In metallurgical processes, packed beds of biochar are subjected to thermal gradients and heterogeneous reactions. In addition, densified biochar particles emerge as a promising carbon-negative insulation solution for building constructions. This makes the effective thermal conductivity (ETC) a critical parameter, but the ETC of biochar particles has not been systematically investigated in dependence of particle size, shape and packed bed porosity. This work aims to partially fill this gap by using densified biochar from spruce bark. The ETC is derived from experiments under non-reactive conditions and numerical analysis including the Zehner–Bauer–Schlünder (ZBS) model. X-ray microtomography was used to quantify particle structure, including sphericity and void fraction. Thermal properties were measured experimentally, and ETC was determined for various particle size fractions between 0.315 and 6.3 mm. The smaller particle fractions exhibited similar bed porosities and ETCs, whereas the largest particle fraction (4-6.3 mm) showed slightly lower ETC and higher bed porosity. Analysis with the ZBS model revealed the need to include an additional thermal resistance in the gap between the irregular particles due to the imperfect contact of particles. The results highlight the importance of manipulating inter-particle thermal contacts in altering the ETC of packed beds of irregular particles.