Abstract: A combustor for burning a feedstock includes a cylindrical body having a tangential feedstock inlet and an exhaust outlet. A catchment chamber of hollow cylindrical form intersects the wall of the body and opens into the cavity to provide a common boundary. Swirling or vortical flow is created in the cavity and a secondary vortex is generated and driven within the chamber by the main flow. In operation contaminant matter is sheared off from the flow in the body into the chamber and is entrapped by the secondary vortex from which it is precipitated, there being no net gas flow across the boundary.