Porosity measurement of biomass char by a novel combination of adsorption measurements and an optical method

As pores in biomass char can differ in size significantly, a method is developed to combine two comple- mentary methods (adsorption and optical) to cover a wide range of different pore size regimes. The pore size distribution for pore classes in the range of 0.33 nm to 50 nm is determined with 2D nonlocal density functional theory calculations, relying on N2 and CO2 adsorption isotherms. For the optical method, polished thin film char samples are evaluated under a scanning electron microscope investi- gating larger pores (with diameter dp > 600 nm). The novel combination of methods has been applied exemplarily to the porosity change of pulverized walnut shells. Therefore the walnut shell particles (90–125 µm) were pyrolyzed in a drop tube reactor with high heating rates (> 1000 K/s) and short residence times (max ~ 400 ms) at two different temperatures (1125 K and 1300 K) in a N2 atmosphere. Samples were taken at different residence times to produce particles of different pyrolysis degrees. The results of both methods (adsorption and optical) were combined and compared with the volatile yield. The results clearly show that the pores with diameter dp > 600 nm increase significantly compared to the small nanometer pores and have the largest influence on the overall porosity of the walnut shell particles through pyrolysis. Furthermore, changes in the cell structure were recognized, which align with the plastic deformation and overall changes in the particle morphology.