GHG quantification

Calculations of GHG emissions for the baseline and project scenarios shall follow a robust, recognized method and good practice guidance. The overall methodological approach is a comparative life cycle assessment (LCA) at the project-scale, based on ISO 14064-2:2019.

This methodology shall be used in conjunction with the Rainbow modules listed below. Modules are like mini-methodologies that only cover a part of the project life-cycle. Combining the relevant modules for a project results in a complete picture of the required data, calculations, monitoring plans, and other information needed for a full GHG quantification.

Processing and energy use

Transportation

Infrastructure and machinery

Quantification shall be done separately for each biochar production batch, since each batch by definition has distinctly measured biochar carbon characteristics. The GHG quantification results of multiple batches may be combined for one monitoring period. The duration of the monitoring period is chosen by the Project Developer, is typically one calendar year, but may be any other duration with a maximum of 18 months.

System boundary

The system boundary of this quantification section starts at the procurement of biomass feedstock, and ends at the biochar end of life, after accounting for decay and re-emission in its end use application. Biomass feedstock production impacts are excluded because biomass is required to be waste, and therefore not allocated any emissions. The system boundary includes the following key steps:

• biomass collection

• biomass transport to the kiln

• biomass processing (including but not limited to drying and chipping)

• energy used to start the kiln (including high-quality wood, and any associated leakage emissions)

• energy used to combust methane emissions within the reactor

• embodied emissions from manufacturing and shipping the kiln/s to the pyrolysis site

• methane emissions from the pyrolysis process

• biochar transport to the site of use

Any steps that are fully manual (which are largely expected for this technology type) do not incur any GHG emissions. Any steps that would have occurred anyway in the baseline scenario shall be excluded from the system boundary.

The following high-level equations shall be used to calculate carbon removals from distributed biochar projects.

Calculations: Removals

$\textbf{(Eq.1)}\ Net\ Removal = R_{baseline}-R_{project}-E_{project}$

where,

• $Net\ Removal$ represents the net removals from the project during the verification period, in tonnes of CO$_2$eq. Its sign is positive.

• $R_{baseline}$ represents any baseline GHG removals from the capture module(s), representing permanent storage that would have occurred in the absence of the project, in tonnes of CO$_2$eq. Its sign is negative.

• $R_{project}$ represents the project's gross GHG removals from the storage module(s) used by the project, in tonnes of CO$_2$eq. Its sign is negative.

• $E_{project}$ represents the project's total induced GHG emissions across the project life cycle, in tonnes of CO$_2$eq. Its sign is positive.

$\textbf{(Eq.2)}\ E_{project} = {E}_{project,\ biomass} + {E}_{project,\ Transformation}+ {E}_{project,\ Storage}$

where,

• $E_{project}$ was described in Eq. 1.

• $E_{project,\ biomass}$ represents the project's GHG emissions from the capture module(s) used by the project.

• $E_{project,\ Transformation}$ represents the project's GHG emissions from the transformation module(s) used by the project.

• ${E}_{project,\ Storage}$ represents the project's GHG emissions from the storage module(s) used by the project.

Data sources

The required primary data for GHG calculations from projects are presented in Table 1. These data shall be aggregated for all kiln runs within a production batch, after being measured and reported in dMRV at the frequencies summarized in the Monitoring section, and made publicly available.

Note that the table does not include all information needed for project monitoring and verification— only the data inputs for ongoing GHG quantification. The full list of information is provided in the minimum requirements for a Monitoring Plan.

Table 1 Summary of primary data needed from projects and their source for GHG quantification. All primary data sources listed here are required to be monitored and updated during verification. *Note that only one approach is required for reporting transport data. See the Transportation module for more details.

