Subsurface Biomass Storage

1.0 Introduction

Plants are photosynthetic, autotrophic organisms that use energy from sunlight and reducing equivalents from water to convert atmospheric CO₂ into biomass. Plant biomass represents a standing stock of carbon on the order of several hundred gigatonnes ¹. Net primary productivity, or the net amount of new carbon fixed into biomass by autotrophic organisms annually, is a flux of approximately 100 gigatonnes of carbon per year, with about half of this flux occurring on land ². Every year, approximately the same mass of carbon is returned to atmospheric CO₂ (and other GHGs such as methane) through microbial degradation and other biogeochemical reactions. At steady state, this suggests that the typical residence time of carbon in terrestrial biomass is about 10 years (assuming 500 Gt standing stock and 50 Gt/year flux). Biomass storage for carbon removal aims to decrease the amount of carbon dioxide in the atmosphere through project (An activity or process or group of activities or processes that alter the condition of a Baseline and leads to Removals or Reductions.)s that act to prolong the residence time of carbon in stored biomass far beyond this typical residence time.

This Module (Independent components of Isometric Certified Protocols which are transferable between and applicable to different Protocols.)  provides guidance on the storage (Describes the addition of carbon dioxide removed from the atmosphere to a reservoir, which serves as its ultimate destination. This is also referred to as “sequestration”.) of biomass in the shallow subsurface for the purpose of carbon dioxide removal. A net decrease in atmospheric CO₂ will occur when biomass is processed and stored in a way to substantially slow or arrest microbial degradation. When these carbon cycle interventions are combined with engineered controls and favorable site characteristics to limit disturbance, this stored carbon may persist for long periods of time.

While this Module provides specific requirements on how biomass can be stored to significantly delay the degradation of biomass, it is noted that adherence to the storage provisions outlined in this Module does not guarantee durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) of carbon storage. The first version of this Module primarily focuses on highly controlled subsurface storage settings, such as a dedicated burial site, where external influences like groundwater and soil biogeochemical cycling will have limited interference to the ability to monitor for and detect reversal (The escape of CO₂ to the atmosphere after it has been stored, and after a Credit has been Issued. A Reversal is classified as avoidable if a Project Proponent has influence or control over it and it likely could have been averted through application of reasonable risk mitigation measures. Any other Reversals will be classified as unavoidable.)s. Given the significant diversity of biomass chemical characteristics, processing prior to storage, storage environments, and other site-specific considerations, it would be inappropriate to claim a minimum, maximum, or average durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.). Instead, durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) claims associated with removals utilizing this Module must provide direct, time-series evidence of the underlying claims and a detailed, quantitative description of how those storage conditions will be maintained throughout the project lifetime. Additionally, this Module describes the requirements of monitoring and reporting of fugitive gasses as well as buffer pool (A common and recognized insurance mechanism among Registries allowing Credits to be set aside (in this case by Isometric) to compensate for Reversals which may occur in the future.) requirements. Any biomass processing and carbon quantification must be conducted in accordance with other Isometric Protocol (A document that describes how to quantitatively assess the net amount of CO₂ removed by a process. To Isometric, a Protocol is specific to a Project Proponent's process and comprised of Modules representing the Carbon Fluxes involved in the CDR process. A Protocol measures the full carbon impact of a process against the Baseline of it not occurring.)s and Modules.

2.0 Structure and Future Versions

Biomass is inherently labile with respect to degradation by fungi and microorganisms and other biogeochemical redox reactions. While there are many biomass processing technologies and storage options present in the market today, scientific literature has not yet reached consensus on the carbon storage durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) of different processing and storage techniques.

Given this current state of understanding of shallow subsurface biomass burial for carbon dioxide removal, Isometric takes a restrictive approach to project eligibility. Specifically, Isometric will assess the current state of scientific understanding and consensus on shallow subsurface biomass burial technologies on an individual basis. Once such technologies have demonstrably met a reasonable burden of scientific evidence, they will be added to the Eligible Biomass Processing and Storage Option list below as the Module is updated in line with the Public Consultation process. Isometric will update this list as frequently as necessary, guided by scientific research and market learnings.

