Manganese ores as oxygen carriers for chemical-looping combustion

Doctoral thesis, 2017

Today, there is a need for rapid and substantial decrease in emissions of carbon dioxide to the atmosphere, especially from the combustion of fossil fuels. This is needed in order to combat climate change and stabilize the surface temperature increase to below 2°C. This is a challenge, as there is a rapid economic development in many populated areas, combined with a dominating use of fossil fuels for energy conversion. Thus, our dependence on fossil fuels cannot continue if we are to avoid serious climate change. There are several options available, such as increasing the efficiency of power production, energy utilization, and switching to renewable source, such as wind and solar, conversional alternatives with no or small emissions. However, while these are important parts of the solution, these methods alone will most likely not be enough to achieve emission reductions needed for the 2°C target. . Another option, which is the basis of this thesis, is carbon capture and storage (CCS).

In carbon capture and storage (CCS), the carbon dioxide (CO2) is captured from big point emission sources, such as power plants, and then stored in the ground for significant periods of time. This would prevent the emission of greenhouse gases to the atmosphere and still make it possible to use fossil fuel while at the same time switching to a more sustainable way of converting energy.

One method of capturing the CO2 is chemical looping combustion (CLC) which is a combustion process which splits the reaction in two parts, each one taking place in a different chamber. The combustion of fuel takes place in the fuel chamber where an oxygen carrier supplies the oxygen to the fuel, in the absence of air. The oxygen carrier is normally a metal oxide, which takes up oxygen from the air in the other chamber, the so called air reactor. The metal oxide is then transported back to the fuel chamber where the oxygen can take part in the combustion reaction again. The overall heat release over the system is the same as for normal combustion, but the advantage is that CO2 and H2O is obtained in pure form, without the need for any gas separation step. This is one benefit with CLC compared with other methods of CCS, which all requires some sort of gas separation, and such separation always has an energy penalty associated with it. The penalty can be as high as 30% of the power produced, which would drastically increase the cost of electricity or heat

One big difference between other CCS technologies and CLC is of course the use of a metal oxide as an oxygen carrier. This is one of the key aspect of the process, and it is important to find cheap and functional oxygen carrier materials. This thesis will presents a comprehensive work on natural manganese ore as oxygen carriers in CLC. Such natural minerals have the benefit of being readily available at low to moderate cost. Manganese ores are especially interesting, as they often contain active phases which have been found to be very reactive for fuel combustion, and this is the topic of this thesis.