(635d) Dynamic Simulation of a Natural Gas-Fired Combined Cycle Power Plant Integrated with Chemical-Looping Combustion

Authors: 
Han, L., University of Connecticut
Zhou, Z., University of Connecticut
Bollas, G. M., University of Connecticut
Chen, C., University of Connecticut
Such, K. D., University of Connecticut

Dynamic Simulation of a Natural Gas-Fired Combined Cycle Power Plant Integrated with Chemical-looping Combustion

Lu Han, Chen Chen, Kyle D. Such, Zhiquan Zhou, George M. Bollas

Department of Chemical & Biomolecular Engineering, University of Connecticut, Storrs, CT

Abstract

Chemical-looping combustion (CLC) offers a promising method to reduce the CO2 emissions from power generation, at minimal energy penalty.1 The fixed-bed reactor is recognized as a promising reactor configuration for CLC because it avoids the problems with gas/solid separation and particle attrition commonly reported in circulating processes.2 Also, the fixed-bed is more compact and easier to scale-up and pressurize. The major disadvantage of the fixed-bed reactor is its dynamic operation, leading to fluctuations in temperature and flow rate of the streams exiting the reactor. This is challenging when the CLC reactor is integrated into a gas turbine-based combine cycle, because the downstream gas turbine is most efficient when operated at the nominal steady-state conditions.

In this work, we evaluate the dynamic operation of fixed-bed reactors and their impact on the power plant performance. Nickel- and copper-based oxygen carriers are exploited for their high reactivity and stability in the CLC cycles.3,4 First, a detailed kinetic model is needed to capture the reactivity of the oxygen carriers at the high-pressure operation of modern power cycles. Here, the kinetics of the oxygen carrier reduction and oxidation reactions are obtained from high pressure experiments conducted in a bench-scale fixed-bed reactor under methane and syngas fuel and various pressures from 1 to 10 bar.5 An empirical model6 is used to describe the effect of pressure on the kinetic rates, leading to accurate predictions of the data. Subsequently, the design of the CLC reactor is treated as a dynamic optimization problem, in order to maximize the efficiency of the power plant. The temperature fluctuations of the reactor exhaust are minimized to satisfy the standard operating conditions of commercial gas turbines, by manipulating the mass flows to the reactor, cycle times, and active metal loading of the oxygen carriers. In addition, the CLC reactor must exhibit a high fuel conversion (>98%), sufficient CO2 capture efficiency (>90%), and low pressure drop (<8%), which are set as constraints within the optimization problem.

We propose optimal designs for the CLC reactor utilizing nickel and copper oxygen carriers and explore the effect of the residual dynamics on the performance of the combined cycle power plant. A power plant model is developed in Dymola,7 consisting of a gas turbine and a steam cycle. The components of the model are separately validated with steady-state data from an existing plant.8 The system variables predicted by the model are within a narrow range of the reported values, and therefore the models can be used for dynamic analysis. In this work, the exhaust streams of the CLC reactor are fed to the gas turbine and bottoming cycles, to simulate an integrated natural-gas fired power plant with CLC. The feasibility of the combined cycle power plant to reach continuous operation is demonstrated, despite the inherent dynamics of the cyclic operation of the alternating-flow CLC reactor. By applying this model-based approach to the CLC power plant design, we are more equipped to examine the real efficiencies of the plant and propose methods to increase the heat recovery cycle for higher efficiencies.

Acknowledgement

This material is based upon work supported by the National Science Foundation under Grant No. 1054718. Financial support by Alstom Power, Inc. is gratefully acknowledged.

References

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(8)      Singer, J. G. Combustion Fossil Power: A Reference Book on Fuel Burning and Steam Generation; Combustion Engineering, 1991.