(212f) Efficiency Evaluation of Combined Cycle Power Plants Integrated with Fixed-Bed Chemical-Looping Combustion Reactors

CO₂
produced from the combustion of fossil fuels for power generation is the main
greenhouse gas contributing to climate change. The mitigation of CO₂
emissions in power generation can be achieved by increase in renewable industry
penetration, improvement of power plant efficiency, and integration of carbon
capture and sequestration (CCS) technologies. Specifically, chemical-looping
combustion (CLC) is a promising CCS technology for power generation with low
energy penalty and inherent CO₂ separation.¹ CLC involves
the use of a metal oxide as an oxygen carrier which transports the oxygen from
the air to the fuel, and avoids direct contact between fuel and air. Commonly,
CLC is realized as two interconnected reactors, a Reducer and an Oxidizer. In
the Reducer, fuel is oxidized by the metal oxide into combustion products (CO₂,
H₂O), and pure CO₂ can be captured after the stream of
combustion products is condensed. In the Oxidizer, the reduced metal oxide is
reacted with air. Other CLC reactor designs include fixed bed reactors²
and reverse flow fixed bed reactors,³ which provide process
intensification and reactor modularization advantages compared to traditional
fluidized beds.

The focus of this
work is on combined cycle (CC) power plants integrated with fixed-bed CLC
reactors, which consist of a fixed bed reactors island, an open cycle air gas
turbine and a water/steam cycle. This plant configuration is of interest due to
its high efficiency, low installation cost, and faster commercialization time.^4,5
We explore the feasibility of fossil-fueled power plants with CLC on the basis
of “virtual” power plant prototypes, as shown in Figure 1. Prototype plant
models were first validated against static, full load data from an existing
natural-gas-fueled supercrictical power plant with an efficiency of 57.9%, and
then the conventional combustion chamber was replaced by a high-pressure
fixed-bed CLC island. Dynamic optimization of high-pressure fixed-bed reactors
was performed to improve the utilization and efficiency of the gas turbine of
the power plant.⁵ The main factors impacting the integrated CLC-CC
power plants efficiency were explored, including the CLC reactor operating
strategy, pressure ratio, gas turbine inlet temperature and temperature of
ambient air. This analysis showed an optimal efficiency of up to 49% being
feasible for the integrated CLC-CC plant. This was accomplished by utilizing
all the reactor exhaust streams to improve the Brayton and Rankine cycles
efficiency. Dynamic simulation of the integrated CLC-CC plant showed that the
integrated CLC-CC power plant generates a relatively flat power output and CC is
only slightly affected by the discontinuous batch nature of the operation of
the fixed-bed CLC reactors. Plant simulation under part-load and transient load
operation was also studied and a plant level control architecture was devised
for efficiency optimization. The developed simulation and optimization
framework enabled in-depth studies of stability, feasibility and optimization
of power cycles with CLC technologies embedded.

Figure 1: Simplified diagram of
the CC integrated with CLC. (CPR: Compressor; G: Power generator; GT: Gas
turbine; SH: Superheater; RH: Reheater; EVA: Evaporator; ECO: Economizer; HP:
High-pressure turbine; LP: Low-pressure turbine; CON: Condenser; BFP: Boiler
feed pump; PH: Preheater; OX: Oxidation; RED: Reduction; HR: Heat removal).

References

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Kehlhofer, R., Hannemann, F., Rukes, B. & Stirnimann, F. Combined-Cycle Gas
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Acknowledgements

This material is based upon work
supported by the National Science Foundation under Grant No. 1054718. CC
gratefully acknowledges support by the GE Graduate Fellowship for Innovation.
This work was partially sponsored by the United Technologies Corporation
Institute for Advanced Systems Engineering (UTC-IASE) of the University of
Connecticut. Any opinions expressed herein are those of the authors and do not
represent those of the sponsor.