Category	Parameter	Unit	Source
Carbon storage	Volume or mass of biochar applied to soil	m3 or tonnes of fresh biochar	Measured onsite, dMRV
Carbon storage	Bulk density of biochar (only if using volume)	tonne of fresh biochar/m3	Measured onsite, dMRV
Carbon storage	Biochar moisture content () (only if using mass)	Percent	Elemental analysis by accredited laboratory
Carbon storage	Biochar $H/C_{\text{org}}$	Ratio	Elemental analysis by accredited laboratory
Carbon storage	Biochar organic carbon content	Percent	Elemental analysis by accredited laboratory
Carbon storage	GPS coordinates of biochar spreading sites (for determining soil temperature)	coordinates	dMRV
Pyrolysis process	Methane emissions rate	g CH4/kg dry biochar	Analyses from accredited independent provider
Pyrolysis process	Energy or wood for starting pyrolysis (amount and type)	MJ, kWh, liters fuel, kg wood	Measured onsite, dMRV
Transport of biomass	Distance biomass transported by motorized vehicle*	km	Operational records, conservative justified estimates
Transport of biomass	Weight of biomass transported*	tonne	Operational records, conservative justified estimates
Transport of biomass	Vehicle type for biomass transport*	category	Operational records, conservative justified estimates
Transport of biomass	Fuel quantity consumed for biomass transport*	liters fuel	Operational records, conservative justified estimates
Transport of biochar	Distance biomass transported by motorized vehicle*	km	Operational records, conservative justified estimates
Transport of biochar	Weight of biomass transported*	tonne	Operational records, conservative justified estimates
Transport of biochar	Vehicle type for biomass transport*	category	Operational records, conservative justified estimates
Transport of biochar	Fuel quantity consumed for biomass transport*	liters fuel	Operational records, conservative justified estimates

The ecoinvent database version 3.12 (hereafter referred to as ecoinvent) shall be the main source of emission factors unless otherwise specified. Ecoinvent is preferred because it is traceable, reliable, and well-recognized. The ecoinvent processes selected are detailed in Appendix 1.

No other secondary data sources are used in this module.

Assumptions

1. All biochar from the same production batch has the same characteristics (e.g. , $H/C_{\text{org}}$).

2. All biochar made from the same feedstock has the same methane emission rate from pyrolysis.

3. The permanent carbon sequestration rate from biomass leakage, where the alternate fate is being left on the field to decompose, is 0.5%.

Baseline scenario

For removal RCCs, there is no baseline from this module because it is assumed that there is no significant share of the project activity already occurring in business-as-usual. Therefore, the baseline for removal credits is zero and is omitted from calculations.

According to the Rainbow Standard Rules, this assumption shall be re-assessed at a minimum every 5 years, and any changes to this assumption would be applied to existing projects.

Project scenario

Biomass leakage

This section is only required if the feedstock's alternative use was to be left on the soil or reapplied to soils for nutrient recycling. This includes but is not limited to:

• mulching

• composting

• spreading fast-decaying cellulose-based residues (e.g. decay within 5 years)

Project leakage shall account for permanent carbon storage that would have occurred anyway in the absence of the project.

Although most biomass carbon would be released before the project's permanence horizon, a small fraction is stabilized permanently as soil carbon. This portion is counted as leakage and deducted from the project's carbon removal capacity.

The uncertainty around biomass carbon being 1) naturally incorporated into the soil and 2) converted to a stable carbon form is high, influenced by factors such as climate, soil type, soil health, and land use, making it hard to estimate for individual projects. Thus, it is assumed that a default 0.5% of the carbon in the biomass feedstock left on the soil, or reapplied to soil, will be permanently stored in soils.

Calculations: Biomass leakage

$\textbf{(Eq.3)}\ {E}_{project\ biomass}= A_{feedstock}* C * S$

Where,

• $E_{project,\ biomass}$ represents the permanent carbon removal in the baseline scenario in the monitoring period, in tCO$_2$eq. This value shall be applied to Equation 1.

• $A_{feedstock}$ represents the amount of biomass feedstock used in the monitoring period, in tonnes of dry matter.

• $C$ represents the concentration of carbon in the biomass feedstock, in tonnes of carbon per tonne of dry matter.

• $S$ represents the permanent sequestration rate of carbon applied to soils, which is 0.5%, as described in the Assumptions section.

Transport of biomass and biochar

If biomass is transported to a pyrolysis site, or biochar is transported to its end-use point, using a vehicle that is not manually powered, transport emissions shall be accounted for using the Transportation module.

