A method and an apparatus for manufacturing, with thermal treatment, biocoal which is non-energent, such as functional as a heat sink, by using a conveyor arrangement housed in an essentially Thompson Converter type process space. A to-be-processed feedstock is conveyed in the process space with the conveyor arrangement, which is closed relative thereto, in a longitudinal direction of the process space. A pyrolysis gas, generated from the to-be-processed feedstock present inside the conveyor arrangement as a result of heat transferring from the process space thereto, is conducted into a combustion chamber included in the process space for burning the gas, a thereby generated flue gas being removed from the process space by a discharge arrangement and a resulting non-energent biocoal being removed from the conveyor arrangement for further processing.