Screening of copper based particles supported on bentonite for chemical looping with oxygen uncoupling

The vast majority of the scientific and political communities agree that the release of greenhouse gases from anthropogenic sources is leading to an increase of global average temperature. One of the main gases involved in this effect is carbon dioxide (CO2). International scientific and governmental bodies such as the IPCC agree that CO2 release must be reduced and targets for the reduction of the amounts of gas that are released have been ratified by national governments across the world. One of the largest sources of anthropogenic CO2 is the combustion of fossil fuels for electricity production. Several methods of capturing the CO2 produced by combustion have been developed, with chemical looping combustion (CLC) being suggested by the IPCC as a promising method to achieve the targets in reducing the release of this gas. The combustion of solid fuel in a CLC reactor is not reasonable without modification of the process and chemical looping with oxygen uncoupling (CLOU) is the most promising method of achieving this goal. The CLC process use oxygen carriers to transport oxygen from the air to the fuel in a separate chamber for combustion. CLOU is a form of CLC where the oxygen carrier releases gaseous oxygen. The development of suitable oxygen carrier materials is important in order to progress CLC and CLOU to usable industrial scale technologies and much research has been done into developing them. One of the most widely studied oxygen carrier materials for CLOU is copper oxide supported on magnesium aluminate, however it has been found that this material degrades over many redox cycles, giving the particles a short usable lifetime. This study aims to determine the potential of a different material, with a long usable lifetime, for use in CLOU systems. This material again uses copper oxide, but now supported on bentonite and produced by mechanical mixing and pelletisation. This material is tested in terms of its initial characteristics consisting of crystalline phases determined by XRD, crushing strength and SFEG imaging. The ability of the material to be reduced in an inert environment, and oxidised in air has also been tested in a fluidised bed. The material was subjected to multiple reduction and oxidation cycles and the characteristics will be tested again. The study has endeavoured to determine if the material meets the requirements of oxygen carriers in terms of reducibility and oxidisibility, and whether the material maintains these characteristics over many redox cycles. The results of this study indicate that bentonite acts as a tough support for CuO particles, able to withstand use in a fluidised bed without significant attrition. XRD results show that the particles designated Cu60B40 actually contained the highest amount of CuO of all particles produced, despite having the smallest proportion of CuO added during production. Fluid bed testing shows that the Cu60B40 particles were also able to give off the most oxygen during CLOU testing. These results combine to indicate that sintering occurred in the particles containing the least amount of support, and that the support phase is necessary to maintain oxygen carrier performance, which is in line with previous research. TGA testing with reducing gas showed a much higher degree of conversion than the fluid bed testing in all samples. This indicates that these copper oxide- bentonite particles are better suited to use with gaseous fuels than to solid fuel. This study has followed on from work such as Tian et al(2008) and Arjmand, M. et al (2013) in looking at copper based particles, but takes a more fully rounded approach looking at both reduction in reducing gas, and in inert atmosphere, as well as performance while fluidised. This has shown unexpected pitfalls in oxygen carrier selection. In order to find copper based particles that are better suited to CLOU activity, it is suggested that further work be carried out into the use of cement as a support phase. It is important to retain the full range of tests carried out here to rigorously test these particles.