The mass of the biomass feedstock can be directly measured and converted to CO₂e using a default factor or direct measurements of biomass carbon and moisture content. The feedstock quantity can be checked for consistency against operational records, and will form the basis for estimating feedstock-related upstream emissions (see Carbon storage counterfactual, Feedstock use counterfactual, and Market effects) as well as understanding the removal efficiency of the process. In addition to characterizing carbon content, a feedstock sampling protocol should characterize embodied nutrients such as nitrogen, phosphorus, and potassium. This enables an assessment of potential nutrient depletion and may inform the estimation of upstream emissions (see Feedstock use counterfactual). Any emissions associated with the process of biomass cultivation or procurement must be included in the project’s lifecycle assessment (see Materials, Energy, and Non-CO₂ biomass emissions).

Some biomaterial injection approaches store carbon-containing materials like biosolids or paper sludge with minimal processing. Others rely on processes like pyrolysis to transform biomass into an injectable form like bio-oil, resulting in the direct emission of some portion of the carbon originally contained in the biomass feedstock. Quantifying emissions from biomass transformation supports the assessment of how efficiently a process converts biomass into long-term carbon storage or useful co-products. Biomass carbon lost during biomass transformation can be estimated by comparing the mass of carbon in the stored biomaterial to the mass of carbon contained in the biomass feedstock (see Biomass procurement) or via direct monitoring of the transformation process. For a transformation process like pyrolysis, the conversion efficiency and characteristics of output is contingent on the specific feedstock and operating conditions of the pyrolyzer, and a sampling regime should be designed accordingly. If the transformation process produces other carbon-containing products, such as syngas, tar, or char, we recommend the embodied carbon be conservatively classified as “emitted” unless the product has a clearly defined path to durable storage. Any emissions from the biomass transformation process beyond biogenic CO₂ must be included in the project’s lifecycle assessment (see Materials, Energy, and Non-CO₂ biomass emissions).

The mass of carbon-containing biomaterial injected for geologic storage can be measured directly as a metered input to the storage system. This quantity can also be checked for consistency against the measured mass of biomass feedstock input to the system and operational data.

Injected biomaterial could leak from the storage reservoir if the reservoir loses integrity and the biomaterial migrates, or if the biomaterial degrades and releases greenhouse gasses that escape the reservoir via the well head or surroundings. These risks are contingent upon the characteristics of the injected biomaterial (e.g. pH), the operation and maintenance of the well (e.g. pressure, temperature, and ultimate well capping), and the depth and geology of the storage reservoir. Monitoring for leakage will likely require a combination of direct measurements and modeling. This approach may be partially dictated by the relevant permitting authority (e.g. EPA UIC), but characterizing leakage risks is still an area of active research. Any GWP assumptions used to incorporate the leakage of non-CO₂ gasses into the calculation of net carbon removal should be transparently disclosed.

While nearly all biomass carbon eventually makes its way back to the atmosphere, the effective drawdown from more durably storing biomass carbon occurs when the biomass feedstock would otherwise have released CO₂ into the atmosphere. If the counterfactual fate of the biomass feedstock would have resulted in little to no carbon storage — for example, burning or rapid decomposition — any durable storage achieved represents additional carbon removal. If, however, the counterfactual fate of the biomass feedstock would have resulted in medium-to-long term carbon storage — for example in soils, slow-degradation environments like some landfills, or living ecosystems — the mass of stored carbon in the counterfactual should not count as additional carbon removal unless and until counterfactual emissions would have occurred. This component may not apply in cases where carbon is purpose grown.

If a biomass feedstock currently serves a function that will need to be replaced if the feedstock is used for CDR, any emissions associated with the replacement must be considered. For example, if agricultural waste is currently used as animal feed or left on the fields to contribute to nitrogen, phosphorus, and potassium in soils, using that agricultural waste for CDR could result in new demand for feed or fertilizer, respectively. Current feedstock uses can be evaluated on a project-by-project basis, and the carbon impact of replacements can be estimated via lifecycle assessment. We recommend accounting for feedstock replacement emissions that involve existing feedstock uses rather than potential future uses. For example, agricultural feedstocks used for CDR should be evaluated based on their current uses (e.g. for animal feed or soil nutrients) rather than potential future uses that are not practiced today. The counterfactual for existing utilization should be flexible, and re-evaluated in the future if prevailing practices change. This component may not apply in cases where carbon is purpose grown.

Any emissions associated with market effects as a result of feedstock demand must be considered. For example, paying farmers for agricultural waste will likely increase the profitability of their operations and could result in an increase in acres planted. Directly growing crops or creating plantations could displace food production onto other lands and contribute to deforestation. Estimating the system emissions resulting from new feedstock demand will likely be difficult in practice and require careful economic modeling that reflects the overall scale of new biomass demand.

Using a biomass feedstock for CDR could potentially avoid non-CO₂ emissions such as methane or nitrous oxide that would have occurred in the counterfactual, for example during biomass decay. We recommend that these avoided emissions be considered a co-benefit of the project, and not be included in the accounting of net carbon removal. GWP assumptions used to characterize any avoided non-CO₂ emissions should be transparently disclosed.

The embodied emissions of all non-consumable and consumable materials that enable biomaterial injection — including materials, chemicals, and biomass feedstocks — should be estimated using a cradle-to-grave lifecycle assessment (LCA). If biomass is purpose-grown, emissions from biomass cultivation and direct land use change must be taken into account. Emissions associated with any built infrastructure, including monitoring and maintenance equipment, should include both construction emissions and material embodied emissions. There are not yet consistent best practices around whether or how to account for the embodied emissions of equipment or infrastructure of existing industrial projects that is used but not owned by the project. Transparency around boundary assumptions, data sources, and uncertainties is critical for LCA consistency and comparability.

The emissions associated with energy use for project activities should be estimated using an assessment of lifecycle emissions for the specific electricity or other energy sources required by the project. This should include the energy use associated with transporting biomass feedstock and processed biomaterial, combustion emissions associated with fuel use during the project, and energy use associated with monitoring and maintenance.

If a project’s management of the biomass feedstock before long-term storage leads to non-CO₂ emissions, these must be accounted for. Non-CO₂ emissions such as methane or nitrous oxide could be produced via processes like anaerobic decomposition during feedstock storage and transport, or during the management of biogenic waste products. Venting, leaking, or incompletely flaring methane produced during the biomass transformation process must also be considered. These emissions are considered separately from any non-CO₂ gasses that are emitted after biomass storage (see Leakage). GWP assumptions used to incorporate these emissions into the calculation of net carbon removal should be transparently disclosed.

The dynamics of biomaterial in geologic storage is still an area of active research, and is likely specific to the storage site and biomatieral. Monitoring of the geologic storage site will likely include a mix of direct measurement and modeling until the well is capped. An evaluation of durability claims associated with biomaterial injection must consider the monitoring and maintenance plan for the well, as well as any applicable regulatory structure that assigns ongoing liability for storage integrity. One important consideration is whether or not there is a track record of sufficient administrative capacity to guarantee execution of monitoring, maintenance, and liability arrangements, and whether or not the relevant authorities are permitting geologic storage monitoring plans with carbon storage in mind. This uncertainty is not included in the calculation of this pathway's Verification Confidence Level (VCL), because it is also captured by the Leakage component.