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Investigations for the calculation and optimization of the heat transfer in the steam fluidized bed drying using the example of Lusatian lignite

Investigations for calculation and improvement of heat transfer in steam fluidized bed drying, using the example of Lusatian lignite

Lechner, Stefan

  • The present work deals with the heat transfer in fluidized beds with horizontal heat exchanger surfaces in the form of single tubes or tube bundles. The underlying application is the (pressurized) steam fluidized bed drying of lignite, which is ground to a grain size of 0-2 mm ("fine grain", Sauter diameter <85 micrometers), which meets the requirements of dust burners in power plants. In these fluidized beds, the particle convective heat transfer mechanism clearly dominates. The various (D) DWT and other drying processes are compared and evaluated on the basis of energy efficiency and large-scale applicability. There are currently great uncertainties and contradictions in the literature regarding the prediction of the heat transfer for different individual tube and tube bundle geometries. Areas of investigation in the work are the heat exchanger geometry, the influence of grain size distribution and heated neighboring tubes in the tube bundle as well as the effects of the presence and simultaneous The present work deals with the heat transfer in fluidized beds with horizontal heat exchanger surfaces in the form of individual tubes or tube bundles. The underlying application is the (pressurized) steam fluidized bed drying of lignite, which is ground to a grain size of 0-2 mm ("fine grain", Sauter diameter <85 micrometers), which meets the requirements of dust burners in power plants. In these fluidized beds, the particle convective heat transfer mechanism clearly dominates. The various (D) DWT and other drying processes are compared and evaluated on the basis of energy efficiency and large-scale applicability. There are currently great uncertainties and contradictions in the literature with regard to the prediction of the heat transfer for different individual tube and tube bundle geometries. Areas of investigation in the work are the heat exchanger geometry, the influence of grain size distribution and heated neighboring pipes in the pipe bundle as well as the effects of the presence and simultaneous evaporation of water. The predictive quality of the recognized theories of Martin and Molerus / Dietz is checked using the (D) DWT method with regard to the heat transfer from the heating pipe into the fluidized bed. The use of the Martine diameter for describing the highly polydisperse fluidized material turns out to be suitable. The additionally introduced 40-my criterion was identified as a suitable standard for comparing fine-grained, ground brown coals of different origins. The effects of the pipe diameter on the heat transfer were examined over the wide range of 12 - 200 mm and implemented in theory. The focus of the work is the investigation of different tube bundle geometries, especially with staggered tube arrangements, to derive a tube bundle factor based on geometric, dimensionless key figures, which predicts the reduction of the heat transfer coefficient at the tube bundle compared to the individual tube. The tube bundles used comprised tube diameters from 12 to 35 mm and tube pitches from 1.5 to 4.0. The main influencing variables found on the tube bundle factor are the horizontal and diagonal division as well as the smallest free tube distance between the tubes. Important marginal effects were also considered and discussed. In addition, the tube bundle geometries were compared on the basis of the specific, volume-related heat transfer coefficient. In the thesis, relatively precise and detailed design principles for the construction of tube bundle heat exchangers in fluidized beds could be worked out. These include the application of the (D) DWT, but can also be transferred to other fine-grain fluidized beds. ...
  • This work examines the phenomenon of heat transfer in fluidized beds equipped with horizontal heat exchanger surfaces such as single tubes and tube bundles. The underlying application is the (Pressurized) Steam Fluidized Bed Drying (P) SFBD of lignite ground to a grain size of 0-2 mm (“fine grain”, d <85 micron), suitable for a pulverized-fuel burner, such that the dominant heat transfer mechanism is heat transport through particle convection. The various (P) SFBD and other drying processes are ranked against each other according to energy efficiency and large scale applicability. There are currently large gaps and contradictions in the state of knowledge of heat transfer phenomena and its ability to predict the influence of single tube and tube bundle geometries. This, as well as the impact of grain size distribution, heated adjacent tubes and the presence and evaporation of water, is analyzed here. The predictive quality of the well-developed theories of Martin and Molerus / Dietz are tested in their application to the (P) SFBDThis work examines the phenomenon of heat transfer in fluidized beds equipped with horizontal heat exchanger surfaces such as single tubes and tube bundles. The underlying application is the (Pressurized) Steam Fluidized Bed Drying (P) SFBD of lignite ground to a grain size of 0-2 mm (“fine grain”, d <85 micron), suitable for a pulverized-fuel burner, such that the dominant heat transfer mechanism is heat transport through particle convection. The various (P) SFBD and other drying processes are ranked against each other according to energy efficiency and large scale applicability. There are currently large gaps and contradictions in the state of knowledge of heat transfer phenomena and its ability to predict the influence of single tube and tube bundle geometries. This, as well as the impact of grain size distribution, heated adjacent tubes and the presence and evaporation of water, is analyzed here. The predictive quality of the well-developed theories of Martin and Molerus / Dietz are tested in their application to the (P) SFBD process, specifically the heat transfer from heating tubes to the fluidized bed, which is the focus of this work. Here the use of the Martin diameter proves useful in describing the highly polydisperse coal. The additional introduction of a 40-micron criterion creates an efficacious standard of comparison for various fine-grained lignites. The effect of tube diameter on performance has been examined over a large range of values ​​(12 - 200 mm) and integrated into the theory. The following study of tube bundle geometry, in particular of staggered horizontal tubes, leads to the derivation of the tube bundle factor, based on dimensionless geometric numbers, which then predicts the reduction of the heat transfer coefficient compared to that of the single tube. The investigated bundles were composed of tubes of diameters 12 - 35 mm and pitch / diameter ratios of 1.5 - 4.0. The main factors influencing the tube bundle factor for a given geometry were found to be the horizontal and diagonal tube pitch as well as the minimum distance between tubes. Important edge effects are also discussed. Finally, the tube bundles are evaluated using the volume-related heat transfer coefficients. In summary, rather precise and detailed design principles are developed for the configuration of tube bundle heat exchangers, including those also used in (P) SFBD processes. ...