SOLID-STATE NUCLEAR-MAGNETIC-RESONANCE STUDIES OF PHOSPHORUS AND BORON IN COALS AND COMBUSTION RESIDUES

Solid-state nuclear magnetic resonance spectroscopy with magic angle spinning (MAS-n.m.r.) was used to study the occurrence of phosphorus and boron in coal, and their fate on combustion. These elements are only minor components of coal, but may significantly influence the utilization properties. 31P MAS-n.m.r. spectroscopy has confirmed that phosphorus is present in coal predominantly as apatite. This mineral is thermally stable under oxidizing conditions, and survives largely unaltered in high temperature ashes. However, under the semi-reducing bed conditions of certain stoker-fired boilers, it may be decomposed, volatilizing the phosphorus. The 31P MAS-n.m.r. spectra of bonded deposits show phosphorus in a markedly different coordination environment to that in apatite, the chemical shift suggesting aluminium phosphate or boron phosphate. 11B MAS-n.m.r. spectra of coals exhibit resonances due to both trigonal and tetrahedrally coordinated boron. Trigonal boron is probably present as tourmaline, but the nature of the tetrahedral boron is less certain; it may be held in tetrahedral sites within certain clay minerals. In common with phosphorus, boron may be volatilized during combustion. The 11B MAS-n.m.r. spectra of bonded deposits show a tetrahedral resonance with a chemical shift quite consistent with that of boron phosphate. © 1990.