Screening of oxygen-carrier particles based on iron-, manganese-, copper- and nickel oxides for use in chemical-looping technologies

Doktorsavhandling, 2007

Capture and storage of carbon dioxide from combustion will likely be used in the future as a method of reducing emissions of greenhouse gases and thus be part of the overall strategy to stabilize the atmospheric levels of CO2. Chemical-looping combustion is a method of combustion where CO2 is inherently separated from the non-condensable components in the flue gas without the need for an energy intensive air separation unit. 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 heterogeneous gas-solid redox reactions. The technology could also be adapted for the production of hydrogen from fossil fuels with CO2 separation, i.e. chemical-looping reforming. 108 different oxygen-carriers based on iron-, manganese-, copper- and nickel oxides have been investigated. These carriers are prepared with inert material to increase the lifetime and performance of the particles. All particles but one have been produced by a freeze-granulation method. In order to optimize the performance of the particles, the sintering temperature of the particles was varied between 950°C and 1600°C. Normally particles of the size range of 125-180 m have been used for the reactivity investigations. Screening tests were performed in a laboratory fluidized-bed reactor of quartz placed in a furnace. 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). The temperature was varied in the range 600 – 950°C with most experiments conducted at 950°C. In addition the particles were characterized with respect to strength, physical appearance and chemical structure before and after the experiments. Some suitable oxygen carriers were investigated in more detail in the fluidized bed, and parameters such as reaction temperature, particle size, reducing gas and experimental method were varied. With respect to reactivity with methane, the different oxygen carriers can generally be ranked in the order nickel> copper> manganese> iron whereas the crushing strength roughly follows the opposite order. Several types of inert material were used in this work, and this was found to be a very important parameter. It was found that inert material based on alumina and zirconia in general resulted in promising oxygen carriers, whereas titania, silica and magnesia were less promising with respect to reactivity or lifetime of the particles. Using a low sintering temperature in preparation 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. Twelve out of the initial 108 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 reactivity, high conversion of the fuel, relatively high strength and ability to withstand de-fluidization, agglomeration, fragmentation and abrasion.