Manganese Combined Oxides as Oxygen Carriers for Chemical-Looping Combustion
The global emissions of greenhouse gases are increasing and the development of mitigation measures is becoming more important. One of the alternatives proposed is carbon capture and storage, where the carbon dioxide emitted from large point sources is captured, compressed and stored in underground storage sites. Many of the largest point sources of carbon dioxide are power plants fuelled by fossil fuels. There are several technologies for adapting the combustion process to capture carbon dioxide. Chemical-looping combustion is one such option and has the advantage of keeping the fuel and the combustion air apart, thus avoiding energy consuming carbon dioxide-nitrogen separation. This is achieved by transferring oxygen from the air to the fuel by a cyclic oxidation and reduction of a solid metal oxide. The oxygen-carrying material needs to meet a number of requirements in order to achieve an efficient combustion process.
Manganese oxides have promising properties as oxygen-carrier material and these can be further improved by combining the manganese with for example iron, silica and calcium. Chemical-looping combustion is mainly developed as a technology for fluidised-bed combustion with the oxygen carrier present as the bed material in the form of small particles. To perform well in a circulating fluidised bed the oxygen carrier needs to be mechanically stable as well as have good reactivity with the fuel. During the development of manganese combined oxides, materials with such properties have been identified.
The work presented in this thesis examines the performance of manganese combined oxides as oxygen carriers in interconnected fluidised beds with continuous circulation. The operation has been carried out in two reactor systems with gaseous fuels, in which the properties of the materials have been evaluated. It has been shown that full conversion of the fuel can be achieved during chemical-looping combustion in a 10 kWth reactor unit with a calcium manganite of perovskite structure as oxygen carrier. Furthermore, combined oxides of iron-manganese-silica and manganese-silica have been examined in a 300 Wth reactor unit. High fuel conversion was achieved with both combined oxides systems, but the mechanical stability of these materials was not satisfactory. It was found that the mechanical stability of combined oxides of manganese-silica could be improved by adding titania to the material. Future work would include further investigation regarding the effect of the material composition on the performance.