Principles & requirements

Project Developers shall demonstrate that they comply with all principles and requirements outlined in the Rainbow Standard Rules, and described below with a specific focus on distributed biochar production.

Rainbow Standard Rules

Durability

Durability threshold

All projects certified under this methodology shall prove durable carbon removals for at least 100 years. All credits issued shall be labeled as 100-year durability on the Rainbow Registry.

Reversal risk assessment

The major carbon reversal risks from biochar carbon storage are:

1. Insufficient biochar stability, where biochar carbon is not sufficiently carbonized and is decomposed by microbes and soil organisms, resulting in re-emission of CO₂.

2. Failure to durably incorporate into a permanent matrix (e.g. soil or soil-like material) and is instead burned or destroyed, intentionally or unintentionally (e.g. as fuel, in storage fires, or via waste incineration).

This methodology establishes the following mandatory project design requirements to mitigate these risks, detailed in the following sections:

• measuring the durable carbon fraction

• verification of biochar end use

Upon meeting these requirements for each verification and credit issuance, the risk of reversal is considered negligible for biochar application to soils. There are no further project requirements to assess reversal risks or conduct post-crediting monitoring for reversals.

All projects certified under this methodology shall contribute the default minimum 2% of their verified removal RCCs to the Rainbow Buffer Pool, as defined in the Rainbow Standard Rules.

Risk mitigation: Measuring permanent carbon fraction

Not all biomass carbon that is converted to biochar is expected to remain durably stored. The durability of biochar carbon depends on the its physicochemical stability, which is influenced by factors such as carbonization temperature and biomass feedstock.

Project Developers shall measure and prove that the H/C ratio ($H/C_{\text{org}}$) of biochar is <0.7 for each production batch. The H/C ratio is a well-known proxy indicator for biochar durability, and serves both as an eligibility threshold, and as input to quantify the permanent fraction of carbon ($F_{perm}$) expected to remain durably stored beyond the applicable durability threshold.

The fraction of permanently stored carbon shall be quantified using the models and equations specified in the GHG quantification section. Only this fraction shall be issued as removal RCCs.

The H/C ratio durability indicator shall be monitored for each production batch according to the Sampling Requirements.

Risk mitigation: Proof of biochar end use

Project Developers shall prove that all biochar has been used in the intended durable storage application and eligible biochar end use (e.g. incorporated into soils, added to fertilizer mixes, mixed in concrete). This shall be done by documenting all of the following information in dMRV:

• Proof of delivery of the biochar to its end use point of use, specifying the date, GPS coordinates, amount of biochar and Production Batch ID.

• Name and individual contact information for each buyer/user of biochar, for traceability and random checking by VVBs.

• Photo diary of biochar application, including photos of the process of biochar spreading and mixing.

No double counting

Project Developers shall sign the Rainbow MRV & Registry Terms & Conditions, committing to follow the requirements outlined in the Rainbow Standard Rules, including not double using or double issuing carbon credits.

The nature of distributed biochar projects means that biochar is applied to soils locally, as part of the same network of farmers that provide biomass and operate kilns. The biochar stays within the project scope all the way through application to soil, and there is no risk of downstream users issuing carbon credits for biochar use under another standard. Therefore, there are no further project-specific double counting requirements.

Co-benefits

Projects should support at least two quantifiable and verifiable environmental or social co-benefits, aligned with the UN Sustainable Development Goals (SDGs) framework. Any co-benefits claimed by the Project Developer shall be quantified, monitored, and audited for each verification and credit issuance.

Common co-benefits under this methodology are detailed in the table below. Project Developers may suggest and prove other co-benefits not mentioned here.

SDG 13 on Climate Action by default is not considered a co-benefit here, since it is implicitly accounted for in the issuance of carbon credits. If the project delivers climate benefits that are not accounted for in the GHG reduction quantifications, then they may be considered as co-benefits.

