Laboratory studies on the ageing behaviour of waste incineration superheater ash deposits

Combustion of biomass for heat and power generation is typically associated with the formation of highly corrosive ash deposits on heat transfer surfaces. To get a better understanding of how these deposits change in their local chemical composition and morphology with time, synthetic deposit mixtures containing different proportions of NaCl, KCl, CaCl2, and CaO were exposed to a temperature gradient utilising a novel experimental approach. In the experiments, exposure time, steel temperature, and furnace temperature were varied. Temperature gradient induced changes in the local deposit composition, and connected to that, the deposit corrosivity were identified. Melt formed within the deposit moved toward the steel and the subsequent dissolution of formed corrosion products into the molten phase reduces its first melting temperature resulting in the direct contact between the melt and steel. CaO was shown to have a dual role in the corrosion process. CaO reacts with Cr2O3 and Fe2O3 forming CaCrO4 and Ca2Fe2O5. Especially the reaction with Cr2O3 destroys its protective properties toward Cl-induced corrosion. Furthermore, CaO reduces the amount of melt formed within the deposit at the prevailing temperatures, generating a porous deposit morphology enabling the diffusion of oxygen through the deposit toward the steel surface, further propagating corrosion.