By Shaoping Shi, Mehrdad Shahnam, and Madhava Syamlal, Fluent Inc.; Stephen E. Zitney and William A. Rogers, National Energy Technology Laboratory, Morgantown, WV

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Because of deregulation, rapidly changing market demands, fluctuations in natural-gas prices, and increased environmental concerns, gasification will become the centerpiece of tomorrow’s advanced power plants. Large improvements in the efficiency, reliability, and feedstock flexibility of gasification systems are necessary for the success of such power plants. To address these challenges, the U.S. Department of Energy (DOE) is sponsoring a broad spectrum of research and demonstration projects for gasification technologies and gasification-based power plants. For example, the DOE’s $1 billion, 10-year demonstration FutureGen project is aimed at creating the world’s first coal-fired, gasification-based, near-zero emissions electricity and hydrogen production power plant.

At the DOE’s National Energy Technology Laboratory (NETL), scientists and engineers are focusing on the need for high-fidelity gasifier modeling. Gasifiers involve complex physical and chemical phenomena including fluid flow, heat and mass transfer, and chemical reactions. Through gasification, coal is reacted to form CO/H2 rich syngas that undergoes processing before entering gas turbines or fuel cells. Combined with data from existing gasifiers, CFD offers a powerful method for understanding and improving the operation of these devices, especially those of industrial scale. As part of developing a comprehensive simulation of a potential FutureGen power plant configuration, a CFD model of a two-stage, oxygenblown, entrained-flow, coal slurry gasifier, a key component in the configuration, has been developed. This is a prototype gasifier design, which is not intended to represent any existing device, commercial or otherwise. The CFD-based gasifier model was integrated with a flowsheet model of the entire FutureGen power plant to perform the systems analysis of the power plant concept.

FutureGen power plant with gasifier [2]

Contours of temperature (left) and mole fraction of CO (right) on the center plane of the gasifier

The two-stage up-flow gasifier consists of a horizontal section (first stage) and a vertical section. Coal slurry and air are injected into the two side inlets of the first stage, which is mainly a coal combustor that provides hot gases to the second stage, where only coal slurry is injected and most of the coal gasification occurs.

The discrete phase model (DPM) in FLUENT is used to simulate the coal slurry flow as separate water droplets and coal particles. The coal slurry gasification process is implemented through user-defined functions (UDFs). Following injection, the coal particles undergo heating and devolitilization, releasing gases that include CH4, CO, CO₂, H₂, H₂S, O₂, N₂, and H₂O. Char combustion and gasification occur, according to an unreacted-core shrinking model [1]. The products of these reactions are primarily CO, CO₂, and H₂. Reactions in the gas phase, modeled using the eddy dissipation model along with an Arrhenius rate law, produce CO₂, H₂O, and H₂.

The predicted syngas composition (mole percentage) is 39.2% CO, 23.7% H₂, 23% H₂O, 10.3% CO₂, 1.5% CH₄, 0.7% H₂S, 0.8% Ar, and 0.8% N₂, which is very close to the values calculated by a 0-D Aspen Plus® model representing typical experimental data. The char conversions are 100% and 86% for the first stage and second stage, respectively. The benefit of using CFD is that the model accounts for the geometry of the gasifier and provides information on the distribution of species and temperature within the device. When combined with the process simulation calculation, CFD offers a way for investigating the impact of the gasifier design on the overall performance of the gasification-based power plant.

References:

1. Wen, C. Y. and Chaung, T.Z. (1979) “Entrainment Coal Gasification Modeling”, Ind. Eng. Chem. Process. Dev., Vol. 18, No. 4, 684-695.
2. www.netl.doe.gov, 2004.

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