Capture of Alkali Metals by Kaolin

Alkali metals present in biomass fuels may cause increased bed agglomeration during fluidized bed combustion. In worst case this may lead to complete defluidization of the bed. Other problems caused by alkali metals include increased fouling and slagging. One possibility to reduce the impact of alkali metals is to add sorbents, e.g. aluminosilicates, to the bed for the capture of alkali metals. In the current investigation, the capture of vapor phase potassium compounds by kaolin was investigated in a fixed bed reactor. The reactor consisted of an alkali metal source placed at a variable temperature from which gaseous potassium compounds were generated, a fixed bed holding the kaolin, and an on-line detector for the alkali metal concentration. The on-line alkali metal detector was based on ionization of alkali metals on hot surfaces and is capable of detecting alkali metals down to ppb levels. This makes it possible to perform experiments at alkali metal concentrations relevant to fluidized bed combustion of biomass fuels. In the experiments, KCl was used as the alkali metal source with inlet concentrations of 0.5–3.5 ppm. The experiments were performed at reactor temperatures of 800–900°C and a contact time of 0.26 s. The capture efficiencies of KCl were always above 97%. The capture efficiency was somewhat higher in oxidizing than in reducing gas atmospheres. In the oxidizing gas atmosphere, the conversion was slightly higher with H₂O addition than without. The capture efficiency decreased slightly as temperature or KCl concentration was increased.