The increasing rate of emissions of carbon dioxide (CO2) from combustion of fossil fuels is widely accepted as the cause of global warming. The power generation sector alone is responsible for 40% of the total anthropogenic CO2 emissions.1 Therefore, interest exists in controling the CO2 emissions through carbon capture and sequestration technologies. The available methods entail the separation of the CO2 from flue gases (post-combustion), de-carbonization of the fuel (pre-combustion), or use of pure oxygen for combustion (oxy-fuel combustion).2 One of the greatest barriers facing the implementation of power generation technologies with CO2 capture is the high capital cost and high energy penalty associated with gas separation.3 Thus, considerable research and investment are aimed at developing new strategies that enable CO2 separation to be accomplished at low-cost.
Chemical-looping combustion (CLC) has been identified as one of the most promising strategies to reduce CO2 emissions from power plants. The principle of CLC is that combustion of a hydrocarbon fuel is carried out in two stages, so that the fuel and air streams are not mixed. Thus, CLC intrinsically eliminates the need for additional CO2 separation equipment and the associated energy penalty. Fixed-bed reactors are one possible option for CLC, because it can operate at high pressures (10-20 bar), which is a requirement for the modern combined cycle-based power plants. To implement CLC in the fixed-bed reactor, the gas flows dynamically alternate between reducing, oxidizing, and purge conditions. This concept takes advantage of the different reaction and heat fronts moving along the axial direction of the bed. After the oxidation cycle, the heat stored in the bed is used to produce a high temperature gas stream, which can be effectively converted into mechanical work in a combined cycle. The gas turbine, however, poses strict constraints to the variability of the purge gas temperature, which is challenging for CLC because of its transient, batch-type cyclic operation.
In this work, an optimal design of the CLC reactor system based on dynamically operated fixed-bed reactors is proposed. An optimization problem is formulated to maximize the efficiency of the power plant. The fluctuations of the exhaust gas stream of the CLC reactor are minimized to satisfy the standard operating conditions of commercial gas turbines. The set of manipulated variables consists of the cycle configuration (i.e., time interval and mass flows), number of reactors, and properties of the CLC oxygen carrier. Additionally, the CLC reactor needs to operate at a high conversion of the fuel (<98%), high CO2 capture efficiency (>90%), and low pressure drop (<8%).4–6 The temperature inside the reactor must always be below the melting point of the CLC oxygen carrier. Thus, the design of the CLC reactor system presents a challenging, heavily constrained non-linear mixed integer problem (MINLP). In this work, we show that at the optimal process conditions, the fixed-bed reactor performs significantly better than the conventional configuration, with minimal transient impact on the gas turbines downstream the reactor. Furthermore, we demonstrate the feasibility of the integrated CLC power plant for continuous operation at different loads, despite the inherent batch-type operation of the CLC reactor.
This material is based upon work supported by the National Science Foundation under Grant No. 1054718.
(1) Department of Energy and Environmental Protection Agency, Carbon dioxide emssions from the generation of electric power in the United States; 2000.
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