly.
The H/Corg ratio - the primary permanence indicator
The hydrogen-to-organic-carbon ratio (H/Corg) is the most widely used proxy for biochar permanence. It measures the degree of aromatisation: lower H/Corg means more aromatic, more stable carbon. The relationship is well-established in the scientific literature and forms the basis of permanence assessment in both Puro.earth and Verra methodologies.
H/Corg is measured by elemental analysis of the biochar - a standard laboratory technique. The result is used to assign the biochar to a permanence class:
| H/Corg ratio | BC+100 (fraction retained) | Puro.earth class | Permanence discount |
|---|---|---|---|
| < 0.4 | > 95% | Class 1 (Highest) | 5% |
| 0.4 – 0.6 | 80 – 95% | Class 2 | 10 – 20% |
| 0.6 – 0.7 | 60 – 80% | Class 3 | 20 – 40% |
| > 0.7 | < 60% | Not eligible | N/A - rejected |
The permanence discount is applied to the gross credit yield: if your biochar stores 1 tCO₂e but has an H/Corg of 0.55 (Class 2, BC+100 = 87%), the verifiable credit yield is 0.87 tCO₂e, not 1 tCO₂e. The remaining 0.13 tCO₂e is discounted to account for expected decomposition.
Once the permanence picture is clearer, the next practical question is which methodology route best fits the project and whether the expected durability is strong enough to support premium buyer positioning.
What drives the H/Corg ratio?
The H/Corg ratio of the produced biochar is primarily determined by pyrolysis temperature. Higher temperatures drive off more hydrogen and oxygen, leaving a more aromatic, more stable carbon structure. In practice:
- Above 700°C - typically produces Class 1 biochar with H/Corg below 0.4. High-temperature pyrolysis using gasifiers or kilns designed for biochar CDR.
- 500–700°C - typically Class 1 or 2 depending on residence time and feedstock. Most modern biochar kilns operate in this range.
- 400–500°C - Class 2 or 3. Lower-temperature pyrolysis produces more biochar by weight but of lower permanence. May still be economic at Class 2 prices.
- Below 400°C - typically Class 3 or ineligible. Slow pyrolysis at low temperatures or traditional charcoal production often produces high H/Corg biochar with limited permanence.
Feedstock also matters, though less than temperature. Woody feedstocks (hardwood, forestry residues) generally produce more stable biochar than high-ash agricultural residues (rice husk, straw) at the same temperature. Sewage sludge biochar tends toward higher H/Corg due to its inorganic content interfering with carbon analysis - additional characterisation is often required.
O/Corg and additional characterisation
Some standards also use the oxygen-to-organic-carbon ratio (O/Corg) as a supplementary permanence indicator. Gold Standard requires O/Corg below 0.4 for biochar credit issuance. High O/Corg indicates incompletely pyrolysed material with more reactive oxygen-containing functional groups - less stable carbon.
For biochar with borderline H/Corg ratios, additional characterisation methods are sometimes used:
- Thermogravimetric analysis (TGA) - measures mass loss at elevated temperatures, an indicator of thermal stability
- Recalcitrance index (R50) - measures resistance to oxidative decomposition, an alternative permanence proxy
- Volatile matter content - simpler proxy, inversely correlated with stability
Puro.earth permanence classes and credit pricing
Puro.earth assigns biochar projects to permanence classes based on average H/Corg measured across certified batches. Projects must maintain consistent production conditions to maintain their class - significant changes to feedstock, temperature, or process require re-characterisation.
Permanence class affects both credit eligibility and, in some cases, price. Class 1 biochar commands the highest prices from buyers specifically targeting high-permanence CDR - typically £80–£120/t in 2026. Class 2 trades at a modest discount (£60–£90/t). Class 3 has limited buyer demand outside specialised markets.
Verra VM0044 approach
Verra's biochar methodology (VM0044) uses the H/Corg ratio for permanence assessment but structures the discount differently to Puro.earth. Instead of fixed permanence classes, VM0044 applies a continuous permanence fraction based on the H/Corg measurement, multiplied by a climate-dependent soil stability factor (warmer, wetter climates have slightly faster decomposition rates).
VM0044 also requires tracking and reporting of all biochar applications, with GPS coordinates for field applications where possible. This makes traceability more demanding than Puro.earth but also provides stronger defensibility against reversal claims.
Practical implications for project design
If you are designing a biochar project, H/Corg optimisation should be part of your process design from the start:
- Design your kiln for Class 1 production - if your process temperature can reliably achieve H/Corg below 0.4, you maximise both credit yield and price. The cost of achieving higher temperatures is usually offset by the higher credit values.
- Test every batch - standards require batch-by-batch H/Corg testing. Budget for this in your cost model. Typical lab costs are £50–£150 per sample for elemental analysis.
- Characterise your feedstock first - different feedstocks produce different biochar at the same temperature. Test your specific feedstock at your intended process temperature before committing to production targets.
- Account for the permanence discount in your credit calculations - many first-time developers calculate gross tCO₂e without applying the permanence discount, then are surprised when net credits are 5–20% lower than expected.
The Carbon Workbench biochar calculator includes a permanence discount slider so you can model the effect of different H/Corg outcomes on your net credit yield:
Use the full tool in The Carbon Workbench for saved calculations, PDF reports, and a quicker route into methodology, pricing and verification once permanence assumptions are clearer.
Use full tool in The Carbon Workbench →Permanence in other carbon project types
Permanence is not unique to biochar - it is a concern across all carbon project types, addressed differently in each:
- Forestry / ARR - forests can be logged, burned, or degraded. Addressed through buffer pools (a percentage of credits withheld in reserve) and reversal monitoring. Verra maintains a pooled buffer account across all forestry projects.
- REDD+ - similar to forestry, with additional leakage and baseline risks. Buffer pools of 10–30% are typical.
- Soil carbon - highly impermanent; soil carbon can be released by tillage, erosion, or land use change. The least permanent major project type.
- Cookstoves / solar - these are avoidance credits, not removal credits. Permanence in the conventional sense does not apply - the avoided emission is permanent by definition.