Experimental study on the vaporization and condensation of alkali metal chlorides in biomass ash under pressurized conditions

The aim of this work was to understand the vaporization and condensation of alkali chlorides in biomass ash under elevated pressure, a series of experiments have been conducted on a horizontal high-pressure tube furnace. Stainless steel sheets were located in the tube furnace to condense inorganic species. Scanning electron microscopy-energy dispersive spectrometry (SEM-EDS), X-ray diffraction (XRD), X-ray fluorescence (XRF), and ImageJ software were employed to characterize and analyze the condensates and solid refractory residues. The results demonstrated that at 4.0 MPa, the relative change rates( λ ) of K 2 O and Cl in the solid residues reached 20.89% and 726.44%, respectively. Elevated pressure inhibited the vaporization of alkali metal chlorides in biomass ash, and high pressure further promoted the formation of mullite (Al 2.4 Si 0.6 O 4.8 ) and potassium calcium phosphate (KCaPO 4 ) in the solid residues. The condensates formed under different pressures were mainly white, square or spherical KCl and NaCl particles; when the pressure increased from 0.1 MPa to 2.0 MPa, the particle size of the condensates decreased to 18.8% of the initial value. In conclusion, pressure exerted a crucial regulatory effect on the vaporization and condensation processes of gaseous chlorides throughout the entire thermal conversion process. • Increasing reaction pressure inhibited the vaporization of alkali chlorides in corn straw ash. • Mullite (Al 2.4 Si 0.6 O 4.8 ) diffraction peaks detected in solid residue under pressure (2 MPa and 4 MPa). • The average size of condensates reduced from 3.733 μm at 0.1 MPa to 0.723 μm at 4.0 MPa. • The condensate was mainly white, spherical and square KCl and NaCl particles.