Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Hugo I. de Lasa

Abstract

In last decades, significant concerns have been raised regarding the global warming effect. To date, about one-third of the total anthropogenic CO2 emission results from power generation using fossil based fuel and CO2 is the main contributor to global warming. Therefore, technologies for efficient capture of CO2 are becoming of great value. In this respect, Chemical-Looping Combustion (CLC) has received significant attention as a promising technology facilitating concurrent CO2 capture and power generation. This non-conventional technique employs a solid carrier, known as oxygen carrier, to supply oxygen and facilitates combustion process in absence of N2 diluted air. Therefore, the combustion products (CO2 and water) are easily separable without any extra downstream processing cost involved in other available alternatives.

However, the non-availability of suitable oxygen carriers still hinders the commercialization of CLC. This study, thus, deals with the development of a new mixed metallic oxygen carrier, Ni-Co/La-γ-Al2O3. Several characterization techniques are used to evaluate the reactivity and stability of the prepared oxygen carriers under the industrial-scale conditions of a CLC processes. Apart from the beneficial effects of La and Co, the reducibility and the structural properties of the prepared oxygen carriers are found to be influenced significantly by the different preparation methods used.

N2 adsorption isotherms show that γ-Al2O3 retains its structural integrity under some specific preparation conditions. Reducibility as determined by consecutive temperature programmed techniques resembles the chemical properties of δ- and θ-Al2O3 for the other preparation techniques. However, no bulk phase change is detected for all the oxygen carriers studied using XRD. The SEM/EDX and H2 chemisorption analyses show the absence of metal agglomeration and suggest that the prepared oxygen carriers are highly stable under CLC operating conditions. The prepared oxygen carriers are also tested for reactivity, stability and fluidizability in the CREC Riser Simulator using multiple reduction/oxidation cycles with CLC fuel. Results obtained show expected reducibility, oxygen carrying capacity and stability. The solid-state kinetics of the reduction processes are developed using nucleation and nuclei growth model (NNGM) and unreacted shrinking core model (USCM). The NNGM model shows better adequacy over USCM in describing the mechanism of reduction process.

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