3.0 Eligible Biomass Processing and Associated Storage Options

3.1 Biomass drying

When biomass is dried to sufficiently low water activity, biological activity is severely inhibited. It has been shown that production of carbon dioxide from labile organic matter will be severely inhibited below 12% water content (corresponding to a water activity of approximately 0.6) ^3,4. While 12% water content is considered a minimum threshold in this Module, it is recommended that the Project Proponent include a safety factor of an additional 2% (or 10% water content in biomass) to mitigate the risk of water uptake during processing or storage. Project Proponents conducting biomass drying must sample and measure biomass water content at the same frequency and cadence of carbon content sampling.

3.2 Dried biomass storage options

3.2.1 Dedicated burial storage

Dried biomass is eligible for storage in a dedicated burial site (i.e., dried biomass and associated storage materials are the only constituents of the burial site. The burial site must consist of multiple impermeable layers, including, at a minimum, a synthetic liner consistent with durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) claims and compacted earthen material with the hydraulic conductivity specifications outlined in Section 5.1. The Project must also include measures for maintaining biomass water activity within the site. Examples of measures that may be used to maintain biomass water activity include biomass salting (20% salt or higher) or wrapping in low permeability materials ⁵.

4.0 Evidence of Biomass Durability

Projects utilizing shallow subsurface biomass burial have diverse characteristics, using a range of feedstocks, processing technologies, and storage solutions. Additionally, the environmental context will differ from site to site. Given this diversity in project level characteristics, and the potential diversity in true durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) of the associated removal, suppliers of removals utilizing this Module must provide evidence of their durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) claims with the following requirements. These requirements are in place to assess durability of biomass and any encapsulating storage wrapping, as well as microbial degradation potential.

• [R-8HFE-0, Please provide details and evidence that the feedstock durability has been demonstrated using incubation studies for a minimum of 6 months.]Direct, time-series observation of GHG evolution from representative biomass that has been processed in the same manner as the feedstock and incubated in a context analogous to the storage conditions. A detailed description and quantitative justification as to how the storage conditions will be maintained for a minimum of the claimed durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) period is required. In most cases, this will include direct calculation of time integrated permeation of chemical species that may compromise biomass (e.g., if biomass durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) is predicated on low water activity, time integrated diffusion of water into stored biomass must be determined). [/R-8HFE-0]
• [R-3G2D-0, The Project Proponent must provide incubation data, including the performance of any encapsulation matrix, demonstrating the GHG evolution rate when extrapolated to the claimed durability period, demonstrates the biomass storage technology will facilitate the durability period claimed. ]Time-series evidence of biomass stability must be included in the PDD with a minimum of six months of observation demonstrating biomass stability compared to control observations (longer observation windows are recommended). Suitable tests could include standard ASTM tests for aging of polymers in landfill conditions in both anaerobic and aerobic conditions (ASTM D5526-18: accelerated anaerobic degradation in landfills; and ASTM D7475D747 5-20: accelerated aerobic degradation in landfills). This should ideally include microbial inocolum representative of the storage site. Where biomass is being wrapped, there will be minimal exposure to microbes and moisture once sites are closed. Including a microbial inoculum during testing provides a conservative (Purposefully erring on the side of caution under conditions of Uncertainty by choosing input parameter values that will result in a lower net CO₂ Removal or GHG Reduction than if using the median input values. This is done to increase the likelihood that a given Removal or Reduction calculation is an underestimation rather than an overestimation.) scenario forecast of expected durability. The Project Proponent must demonstrate that the observed GHG production rate, when extrapolated to the time horizon of the durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) claim, is consistent with the durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) requirements of Section 2.5.8 of the Isometric Standard. It may be the case in some instances that no GHG evolution is observed over the period of time-series analysis. In such instances, the Project Proponent must include either a theoretical or empirical description of the minimum detection limit of the methods and analyses used, and extrapolate that minimum detection limit to the claimed durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) period, with appropriate compounding of error. The evidence used to support a durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) claim must be specific to a feedstock and a processing method. Evidence of durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) may be reused for multiple projects that utilize the same feedstock and processing methods. [/R-3G2D-0]
• [R-W6B3-0, Projects must demonstrate how the burial conditions facilitating the intended durability of the credits will be maintained.]The Project Proponent must also include a detailed description of how the burial conditions facilitating biomass preservation will be maintained for the claimed durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) period, including details of natural and engineered controls that will be considered. Wherever engineered controls are used to maintain storage conditions (e.g., low permeability/high plasticity clay and synthetic liner to impeded liquid water), the Project Proponent must provide quantitative characterization of the durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) and physical/chemical properties underpinning these controls (e.g., permeability of water and oxygen through synthetic wrapping material). The Project Proponent must provide calculations extrapolating the storage conditions of the buried biomass to the claimed durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) period.[/R-W6B3-0]
• [R-FV6R-0, Projects must demonstrate that any sealing/encapsulation matrix will facilitate the intended durability of the credits.]The Project Proponent must provide a detailed plan for seal integrity testing of wrappers chosen for the Project, in conditions representative of the storage site. A suitable pressure test should be chosen (e.g. Haug test) and choices on how that test will be carried out to be representative of the storage site conditions must be justified in full in the PDD. The passing rate for quality testing must be >99%. [/R-FV6R-0]
• [R-Q4Y2-0, Projects must demonstrate the shear force that would cause slope collapse and how the storage site is designed to reduce the risk of slope collapse and subsequent tearing of encapsulated biomass.]Testing must also be carried out to determine the shear force that causes slope collapse, a condition which would expose wrapped biomass to tearing risk. The storage site should be designed to avoid tearing risk as identified in this aspect of testing.[/R-Q4Y2-0]

