Alkali chloride transport within superheater deposits due to temperature gradients

A novel laboratory method was used to study the effects of temperature gradients on synthetic superheater ash-deposits. The set-up includes an air-cooled probe inserted into a hot tube furnace, resulting in a temperature gradient over the deposit. Granular synthetic ash-deposits are applied on exchangeable steel sample rings that are a part of the probe. KCl+K2SO4 and NaCl+Na2SO4 mixtures were studied, chosen to simulate deposits in biomass fired boilers. The cross-sections of the deposits were analyzed using SEM/EDXA.Distinct multilayer morphologies were observed to form in the deposits during experiments. The outer layer of the deposit was sintered and agglomerated. The inner layer of the deposit was observed to remain porous. Within the porous layer, enrichment of pure alkali chloride was observed on the furnace-facing side of the ash particles and on the steel surface. In addition, alkali chloride depletion was observed on the steel-facing side of the particles.The thicknesses of the deposited alkali chloride layers were measured and theoretical gas phase diffusion rates for different alkali chloride species were calculated. The agreement between measurements and calculations support the hypothesis that the alkali chlorides evaporate from hotter particles, diffuse towards the lower temperature and deposit on the colder particles or steel surface. The alkali chlorides are transported due to concentration diffusion, induced by the temperature gradient. Within the deposit, the transport rate increases as a function of temperature. Alkali chloride transport was not observed in temperatures lower than 480 °C after exposure times of 72 h.The results from the laboratory study suggest that temperature gradients can affect the morphology and chemistry of superheater deposits. Depending on the deposit porosity, gas-phase transport can lead to local enrichment of alkali chloride within the deposit, resulting in deposit sintering and increased risk of corrosion.