Coking by metal dusting of steels

In process industries coking is an annoying phenomenon, the carbon deposition causes decrease of heat transfer and hinders gas flow. Coking in a process may indicate metal dusting, i.e. the disintegration of metals and alloys in carbonaceous atmospheres under formation of graphite and fine metal particles. The metal particles act as catalysts for vast coke formation. The thermodynamics, mechanisms and kinetics of metal dusting have been studied on iron and steels in synthesis respectively reduction gas CO-H₂- H₂O, here the aspects are presented of coking due to metal dusting. From the interplay of the metal disintegration and carbon deposition rather complex coupled kinetics are resulting, even different in a low temperature range where the decomposition of the intermediate cementite is rate determining and in a higher temperature range where the carbon transfer from the atmosphere is rate controlling. Coking by metal dusting can be suppressed in the same way as metal dusting, by sulfur addition to the atmosphere and/or by a stable dense protective oxide layer.