6.4.2.3.1 Alkali metal compounds

Alkali metal compounds may be found in the vapor phase at high temperatures and will therefore pass through particulate removal devices unless the gas is cooled. The maximum temperature considered to be effective for condensing alkali metal species is around 600°C. Tests have shown that their gaseous concentrations fall with temperature so much that the concentrations are close to turbine specifications at temperatures 500-600°C. Thus, gas cooling down to these temperatures may result in condensation of the alkali metals on the entrained solids and thus be removed with the overall particulate removal.

Alkali metal vapors may damage ceramic filters at high temperatures, and the gas thus has to be cooled before passing though the hot gas filter. Alkali metals may cause high-temperature corrosion of turbine blades, stripping off their protective oxide layer and for this reason, it is believed that the alkali concentration should not exceed 0.1 ppm at entry to the turbine. There is no experience with modern coated blades in such an environment. Alternatively, or additionally, water scrubbing can be employed for alkali removal.