Aluminosilicate Sorbents for Control of Alkali Vapors during Coal Combustion and Gasification

Kaolinite is found to be a suitable sorbent for the removal of alkali from hot flue gases. The kinetics and mechanism of adsorption of NaCl vapor on kaolinite were studied at 800 °C under both nitrogen and simulated flue gas (SFG) atmospheres. Under nitrogen, both chlorine and sodium were retained by the sorbent; however, under the simulated flue gas, only sodium was retained. In both cases the adsorption was irreversible. High-resolution scanning Auger analysis of kaolinite particles indicated the formation of a product layer during adsorption. Under the SFG atmosphere the product layer appears to be nephelite, a stable sodium aluminosilicate compound. The rate of adsorption dropped with the increase in alkali loading, and a maximum saturation limit was observed. In the SFG environment this saturation capacity was approximately 5 times greater than that under the nitrogen atmosphere. An analytical model is presented that facilitates the extraction of fundamental kinetic information from experimental results. The model allows for surface adsorption as well as diffusion through both the saturated product layer and pores of the active sorbent.