Table 1 Common co-benefits that projects under this methodology may provide are detailed, including types of proof that can be used to justify each co-benefit.

UN SDG	Example	Proof
SDG 1.5: Build the resilience of the poor and those in vulnerable situations, ensure significant mobilization of resources from a variety of sources	Kiln operators, who are often smallholder farmers, receive direct payments from carbon finance generated.	Payment slips, bank transaction records, contracts.
SDG 2.4: Ensure sustainable food production systems, increase productivity, help maintain resilient ecosystems, improve land and soil quality.	Biochar application to agricultural soils can increase crop yields.	Proof of biochar sales to farmers for agricultural use, invoices, receipts of sale of biochar to farmers.
SDG 3.9: reduce the number of deaths and illnesses from hazardous chemicals and air, water and soil pollution and contamination	Agricultural residues (feedstock) are pyrolyzed instead of open-field burning, improving air quality.	Biomass type records, local data on traditional burning practices.
SDG 12.2: Sustainable management and efficient use of natural resources	Biomass waste is converted into valuable biochar instead of being burned or discarded.	Biomass type records, local statistics on traditional biomass disposal methods.

Environmental and social safeguards

Project Developers shall prove that the project does not contribute to substantial environmental and social harms.

Projects must follow all national, local, and European (if located in Europe) environmental regulations related to, for example, pyrolysis, gasification, waste feedstock management, and biochar spreading on soils.

Biomass requirements

Waste status

Project Developers shall provide proof that the biomass feedstock is classified as waste. This can be done via any one of the following three methods:

• Price: if Project Developers did not pay for the biomass, or if they were paid to handle it, the biomass can be considered waste. Acceptable proof includes invoices, receipts, or contracts.

• Contextual analysis: Project Developers may submit an analysis supported by reputable sources that the biomass 1) could not be used as main material products, and 2) was not grown for the purpose of CDR.

• Positive list of wastes: if the biomass is included in the following list, it can be considered waste. Acceptable proof includes invoices, receipts, contracts, or photographic evidence and is required for validation:

  • sawmill residues

  • sawdust

  • shavings

  • bark

  • forestry tops and branches

  • wildfire management residues

  • straw

  • husks

  • corn cobs

  • wood from horticulture (trimmings or whole plants)

  • nut shells

  • bagasse

  • sugar beet pulp

  • invasive species

Alternative use

Forestry certification

Biochar requirements

Biochar applied to soils must be below the pollutant concentration thresholds outlined in Table 2, defined by the World Biochar Certificate Guidelines (for WBC-Agro). This shall be measured on a representative composite sample of biochar anytime biomass feedstock changes, or annually, if biomass feedstock does not change. PAH measurements have been excluded from this list due to their low bioavailability when in biochar.

Table 2 The thresholds for pollutant concentrations allowed in biochar, as detailed in the World Biochar Certificate Guidelines.

Substance	Limit amount (g/tonne dry matter)
Pb	300
Cd	5
Cu	200
Ni	100
Hg	2
Zn	1000
Cr	200
As	20

Disclaimer: The European Biochar Certificate (EBC) and the World Biochar Certificate (WBC) are independent certification programs designed to ensure the quality of biochar products. These certifications are administered and trademarked by Carbon Standards International (CSI) and are distinct from the Rainbow certification and the issuance of carbon credits.

The threshold values provided here are based on the voluntary guidelines of the World Biochar Certificate (WBC), reproduced with permission. While these values have been adopted by the Rainbow standard as pollutant thresholds, they are only indicative. Meeting these thresholds for Rainbow certification does not imply eligibility for or any association with the EBC or WBC programs. Project Developers certified under the Rainbow standard shall not make claims or use any trademarked materials from CSI, unless explicitly allowed by CSI.

Voluntary certification under the WBC and EBC schemes is overseen by CSI and includes additional requirements beyond pollutant thresholds.

