The ash deposition behaviors of co-combustion of three-fuel blends of white pine pellet (WPP), peat pellet (PP), and crushed lignite (CL) coal were studied on a pilot-scale bubbling fluidized-bed combustor operated at 40% excess air ratio. Reference tests with individual fuel (pine, peat, or lignite) and two-fuel blends of lignite and pine or peat were also performed and discussed in this study. Fly ash deposits were collected with an air-cooled probe installed in the freeboard zone of the reactor. The collected deposits were comprehensively characterized by X-ray fluorescence (XRF), X-ray powder diffraction (XRD), ion chromatography (IC), and scanning electron microscopy (SEM) for their chemical compositions, mineralogical compositions, Cl/S concentrations, and morphology, respectively. As a very interesting finding from this work, co-combustion of the three-fuel blends at 50% lignite/25% peat/25% pine resulted in a higher ash deposition rate than co-combustion of two-fuel blends of either 50% lignite/50% peat or 50% lignite/50% pine. In contrast, co-combustion of three-fuel blends at 20% lignite/40% peat/40% pine resulted in the lowest deposition rate and the least deposition tendency among all of the combustion tests with various mixed fuels or individual fuels. The greatly decreased ash deposition tendency of co-firing three-fuel blends of 20% lignite/40% peat/40% pine might be accounted for by the formation of more minerals containing CaO, MgO, Al2O3, and SiO2 with high ash melting points and high crystallinity. The chemical compositions of deposits obtained from the co-combustions of three-fuel blends were apparently enriched with the elements Si and Al and depleted of the elements P, S, and K.
The objective of this study was to examine the ash deposition tendencies of biomass fuels and the biomass−coal blended fuels against the base fuel (coal) during co-firing and how the operating parameters influence the ash deposition tendencies. In this study, ash deposition behaviors during combustion and co-combustion of white pine pellets (WPP) and lignite coal were investigated in a pilot-scale, fluidized-bed combustor. Employing a custom-designed, air-cooled probe installed in the freeboard zone of the reactor to simulate a heat-transfer surface, effects of various operating parameters on the ash deposition rate and compositions of the ash deposits were studied, including the fuel type, fuel blending ratio (0−100% biomass on a thermal basis), moisture content, and air/fuel ratio. A new parameter, "relative deposition rate" (RDA), was proposed in this study to evaluate the relative deposition tendencies of biomass fuels and biomass−coal mixed fuels against the coal as the base fuel for co-firing. As expected, co-firing of the lignite and the wood pellets (with a much lower ash content than the lignite) resulted in a decreased superficial rate of ash deposition. However, co-firing of WPP and crushed lignite (CL) did not significantly increase the ash deposition tendency in terms of the values of RDA, and more interestingly, co-firing of the 50% CL−50% WPP fuel blend produced a lower RDA. Another new and interesting discovery of this study was that fluidized-bed combustion of an individual fuel or a mixture fuel with a higher moisture content produced not only a more uniform temperature profile along the fluidized-bed column but also a reduced ash deposition rate.
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