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The impact of fine fuel particles on ash deposition in solid biomass combustion: Experiments and CFD modelling
347 Seiten, Dissertation Universität Erlangen-Nürnberg (2019), Softcover, A5
This thesis addresses one of the major challenges for the stable operation of biomass-fired industrial grate and fluidised bed combustion chambers: ash deposition. The main scope of this work is the investigation of the impact of fine fuel particles, i.e. undesired comparably small fuel particles within a typically chipped feedstock, on the ash deposition characteristics of industrial biomass furnaces in the megawatt range.
In the first step, lab-scale experiments serve the investigation of the deposition propensity of biomass feedstock. The plant features an isothermal vertical deposition probe in order to provoke ash accretions and facilitate the assessment of axial deposition profiles. Subsequent analysis by means of laser diffraction measurement and SEM/EDS reveal the condensation of inorganic matter, deposition of aerosols and the inertial impaction of either (partially) molten or coated coarse fly ash as dominating underlying deposition mechanisms with varying intensity, depending on the ash composition. The experiments with additional supply of fine fuel particles lead to a disproportional increase of collected deposits, especially due to inertial impaction.
The experimental results define the requirements for numerical ash deposition predictions based on burning fine fuel particles. This leads to the development of a CFD framework for biomass power plant simulations including gas phase and particle combustion, ash softening and sticking sub-models. The application to the lab-scale plant and three industrial plants in the 5-40 MW range leads to plausible and realistic results for gas phase and particle combustion as well as regarding the locations and severity of predicted ash deposits with respect to operating point and feedstock properties.