There is described a pyrolysis system for converting carboneous materials into biochar including a kiln, a retort, and a gas recovery system including a combustor supplying hot air to the kiln, a conduit fluidly connected between the retort and the combustor, the conduit conveying the pyrolysis gas and residues from a chamber of the retort to the combustor, and a filter cartridge fluidly connecting the chamber of the retort to the conduit for filtering the pyrolysis gas and the residues conveyed from the chamber of the retort to the combustor, the filter cartridge removably located inside the conduit and extending at least partially inside the chamber of the retort. Methods for operating the pyrolysis system are also described. The pyrolysis system and methods described herein produce biochar with improved resistance to self-heating.