It is well known that carbon dioxide (CO2) is a greenhouse gas that contributes to global warming. Nowadays, a third of the worldwide anthropogenic CO2 emissions arise from fossil fuel fired power production. Meanwhile, fossil fuels continue to be the main source of energy for the foreseeable future. The increasing threat posed by enhanced global warming, as well as the requirement for sustainable energy supplies around the world, have led to the development of several novel technologies to produce clean energy from fuels. Among these new technologies is chemical looping combustion (CLC) that uses a solid metal oxide (oxygen carrier) to react with fuels. This technology has the potential advantage that it produces a pure stream of CO2 that can then be sequestrated. In a CLC process, the oxygen carrier is reduced by fuels in one reactor while being oxidised by air in a separate reactor. As the oxygen carrier circulates through the system, it is subjected to morphological and compositional changes such as sintering, attrition and reactions between various metal oxides and fuels. These changes tend to cause the reactivity of the oxygen carrier to decrease over time. The main objective of this PhD study was to investigate and characterise the morphological and compositional changes of the oxygen carrier particles after they have undergone multiple reduction oxidation cycles in a CLC system. A single fluidised bed system was used in this study. Fuel was fed into a bed of oxygen carrier consisting of mechanically mixed iron oxide or wet impregnated copper oxide supported on alumina. The bed was fluidised by a stream of CO2 and/or steam. Pyrolysis gases from the fuel gasification process reduced the oxygen carrier while forming char in the bed. Thus char was oxidised and the oxygen carrier was regenerated when the fluidising gas was switched to air. Five different types of fuels were initially used in the tests. They were lignite coal, lignite char, activated carbon, US bituminous coal and Taldinski bituminous coal. The rates of gasification of the bituminous coal and activated carbon were much slower than those of the lignite coal and lignite char, resulting in an unfavourably large accumulation of char during the reduction stage. Subsequent experiments were conducted with UK bituminous coal to determine the effect of ash on the oxygen carrier particles over a long operational period. The series of analytical tests included; stereo microscopy, porosimetry analysis, X-ray diffraction (XRD), X-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy with an energy dispersive system (SEM/EDS) and Page ii X-ray photoelectron spectroscopy (XPS). Tests were performed on both the fresh and reacted oxygen carriers. Analytical results showed that when the pyrolysis gases react with the oxygen carrier, mineral matter left behind from the gasification process will deposit on the surface of the particles and diffuse into the core of the particles. This is due to the fact that mineral matter has a higher melting point compared to iron oxide and copper oxide. Iron oxide and copper oxide diffusing to the surface of the particles will replace those that are lost via attrition. As a result, the composition of the surface of these particles remains relatively unchanged. As more mineral matter diffuses into the core of the oxygen carrier particles, they can segregate metal oxide molecules located at the surfaces from those located at the core of the particles. When this occurs, there is a possibility that the segregated material formed will reduce the ability of oxygen to diffuse to the surface of the oxygen carrier particles. Hence this will reduce the conversion of the pyrolysis gases. This will thus lead to the reduction in the conversion of the pyrolysis gas and possibly in the deactivation of the oxygen carrier. It was found that the support structure played a key role in maintaining the structural integrity of the metal oxide particles during repeated reduction and oxidation cycles. Experimental results showed that the rate of attrition initially increases with time indicating that the oxygen carrier structure weakens as it interacts with the mineral matter in ash. Results from this research study have shown that the semi-batch chemical looping combustion of solid fuels is feasible provided reactive fuels that produce a large quantity of pyrolysis gases are used. Less reactive fuels will lead to the accumulation of a large inventory of char in the bed. The slow rate of gasification of char will then result in a lower carbon capture efficiency. In order to operate a semi-batch chemical looping combustion system with solid fuel, temperatures of above 1000°C are most likely required. However, this would exclude metal oxides with low melting points to act as potential oxygen carriers and may cause other problems such as ash fusion. A possible solution is to gasify the solid fuels in a separate reactor and channel the resulting pyrolysis gases into the chemical looping combustion system.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:570165 |
| Date | January 2013 |
| Creators | Sim, Chern Yean |
| Contributors | Sharifi, Vida ; Swithenbank, Jim |
| Publisher | University of Sheffield |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://etheses.whiterose.ac.uk/3787/ |
Page generated in 0.0631 seconds