AbstractsChemistry

Research Doctorate - Doctor of Philosophy (PhD) Oxy-fuel combustion, which refers to the combustion of coal in the presence of oxygen rather than air, is one of the key technology options among the portfolio of carbon capture and storage (CCS) technologies. Oxy-fuel combustion captures carbon dioxide in-situ, producing a CO2-enriched flue gas stream (95 vol% CO2) ready for storage. The main shortcoming of the oxy-fuel combustion, however, is the high cost and energy intensity of its oxygen plant. At present, the main technology option for the oxygen plant is cryogenic air separation which typically consumes about 20% of the total electricity produced in oxy-firing mode and, thus significantly reduces the thermal efficiency of the oxy-fuel power plant. The oxygen plant also accounts for 30% of the total capital investment. In view of the above, a comprehensive program of study on alternative air separation technology - chemical looping air separation (CLAS), which offers a cost effective method for large-scale oxygen production, has been systematically conducted in the current study. The present study aims to identify and characterise the suitable metal oxide oxygen carriers for CLAS applications. The broad objectives of the project have been achieved using a combined theoretical and experimental approach. A comprehensive review of the current state of oxygen carrier developments and utilisation was carried out. Other emerging air separation methods have also been reviewed in detail as part of this literature review. A thermodynamic method of identifying the feasible metal oxide oxygen carriers for high temperature air separation was developed in the present study. The energy cost associated with oxygen production using CLAS and its comparison with an advanced cryogenic air separation unit has been investigated in detail. The reaction mechanisms, underpinning the oxidation and oxygen release processes under conditions pertinent to CLAS, were also determined as one of the main objectives for the current study. The relevant experiments have been carried out using a variety of experimental setups, including thermogravimetric analysis (TGA), packed-bed, and interconnected circulating fluidised beds (ICFB) system. Numerous oxides of metal elements from the periodic table were systematically investigated as potential oxygen carrier candidates for CLAS based on a thermodynamic approach in the current study. The majority of the metal oxides exhibit the capability of releasing oxygen. However, most of them cannot be used in CLAS considering the oxygen equilibrium partial pressure (EPP) lower than 0.015, which may increase the use of sweep gas during the oxygen releasing process and correspondingly the energy consumption. Moreover, the likely carbonation and formation of hydrides on exposure to CO2/steam enriched conditions for metal oxides, such as the Ca and Na-based oxides, limit their application in the preparation of oxygen carriers. Only Mn3O4/Mn2O3, CoO/Co3O4, and Cu2O/CuO are found to be the most suitable oxidation…