IRON-SULFIDE CRYSTALS IN PROBE DEPOSITS

Iron-sulfides were observed in deposits collected on a probe inserted at the top of the furnace of a coal-fired power station in Denmark. Th e chemical composition of the iron-sulfides is equivalent to pyrrhotit e (FeS). The pyrrhotites are present as crystals and, based on the sha pe of the crystals, it was deduced that they were not deposited but in stead grew within the deposit. The presence of unburned char particles within the deposits supports the concept that a reducing environment existed in the deposits. Two processes are proposed for explaining the existence of pyrrhotite crystals within a deposit: (1) impact of low viscous droplets of iron sulfide; and (2) sulfur diffusion. Previous r esearch on the influence of pyrite on slagging focused on the decompos ition of pyrite into pyrrhotite and especially an the oxidation stage of this product during impact on the heat transfer surfaces. This rese arch shows that the influence of pyrite and its derivatives is also st rongly controlled by the flue gas composition in the deposits. (C) 199 8 Published by Elsevier Science B.V. All rights reserved.