Effect of temperature gradient on composition and morphology of synthetic chlorine-containing biomass boiler deposits

•A method for studying the effect of temperature gradient on deposit chemistry has been developed.•The deposit chemistry and morphology in temperature gradients are explained by phase equilibrium considerations.•CFD was used to simulate alkali chloride buildup in porous regions of the deposits.•Chlorine-enrichment in the deposits due to temperature gradients can lead to increased corrosion.

A novel laboratory method has been developed to study the chemical and physical behavior of ash deposits in a temperature gradient. Experiments with synthetic alkali salt mixtures similar to biomass boiler deposits show that alkali chlorides evaporate from hotter particles in the deposit and condense on colder particles closer to the cooled metal surface or even condense on the metal surface. Formation of a partially or completely molten layer in the outer hotter region closer to the flue gas is also observed in the experiments.The effect of time is shown to be significant for the enrichment of chlorides as longer experiment time leads to higher amounts of vaporization, transport and condensation within the deposits.These effects are quantitatively verified using Computational Fluid Dynamics modeling.The transport of alkali chloride vapors becomes negligible if the deposit and metal temperature is cold enough. An enrichment of alkali chlorides towards the cooled metal surface occurs and can increase chlorine-induced corrosion of superheaters as the deposits mature over time.The experimental observations are similar to superheater deposit morphologies observed in biomass boilers, such as straw-fired grate boilers.