Micropore Analysis of Biomass Chars by CO2 Adsorption: Comparsion of Different Analysis Methods

Detailed knowledge of the pore structure evolution of fuel chars is essential for a sound description of the conversion process. The analysis of the chars by N-2 adsorption at 77.36 K according to the theory of Brunauer, Emmett, and Teller has been established as a standard in pore analysis. However, at this low temperature, the diffusion of N-2 in micropores is limited, and N-2 is therefore not suitable for an analysis of the microporous structure. For micropore analysis, CO2 adsorption at 273.15 K can be used instead since CO2 shows no limitation with regard to diffusion into micropores. For the evaluation of the CO2 adsorption data, various analysis methods are available, which are commonly used in the literature. However, no consistent method has been established in the literature to determine micropore volume and surface area, and the used methods are often not described in a traceable way, which makes a comparison of results based on different analysis approaches hardly possible. To show and discuss the differences, CO2 adsorption experiments were conducted in this study for four biomass chars. The data were analyzed according to Dubinin-Astakhov (DA), Dubinin-Radushkevich (DR), and nonlocal density functional theory (NLDFT) analysis. Because DA and DR are only able to determine the pore volume of the chars, an additional correlation is required to determine the surface area. Three different correlations for the relation between pore volume and surface area were compared within this study as well.

Automated summary

Detailed knowledge of pore structure evolution of fuel chars is essential for a sound description of the conversion process. N-2 adsorption is standard for pore analysis, while CO2 adsorption is more suitable for micropore analysis as it shows no limitation with regard to diffusion. Despite various analysis methods for CO2 adsorption data, no consistent method has been established in the literature for determining micropore volume and surface area.