Manganese Materials in Solid Fuel Chemical Looping Combustion

Licentiate thesis, 2016

Against the background of rising anthropogenic greenhouse gas emissions, carbon capture and storage (CCS) has been proposed as a means of mitigating climate change by storing CO2 captured from large point sources in underground geological formations such as aquifers or depleted oiland gas fields. While first generation CCS technologies rely on active gas separation with an associated energy penalty, chemical looping combustion (CLC) can avoid that step by utilizing solid metal oxide particles to transfer oxygen from combustion air to fuel, making CO2 capture inherent to the process. The concept can be realized using two interconnected fluidized bed reactors, the air and the fuel reactor, to ensure that the two reactions incorporating the oxygen carrier, i.e. oxidation with combustion air and reduction with fuel, are unmixed. A key research issue is to find oxygen carriers which perform satisfactorily with respect to fuel conversion and lifetime. Manganese materials are promising candidates from a thermodynamic point of view, potentially cheap and environmentally benign. In this study, the performance of four different oxygen carrier materials was investigated in a 10 kW pilot using both biochar and petroleum coke as fuel. Performance - with respect to fuel conversion - of all oxygen carriers was higher than that of the state-of-the-art material, i.e. ilmenite. CaMn0.9Mg0.1O3-δ, a manufactured perovskite material, showed the best results of all materials but was susceptible to sulphur poisoning, which was proven to be at least in parts reversible. The three other materials, all of them natural manganese ores, showed high gas conversion as well and offer the advantage of low oxygen carrier cost. The lifetime of the most durable manganese ore reached 284 hours, which is deemed sufficient in the context. The results suggest that manganese materials, both manufactured and of natural origin, can be a feasible, efficient and cost-effective alternative as oxygen carrier in chemical looping combustion.