Ash Deposition during Co-firing Biomass and Coal in a Fluidized-Bed Combustor^†

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.