Ascertaining the role of mesopores and macropores in capturing carbon dioxide in multi-hierarchical biochar sorbent: a theoretical and experimental approach

Abstract

Biochar is increasingly studied as a potentially carbon-negative material that can also capture and sequester carbon dioxide (CO₂) from air or from a CO₂ stream. Physisorption of CO₂ is identified as the dominant process of CO₂ capture in biochar; it is also widely accepted that micropores play the main role in capturing and storing CO₂ molecules, whereas mesopores and macropores play a secondary role of providing a passageway for CO₂ molecules. This study aims to critically revisit the roles of mesopores and macropores in CO₂ capture. The research objectives include proposing improved mathematical models for calculating fractal dimensions for micropores, mesopores and macrospores in sawdust and biochar samples produced at different temperatures (300 °C, 500 °C, 700 °C, and 1000 °C); the CO₂ capture capability of these sorbents was then correlated with and predicted from their fractal dimensions, pore volume, pore area, pore diameter/width, permeability, and porosity. For mesopores/macropores, the improved fractal dimension model expressed the mercury contact angle on the sorbent pore wall with respect to the surface area covered by mercury. For micropores, the improved model describes the CO₂ adsorption cross-section of the sorbent within the formulation of the Brunauer–Emmett–Teller (BET) theory. It was found that these two models provided more physically realistic values for fractal dimensions (between 2.81 and 3.00) for all the sorbents, compared to existing models. The correlation between CO₂ capture capability and mesopore/macropore permeability (R² = 0.7148) is lower than the correlations with total pore volume (R² = 0.7894) and fractal dimension (R² = 0.7433). Multiple linear regression analyses also showed that only total pore volume and porosity are statistically significant in affecting CO₂ capture by these pores. Scanning electron microscopy revealed “in-foldings” of pore wall in biochar samples produced under high temperature, which can help explain the relatively low correlation between CO₂ captured and pore permeability.

Graphical Abstract

Keywords

Biochar / Carbon capture / Fractal dimension / Mesopores / Macropores / Mercury contact angle

Highlight

•	CO2 capture is correlated with total mesopore/macropore volume (R2 = 0.7894) and fractal dimension (R2 = 0.7433)
•	CO2 capture’s correlation with mesopore/macropore permeability is lower (R2 = 0.7148)
•	This is due to “in-foldings” on mesopore/macropore pore walls observed in biochar produced at higher temperatures

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Funding

College of Design and Engineering Dean’s Chair fund(E-471-00-0010-03)

Research Support Fund(E-471-00-0009-02)

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The Author(s)