How the carbon credit estimate is worked out
BNNZ shows an estimate of the carbon that Producers have stored in New Zealand soils. Every batch is worked out the same way. What differs is whether its key numbers were measured in a laboratory or taken from cautious standard values. This page shows exactly how, and where each number comes from.
These are estimates to show what a New Zealand voluntary carbon scheme could recognise. They are not certified, verified or issued credits, and must not be used as carbon claims.
The steps
- How much biochar was made. The weight, or the volume multiplied by a bulk density (200 kg per cubic metre is assumed if none was measured).
- How much of it went onto or into soil. Carbon only counts as stored once it has been applied. This comes from the applications a Producer has recorded against the batch, or the percentage of the batch they say was applied.
- How much carbon that is, and how stable.
Carbon stored = Biochar applied (tonnes) × F_C × F_perm
This is the IPCC 2019 Refinement Tier 2 method as adapted for New Zealand in MPI Technical Paper 2026/11. F_C is the organic carbon fraction and H:Corg is a stability ratio (lower is more stable).
F_perm = min(1.075 − 0.61 × H:Corg, 1.0) - Carbon to carbon dioxide equivalent. Multiply by 44/12 (about 3.67).
- Be cautious unless the batch is verified. F_C and H:Corg come from a laboratory test when the batch was made under a production profile that BNNZ has verified (the lab certificate was checked). Lab values typed onto a single batch are not relied on. Otherwise the standard values in the tables below are used, and 10% is taken off the carbon counted. This is why verification earns more.
- Subtract the emissions of making it. The fuel, electricity and transport recorded for the batch are converted to CO₂e with the factors below and deducted.
- Result: the estimated carbon credit in tonnes of CO₂e.
Feedstock → standard organic carbon fraction (F_C)
| Feedstock type | F_C | Source |
|---|---|---|
| Herbaceous | 0.6500 | Verra VM0044, Table 4 (default organic carbon fraction for herbaceous feedstock, pyrolysis). |
| Wood - mixed softwoods | 0.7700 | Verra VM0044, Table 4 (default organic carbon fraction for wood feedstock, pyrolysis) — VM0044 does not distinguish softwood from hardwood. |
| Wood - mixed hardwoods | 0.7700 | Verra VM0044, Table 4 (default organic carbon fraction for wood feedstock, pyrolysis) — VM0044 does not distinguish softwood from hardwood. |
| Wood - mixed | 0.7700 | Verra VM0044, Table 4 (default organic carbon fraction for wood feedstock, pyrolysis). |
| Nut shells/pits/stones | 0.7400 | Verra VM0044, Table 4. |
| Mixed/Other | 0.3500 | No single feedstock — VM0044 requires using the most conservative (lowest) value among the defined categories when feedstock is mixed or unknown; currently equal to biosolids. |
| Manure | 0.3800 | Verra VM0044, Table 4. |
| Biosolids | 0.3500 | Verra VM0044, Table 4. |
| Hemp | 0.4900 | Verra VM0044, Table 4 (listed as rice husks/straw). |
| Bones | 0.3500 | Lowest value until better value found |
| Textiles | 0.3500 | Lowest value until better value found |
| Crop residue (straw, corn cobs, rice husks, etc.) | 0.4900 | Verra VM0044, Table 4 (listed as rice husks/straw). |
Production method → standard H:Corg ratio
| Method | H:Corg | Source |
|---|---|---|
| Open pit / trench burn | 0.6000 | No direct H:Corg measurement found for open pit/trench burns. Extrapolated conservatively (close to the 0.7 hard-reject threshold) from Namaswa et al. 2023 (Bioresource Technology Reports), who report low fixed-carbon (28-44%) and low peak temperatures (200-500C) for this method. |
| Flame cap kiln (bath, kontiki, warmheart, drum, ring of fire) | 0.2800 | Pandit et al. 2017, PLOS ONE 12(4):e0176378 — groups flame-curtain/TLUD/gasifier kilns together at ~90% modelled 100-year stability (similar peak temperatures). |
| TLUD (top-lit updraft) stove / metal container | 0.3200 | Pandit et al. 2017, PLOS ONE 12(4):e0176378 — groups flame-curtain/TLUD/gasifier kilns together at ~90% modelled 100-year stability (similar peak temperatures). Set slightly less favourable than flame-curtain given TLUDs showed wider variability in Namaswa et al. 2023's fixed-carbon rankings depending on draft/insulation. |
