Advantages of dual CO2 & O2 adsorption model for assessment of micropore development in biochar during two-stage gasification

Residual biochar has the potential to replace commercial carbons even in highly specialised applications, presuming further advances in the engineered biochar production. Optimising biomass conversion requires dynamic feedback on the resultant char porosity, but investigation of pore size distribution (PSD) in pyrogenic carbons is challenging due to their extremely ultramicroporous nature. The most common probe molecule used in gas adsorption methods, N2, is often unable to access the narrowest pores, while CO2 can analyse only pores <10 Å. We propose an approachable way to evaluate PSD of ultramicroporous carbons by simultaneously fitting the 2D-NLDFT (two-dimensional non-local density functional theory) kernels to the CO2 and O2 isotherms. Using O2 as the probe molecule allowed collecting isotherms for the ultramicroporous pyrolytic char, for which a negligible adsorption of N2 was observed. For the more activated, gasification chars, reaching equilibrium for N2 adsorption took up to 8 h for some datapoints. O2 adsorption was much faster, resulting in the total runtime more than two times shorter than for the N2 tests. By analysing chars from the semi-pilot scale gasifier, we confirmed the dependence of the char structure on the process parameters, but we also suspect a strong influence of the reactor design.