Rotary (redox) reactor-based oxy combustion chemical looping power cycles for CO₂ capture : analysis and optimization

Abstract

A number of CO₂ capture-enabled power generation technologies have been proposed to address the negative environmental impact of CO₂ emission. An important barrier to adopting these technologies is the associated energy and economic penalties. Chemical-looping (CLC) is an oxycombustion technology that can significantly lower such penalties, utilizing a redox process to eliminate the need for an air separation unit and enable better energy integration. Conventional CLC employs two separate reactors, with metal oxide particles circulating pneumatically in-between, leading to significant irreversibility associated with reactor temperature difference. A rotary reactor, on the other hand, maintains near-thermal equilibrium between the two stages by thermally coupling channels undergoing oxidation and reduction. In this thesis, a multiscale analysis for assessing the integration of the rotary CLC reactor technology in power generation systems is presented. This approach employs a sequence of models that successively increase the resolution of the rotary reactor representation, ranging from interacting thermal reservoirs to higher fidelity quasi-steady state models, in order to assess the efficiency potential and perform a robust optimization of the integrated system. Analytical thermodynamic availability and ideal cycles are used to demonstrate the positive impact of reactor thermal coupling on system efficiency. Next, detailed process flow-sheet models in which the rotary reactor is modeled as a set of interacting equilibrium reactors are used to validate the analytical model results, identify best cycle configurations and perform preliminary parametric analysis for between the reactor and the system while maintaining computational efficiency, an intermediate fidelity model is developed, retaining finite rate surface kinetics and internal heat transfer within the reactor. This model is integrated with a detailed system model and used for optimization, parametric analysis and characterization of the relative techno-economic performance of different oxygen carrier options for thermal plants integrated with the rotary CLC reactor. Results show that thermal coupling in the redox process increases the efficiency by up to 2% points for combined, recuperative and hybrid cycles. The studies also show that the thermal efficiency is a function of the reactor purge steam demand, which depends on the reactivity of the oxygen carrier. While purge steam constitutes a monotonic parasitic loss for the combined cycle, for recuperative and hybrid cycles, it raises the efficiency as long as the steam demand is less than a threshold value. This relationship between reactivity and system efficiency provides a useful selection criteria for the oxygen carrier material. Optimization results based on efficiency and levelized cost of electricity (LCOE) identify nickel-based oxygen carriers as the most suitable for the rotary reactor because its high reactivity ensures low steam demand and reactor cost. Compared to nickel, maximum efficiency and minimum LCOE are respectively 7% lower and 40% higher for a copper-based system; iron-based systems have 4% higher maximum efficiency and 7% higher minimum LCOE. This study also showed that optimal efficiency generally has an inverse profile to that for the optimized LCOE.