In an attempt to understand the problem of severe slagging in biomass-fired boilers, slags formed on the fourth-, second-, and first-stage superheaters in a 12 MW biomass-fired grate furnace have been collected, sampled, and analyzed using X-ray fluorescence (XRF) and X-ray diffraction (XRD) techniques. The slag collected on each superheater has a unique morphology and structure. The slag on the fourth-stage superheater is loose and porous, while on the secondary and the primary superheaters, the slag shows a clear layered structure of different colors and levels of hardness. Aluminosilicate is the major component in the slag on the fourth-stage superheater, which is formed by Si, Al, and alkali species. On both the secondary and primary superheaters, the sticky sylvine, potassium calcium sulfate, and aphthitalite in the bottom layer trap coarse, large particles, resulting in further slagging. In the alternating layers, the particles containing high concentrations of K, Na, and Cl form the initial deposit layer, the yellow layer in the secondary superheater. Then, it traps the coarse, large particles containing high Si, Al, Ca, Mg, and Fe to form the brown layer. Similarly, the particles in the pale-yellow layer of the primary superheater containing high concentrations of K, Na, and Cl form the initial deposit layer, and then, it traps the coarse, large particles containing high Si, Al, Ca, Mg, and Fe to form the black layer. Accompanied by the periodic blowing, the alternating layer is formed and the slag gradually becomes thicker.
