Ash deposition in Co-firing three-fuel blends consisting of woody biomass, peat, and lignite in a pilot-scale fluidized-bed reactor

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.