(372u) Simulated Moving Bed Chemical-Looping Combustion Reactor for Power Generation in Combined Cycle Power Plants

The impact of anthropogenic
carbon dioxide on the environment necessitates the deployment of Carbon Capture
and Sequestration (CCS) technologies to mitigate CO₂ emissions from
fossil fuel combustion. Amongst those, chemical-looping combustion (CLC) is one
of the most efficient processes for power generation with low energy penalty
and inherent CO₂ separation.¹ In CLC, metal oxides are used as oxygen carrier to avoid the
mixing of fuel and air in the combustor; thus, avoiding additional equipment or
significant energy penalty for the separation of CO₂ from N₂
at the combustor exhaust. CLC with natural gas is of particular interest, as the
latter is cleaner and more efficient than coal, due to its higher H/C ratio,
lower impurity content and higher combustion efficiency. In this presentation,
process intensification options will be explored for the CLC island, wherein
the conventional combustor is replaced by an arrangement of fixed bed reactors
that emulate a simulated moving bed reactor, as shown in Figure 1. This
integrated CLC-CC plant configuration is simulated using models developed
previously for the power plant transient operation and control, exploring CLC reactor
design configurations that enable overall CLC-CC efficiency optimization.^2–5

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).

Simulated
moving bed (SMB) reactors have been shown to increase efficiency and overcome
equilibrium-restricted reactions in absorption, adsorption and extraction
processes, such as reactive chromatography.⁶ To explore the concept of SMB
reactors for chemical-looping combustion applications, we developed a dynamic
model for this reactor configuration and explored options for its design,
sequencing and control. The design of the SMB reactor comprises fixed-bed reactors
operating in a loop arrangement, as shown in Figure 1. The principle of
operation of this arrangement of heterogeneous reactors is based on switching
the inlet and outlet ports simultaneously along the axial dimension, to
simulate the countercurrent movement of solids. Figure 2 shows an example of the
port switching sequence of a three-bed SMB reactor design for one complete CLC
cycle. At cyclic steady state, we capture the SMB operation starting from the
CLC oxygen carrier reduction stage, with the inlet and outlet gas valves turned
on for each reactor, as indicated in Figure 2. Oxidation commences when
reduction is complete in the last reactor of the SMB configuration. Heat is
removed from the reactors through a heat removal stage where air is flowing
through the already oxidized bed. This configuration allows for better bed (oxygen
carrier) utilization and better heat management inside each reactor. This is
illustrated through simulation case studies, using a one-dimensional,
homogeneous fixed-bed reactor model with coupled mass, energy and momentum
balances, in concert with CLC reduction and oxidation kinetics derived
previously.^7–9 The performance of this SMB CLC
reactor integrated with a model of a commercial CC power plant is explored in
terms of thermodynamic efficiency, heat management and carbon capture, similar
to prior work that studied CLC-CC intensification options that were based on
simple fixed bed or reverse flow CLC reactors. ^10,11

Figure 2: Operation
sequence of the simulated moving bed reactor during one complete CLC cycle
time.

Acknowledgements

This material is based upon work
supported by the National Science Foundation under Grant No. 1054718. 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.

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