Nicolai, Hendrik (2022)
Towards Predictive Simulations of Low-Emission Reactive Solid Fuel Systems.
Technische Universität Darmstadt
doi: 10.26083/tuprints-00021079
Dissertation, Erstveröffentlichung, Verlagsversion

Kurzbeschreibung (Abstract)

The increasing worldwide energy demand presents a strong contrast to the required CO2 reduction to limit global warming. Although renewable energy sources are developing rapidly, fossil solid fuels are expected to still play an essential role in worldwide electricity production in the foreseeable future. Therefore, the decarbonization of the power sector presents a major task for reducing industrial emissions. For this purpose, carbon capture, utilization and storage is considered a key technology for carbon-neutral power generation. Among the possible carbon capture procedures, the oxy-fuel process is one promising technology for rapid CO2 cutbacks. In the oxy-fuel process, the nitrogen fraction of air is substituted by the much more chemically reactive and radiation-absorbing molecules CO2, resulting in significant changes in the combustion characteristics. One major advantage of oxy-fuel processes is that flue gases consist almost entirely of CO2 facilitating its utilization and storage. To exploit the high potential of retrofitting existing plants for applying the oxy-fuel process, comprehensive simulation tools that can accurately predict such systems are required. Considering the combustion part of the oxy-fuel process, three main modeling pillars can be identified: 1) turbulent mixing and heat transfer, 2) turbulent chemistry interactions, and 3) solid fuel kinetics. Due to the strong coupling of all these processes, the weakest model determines the overall error. Hence, all three modeling pillars must be addressed adequately to contribute to an efficient and predictive holistic model for simulating pulverized solid fuel combustion in air and oxy-fuel conditions. In this work, Large-Eddy Simulation with a detailed chemical description of the gas phase using flamelet-based chemistry tables accounts for the first two pillars. With respect to solid fuel kinetics, besides state-of-the-art coal conversion models, a recently developed seamless multi-step mechanism is used to describe the particle conversion. A series of different configurations with increasing size and complexity are investigated to validate the developed modeling strategies. First, investigations of particle groups in a laminar flow reactor enable detailed analyses of the ignition and combustion process. Subsequently, at the laboratory burner scale, investigations focus on flame stabilization in complex turbulent flows. Finally, in a pilot-scale burner, the investigation of the overall conversion process is carried out. This step-wise increase of complexity allows for a seamless validation of models under practical relevant conditions. Furthermore, the validated numerical simulations are used to gain new insights into the complex coupled phenomena occurring during solid fuel combustion in an oxy-fuel atmosphere.

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