Numerical modeling of moving carbonaceous particle conversion in hot environments

• Main Author: Kestel, Matthias
• Other Authors: TU Bergakademie Freiberg, Maschinenbau, Verfahrens- und Energietechnik
• Format: Doctoral Thesis
• Language: English
• Published: Technische Universitaet Bergakademie Freiberg Universitaetsbibliothek "Georgius Agricola" 2016
• Subjects: Vergasung; CFD; Partikel; Verbrennung; Kohlenstoffpartikel; Flugstromvergaser; Kohlepartikel; Stefan-Strom; Nußelt Korrelation; Korrelation Widerstandskoeffizient; Numerische Simulation; 0-D Modell; Einzelpartikel; Umströmtes Partikel; Wärme- und Stofftransport; Reaktive Strömung; Gasification; Combustion; Particle; Single Particle; Moving Particle; Coal Particle; Carbon Particle; Carbon Conversion; Entrained Flow Gasifier; Stefan Flow; Nusselt Correlation; Drag Correlation; Reactive Flow; Numerical Simulation; 0-D Model; Heat and Mass Transfer; ddc:620; Kohlenstoff; Numerische Strömungssimulation; Stoffübertragung; Flugstromreaktor; Kohle; Stefan-Problem; Korrelation; Strömungsmechanik; Mathematisches Modell
• Online Access: http://nbn-resolving.de/urn:nbn:de:bsz:105-qucosa-204732; http://nbn-resolving.de/urn:nbn:de:bsz:105-qucosa-204732; http://www.qucosa.de/fileadmin/data/qucosa/documents/20473/Kestel_PDF-A-1b.pdf

The design and optimization of entrained flow gasifiers is conducted more and more via computational fluid dynamics (CFD). A detailed resolution of single coal particles within such simulations is nowadays not possible due to computational limitations. Therefore the coal particle conversion is often represented by simple 0-D models. For an optimization of such 0-D models a precise understanding of the physical processes at the boundary layer and within the particle is necessary. In real gasifiers the particles experience Reynolds numbers up to 10000. However in the literature the conversion of coal particles is mainly regarded under quiescent conditions. Therefore an analysis of the conversion of single particles is needed. Thereto the computational fluid dynamics can be used. For the detailed analysis of single reacting particles under flow conditions a CFD model is presented. Practice-oriented parameters as well as features of the CFD model result from CFD simulations of a Siemens 200MWentrained flow gasifier. The CFD model is validated against an analytical model as well as two experimental data-sets taken from the literature. In all cases good agreement between the CFD and the analytics/experiments is shown. The numerical model is used to study single moving solid particles under combustion conditions. The analyzed parameters are namely the Reynolds number, the ambient temperature, the particle size, the operating pressure, the particle shape, the coal type and the composition of the gas. It is shown that for a wide range of the analyzed parameter range no complete flame exists around moving particles. This is in contrast to observations made by other authors for particles in quiescent atmospheres. For high operating pressures, low Reynolds numbers, large particle diameters and high ambient temperatures a flame exists in the wake of the particle. The impact of such a flame on the conversion of the particle is low. For high steam concentrations in the gas a flame appears, which interacts with the particle and influences its conversion. Furthermore the impact of the Stefan-flow on the boundary layer of the particle is studied. It is demonstrated that the Stefan-flow can reduce the drag coefficient and the Nusselt number for several orders of magnitude. On basis of the CFD results two new correlations are presented for the drag coefficient and the Nusselt number. The comparison between the correlations and the CFD shows a significant improvement of the new correlations in comparison to archived correlations. The CFD-model is further used to study moving single porous particles under gasifying conditions. Therefore a 2-D axis-symmetric system of non-touching tori as well as a complex 3-D geometry based on the an inverted settlement of monodisperse spheres is utilized. With these geometries the influence of the Reynolds number, the ambient temperature, the porosity, the intrinsic surface and the size of the radiating surface is analyzed. The studies show, that the influence of the flow on the particle conversion is moderate. In particular the impact of the flow on the intrinsic transport and conversion processes is mainly negligible. The size of the radiating surface has a similar impact on the conversion as the flow in the regarded parameter range. On basis of the CFD calculations two 0-D models for the combustion and gasification of moving particles are presented. These models can reproduce the results predicted by the CFD sufficiently for a wide parameter range.