Knowledge of the
effect of aluminosilicates in fly ash on slagging
is very important for the prevention and control of slagging in coal-fired
boilers. To understand hydrochloric acid dissolution of the aluminum
element of fly ash aluminosilicates at a chemical coordination level
and establish a reliable method to characterize fly ash aluminosilicates
that easily cause slagging, we performed hot hydrochloric acid separation
for fly ashes with different melting points obtained from four power
plants in China and measured the acid-solubility of major elements
in the fly ashes. The fly ashes and their acid separation residues
were analyzed using X-ray diffraction and 27Al magic-angle
spinning nuclear magnetic resonance (27Al MAS NMR). The
results show that the 6-fold coordinated aluminum (denoted as Al(VI))
in the fly ashes does not easily dissolve in hot hydrochloric acid.
One part of the 4-fold coordinated aluminum (denoted as Al(IV)) easily
dissolves in hydrochloric acid. The acid-soluble Al(IV) in ash has
stronger quardapolar interactions than the Al(IV) of mullite, and
the excess negative charges of the AlO4 tetrahedron are
compensated by active metal cations. The aluminosilicates containing
a large amount of acid-soluble Al(IV) are easily slagged. In the fly
ashes with high melting point, the mullite and corundum contents are
high; the Al(IV) content is low, and the acid-soluble Al(IV) is minor.
In the fly ashes with low melting point, the aluminosilicates are
mainly amorphous substances; the Al(IV) content is high, and the acid-soluble
Al(IV) content is more. Therefore, the acid-soluble aluminum fraction
of fly ash can be used to characterize the slagging propensity of
aluminosilicates in fly ash.