Project Proponents must submit the required evidence of biomass durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) and burial setting stability set out in this section to the Isometric Science Team for review and approval prior to crediting.

Hypothetical example of a Project Proponent meeting the above requirements

A Project Proponent supplies removals that involve drying biomass to a water activity that is sufficiently low to inhibit all biological activity. The Project Proponent incubates the dried biomass under controlled laboratory conditions for six months, and finds that no measurable greenhouse gasses have been produced. The Project Proponent uses the extrapolation of the minimum detection limit to evidence that < 1% of the biomass is expected to decay in a 1,000 year time horizon. The Project Proponent seals the biomass in synthetic wrapping before aggregating the wrapped biomass in a landfill that includes a sealed synthetic liner and a clay layer. The Project Proponent in this example must include in the PDD the water and gas permeability and other relevant material properties of the wrapping, landfill liner, and clay layer. The Project Proponent must provide calculations using these material properties to demonstrate that the dry storage conditions can be maintained over the claimed durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) period. The Project Proponent calculates that under a worst case scenario, the water activity will increase by 2% in the vicinity of the biomass over a 1000-year period. The Project Proponent adjusts their biomass processing operations to ensure a 2% safety factor in water activity (i.e., stored biomass has a water activity 2% below that which has been shown to fully inhibit biological activity).

5.0 Site Characterization, Permitting, and Land Tenure

5.1 Site Characterization

For underground biomass storage site selection, several site attributes need to be assessed for their suitability as a storage site according to the criteria listed below ⁶. These categories include surface water, groundwater, rock and soil, and regional seismicity. Project Proponents must provide detailed information on the following site characteristics and demonstrate that they pose limited risk to the Project.

