CFD and CPFD modeling of hydrogen production from coal via Lewis Ultra-Superheated Steam (USS) fluidized bed gasification

Developmental work on the Lewis Ultra-Superheated Steam (USS) bubbling bed coal gasification is being performed as a joint venture between Ohio University (OU) and the University of North Dakota Chemical Engineering Department (UND). The research objective of this joint venture is to develop a low capital and operating cost method for gasification of coal into tar-free and hydrogen enriched synthesis gas without the need for expensive catalysts.;Ultra-Superheated Steam (USS) gasification is used to convert coal to hydrogen. The process, known as Lewis USS reforming, creates an extremely hot steam flame, which is formed at the tips of the tuyeres as the gases enter a fluidized bed containing coal. A major process advantage of steam-only gasification is the absence of the oxidation of the feedstock material. The conversion efficiencies and product streams are being quantified in a bench-scale unit at OU, and comprehensive modeling is being used to further develop an understanding of the processes. A thermo-equilibrium model of the gasifier was developed at UND using ASPEN PLUS. Results from the thermo-equilibrium modeling proved the two hypotheses. The H2 composition in the syngas exceeds the original amount of H2 in the coal with steam as gasifying agent and the Lewis USS gasifier produces a syngas (254 Btu/scf) having higher heating value than common air blown gasifiers.;Comparing the thermo-equilibrium model results to experimental data indicated that the fast water-gas shift reaction is not in equilibrium and there may be transport/diffusion limitations in the pilot-scale gasifier.;The objective of the current work was to develop (1) a CFD and CPFD model of an advanced Lewis USS coal gasifier that can determine syngas composition for various ranks of coal based on operational conditions, and (2) a CFD/CPFD model that will interpret the transport/diffusion limitations to describe the mechanisms for the water-gas shift reaction and determine reactor configurations that will allow it to reach equilibrium.;The hypothesis of the current work is that CFD and CPFD modeling based on first principles for mass/heat transfer and hydrodynamic behavior in bubbling beds can model the Lewis USS gasifier syngas composition. A CFD and CPFD model of the Lewis USS gasifier was developed at UND using ANSYS FLUENT 12.1.4 and BARRACUDA 14.5. Results from the CFD and CPFD modeling proved the hypothesis. Hydrodynamic and transport limitations within the bubbling bed and freeboard dominate over the reaction kinetics, preventing the produced syngas to be at equilibrium within the entire freeboard.;The CFD and CPFD models also represented the hydrodynamic behavior of a Geldart B particle size bubbling bed, and CFD modeling represented how the bubbling bed acts as a flame arrester between the temperatures of 750°C and 900°C.;Keywords: Ultra-Superheated Steam; USS; Lewis USS; Modeling; ANSYS FLUENT; CFD; BARRACUDA; CPFD; Coal/char gasification; Fluidized bed gasifier...