Characterization of Aluminosilicates in Fly Ashes with Different Melting Points Using ²⁷Al Magic-Angel Spinning Nuclear Magnetic Resonance

Abstract

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.

Evolution of Fly Ash Aluminosilicates in Slagging Deposition during Oxy-Coal Combustion Investigated by 27Al Magic Angle Spinning Nuclear Magnetic Resonance

“…A small amount of Al­(VI) (chemical shift: 2.8 ppm) is also present in fly ash, and its NMR center peak is less intense than the spinning sideband of Al­(IV) on the high-field side. According to the corresponding research, Al­(VI) with this chemical shift value originates from aluminosilicates in fly ashes, whereas the chemical shift of corundum Al­(VI) is approximately 14.7 ppm. The XRD and NMR results indicate that the Al­(VI) of kaolinite in the coal was mainly transformed into Al­(IV) in aluminosilicates during fly ash formation.…”

“…Because of the complexity of fly ash, the difficulty of its spectral resolution increases when 27 Al MAS-NMR is used to analyze it. Although many researchers in the field of fly ash utilization have used 27 Al MAS-NMR to analyze chemically treated fly ash, this technology has rarely been used to analyze fly ash and ash deposits on-site to study the boiler slagging mechanism. − Tian et al used 27 Al MAS-NMR to study fly ash samples with different ash melting points before and after a hydrochloric acid separation process. They reported that higher acid-soluble fractions of fourfold-coordinated aluminum [denoted as Al­(IV)] indicated that the aluminosilicates contained more active metal elements and were easily melted.…”