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 (CO 2 ) from air or from a CO 2 stream. Physisorption of CO 2 is identified as the dominant process of CO 2 capture in biochar; it is also widely accepted that micropores play the main role in capturing and storing CO 2 molecules, whereas mesopores and macropores play a secondary role of providing a passageway for CO 2 molecules. This study aims to critically revisit the roles of mesopores and macropores in CO 2 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 CO 2 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 CO 2 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 CO 2 capture capability and mesopore/macropore permeability ( R 2 = 0.7148) is lower than the correlations with total pore volume ( R 2 = 0.7894) and fractal dimension ( R 2 = 0.7433). Multiple linear regression analyses also showed that only total pore volume and porosity are statistically significant in affecting CO 2 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 CO 2 captured and pore permeability. Graphical Abstract