| Retort / batch pyrolysis kiln | 0.2700 | Pandit et al. 2017, PLOS ONE 12(4):e0176378 — groups flame-curtain/TLUD/gasifier kilns together at ~90% modelled 100-year stability (similar peak temperatures). Set slightly less favourable than flame-curtain given TLUDs showed wider variability in Namaswa et al. 2023's fixed-carbon rankings depending on draft/insulation. |
| Continuous / industrial pyrolysis reactor | 0.2500 | Weakest sourcing in this table: no reliably-verifiable direct citation found for industrial continuous pyrolysis H:Corg. Reasoned from the general, well-supported principle that higher, more consistently controlled pyrolysis temperature produces more stable (lower H:Corg) biochar (e.g. Hagemann et al. 2025, PLOS ONE 20(9):e0330206, wood biochar trending from H/C 0.43 at 400C to 0.10 at 800C) — kept conservative rather than assuming the lowest end of that range. |
| Wood pile / stack - extinguished when flames finished | 0.2500 | Weakest sourcing in this table: no reliably-verifiable direct citation found for industrial continuous pyrolysis H:Corg. Reasoned from the general, well-supported principle that higher, more consistently controlled pyrolysis temperature produces more stable (lower H:Corg) biochar (e.g. Hagemann et al. 2025, PLOS ONE 20(9):e0330206, wood biochar trending from H/C 0.43 at 400C to 0.10 at 800C) — kept conservative rather than assuming the lowest end of that range. |
| Other / unspecified | 0.6500 | Method unknown or not listed — assumed close to the least-favourable credible case (similar to open pit/trench burn), since there is no basis to assume a better outcome. |
Life-cycle emission factors
| Step | Item | Unit | kg CO₂e per unit | Source |
|---|---|---|---|---|
| Feedstock | Feedstock collection - diesel | litres of diesel | 2.67177 | MfE 2026 emission factors: Transport Fuel, Diesel (kg CO2e per litre) |
| Feedstock | Feedstock collection - petrol | litres of petrol | 2.36143 | MfE 2026 emission factors: Transport Fuel, Regular Petrol (kg CO2e per litre) |
| Feedstock | Feedstock processing - electricity | kWh | 0.07866 | MfE 2026 emission factors: Purchased Electricity, NZ grid annual average 2025 (kg CO2e per kWh, excludes line losses) |
| Feedstock | Feedstock processing - diesel | litres of diesel | 2.67177 | MfE 2026 emission factors: Transport Fuel, Diesel (kg CO2e per litre) |
| Feedstock | Feedstock transport - road freight | tonne-km | 0.135 | MfE 2026 emission factors: Freighting Goods by Road, All trucks (kg CO2e per tonne-km) |
| Making the biochar | Start-up fuel - diesel | litres of diesel | 2.67177 | MfE 2026 emission factors: Transport Fuel, Diesel (kg CO2e per litre) |
| Making the biochar | Start-up fuel - LPG | kg of LPG | 2.96632 | MfE 2026 emission factors: Stationary Combustion, Industrial Use, LPG (kg CO2e per kg) |
| Making the biochar | Start-up fuel - kindling wood | kg of wood | 0.11466 | MfE 2026 emission factors: Biomass Commercial Use, Wood chips (kg CO2e per kg, methane and nitrous oxide only, biogenic CO2 excluded) |
| Making the biochar | Process energy - electricity | kWh | 0.07866 | MfE 2026 emission factors: Purchased Electricity, NZ grid annual average 2025 (kg CO2e per kWh, excludes line losses) |
| Making the biochar | Process energy - diesel | litres of diesel | 2.67177 | MfE 2026 emission factors: Transport Fuel, Diesel (kg CO2e per litre) |
| Processing the biochar | Biochar crushing / screening - electricity | kWh | 0.07866 | MfE 2026 emission factors: Purchased Electricity, NZ grid annual average 2025 (kg CO2e per kWh, excludes line losses) |
| Processing the biochar | Biochar crushing / screening - diesel | litres of diesel | 2.67177 | MfE 2026 emission factors: Transport Fuel, Diesel (kg CO2e per litre) |
| Processing the biochar | Biochar crushing / screening - petrol | litres of petrol | 2.36143 | MfE 2026 emission factors: Transport Fuel, Regular Petrol (kg CO2e per litre) |
| Delivery and application | Biochar transport - road freight | tonne-km | 0.135 | MfE 2026 emission factors: Freighting Goods by Road, All trucks (kg CO2e per tonne-km) |
| Delivery and application | Application machinery - diesel | litres of diesel | 2.67177 | MfE 2026 emission factors: Transport Fuel, Diesel (kg CO2e per litre) |
Want to help refine these figures?
These starting figures are a first pass and are expected to be refined with input from biochar production experts over time. If you have published data, lab results, or direct experience that would improve a figure on this page, please get in touch with BNNZ.