Optimierung der Prozessführung eines Wirbelschichtkessels durch Störfallanalysen

The present thesis analyses the operation of a bubbling fluidized bed boiler for the thermal utilisation of residual materials at the Norske Skog Bruck site. The objective of this study is to identify the causes of recurring disturbances and to develop optimisation approaches for stable plant operation. The primary focus of this study is on operating conditions characterised by unstable fluidization of the fluidized bed, as well as pressure fluctuations in the freeboard of the combustion chamber. The analysis is based on measurement data retrieved through a process control system combined with supplementary calculations of process parameters. The definition of a stable operating state is provided, and this is then systematically examined by evaluating deviations from this reference state. The evaluation of measurement data reveals that particularly uneven fuel distribution and the accumulation of foreign materials lead to instability in the fluidization of the bubbling bed. Localised accumulations of contaminants have been observed to contribute to the formation of ash agglomerates, which have been shown to result in enduring disruptions to the process. The analysis of pressure peaks indicates that these are not attributable to fluctuations in fuel quality but are instead significantly influenced by the current control system for combustion air supply. The utilisation of the average value of the oxygen measurements has been demonstrated to be associated with elevated sensitivity to the malfunction of individual measurements. This has the potential to trigger an unfavourable response in the control system, thereby leading to the destabilisation of the combustion process. The evaluation of an unplanned plant shutdown corroborates these findings and underscores the significance of a stable fluidized bed for ensuring efficient operation. Conversely, the analysis of a constructive modification to the fuel feeding system does not demonstrate any substantial enhancement in plant performance. This thesis contributes to a more profound understanding of the interrelationships within bubbling fluidized bed boilers and presents practical optimisation measures aimed at process stabilisation.