Carbonaceous Material Gasification: Zero-Dimensional Analysis and Development of Computational Fluid Dynamics Model for an Entrained Flow Gasifier

  • Ilham Talab

Student thesis: Master's Thesis

Abstract

Gasification is an oxygen-deficient thermochemical conversion process of solid fuels (coal, petroleum coke, biomass, municipal solid waste…etc) into a mixture of gaseous energy carriers (mainly hydrogen and carbon monoxide). The US DOE estimate for the world gasification capacity as of 2010 is 70 GWth of syngas output with a projected increase of 70% by 2016. In addition to providing a solution to the 'bury or burn' practice for solid waste, the produced syngases from gasification can be used for chemical manufacturing or as a fuel for power generation. Significant developments in gasification technologies since the early 1800's has made the integration of gasification in combined power cycles a key enabling technology for advanced, high-efficiency power generation where efficiencies approaching 60% have been reported. This work aims at developing the fundamental tools used to assess the performance of entrained flow gasifiers using a zero-dimensional model and a detailed flow analysis. The first approach assumes chemical and thermodynamic equilibrium and uses the concept of 'element potential' to determine the resulting composition of syngas at varying operating conditions. The developed model correlates well against data obtained from actual operating gasifiers (Texaco gasifier) using three different coals. Although entrained flow gasifiers are amenable to equilibrium modeling, there exists a need for a more detailed high-fidelity analysis that can generate localized information such as spatial temperature distribution, species distribution in addition to include the effect of reaction kinetics and turbulence thus suggesting the use of Computational Fluid Dynamics (CFD). CFD, which can be defined in its simplest form as the art of replacing the governing partial differential equations of fluid flow with numbers, is a powerful tool that has been used to model reactive flows with reasonable accuracy and minimum cost especially given the advances in computational power in terms of speed and memory. Despite the overwhelming number of publications in the field of reactive (gasification) flow CFD modeling, the literature review revealed a lack of emphasis on the detailed chemical and physical processes that characterize gasification such as the evolution of light gases (devolatilization) and tar formation which were reported to have significant effect on the temperature profile and thus are specifically targeted in this study. The second part of this work focuses on developing a CFD model for a pilot-scale entrained-flow air-blown gasifier tested in Japan with a throughput of 200 ton of 'Taiheiyo' bituminous coal/day. Boundary conditions resembling the pilot-scale gasifier operation are applied to the three-dimensional domain which consists of 1.5 million finite volume cells. The micron-sized particles are modeled using the Discrete Phase Model with a Lagrangian-Eulerian scheme. Non-reactive flow simulation was performed first followed by reactive flow where gasification models including, devolatilization and global homogeneous and heterogeneous reaction kinetics are developed and implemented. The computed solution agrees favorably with literature with an improved temperature profile especially near the burners.
Date of Award2011
Original languageAmerican English
SupervisorIsam Janajreh (Supervisor)

Keywords

  • Energy Conversion
  • Fluid Dynamics
  • Fluidization

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