Possibilities and Challenges of Using Combined Manganese Oxides as Oxygen Carriers
One alternative proposed to reduce greenhouse gas emissions is carbon capture and storage, where the carbon dioxide emitted from large point sources is captured, compressed and stored in underground geological formations. Many of the largest point sources of carbon dioxide are power plants and industries fuelled by fossil fuels or biomass. There are several technologies for adapting the combustion process to capture carbon dioxide and chemical-looping combustion is one such option. It has the advantage of keeping the fuel and the combustion air apart, thus avoiding energy consuming and costly separation of carbon dioxide and nitrogen. 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-carrier material needs to meet several 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 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 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.
The work presented in this thesis examines the performance of manganese combined oxides as oxygen carriers in chemical-looping combustion units and in a conventional circulating fluidised bed. The operation has been carried out in two reactor systems with gaseous fuels and in a large-scale biomass boiler, in which the properties of the materials have been evaluated. It has been shown that full conversion of the fuel can be achieved in chemical-looping combustion with calcium manganites as oxygen carrier. Furthermore, combined oxides of iron-manganese-silica and manganese-silica have been examined. High fuel conversion was achieved with both combined oxide systems, but the mechanical stability of these materials was poor. It was found that the mechanical stability of combined oxides of manganese-silica could be improved by adding titania to the material. Interactions between a manganese ore and biomass ash were studied and it was found that ash components accumulated in the particles during operation in the biomass boiler. The reactivity of the ore decreased during operation which could be an effect of deactivation by the ash elements.