Inhibition of lignite ash slagging and fouling upon the use of a silica-based additive in an industrial pulverised coal-fired boiler: Part 2. Speciation of iron in ash deposits and separation of magnetite and ferrite

This study for the first time reported the speciation of iron in ash deposits collected from the combustion of a pulverised lignite mixed with external silica in an industrial 30 MWth boiler. The iron speciation was conducted by the use of Mössbauer spectroscopy, Quantitative XRD, and SEM-EDX couple with elemental mapping. In addition, the magnetic particles were separated from fly ash deposits, and their yields and properties were compared with those reported in the literature. As has been confirmed, the addition of external silica caused a remarkable change on the speciation of iron in ash deposits. In the fireside slag, the quantities of highly molten Fe2+-slag and Fe3+-slag were reduced, whereas the contents of fully oxidised hematite and Fe3+-silicate were improved remarkably. The latter two species possesses high melting points and viscosity, and are therefore more difficult to melt to trigger the slagging. These changes are thought to be mainly contributed from the reactions between external silica additive and ash-forming metals, as well as the variation of fluid dynamics and particle residence time in the boiler. The formation of magnetite was also promoted noticeably in the silica additive case, due to a gradual oxidation of Fe2+ cation and precipitation of Fe3+ - bearing magnetite/ferrites out of the high-viscosity silica matrix, upon a gradual cooling and oxidation in flue gas. The highly molten species were agglomerated with external silica to form coarse clusters that rarely underwent supercooling when the external silica was added into coal. The ferrites achieved after the use of external silica contain less impure SiO2 and Al2O3 than those separated from raw coal fly ashes in the literature. The external silica cannot be simply treated as the inherent silica in a high-rank bituminous coal. Its properties such as contents of impurities, structure and size are apparently pivotal in the capture of Fe2+/Fe3+ cations and their speciation.