Micropore analysis of biomass chars: Physisorption measurements of N2, CO2, O2, and H2 and application of different theoretical methods

The pore structure of biomass particles is a key property governing their reaction behavior during oxyfuel conversion. This study aims to assess the applicability of different physisorption measurement protocols and analytical approaches in pore structure analysis. Therefore, three samples produced in different conversion processes are investigated. The physisorption of N2, CO2, and H2 is measured at 77 K and CO2 at 273 K using a commercial volumetric sorption instrument. The four probe gases exhibit distinct adsorption behavior on the investigated chars due to their physicochemical properties under the respective adsorption conditions. This study highlights the suitability of probe gases for investigating complex pore structures in biomass chars. The applied and compared analytical methods involve the BET theory, the Dubinin-Astakhov (DA) theory, and different non-local density functional theory (NLDFT)-based models. For microporous biomass chars, the applicability of the BET theory is limited by both the feasibility of reliable N2 adsorption measurements and the inadequacy of the BET assumptions for adsorption in microporous carbons. The DA theory, based on CO2 isotherms, provides a simple and straightforward basis for micropore analysis. The most comprehensive characterization is obtained from two-dimensional (2D)-NLDFT analysis resolving the pore size distribution (PSD) of the samples. Provided that a reliable adsorption isotherm is available, 2D-NLDFT analysis is considered the most robust method for determining pore properties compared to BET and DA. Dual-gas 2D-NLDFT analysis expands the analyzable pore width range. However, this study highlights the sensitivity of correlated models when adsorption data are affected by pore blocking or diffusion limitations.