High temperature corrosion of boiler waterwalls induced by chlorides and bromides. Part 2: Lab-scale corrosion tests and thermodynamic equilibrium modeling of ash and gaseous species

In Part 1 of the work a measurement campaign was carried out to determine the occurrence of corrosive ash components in a bubbling fluidized bed (BFB) boiler combusting solid recovered fuel (SRF). It was found out that the main ash-forming elements that had been vaporized and subsequently condensed from the gas phase were Na, K, S and Cl together with minor amounts of Zn, Pb, Cu, and Br. Both, Cl and Br were found in the corrosion front of the waterwalls prior to the measurement campaign. In this work, the forms of ash forming elements in the combustion gases and in the waterwall deposits of the examined boiler were predicted by means of thermodynamic equilibrium modeling. Laboratory tests were also carried out to estimate the degree of corrosion of boiler steels under bromine containing deposits. A mixture of ZnBr₂ and K₂SO₄ was selected for high temperature lab-scale corrosion testing since Zn, Br and K as well as sulfates were present in the deposit and it was possible to compare these results to previously published results with ZnCl₂–K₂SO₄ mixtures. The comparison showed that more severe material degradation was caused by the ZnBr₂ containing salt than by the corresponding chloride and corrosion was significant already at 400 °C.