Experimental and modeling approaches to simulate temperature-gradient induced intradeposit chemical processes with implications for biomass boiler corrosion

The heterogeneous nature of the ash chemistry of biomass fuels gives rise to challenges in predicting the intradeposit chemical processes relevant for deposit melting, sintering and enrichment of corrosive ash species.

An experimental method has been developed to study the evolution of ash deposit chemistry and morphology in temperature gradients simulating conditions similar to real superheater deposits. The method is based on applying synthetic ash mixtures on an air-cooled corrosion probe, which is inserted into a tube furnace in air or synthetic flue gas. Gas temperatures vary between 700 and 900 °C and probe temperatures vary between 300 and 600 °C. Focus has been on how melting behavior of alkali salt-rich deposits, i.e. KCl-K₂SO₄-NaCl-Na₂SO₄ mixtures, affects the sintering of the deposits, as well as on studying vaporization-condensation of KCl and NaCl within the deposits. The interaction of reactive gas components, such as SO₂ and gaseous KCl, with the deposits was also studied.

The vaporization-condensation mechanism was shown to lead to enrichment of alkali chlorides towards colder surfaces within the porous parts of the deposit. It leads to condensation and build-up of chlorides on the steel surface, which causes accelerated corrosion, due to the formation of low-melting FeCl₂ mixtures. Liquid-phase sintering and temperature gradient zone melting were shown to be the main mechanisms for the supersolidus sintering of the deposits.

The vaporization and condensation of alkali chlorides within the deposits was modelled to explain the time-dependent build-up of the chlorides using both CFD and thermodynamic modeling. Temperature gradient induced Fickian concentration diffusion was shown to be the main mechanism and accurately predicted the alkali chloride build up as a function of deposit composition, local temperature and time. A CFD-model for predicting the alkali chloride enrichment in superheater deposits in full-scale boilers has also been developed.