Environmental and social risk evaluation

Project Developers shall fill in the Rainbow Distributed biochar risk evaluation, to evaluate the identified environmental and social risks of projects. The identified risks include:

• Heavy metal or other pollutants in biochar applied to agricultural soils

• Disruption of soil health when collecting and exporting organic matter

• Presence of heavy metals, toxins or other chemical pollutants in the biomass⁠

• Spread of diseases or invasive species

• Cultivation of feedstock

• Deforestation from use of forestry products as feedstock

• Distant transport of feedstock inputs (>100 km)

• Timely and fair payments to smallholder farmers

• Safe kiln operation and training events to prevents burns

• Use of large amounts of water, if water is scarce, for quenching

Project Developers shall assign a likelihood and severity score of each risk, and provide an explanation of their choices. The VVB and Rainbow’s Certification team shall evaluate the assessment and may recommend changes to the assigned scores.

Any identified material risk (defined as issues with a risk score of moderate or higher) shall be subject to a Risk Mitigation Plan, which outlines how Project Developers will mitigate, monitor, report, and if necessary, compensate for any environmental and/or social harms.

Additional proof may be required for certain high risk environmental and social problems.

The Project Developer, the Rainbow Certification Team, or the VVB may suggest additional risks to be considered for a specific project.

Note that the life-cycle GHG reduction calculations account for the climate change impacts of most environmental risks. Nonetheless, Project Developers shall transparently describe any substantial and sensitive GHG emission risks in the risk evaluation template.

All risk assessments must also address the Minimum environmental and social risks defined in the Rainbow Standard Rules.

Project Developers shall follow the requirements in the Rainbow Standard Rules for conducting stakeholder consultation, and where IPLCs are involved, obtaining free prior and informed consent (FPIC) and ensuring benefit sharing.

To ensure kiln operator safety, Project Developers shall comply with the training requirements outlined below.

Leakage

Biomass feedstock sourcing must not contribute to activity shifting leakage.

The requirement that biomass feedstock must be classified as waste helps mitigate activity shifting leakage. Consequently, the evidence provided in the "Environmental and Social Do No Harm" section shall also be applied here to verify that the feedstock is waste.

Project Developers shall justify the alternative use of biomass, as outlined in the Environmental and social safeguards section. If the waste or residue has a valuable existing use, Project Developers shall assess the impacts of diverting it for biochar production. This assessment must address, at a minimum:

• The availability of substitute materials (either the same material, if locally abundant, or alternative materials that fulfill the same function).

• The risk of indirect land use change or deforestation, particularly if diverting the biomass creates demand for new materials to replace its original function.

Where leakage risks are identified, Project Developers shall make and document their efforts to mitigate the risk. Where leakage cannot be fully avoided, leakage emissions shall be counted towards the project induced GHG emissions and therefore deducted from project removals.

Several other types of leakage risks are already covered by other components of this module:

• Risk of deforestation as a direct consequence of biomass sourcing (e.g. cutting trees for biochar production), which is mitigated by requiring biomass to be waste, residue, or invasive species.

• Displacement of soil carbon storage: a small amount of soil carbon storage is assumed and modeled in the Baseline Scenario where relevant, effectively deducted from the project's carbon storage.

• Upstream and downstream emissions: considered in the life-cycle based GHG quantifications in companion modules.

Monitoring

Monitoring Plans for this module shall include, but are not limited to, tracking of the following information:

