Oxygen-carrier materials for chemical-looping combustion
For combustion with CO2 capture, chemical-looping combustion (CLC) with inherent separation of CO2 is a promising technology. Two interconnected fluidized beds are used as reactors. In the fuel reactor, a gaseous fuel is oxidized by an oxygen carrier, e.g. metal oxide particles, producing carbon dioxide and water. The reduced oxygen carrier is then transported to the air reactor, where it is oxidized by air back to its original form before it is returned to the fuel reactor.
The feasibility of using both natural iron ore and synthetic oxygen carriers based on oxides of iron, nickel, copper and manganese was determined. Oxygen carrier particles were produced by freeze granulation. They were sintered at 1300°C to 1600°C for 4 to 6 hours and sieved to a size range of 125 - 180 and 180 - 250 m. To be able to study and compare the different types of oxygen carrier particles, a procedure for testing and evaluation was developed. The reactivity was evaluated in both fixed and fluidized bed laboratory reactors, simulating a CLC system by exposing the sample to alternating reducing and oxidizing conditions. In addition, the particles were characterized with respect to crushing strength, agglomeration, tendency for carbon deposition as well as chemical and physical parameters.
The rates of reaction varied and were highly dependent upon the oxygen carrier used. For the natural iron ore it was found that a high yield of CH4 to CO2 was possible although the solid reactivity was relatively low. The reactivity of the freeze granulated particles was considerably higher, with the oxygen carriers based on nickel and copper having the highest reactivity in comparison to Fe and Mn based particles. However all of the investigated samples had a reactivity sufficient for use in a CLC of interconnected fluidized beds. The copper oxide particles agglomerated and may not be suitable as an oxygen carrier.
For the nickel-based particles the formation of carbon was clearly correlated to low conversion of the fuel. For the real application of CLC process, the carbon formation should not be a problem, because the process should be run under conditions of high conversions of the fuel.
Iron oxide with aluminum oxide defluidized and agglomerated only after long reduction periods, in which significant reduction of the magnetite to wustite took place. This is an important observation, because reduction to wustite is not expected in chemical-looping combustion with high fuel conversion.
Carbon Dioxide Capture