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Boss, Mark

Name:Boß, Mark

Staged fuel combustion for industrial water tube boilers - A generic burner family



At the beginning of this thesis an overview of the development of industrial boilers is given. The development path from little more than a large kettle on an open fire to a highly sophisticated and efficient water tube boiler design has been highlighted. In order to predict burner behaviour at these different boiler designs, burner manufactures use empirically derived formulae, which are capable to interpolate the behaviour of a burner within extremes. Examples of these empirically derived parameters have been given. To highlight the development and design process typical for industry and how it compares to development work in the academic sector, the design process of a generic burner family from a practical point of view is given. During the design process essential areas as swirler design, secondary gas nozzle design and prediction of high outlet velocity turbulent free jets are assessed with the help of empirical models and literature data. With the help of a simplified model a parameter study of the generic burner family is presented. To be able to compare the results of the simple models with full 3D CFD calculations the basics of CFD modelling with a strong focus on practicability within an industrial surrounding is given. Thirteen different CFD cases are presented, which focus on the turbulent free jet behaviour of the secondary gas jets, the efficiency of the combustion air geometry, the swirler design and on the combustion behaviour of a 20 MW burner of the generic burner family. It could be shown for the generic burner family that due to the intrinsic unstable combustion regime of the secondary gas jets, especially in case of a highly turbulent flow field inside an industrial water tube boiler, the secondary gas nozzle design is of paramount importance for the operation of this type of burner. Also the distinctive instability of the burner design was detected during the calculations.

The comparison of simple empirical models, non-reacting partial CFD calculations and full 3D calculations including combustion is astonishingly good with regard to the capabilities to describe the behaviour of the secondary gas jets and the attached combustion zone. It could be shown that with the help of empirical models the experienced developer is very able to identify promising geometries during the development process. With a comparison of CFD results and published data of industrial staged burner designs, the key parameters of influence of highly staged burner designs are highlighted. Even with relatively simple models for combustion and NOX formation the CFD is able to deliver acceptable indicative results. The basic description of the flame geometry is possible without any tailored validation process. Only in case a high degree of absolute accuracy with regard to the emission values is aimed for, a validation procedure with measurement results is necessary.

Based on the various empirical formulae describing the behaviour of industrial water tube boilers, the CFD results and measurement data from industrial applications a new heat release parameter (HRPN) has been proposed, which aims to take into account the complete boiler design. A comparison with results from the 20 MW burner of the generic burner family is promising.

The development process of industrial burners is executed under plenty of constraints, which mainly concern time, costs and a specific company history, rather than originating in the combustion theory itself. Each of the methods applied during the development of the generic burner family is valuable for the overall process. Models based purely on empirical correlations and measurements have the advantage of being instantly applicable to a problem. Descriptions based on models incorporating both empirical correlations and a more fundamental approach, describing the physical properties of the motion of fluids, add a degree of freedom. The use of CFD finally removes the need to tailor a model to a specific problem as far as possible. This leads to the final step of any development procedure. Ultimately the success of a development process can only be assessed with the help of full-scale or semi-industrial scale experiments. In the author´s opinion the experience of executing experimental trials is essential for any person who wants to work successfully within an industrial burner development environment. This experience also helps tremendously during the application of models; both from the group of empirical models and also for the effective use of CFD in an industrial combustion environment. 


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