Chemical looping combustion for solid fuels – Development and optimisation of industrial scale flowsheet models for power generation

Energy security and climate change are two significant concerns to the scientific community and policy makers. Given the ever increasing price of oil and the globally rising level of atmospheric carbon-dioxide (CO2), it is necessary to develop a clean and cost effective alternate energy conversion process for fossil fuels such as coal to sustain energy security in the world for many decades ahead. Chemical Looping Combustion (CLC) is considered as a promising novel technology that has inherent properties of capturing CO2 emissions from coal power plants. CLC has been researched forthe past 25 years and considerable progress has been made over the last decade, though further research studies are still required to exploit its full benefits. A CLC system consist of two interconnected fluidised bed reactors, known as air and fuel reactors, between which an oxygen carrier (OC) is circulated to supply oxygen for fuel combustion in the fuel reactor. An OC often is a metal oxide which is regenerated in the air reactor using atmospheric air. This arrangement prevents dilution of flue gas with nitrogen and produces an exhaust stream of mainly steam and CO2 as products from the fuel combustion. The steam can be condensed to obtain a pure stream of CO2 for compression and storage. The coal can be used in two ways in a CLC reactor system by either converting it to syngas (in a separate gasifier) which is then supplied to the fuel reactor, or by supplying coal directly to the fuel reactor. The former method is represented by an integrated gasification combined cycle (IGCC) coupled with chemical looping combustion or IGCC-CLC process whereas the latter is represented by coal-direct chemical looping combustion (CDCLC) process. This thesis aims to explore the potential of CLC for CO2 capture from coal power plants through simulations studies using Aspen plus and IECM tools. The key objectives of the work are (i) to develop and optimise flowsheet models of large-scale IGCC-CLC and CDCLC processes (ii) to compare the performance of five different OCs (cobalt-, copper-, iron-, manganese- and nickel-based) for IGCC-CLC process, (iii) to investigate the benefits of CLC against other capture technologies in terms of net electrical efficiency penalty, CO2 capture efficiency and cost of electricity (COE). The simulation results indicate that IGCC-CLC process can achieve 2.6 - 4.6% higher net electrical efficiency when compared to IGCC with pre-combustion and oxyfuel combustion based CO2 capture processes. Both IGCC-CLC and IGCC with oxyfuel combustion processes can capture 100% CO2, however IGCC with oxyfuel combustion process experiences 9.1% net electrical efficiency penalty against 4.5% for IGCC-CLC. The economic analysis results show that IGCC-CLC process has 7-32% lower COE than IGCC with pre-combustion capture process. Comparison of OCs for IGCC-CLC process leads to the conclusion that OCs with higher enthalpy of oxidation with air (in CLC air reactor) delivers higher net electrical efficiency. It was found that feeding coal directly into the CLC fuel reactor (as in CDCLC process) can significantly reduce the net electrical efficiency penalty, resulting in efficiencies similar to that of a conventional IGCC process without CO2 capture