Applying heat from a heat source to a first region to cause a first pyrolysis process, the first pyrolysis process resulting in a gaseous mixture, and applying heat from the heat source to a second region to cause a second pyrolysis process, the second pyrolysis process being applied to the gaseous mixture, wherein the second region is located closer to the heat source than the first region. Pyrolysis is used to destroy oils, tars and/or PAHs in carbonaceous material.