Quantifying Carbon Sequestration by Enhanced Rock Weathering

Quantifying the carbon sequestration by silicate minerals in Enhanced Rock Weathering (ERW) is a multi-step process that integrates geochemical and agronomic modelling approaches. The ERW process enhances the natural weathering of silicate rocks, leading to stable carbonate formation and long-term CO₂ drawdown. A robust carbon sequestration accounting framework is essential for the credible and effective deployment of ERW as a climate solution. It underpins transparent measurement, reporting, and verification (MRV), assures market and regulatory confidence, and enables ERW projects to credibly qualify for carbon credits and investment.

Steps for ERW Carbon Sequestration Estimation

1. Select and Characterize Silicate Feedstock

The initial step involves selecting minerals with a known Ca, Mg, or K-silicate composition, such as basalt, olivine, feldspar, or wollastonite. These minerals are then analyzed for purity, weathering rate, reactivity, and their potential for carbonate formation through laboratory tests like XRD and calcimetry.

2. Determine Application Rate and Particle Size

To maximize the reaction surface area, the selected rock is milled to sub-millimeter or micron grain sizes. The mass of the rock to be spread per unit area is then calculated, considering factors such as soil type, climatic conditions, and crop management practices.

3. Estimate Theoretical Carbon Sequestration Potential

The theoretical maximum CO₂ capture is quantified by estimating the moles of CO₂ that can be drawn down per mole of mineral based on its chemical formula and the main weathering reaction. For example, in the reaction for wollastonite:

CaSiO3(s) + 2CO2(g) + 3H2O(l) ⟶ Ca2+(aq) + 2HCO3-(aq) + H4SiO4(aq)                     

Each mole of CaSiO₃ sequesters two moles of CO₂. These estimates are then adjusted for incomplete conversion, competing reactions, and mineral impurities.

4. Calculate Realistic Field Uptake

Empirical weathering rate models are applied, taking into account particle size, soil pH, temperature, moisture, and land management. It is crucial to estimate the time to full reactivity, as real-world sequestration can occur over years to decades, not instantly.

5. Monitor and Verify Soil Carbon Change

Measurement, Reporting, and Verification (MRV) methods, such as calcimetry for inorganic carbon and soil sampling at different depths, are used to track changes in both inorganic (carbonates) and organic carbon pools. This helps capture the direct and indirect effects of ERW.

6. Account for Export and Storage Pathways

It’s important to assess how much dissolved inorganic carbon is leached to watersheds and eventually the ocean, and how much precipitates as carbonate in soils. Corrections are made for losses or dilution effects where carbon leaves the field system before mineral carbonation is complete.

7. Process CO2 Emission Correction

Finally, it's important to account for process emissions associated with feedstock production, transport, and spreading to arrive at net carbon sequestration.

Summary of Steps for Quantifying ERW