Manganese ores as oxygen carriers for chemical-looping combustion
Abstract: Chemical looping combustion (CLC) is a fluidized bed system that circulates metal oxide particles in order to transport oxygen from an air reactor to a fuel reactor. In the fuel reactor a fuel reduces the oxygen carrier while producing CO2 and H2O. The oxygen carrier is then transported back to the air reactor and reoxidized. Chemical looping with oxygen uncoupling (CLOU) is similar to CLC, however the oxygen carrier differ as it has the ability to release gaseous oxygen in the fuel reactor. The advantage is that the oxygen released can react with the fuel directly, without the need for an intermediate gasification step. Mainly oxide from Mn, Fe, Ni and Cu has been investigated as oxygen carrier. Of these, manganese oxides could be of interest for CLOU due to the ability to release O2, especially when combined with other metals or metal oxides. Most of the investigated oxygen carriers are produced using various production methods, which could be costly. Another option is to use natural Mn-based minerals, such as manganese ores, which often contains the same elements as used to promote the CLOU effect, e.g. Fe, Si, Ca and Mg. This thesis is a comprehensive study of manganese ore as oxygen carrier for CLC and CLOU. Here, twenty manganese ores have been investigated as potential oxygen carriers. They are evaluated with respect to reactivity with gaseous fuels CO/H2 and CH4 as well as their ability to release gaseous oxygen via oxygen uncoupling. Experiments were conducted with both batch and continuous CLC reactors. From the experiments in this thesis it was found that all manganese ore release oxygen to varying degree, which suggests the presence of combined manganese oxides in the ores, something which was confirmed through particle and phase analysis. By utilizing solid char the extent and rates of oxygen release were determined for several ores at varying temperatures. It was found that most oxygen was released by Ca-rich ores, where up to 1 wt% oxygen could actually be released at higher temperatures. The reactivity with fuel components and the mechanical stability varied depending on the ore utilized. Elemental mapping of individual particles indicate a significant inter-particle heterogeneity, while the intra-particle distribution was more even. Combination of XRD and SEM-EDX confirms a complex mineralogy with the presence of multiple phases which could be involved in CLC and CLOU, including (Mn,Fe)2O3, Mn7SiO12 and CaMnO3. The presence of these phases explains the CLOU effect that has been observed.
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