Measurement frequency	Parameters
Once for each production batch, on a single representative sample of biochar	$H/C_{\text{org}}$ and organic carbon content (full description here)
Three times per production batch	Biochar moisture content (if measuring biochar amount via mass) (full description here)
Once per year, or anytime biomass feedstock changes	Methane emissions, on three kilns representative of all other kilns in the same production batch (full description here) Environmental pollutants of biochar, on one composite sample mixing biochar from each kiln run (full description here) Dry bulk density of biochar, on at least one sample from each kiln (i.e. 20 different kilns, 20 bulk density measurements) (if measuring biochar amount via volume) (full description here)
Each kiln run	Moisture content and type of biomass input (full description here) Mass or volume of fresh biochar produced (full description here) Temperature curve measured in each kiln using real-time sensors, proving pyrolysis has occurred (full description here) Photographs of the pyrolysis process, showing for example a lack of white smoke and ash, and the duration of the kiln run (full description here) Sampling records (proof of setting aside a small sub-sample of biochar, to be combined with sub-samples from all other kiln runs in the same production batch) (full description here)
All biochar used	Proof of eligible, durable biochar end use, with names and GPS coordinates of spreading locations, among other information (full description here)
Once per year	Kiln operator training (full description here) Proof and findings from the visits of kiln supervisor to each kiln operator (full description here) Proof of calibration and accuracy of onsite scales for weighing biochar to determine biochar amount (full description here)
Once per verification and credit issuance	Annual quality oversight report from the Project Developer (full description here) Environmental sustainability and Leakage compliance of biomass feedstock (full description here and here)

The Project Developer is the party responsible for adhering to the Monitoring Plan.

dMRV requirements

Distributed biochar uniquely heavily relies on dMRV platforms to allow for detailed tracking of operations across a network of kiln operators.

Project Developers shall select the dMRV to be used by kilns supervisors and operators to document the operating procedures. The chosen dMRV shall provide step by step procedures to kiln operators and be able to track the following:

• eligible kiln use, via scanned QR codes of individual kilns and photos of the kiln displaying the required technical features

• biomass moisture content, via photos of sensor readings

• biomass preparation and amount, via photos of the pile of biomass before the kiln run

• pyrolysis temperature and duration, via continuous sensor readings

• pyrolysis process quality, via photos taken throughout the pyrolysis process of the intact and clean flame curtain

• rapid quenching, through time-stamped photos or short videos showing the start and end time of quenching

• energy inputs used for process ignition

• amount of biochar produced, via either:

  • Volume of biochar produced from each kiln run, and bulk density measurements conducted by the kiln supervisor, or

  • Mass of biochar produced from each kiln run, and moisture content measurements

• biochar sampling records, via scanned QR codes of the original biochar bags from which the sample is taken, and photos of the sample

• biochar delivery and application, via scanned QR codes of biochar bags for delivery and photos

• methane emissions testing measurement results and photos of the process

• aggregated measurements and data for GHG quantification

All photos must be time-stamped and include GPS coordinates. All records must be kept for at least 5 years after the end of the project's last monitoring period.

Data from dMRV shall be made available and verifiable (by Rainbow, the Project Developer and the VVB) through an oversight-only view, and exportable .csv files.

At project validation, Rainbow will review the proposed dMRV platform to ensure it is capable of providing the above-mentioned data and proof. After a dMRV platform design is validated and approved as eligible, the platform will be added to Rainbow’s published list of eligible platform. For any future projects using the same dMRV platform, the platform shall be considered automatically eligible.

The use of an approved dMRV platform is not sufficient to ensure project compliance with the present methodology. All listed data points and proof shall be uploaded to the dMRV platform in order to issue credits.

If any piece of operational proof or documentation is missing in dMRV, the entire production batch or kiln run is ineligible.

Role	dMRV Responsibility
Kiln operators	Directly upload the required information to the dMRV platform, and flag any known anomalies.
Kiln supervisors	Directly upload any biochar bulk density and/or moisture content measurements to the dMRV platform (depending on the measurement method for the amount of biochar produced), and flag any known anomalies.
Project Developer	Check the information and proof for every kiln run recorded on the dMRV platform, ensuring its accuracy and data preparation. Provide an annual report on oversight findings, listing non-conformities, adjusted data, kiln runs excluded from certification.
VVB	Check the complete raw records in dMRV of a random sample of kiln runs. Check enough raw evidence to cover 10% of all events. Findings shall follow the non-conformity procedure outlined in the Rainbow Procedures Manual. Major non-conformities shall trigger a more thorough audit of a larger sample of events.