For this distributed small-scale technology type, it is expected that direct proof of transport will be unavailable (e.g., distance transported or fuel use during delivery). Therefore, if transport is used, Project Developers may provide justified and conservative estimates of transport distance, fuel consumed, and transport method.

Pyrolysis process

Any energy used to start the kiln (including high-quality wood and any associated leakage emissions), as well as energy used to combust methane emissions within the reactor, shall be included in the project’s total induced GHG emissions quantification. These emissions shall be calculated using the Processing and energy use module.

Methane emissions from pyrolysis shall be accounted for using direct methane measurements on a representative kiln runs, following the Sampling and measurements requirements, and using the following equations.

Calculations: Pyrolysis methane emissions

$\textbf{(Eq.4)}\ {E}_{project,\ methane}= A_{biochar}* E_{bioCH_4}\div 1000*GWP_{bioCH_4}$

Where,

• $E_{project,\ methane}$ represents the total methane emissions from pyrolysis, in tCO$_2$eq. It shall be summed with any emissions from the Processing and energy use module to calculate $E_{project,\ Transformation}$ and used in Equation 1.

• $A_{biochar}$ represents the amount of biochar produced during the verification period, in tonnes of dry biochar. It shall be calculated using either Equation 5, 6, or 7, depending on the project's chosen biochar amount measurement method.

• $E_{bioCH_4}$ represents the emission rate of biogenic methane from the pyrolysis process, in gCH₄/kg dry biochar, measured according to the Sampling and Measurement requirements.

• ${GWP}_{bio\ CH4}$ represents the global warming potential of biogenic CH$_4$ over 100 years, which is 27 tCO$_2$eq/t CH$_4$.

• Divided by 1000 to convert from gCH₄/kg dry biochar to tonne CH₄/tonne dry biochar

$\textbf{(Eq.5)}\ A_{biochar} = V * BD_{quenched}*(1-M_{\%})$

where,

• $A_{biochar}$ represents the amount of biochar produced during the verification period, in tonnes of dry biochar.

• $V$ represents the volume of biochar delivered in m³

• $BD_{quenched}$ represents the bulk density of biochar, considering the mass of quenched biochar, in tonnes biochar/m³

• .$M_{\%}$ represents the moisture content of quenched biochar, on a weight basis (%w/w), so $1-M_{\%}$converts to dry mass of biochar

$\textbf{(Eq.6)}\ A_{biochar} = mass_{quenched}*(1-M_{\%})$

where,

• $A_{biochar}$ represents the amount of biochar produced during the verification period, in tonnes of dry biochar.

• $mass_{quenched}$ represents the mass of quenched biochar directly measured with scales at each kiln, in tonnes.

• $M_{\%}$ represents the moisture content of quenched biochar, on a weight basis (%w/w), so $1-M_{\%}$converts to dry mass of biochar

$\textbf{(Eq.7)}\ A_{biochar} = mass_{dry}$

where,

• $A_{biochar}$ represents the amount of biochar produced during the verification period, in tonnes of dry biochar.

• $mass_{dry}$ represents the mass of dry biochar directly measured with scales at each kiln, in tonnes.

Infrastructure and machinery

The Rainbow Infrastructure and machinery module shall be used to quantify the embodied emissions of kilns.

Biochar carbon storage

Project Developers shall quantify the total carbon removals from their biochar project by modeling 100-year removals using bulk measurements of $H/C_{\text{org}}$. These measurements shall be done once per production which, where a production batch has a maximum validity of 6 months or 75 tonnes of biochar, whichever occurs sooner. The measurements shall be done on a representative composite sample, mixing biochar from each kiln run. See Sampling and measurements for more details.

This approach is based on research from Woolf et al., 2021, and the IPCC modeling method. It is rooted in soil ecology and soil biochemistry disciplines. The permanent fraction of biochar carbon remaining after 100 years ($F_{\text{perm 100}}$) is modeled according to the local average annual soil temperature.

Temperature shall be obtained in the following ways:

• Biochar application to soil or mixing into horticultural products: Soil temperature shall be obtained for the end use location of each biochar spreading or mixing event, using the GPS coordinates provided in the Verification of end use report and the global soil temperature dataset from Lembrechts et al., 2021. The Rainbow Certification Team can provide soil temperature values for Project Developers based on the provided GPS coordinates.