• Permeability
  • Including soil composition and texture, plus bedrock composition and texture
• Soil properties
  • Including pH and clay content
• Hydraulic conductivity
  • Consisting of hydraulic conductivity values for all materials
• Plasticity
  • Consisting of liquid limit and plasticity index values for all materials
• Sorption capacity
  • Consisting of mineral composition, especially abundance of expandable phyllosilicates
• Seismicity
  • The Project Proponent must characterize any material seismic risk in the vicinity of the storage site. This must include the following.
    • Peak Ground Acceleration (PGA) - PGA from shock waves, modeled for 50-year or longer timescale and wave-amplification potential implied by local lithology
    • Peak Ground Displacement (PGD) - mechanical response model for site materials and geologic evidence for absence of land surface disturbance by earthquake activity

Permeability, soil properties, hydraulic conductivity, plasticity, and sorption capacity are included to assess the risk of biomass disturbance by infiltration of surface water and/or groundwater. Peak ground acceleration and peak ground displacement are included to assess the risk of biomass disturbance from seismicity.

5.2 Target Values

Table 1 contains target values against which a potential biomass storage site should be assessed and compared. Any deviations from the ranges specified here must be justified in the PDD. The use of sufficiently non-permeable liners may be a justification for relaxing hydraulic conductivity target values.

Table 1. Design Guidelines and Target Values

These target values were chosen based on conventions and requirements used in municiplemunicipal solid waste landfills ^8,9, as well as parameters that would quantitatively restrict passive water intrusion on sufficentlysufficiently long time scales (e.g., 1000+ years).

6.0 Legal Framework to Ensure Permanence

Table 2. Land Durability Requirements

Sufficient evidence and documentation to demonstrate that the criteria has been met.

The third-party verifier is to assess whether the Project Proponent has sufficiently developed a plan that is likely to ensure that there exists an entity with both the incentive and resources to pursue legal enforcement should such action be necessary.

7.0 Counterfactual Land Use

8.0 Site Closure

The closure standards for municipal solid waste facilities in the United States (MSWLFs) ⁷ require owner/operators to install a final cover system to minimize infiltration of liquids and soil erosion. While biomass storage facilities may or may not be landfills in a legal sense, biomass storage facilities and their contents have many of the same characteristics and will experience the same influences from precipitation, heat and pressure and as such must actively reduce the impact of these influences throughout the project's lifespan. Unlike a landfill for municipal waste disposal, where leachate, odor and methane gas from chemical and biological activities are of major environmental concern, these are not major concerns as long as storage conditions are maintained.

9.0 Monitoring

Incremental changes of CO₂, CH₄, water vapor, temperature and/or pressure may indicate that there is a decay of feedstock within the storage site. If these variables indicate the possibility of decay, additional testing for N₂O must be conducted. Monitoring of gasses, temperature, moisture, and pressure may be conducted through continuous or discrete sampling. If discrete sampling is used, frequent measurements are required over a three-year period to confirm functional stability, with ongoing monitoring for reversals required for 50 years. It is recommended that, if the Project Proponent conducts discrete sampling, sampling be conducted weekly until conditions at the site are shown to be stable and less frequent monitoring is warranted.

The Project Proponent must describe all the methods, equipment, detection limits and any applicable standards that will be used for monitoring. The Project Proponent must also provide either a theoretical or empirical justification that reversals on the order of 1% over the 50 year period of the stored biomass will be detectable.

All equipment used for sampling must be properly adjusted for atmospheric temperature and pressure (ATP) and calibrated per manufacturer requirements, with documentation available upon request. All meters must be calibrated by the manufacturer or a certified third-party calibration service per manufacturer’s guidance or more frequent, if needed. Calibration certificates must be maintained in accordance with Section 11 below.

Projects may utilize tracing techniques (e.g., isotope tracers) in order to quantify how much of the gas sampled is the result of ambient air or soil gas inflow that occurs during the sampling process. If no such methods are used, all increases in greenhouse gas will be considered reversals.

Detection of reversals must be immediately reported to Isometric and will be handled according to Isometric’s reversal policy.

10.0 Risk of Reversal

Projects utilizing this storage Module must assess the Risk of Reversal according to the Isometric Standard Risk Assessment Questionnaire in consultation with Isometric. The Risk of Reversal may be higher due to project specific considerations. The Risk of Reversal will be reassessed every 5 years, aligning with the Crediting Period, or when new scientific research and knowledge are produced.