Quality and oversight

dMRV oversight

The Project Developer shall check records from each kiln run, ensuring:

• duration of run corresponds to amount of biochar produced

• amount of biomass feedstock used corresponds to amount of biochar produced (rough comparison, based on photo of the biomass feedstock pile)

• flame curtain photos show a clean burn and a flame that is not too high (recommended threshold of 0.5 meters above the limit of biomass)

• pyrolysis temperature is consistently high throughout the duration of the kiln run. Project Developers shall suggest their own definition for acceptable temperature curves upon validation. Full details are in the temperature curve section

• quenching was rapid enough to mitigate methane emissions

Any kiln run with incomplete data, or that demonstrates unsatisfactory practices, is rendered ineligible. The Project Developer is responsible for flagging such kiln runs, and the VVB shall check the complete dMRV raw records of a random sample of kiln runs, covering 10% of all events.

Site visits

The kiln supervisor shall visit each kiln operator annually and observe one kiln run, ensuring:

• feedstock is properly added to the kiln

• biomass moisture content measurements are taken appropriately

• storage conditions for biomass and biochar are appropriate

• biochar sampling procedures are followed

If the project is using the volume-approach for measuring the amount of biochar produced, the kiln supervisor shall also take samples for bulk density measurements during this site visit.

Proof of such checks shall be included in the Monitoring Plan and submitted each year, including the name and qualification of the kiln supervisor, and the findings and follow up actions of each kiln operator.

Note that the kiln operator site visits are in addition to the annual VVB site audits, described in the Certification Requirements section.

Annual quality oversight report

Project Developers shall prepare a report, upon each verification, summarizing the findings of dMRV oversight done by the Project Developer and onsite visits conducted by the kiln supervisor. This report shall detail:

• any kiln runs that were excluded from certification and the reason for exclusion

• any non-conformities in data uploaded to dMRV and their remediation

• any non-conformities or failure to adhere to best practices in onsite visits, and their remediation

Temperature curves

All kilns shall be equipped with integrated temperature sensors that take continuous temperature measurements, resulting in a documented temperature curve for each kiln run. The purpose is twofold: to ensure pyrolysis actually occurred, and to assess the quality of the pyrolysis run.

To ensure pyrolysis actually occurred, the temperature curve must show that the temperature remained above 350°C for the entire pyrolysis process (excluding the ramp up phase).

To assess the quality of the pyrolysis run, Project Developers must submit project-specific criteria for evaluating temperature curves during project validation. Developers must use these criteria to exclude ineligible kiln runs. These criteria, approved by Rainbow and the VVB, should focus on red flags indicating:

• Incomplete biomass drying, or

• Improper feed-in rates (e.g., overloading that cools pyrolysis or quenches the flame).

Integrated temperature sensors are a promising and relatively new pathway (for VCM certification) to verify pyrolysis quality, ensuring low methane emissions and proper biochar production. Methodology-level requirements are not set because:

• Thermocouple setups (number, placement, spacing) vary by kiln design.

• Different biomass feedstocks produce distinct temperature curves due to varied energy content.

• Universal temperature thresholds are not yet established.

Trainings

Project Developers are responsible for ensuring regular trainings are provided to kiln operators involved in their project. The kiln supervisor or the Project Developer may be the party providing training. Training shall cover at least the following aspects of kiln operation:

• safety provisions for operating the kiln

• biomass preparation (drying, eligible biomass size, appropriate size of biomass)

• taking biomass moisture content measurements

• feeding biomass into the kiln to ensure consistent pyrolysis

• rapid quenching

• managing the biochar sampling procedure

• use of dMRV

Upon validation, Project Developers shall submit their Training Protocol, describing how they ensure:

• frequency of trainings

• testing or proof of adequate completion of training

• ongoing training and support