The engineering of biochars with desired morphologies and pore structures is a far-reaching objective towards sus‑
tainable pore-dependent environmental technologies, such as water and soil remediation or catalysis. We hereby
report a series of experiments that allow the direct following of the shape and porosity of single biochar particles
during pyrolysis. Particles~1–2 mm in diameter of unwashed and water-washed raw walnut shells were continuously
3D imaged during pyrolysis to 575 ℃ at a 10 K min−1 in Ar to obtain time- and temperature-resolved x-ray micro com‑
puted tomographies to a 0.82 μm resolution. Results showed visual evidence of a 30% and 70% v/v particle shrinkage
for unwashed and washed samples, respectively. Particle swelling between 200 and 300 ℃ in the unwashed sample
provided evidence of the softening of native biopolymers associated with lignin in untreated biomass. A purpose defned parameter Λ shows the temperature-dependence of pore re-distribution towards the center of the particle
to be linear for both samples. Λ was found to be 3.2 × 10−4K−1
in the washed sample, approximately 3.5 times faster
than in the unwashed one. Such linear dependence is signifcantly slower than an exponential Arrhenius-like trend
thereby providing a qualitative measure of the heat and mass transport phenomena limiting the chemical reactions
in the porous medium. This evidence is key to resolving the pathways to the thermochemical decomposition of bio‑
mass leading to preparation of precision-engineered biochars