When quantifying reversals, the Project Proponent must use a 100-year time horizon when selecting GWP. For this Module, GWP100 values provided in the IPCC 6th Assessment Report ¹¹ for CH₄, N₂O, and other relevant non-CO₂ gasses must be used, or most recent updates. For methane, biogenic and non-biogenic GWP differentiation can be applied, where appropriate.

11.0 Record Keeping

All records associated with the characterization, design, construction, burial operations, monitoring, site closure, and site maintenance shall be developed and submitted to proper authorities as required by any applicable permitting authority.

AllRecords recordsof shalllaboratory analyses and relevant permit limitations to demonstrate compliance must be maintained in accordance with the permit and available for review at any point during the Crediting Period or post closure. Where not required by the permit, records of all analyses and injections must be maintained by the storage facility or Project Proponent and provided for verification purposes for a minimum of 10five years after the end of the monitoring period.

All Allclosure and post-closure monitoring records shallmust be maintained by the Project Proponent for a minimum of 10 years after collectionclosure. These records must be available to be consulted by interested parties for future clarifications if needed.

12.0 A Note on Land Security and Removal Durability

There is no single credible mechanism that can ensure, without uncertainty, that biomass buried in the shallow subsurface will remain undisturbed in perpetuity given the relative nascency of such legal mechanisms relative to the time horizons required in the Isometric Standard. Land durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) claims are subject to social and political factors and are thus different in nature from claims regarding physical or geologic durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.). Isometric has developed a set of land security eligibility criteria that align with current best practices for legal strategies to restrict future uses of land. ThisThe Protocol and this Module also considers the risk associated with land ownership as a risk factor in determining the Risk of Reversal and corresponding buffer pool (A common and recognized insurance mechanism among Registries allowing Credits to be set aside (in this case by Isometric) to compensate for Reversals which may occur in the future.).

13.0 Acknowledgements

Isometric would like to thank following contributors to this Module:

• National Renewable Energy Laboratory (NREL)
• Lawrence Livermore National Lab (LLNL)
• EcoEngineers
• Ning Zeng, Ph.D., University of Maryland

NREL and LLNL were supported by the DOE Office of Technology Transitions in collaboration with the Office of Clean Energy Demonstrations (OCED), Office of Fossil Energy and Carbon Management (FECM), Office of Energy Efficiency and Renewable Energy (EERE), and the Bioenergy Technology Office (BETO).

14.0 Definitions and Acronyms

An activity or process or group of activities or processes that alter the condition of a Baseline and leads to Removals or Reductions.

Independent components of Isometric Certified Protocols which are transferable between and applicable to different Protocols.

Describes the addition of carbon dioxide removed from the atmosphere to a reservoir, which serves as its ultimate destination. This is also referred to as “sequestration”.

The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.

The escape of CO₂ to the atmosphere after it has been stored, and after a Credit has been Issued. A Reversal is classified as avoidable if a Project Proponent has influence or control over it and it likely could have been averted through application of reasonable risk mitigation measures. Any other Reversals will be classified as unavoidable.

A common and recognized insurance mechanism among Registries allowing Credits to be set aside (in this case by Isometric) to compensate for Reversals which may occur in the future.

A document that describes how to quantitatively assess the net amount of CO₂ removed by a process. To Isometric, a Protocol is specific to a Project Proponent's process and comprised of Modules representing the Carbon Fluxes involved in the CDR process. A Protocol measures the full carbon impact of a process against the Baseline of it not occurring.

Purposefully erring on the side of caution under conditions of Uncertainty by choosing input parameter values that will result in a lower net CO₂ Removal or GHG Reduction than if using the median input values. This is done to increase the likelihood that a given Removal or Reduction calculation is an underestimation rather than an overestimation.

A set of data describing pre-intervention or control conditions to be used as a reference scenario for comparison.

An assessment of what would have happened in the absence of a particular intervention – i.e., assuming the Baseline scenario.

A common and recognized insurance mechanism among Registries allowing Credits to be set aside (in this case by Isometric) to compensate for Reversals which may occur in the future.