Selection of oxygen carriers for chemical-looping combustion using methane as a fuel
Licentiate thesis, 2005
In order to meet new demands on clean energy producing technologies, chemical-looping combustion could play an important role. With this technique, CO2 from the flue gases can be separated and collected without excessive energy and efficiency penalty. This is because nitrogen from the combustion air is never mixed with the fuel. Instead, oxygen carriers, in the form of metal oxide particles, circulate between two interconnected fluidized reactors and transfer oxygen from the air to the fuel through solid-gas phase redox reactions.
In this thesis, 90 different oxygen-carriers based on nickel-, manganese- and iron oxides have been investigated. These carriers are prepared with inert material to increase the lifetime and performance of the particles. With the exception of one oxygen carrier, all particles have been produced by a freeze-granulation method. The sintering temperature of the particles was between 950°C to 1600°C in order to optimize the strength and porosity. Normally particles of the size range of 125-180 m have been used for the reactivity tests. These tests were performed in a laboratory fluidized-bed reactor of quartz placed in a furnace holding 950°C. There the particles were exposed to an environment simulating a real chemical-looping combustor, by alternating between reducing (50% CH4 / 50 % H2O) and oxidizing conditions (5% O2 in N2). In addition the particles were characterized with respect to strength, physical appearance and chemical structure before and after the experiments.
With respect to reactivity the different oxygen carriers with methane can generally be ranked in the order nickel> manganese> iron whereas the strength roughly follows the opposite order. The addition of inert is made to examine if the properties of the particles can be improved. It was found that inert material based on alumina and zirconia gives good results whereas titania, silica and magnesia were less successful. Another important parameter that affects the reactivity of the particles is the sintering temperature used as the last step in manufacturing. Using a low sintering temperature is associated with a high reactivity, but also a low strength. This is because the higher temperatures provoke a breakdown of the internal porous structure which also makes them denser. Ten out of the initial 90 particles were not useful for different reasons including melting, lack of structure and lack of reactivity due to formation of non-reducible species.
The majority of the investigated oxygen carriers are well suited for chemical-looping combustion taking into consideration the important criteria of high reactivity (solids inventory in the fuel reactor less than 500 kg/MWCH4), high conversion of the fuel, relatively high strength and ability to withstand de-fluidization, agglomeration, fragmentation and attrition.
Carbon Dioxide Capture