• Biochar mixing into concrete: Average annual air temperature at the location where biochar is mixed into concrete shall be used. It shall be taken from reputable public databases.

Table 2 Soil temperature ranges are categorized and their corresponding c and m regression coefficients are presented, which are used in Eq. 8 below to calculate $F_{perm}$. Values are taken from Woolf et al., 2021.

Soil temperature (°C)	c	m
<7.49	1.13	0.46
7.5-12.49	1.10	0.59
12.5-17.49	1.04	0.64
17.5-22.49	1.01	0.65
>22.5	0.98	0.66

Calculations: Biochar carbon storage

$\textbf{(Eq.8)}\ F_{perm\ 100} = c - m*H/C_{org}$

where,

• $F_{perm\ 100}$ represents the fraction of biochar carbon remaining after 100 years

• $c$ and $m$ represent regression coefficients, taken from Woolf et al., 2021, and summarized in Table 2 for the corresponding project location's soil temperature.

• $H/C_{org}$ represents the ratio of molar hydrogen to organic carbon in biochar, measured via elemental analysis by an accredited laboratory for each production batch.

$\textbf{(Eq.9)}\ R_{project}= F_{perm\ 100}*{C_{org}*A}_{biochar}*C\ to\ {CO}_{2}*-1$

where,

• $R_{project}$ represents the total carbon removals from biochar during the verification period, in tonnes of CO$_2$eq. This value shall be applied to Equation 1 to calculate total project removals.

• $F_{perm\ 100}$ is calculated in Equation 8

• $C_{org}$ represents the concentration of organic carbon in biochar, on a dry weight basis.

• $A_{biochar}$ represents the amount of biochar delivered during the verification period, in tonnes of dry biochar. It is obtained using Equation 5, 6, or 7, depending on the project's chosen measurement method.

• $C\ to\ {CO}_{2}$ is 44/12 = 3.67, and represents the molar masses of CO$_2$ and C respectively, and is used to convert tonnes C to tonnes of CO$_2$eq.

• It is multiplied by -1 to obtain a negative sign. Removals are reported as a negative value.

Uncertainty assessment

An uncertainty assessment is presented below for all aspects of GHG quantification set at the methodology level. The findings from this assessment are then applied at the project level, where project-specific GHG quantification also undergoes an uncertainty assessment.

The overall project GHG quantification uncertainty is determined by qualitatively combining both the methodology-level and project-specific uncertainties for each identified source of uncertainty.

The uncertainty of assumptions are assessed below:

Assumption	Uncertainty
All biochar from the same production batch has the same characteristics (e.g. , $H/C_{\text{org}}$).	In principle this assumption has low uncertainty, but the ability of kiln operators to maintain consistent pyrolysis conditions across sites and across kiln runs is moderately uncertain.
All biochar made from the same feedstock has the same methane emission rate from pyrolysis.	In principle this assumption has low uncertainty, but the ability of kiln operators to maintain consistent pyrolysis conditions across sites and across kiln runs is moderately uncertain.
The permanent carbon sequestration rate from biomass leakage, where the alternate fate is being left on the field to decompose, is 0.5%.	High uncertainty, but the total net project removals is not sensitive to this assumption, so a low overall impact.

The equations and models have moderate uncertainty. The model for 100-year permanence from Woolf et al., 2021 has high uncertainty because it is a model fitted to experimental data, which always introduces variability.

Estimates may be used for the amount of processing and energy use inputs and the transport steps, rather than providing direct proof of each step. This is expected to introduce negligible to moderate uncertainty, depending on the level of justification provided for each project. It may be negligible if the process is entirely manual/not motorized, requiring no transport or energy inputs.

The uncertainty at the methodology level of the above-mentioned points are estimated to be moderate. This translates to a minimum discount factor of at least 6% for projects under this methodology.

Uncertainty is measurements is largely evaluated at the project level, based on the specific measurement approach. However, the choice between volume-based and mass-based measurement is critical and defined at the methodology level:

• Mass-based approach: Minimum discount factor of 6%.

• Volume-based approach: Minimum discount factor of 9% (due to higher uncertainty in bulk density measurements).