Combined Iron-Manganese Oxides for Chemical-Looping with Oxygen Uncoupling
Abstract: Abstract The most important factor affecting global warming is the increased concentrations of greenhouse gases in the atmosphere. Carbon dioxide is considered as the most important anthropogenic greenhouse gas. An option which can be employed to reduce the CO2 emissions from combustion is to capture the CO2 and store it in deep geological formations. One innovative technology that can be used for CO2 capture is Chemical-Looping Combustion (CLC). The CLC system is composed of two interconnected fluidized bed reactors. In the fuel reactor the added fuel reacts with an oxygen carrier, usually a metal oxide, to produce CO2 and H2O. The reduced metal oxide is then transported to the air reactor, where it is oxidized back to its original form, and the exit stream from this reactor will contain only nitrogen and some unused oxygen. The advantage of this technology is that carbon dioxide from the combustion is inherently obtained separate from the rest of the flue gases. Chemical-looping with oxygen uncoupling (CLOU) is very similar to CLC, but uses oxygen carriers with the ability to release gas phase oxygen, which can react directly with the fuel, hence avoiding the direct reaction between fuel and oxygen carrier. In this work, CLOU has been studied with gaseous and solid fuels in a small fluidized bed batch reactor, using new Fe-Mn-based oxygen carriers. Particles with different molar ratios of Mn/Fe produced by spray-drying were investigated. They were examined by decomposition in N2 and by reaction with methane and syngas (50/50% CO/H2) at 850˚C, 900˚C and 950˚C. At the higher reaction temperature, 950˚C, the oxygen carriers with a manganese content in the range of 25% to 33%, show both the highest gas conversion of methane as well as the highest concentration of released oxygen. At 850˚C, on the other hand, the best methane conversion and oxygen release was seen for particles with a high manganese content. In fact the oxygen carriers with a manganese content of 67%, 75% and 80% calcined at 950˚C had almost full conversion of methane to CO2 and H2O at 850˚C using an oxygen carrier mass in the batch reactor corresponding to 70 kg/MW. The release pattern of oxygen seen as a function of the Fe/Mn ratio and temperature was explained using the phase diagram of the Fe-Mn-O system. An oxygen carrier with a manganese content of 33% was also tested with solid fuel using inert fluidization gas, N2, at 950˚C. Further, oxygen carriers with a manganese content of 67%, 75% and 80% were investigated at 850˚C. The char originating from the fuel particles effectively removed the oxygen released from the oxygen carrier particles, producing CO2. The tests show that the oxygen carrier with a manganese content of 33% releases oxygen corresponding to approximately 0.5% of its mass and oxygen carriers with a manganese content of 67%, 75% and 80% release oxygen corresponding to approximately 2.7% of their mass. Thus, applying these materials in Chemical-Looping of solid fuels, could contribute both to faster fuel conversion and to higher conversion of gas, as compared to a normal oxygen carrier that does not release oxygen. Due to the low price and favourable environmental properties of manganese and iron oxides, this finding could be of great importance for the development of chemical-looping combustion with oxygen uncoupling.
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