Quantitative analysis of feedstock structural properties can help to produce willow biochar with homogenous pore system

•Possibility to increase structural homogeneity of willow biochar was studied.•Clonal variation in willow affects the micrometre-scale porosity of biochar.•Pyrolysis faded off the structural differences observed in the raw material.•Fibre analysis techniques could be used to predict some biochar pore properties.•Homogenous biochar can be produced by selecting monoclonal willow stands.