Modellbasierte Regelung des Anfahrvorgangs eines Oxyfuel-Prozesses

RWTH Publications (RWTH Aachen)

Abstract

The present work examines the start-up process of the membrane-based oxyfuel process proposed in the OXYCOAL-AC project (project duration 2004 to 2011). The focus of the work is on a model-based control approach investigated as part of the project participation from 2007 to 2010, which can be used to carry out the start-up process of this process, which requires a large number of additional steps compared to the start-up process of conventional coal-fired power plants due to the novel process structure of the flue gas recirculation and its enrichment with oxygen in a membrane system. While the original research focused on the use of oxyfuel combustion in coal-fired power plants, the focus today is increasingly on industrial processes such as the cement industry or waste incineration, where CO2 emissions are generated by material conversions in the process. Building on the findings of the OXYCOAL-AC project, general follow-up activities in the field of oxyfuel combustion (SFB Oxyflame) and oxygen supply using membranes (MEM-OXYCOAL and HETMOC) have taken place. The basis for the analysis of the start-up process of the OXYCOAL-AC process is the extension and modification of a component library so that a detailed non-linear model of the entire process can be created, with which the entire start-up process from power plant shutdown to load increase to 100 % can be simulated. To validate the component models, data from a pilot plant of a project partner is processed and the models are parameterised according to the plant components. The comparison of simulation and measurement data shows that the models can reproduce the real behaviour of the system components well. Based on the detailed process model, a linear, simplified process model is created for use in a model-based predictive controller. For this purpose, the start-up procedure of the process is simulated and the model is linearised along the start-up curve at several transient operating points, reduced and the individual models are combined to form an overall process model. By using a controller with an internal process model to regulate the start-up process, a start-up concept is developed that can take into account the requirements of the temperature-critical ceramic membrane system at all times. Based on the start-up process of a conventional coal-fired power plant, a start-up concept is developed for the OXYCOAL-AC process, in which the start-up process of the flue gas cycle is integrated into the start-up process of the boiler and supplemented by additional measures. As the membrane system only supplies the oxygen required for oxyfuel combustion at high temperatures, the boiler is first started up with ambient air and the resulting flue gas is used to heat up the recirculation circuit before a continuous transition from air operation to oxyfuel operation takes place. In order to achieve the desired OXYCOAL-AC operation as quickly as possible, the maximal allowed temperature change rate is not only maintained, but also maxed out, during the start-up process by using model-based predictive control. For the boiler, taking into account the maximal allowed temperature change rate of the membrane material results in an extension of the start-up time to full load by about 10 percentage points compared to the start-up process in a conventional coal-fired power plant. The full OXYCOAL-AC operation is only reached eight hours later.

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