Influence of heat treatment on manganese ores as oxygen carriers

Artikel i vetenskaplig tidskrift, 2019

Manganese ores have been identified as promising oxygen carriers for chemical-looping processes with solid fuels, including combustion and gasification systems. Such Mn-based minerals are readily available globally and could thus be a cheap alternative to other naturally occurring oxygen carriers, such as ilmenite. These ores are usually highly heterogeneous, with varying phase composition, both on the inter-particle as well as intra-particle level. Further, manganese ores contain Mn in many forms and states, including combined or mixed oxides. The content of Fe, Al, Si, Mg and Ca can be high, but also vary significantly. Thus, it can be expected that there will be transformations of the chemical and physical structures during heating from room temperature to operational conditions. In fact, natural materials are often calcined in air prior to use in CLC systems, to avoid agglomeration and defluidization. In this work the effect of heat treatment is investigated for four different manganese ores that have widely different compositions. The mass losses during heat-up in a batch, fluidized-bed reactor and a thermogravimetric analyzer (TGA) were measured. In addition, the change in reactivity over 21 redox cycles with both methane and a syngas (CO/H2) was investigated for both heat treated and fresh ores. During the batch experiments, the defluidization tendency of the ores was investigated as well. The ores were characterized with X-ray diffraction (XRD) and ICP-SFMS. It was found that there was a release of CO2 and O2 from the ores during heat up in the batch experiment, likely originating from carbonates and thermodynamic unstable manganese oxides respectively. Also, significant mass losses were observed for the untreated ores in the TGA, and in addition to the release of CO2 and O2, a release of H2O was observed at lower temperatures. Reactivity was similar between heat treated and untreated ores, especially when considering the mass loss during heat-up. Still, the reactivity varied significantly among the different ores tested. Defluidization was observed only for one of the ores, when a blend of manganese ore and quartz sand was used as bed material during testing with syngas. However, no defluidization was observed during heat-up. In summary, depending upon the ore used, significant mass losses can be expected, which may need to be accounted for during operation with solid fuels.